Santo Padula, II, Ronald Noebe, and Glen Bigelow

Glenn Research Center, Cleveland, Ohio

Shipeng Qiu and Raj Vaidyanathan

University of Central Florida, Orlando, Florida

Darrell Gaydosh

Ohio Aerospace Institute, Brook Park, Ohio

Anita Garg

University of Toledo, Toledo, Ohio

Effect of Upper-Cycle Temperature on the LoadBiased, Strain-Temperature Response of NiTi

NASA/TM—2011-217408

December 2011
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Santo Padula, II, Ronald Noebe, and Glen Bigelow

Glenn Research Center, Cleveland, Ohio

Shipeng Qiu and Raj Vaidyanathan

University of Central Florida, Orlando, Florida

Darrell Gaydosh

Ohio Aerospace Institute, Brook Park, Ohio

Anita Garg

University of Toledo, Toledo, Ohio

Effect of Upper-Cycle Temperature on the LoadBiased, Strain-Temperature Response of NiTi

NASA/TM—2011-217408

December 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
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7115 Standard Drive

Hanover, MD 21076–1320

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Available electronically at http://www.sti.nasa.gov

Acknowledgments

This work was supported by the Fundamental Aeronautics Program, Supersonics Project, API: Dale Hopkins. The authors 

thank B. Clausen, D. Brown and T. Sisneros at Los Alamos National Laboratory for experimental assistance. This work has 

benefi ted from the use of the Lujan Neutron Scattering Center at LANSCE, which is funded by the Offi ce of Basic Energy 

Sciences (DOE). LANL is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
Abstract

Over the past decade, interest in shape memory alloy based actuators has increased as the 

primary benefits of these solid-state devices have become more apparent. However, much 

is still unknown about the characteristic behavior of these materials when used in actuator 

applications. Recently we have shown [1] that the maximum temperature reached during 

thermal cycling under isobaric conditions could significantly affect the observed 

mechanical response of NiTi (55 wt% Ni), especially the amount of transformation strain 

available for actuation and thus work output. The investigation we report here extends 

that original work to ascertain whether further increases in the upper-cycle temperature 

would produce additional changes in the work output of the material, which has a stressfree austenite finish temperature of 113 oC, and to determine the optimum cyclic 

conditions. Thus, isobaric, thermal-cycle experiments were conducted on the 

aforementioned alloy at various stresses from 50-300 MPa using upper-cycle 

temperatures of 165, 200, 230, 260, 290, 320 and 350 oC. The data indicated that the 

amount of applied stress influenced the transformation strain, as would be expected. 

However, the maximum temperature reached during the thermal excursion also plays an 

equally significant role in determining the transformation strain, with the maximum 

transformation strain observed during thermal cycling to 290 oC. In situ neutron 

diffraction at stress and temperature showed that the differences in transformation strain 

were mostly related to changes in martensite texture when cycling to different uppercycle temperatures. Hence, understanding this effect is important to optimizing the 

operation of SMA-based actuators and could lead to new methods for processing and 

training shape memory alloys for optimal performance.

I. Introduction 

Shape memory alloys (SMAs) are slowly but steadily being viewed as a viable, solidstate replacement for hydraulic and pneumatic actuation systems. However, before 

serious inroads can be made, a better understanding of the macroscopic response of these 

unique materials, especially under constant-stress, thermal cycling conditions is 

necessary. To date, the majority of research has focused on the isothermal, superelastic 

properties of SMAs, as these properties are useful in bio-medical applications. However, 

Santo Padula, II, Ronald Noebe, and Glen Bigelow 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Shipeng Qiu and Raj Vaidyanathan 

University of Central Florida 

Orlando, Florida 32816 

Darrell Gaydosh 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Anita Garg 

University of Toledo 

Toledo, Ohio 43606 

NASA/TM—2011-217408 1

Effect of Upper-Cycle Temperature on the Load-Biased, 

Strain-Temperature Response of NiTi 
transformation temperatures and shape recovery under stress-free conditions, a much 

smaller subset has been investigated to determine transformation strain or work output 

characteristics under bias-stress conditions. Among the materials that have been studied 

for their actuation characteristics, much of the work has focused on the effect of applied 

stress on the observed performance. Various authors have shown that increases in the 

applied stress not only affect the transformation strain [2-7], but also affect the 

transformation temperatures according to the Clausius-Clapeyron relationship [8, 9]. 

However, it has also been reported that the history of the applied stress can affect the 

level of dimensional stability observed [5, 10]. While the magnitude of the applied stress 

is extremely important in determining the actuator-specific characteristics of a given 

material, this work explores the role of upper-cycle temperature (UCT), the highest 

temperature reached during the thermal excursion, on the load-biased thermal-cyclic 

response of an SMA. 

II. Procedures 

A. Material 

The material used in this study was a commercially available, binary NiTi alloy produced 

by Special Metals, New Hartford, New York, with a fully annealed ingot As temperature 

of 95 +/- 5 oC, henceforth designated as 55NiTi. In this case 55 refers to the wt% Ni in 

the alloy. In terms of at%, the stoichiometry of the alloy is Ni49.9Ti50.1, which is optimized 

for high transformation temperature. The material was delivered as 10mm diameter rods 

in the hot-rolled/hot-drawn and hot-straightened condition. DSC measurements on the asreceived material using a heating/cooling rate of 10oC/min indicated stress-free 

transformation temperatures of 47, 75, 89 and 113 oC for the Mf, Ms, As and Af, 

respectively. Cylindrical, uniform gage length tensile specimens having a diameter of 

3.85 mm and a reduced gage length of 25.4 mm were machined from the hot-rolled stock. 

B. Constant-Stress, Thermal-Cycle Testing (Load-biased) 

Subsequent to thermal-cycle testing at constant load (also referred to as load-biased 

testing), two thermal cycles in the absence of any load were performed on the asmachined specimen prior to performing any other operation. The intent of these no-load 

thermal cycles was to relieve any internal stresses resulting from the machining 

operation. Once the no-load cycles were complete, the specimens were tested using a socalled “series” load-biased methodology, as described in the following. 

A “series” load-biased, thermal-cycle test consisted of loading the stress-relieved 

specimen at room temperature, in the martensite state, to a predetermined stress at a strain 

rate of 1x10-4 s

-1 using strain control. Once the desired stress level was achieved, the 

controller was switched into load control and the stress was held constant. The specimen 

was then thermally cycled from room temperature (RT) to the desired upper-cycle 

temperature and back for two complete thermal cycles. After completing the two thermal 

cycles, the controller was switched back to strain control and the same specimen was 

strained until the next higher stress level was achieved. Once the desired stress level was 

achieved, the control-mode was switched back to load control and the thermal cycling 

process was repeated. This process of loading in strain control followed by thermal 

cycling under constant stress was repeated until all desired stress levels were completed 

the isobaric, strain-temperature response of the material is more pertinent when dealing 

with actuation-based applications. Although many SMAs have been studied to ascertain 

NASA/TM—2011-217408 2
using the same specimen (hence the notation “series” load-biased). Stress levels from 50-

300 MPa (in 50 MPa increments) were assessed. To determine the effect of upper-cycle 

temperature on the observed load-biased, strain-temperature response, upper-cycle 

temperatures of 165, 200, 230, 260, 290, 320 and 350 oC were used. In all cases, only the 

second heating and cooling cycle for a given stress was used for analysis. This second 

heating and cooling cycle was chosen because of the nature of the processes involved in 

the response. When a stress is applied to the material or a change in the applied stress is 

made, significant changes in the martensite variant structure results during the first 

thermal cycle. Hence, the first cycle is not representative of subsequent cycles at a given 

stress and is therefore not used for analysis [1]. We refer to the aforementioned 

methodology as “series” to contrast it with a methodology wherein a different starting 

specimen is used for each combination of stress/temperature. Previous work has shown 

that results from the “series” testing methodology are comparable with results obtained 

from individual specimens when testing is limited to a few cycles (less than 10). 

C. In situ Neutron Diffraction 

In situ neutron diffraction measurements were performed in “time-of-flight” mode using 

the Spectrometer for MAterials Research at Temperature and Stress (SMARTS) at Los 

Alamos National Laboratory (LANL). Additional details of the neutron diffraction setup 

can be found in Ref. [11]. Due to the penetration depth of neutrons, any observed 

differences are representative of changes occurring throughout the bulk of the material, 

not merely surface measurements as would be the case if x-ray diffraction from a 

conventional source was utilized. Neutron diffraction measurements were conducted in 

an identical manner to the “series” load-biased experiments described previously with the 

only exception being that, at the extents of the temperature cycle (i.e., whenever the 

upper-cycle temperature was achieved or whenever the specimen reached room 

temperature), the specimens were held for 5 min before taking a measurement to allow 

for the temperature to equilibrate and 30 min were allowed to acquire neutron spectra. 

Both Rietveld and single peak analyses were used, following the methodology 

established previously for analyzing neutron diffraction spectra in NiTi alloys [12, 13], to 

obtain quantitative information on the micromechanical and microstructural evolution, 

e.g., lattice strains, phase volume fractions and texture, in the martensite as well as 

austenite phases that occurred during load-biased, thermal-cycling experiments. 

D. Determination of Transformation-Specific Properties 

The data presented in this paper was taken from the second cycle of the “load-biased”, 

thermal-cycling experiment after each load change, and is in the form of the measured 

axial strain as a function of temperature during that thermal excursion (See Figure 1). 

This strain/temperature data is then used to determine all of the relevant properties for the 

transformation for a given stress/upper-cycle temperature combination. 

NASA/TM—2011-217408 3
Figure 1: Properties relevant to a load-biased, thermal-cycle experiment where Ms, Mf, 

AS and Af are the martensite start, martensite finish, austenite start and austenite finish 

temperatures, respectively. 

To determine transformation temperatures for the alloy, lines were fit through the three 

distinct linear sections of the strain-temperature data associated with the phase 

transformation. The temperatures where these lines intersect were taken to be the 

respective transformation temperatures. Similarly, the transformation strain was defined 

as the difference in strain measured where the lines intersect during the heating portion of 

the cycle. This convention was chosen because this is the most common mode of utilizing 

an SMA for actuation (recovering strain under a bias stress). Finally, the dimensional 

instability or “open-loop” strain was determined by measuring the difference in strain that 

developed from the beginning to the end of the thermal excursion and was taken to be the 

strain difference at room temperature. The aforementioned parameters are schematically 

shown in Figure 1 and were assessed to determine the dependency that each has with 

respect to both the applied stress and the upper-cycle temperature used in the experiment. 

III.Results and Discussion 

Although a working knowledge of the magnitudes of the various transformation-specific 

properties are extremely important to the successful utilization of SMAs, obtaining a 

better understanding of the functional dependency of these properties on the imposed 

conditions (namely, stress and temperature) is equally important. Once a mapping of the 

dependency of the various properties to either the stress and/or temperature has been 

accomplished, regions that provide the best overall material characteristics can be 

identified and used to optimize system performance. The information obtained by 

mapping these stress-temperature-property spaces can also guide work in other areas 

important to SMAs, such as training and processing. Consequently, a large portion of the 

stress-temperature-transformation property space was systematically investigated for 

binary NiTi in the hot-rolled condition. Although the material has not yet achieved a 

stabilized state by the end of the second load-biased thermal cycle, this condition 

incorporates the largest degree of microstructural evolution, which occurs during the first 

cooling portion of the cycle immediately after a change in stress has occurred, and can be 

used as a good gauge for the material’s response to the applied conditions.

NASA/TM—2011-217408 4
A. Load-biased Response and Total Strain Evolution 

Figure 2a shows the development of strain that occurs during various portions of a series, 

load-biased thermal cycling experiment for a UCT of 200 oC. From the figure it can be 

seen that, as the specimen is loaded at room temperature, a small strain develops 

commensurate with the elastic and isothermal de-twinning response of the material 

(Stage 0-1). The presence of de-twinning at these low stresses is confirmed by the 

transient strain response discussed below during heating and is expected given the 

starting hot-worked condition of the alloy. It is also consistent with previous observations 

of de-twinning at very low stresses in NiTi (less than 66 MPa) [14]. At this point, the 

initial fixed stress is maintained and the specimen is thermally cycled between room 

temperature and a fixed upper-cycle temperature, in this case 200 oC. Upon heating, a 

small strain transient is observed as the de-twinned material reverts to the parent austenite 

structure (Stage 1-2). Once the upper-cycle temperature has been reached, the specimen 

is cooled back to room temperature, while still under load. During the forward 

transformation (Stage 2-3), a large strain transient is observed. This strain transient, as 

will be seen later, is in part due to preferred variant selection that occurs when cooling 

from the parent austenite under an applied stress. We note that these transients are very 

important in correlating stability with the internal stress state in shape memory alloys and 

is the subject of an ongoing study. Also important to note is that this same transient 

phenomenon occurs over the first thermal cycle any time the specimen is loaded to a new 

stress level (See Stages 1-3, 6-8, etc.). Since the transient nature of this response makes it 

difficult to determine transformation specific properties like transformation strain, openloop strain and transformation temperatures, the second cycle of the load-biased 

experiments were used to characterize the material. Figure 2b shows only the 2nd cycle 

response for the material shown in Figure 2a, from which all basic shape memory 

properties are extracted using the methodology defined in Figure 1. Using the second 

cycle not only allows for good property determination but also provides the earliest 

condition wherein the greatest amount of evolutionary differences imparted by the 

application of stress and temperature can be observed. 

Figure 2: (a) Strain development in 55NiTi showing all cycles during a series, load-bias 

thermal cycling experiment with a UCT of 200 oC. The red (dashed) and blue (solid) 

portions of the curve indicate the heating and cooling portions of the load-bias cycle 

during the second cycle at various stresses. The black lines represent the initial transient 

response after a stress change. (b) Only data from the second cycle is shown for clarity, 

consistent with the way the data is presented in Figure 3. 

NASA/TM—2011-217408 5
From Figure 3, it is clear that both the stress and the upper-cycle temperature have an 

impact on the amount of strain and the shift in transformation temperatures that occur in 

the system. The underlying reasons for this observation will be discussed in detail in 

subsequent sections; the point that must be made here is that both stress and upper-cycle 

temperature have an effect on the observed responses. As the applied stress is increased, 

the material evolves such that a greater level of strain is achieved. This is apparent by 

looking at any of the individual series plots presented in Figure 3. In fact, the strain that 

results by the end of the load-biased series test can be quite large, on the order of 8% for 

the martensite state when an upper-cycle temperature of 165 oC is used. However, 

performing the identical load-biased series experiment with the UCT being the only 

parameter that is changed can produce dramatic increases in the absolute strains that are 

observed, reaching strains on the order of 20% in the martensite when a UCT of 350 oC is 

used. Thus, overall changes in strain evolution are also dramatically effected by changes 

in the upper-cycle temperature. 

Figure 3: Second cycle data for the series, load-bias experiments conducted using uppercycle temperatures of (a) 165 oC, (b) 230 oC, (c) 290 oC and (d) 350 oC, showing the 

difference in strain evolution that occurs as the upper-cycle temperature is changed. 

It is also important to note that at upper-cycle temperatures above 200 oC, the majority of 

the curves exhibit a segment in the strain-temperature response where the strain change is 

NASA/TM—2011-217408 6
unaffected by temperature increases beyond the apparent Af temperature (except for 

thermal expansion strains). This, plus the observation that the heating and cooling 

portions of the curve are superimposed in this region, indicates that inelastic deformation 

mechanisms such as plasticity, if occurring in the austenite phase, do not clearly manifest 

in these curves. 

B. Transformation Temperatures 

In general, most studies dealing with SMAs in one way or another report on the 

transformation temperatures of the material under investigation. Often times, as 

previously stated, only the “no-load” or “stress-free” temperatures are denoted. In other 

cases, the shift in the transformation temperatures due to the presence of stress has been 

reported [8, 9] and is commonly explained by the Clausius-Clapeyron relationship. 

Figure 4: Effect of applied stress and upper-cycle temperature on the transformation 

temperatures for hot-rolled, binary NiTi. 

Although the application of stress does have an effect on the measured transformation 

temperatures, it is not the only factor that influences these temperatures. As can be seen 

NASA/TM—2011-217408 7
in Figure 4, changing the upper-cycle temperature can also impact the measured 

transformation temperatures. In some cases, this effect can be as important as the effect 

associated with the externally applied stress indicating that the upper-cycle temperature 

must not be neglected when analyzing and comparing results. Even though the ClausiusClapeyron relationship can be readily utilized with reasonable agreement in a number of 

circumstances, the data in Figure 4 indicates that the generalized form of this relationship 

cannot account for all of the factors that govern the true response of the material. As can 

be seen in Figure 4, both the austenite start (As) and martensite start (Mf) show a nonlinear response with stress and upper-cycle temperature (See also Figure 5). It is seen that 

the transformation temperatures are shifted to higher values merely by increasing the 

UCT. This observation can clearly be seen in Figure 5 where the As temperature is shown 

to be shifted by as much as 20 oC or more by increasing the UCT. In general, increasing 

the UCT would not ordinarily have been thought to have a dramatic effect since the 

transformation would have been assumed to have already completed at the austenite 

finish (Af) temperature. This clearly indicates that changes occurring within the material 

as a result of heating the sample under stress to temperatures beyond Af during the 

thermal excursion play a significant role in determining the transformation character of 

the system. This effect is as quantitatively significant on the observed transformation 

temperatures as the effect of stress, a result that is clearly evident from the data presented 

in Figure 5. 

Figure 5: (a) Temperature and (b) Stress slices through the As temperature surface 

showing that the effect of upper-cycle temperature is as important as the effect of stress 

on determining the observed transformation temperature. 

C. Recoverable Transformation Strain and Dimensional Instability (Open-Loop 

Strain) 

When used as a solid-state actuator, SMAs must be able to do work against an externally 

imposed stress. Since the normalized work that a given material is capable of performing 

is directly dependant on the recoverable transformation strain at a given stress, it becomes 

increasingly important to establish the limits of a given material with regard to actuatorspecific properties such as the transformation and open-loop strains. 

NASA/TM—2011-217408 8
Although it is commonly understood that both the transformation strain and open-loop 

strain, which is the component of strain not recovered during the thermal excursion, are 

related to the bias-stress, we are not aware of any literature regarding the dependence of 

these strains on changes in the temperature used during thermal cycling. As was seen in 

the assessment of the transformation temperatures, both the applied stress and the uppercycle temperature had an influence on the measured response. It is therefore expected that 

both the stress and upper-cycle temperature also play a role in governing other actuator 

specific properties like transformation strain and dimensional instability (in this case as 

measured by the open-loop strain). Figure 6 shows the functional nature of these two 

properties for binary NiTi in the hot-rolled condition. As can be seen, the transformation 

and open-loop strains have a strong dependence on both the bias-stress and upper-cycle 

temperature. 

Figure 6: Dependence of the (a) transformation strain and (b) open-loop strain for hotrolled, binary NiTi as a function of applied stress and upper-cycle temperature. Iso-stress 

slices taken through the (c) transformation strain surface and (d) open-loop strain surface 

at different stress levels clearly showing the effect of UCT on the observed quantities. 

NASA/TM—2011-217408 9
The results shown in Figure 6 indicate that a particular level of transformation strain (or 

open-loop strain) can be obtained by operating at more than one point in 

stress/temperature space. Thus, it is important to determine whether these commonalities 

in the response are produced because the underlying microstructures are the same or 

whether differences within the microstructure are independently resulting in similar shape 

memory responses. Such an understanding is important since other properties such as 

fatigue life may be deleteriously impacted even though properties like transformation 

strain and dimensional stability are similar for different stress-temperature combinations. 

D. In situ Neutron Diffraction 

D.1 Martensite Evolution 

In situ neutron diffraction experiments were performed at selected load-biased thermal 

cycling conditions to investigate the effect of stress and upper-cycle temperature on 

underlying microstructural changes that might help explain the shape memory response 

of NiTi. Figure 7 are sections of normalized neutron diffraction spectra recorded at room 

temperature, showing the change in martensite texture with different UCTs and stress. 

The spectra shown here are from diffracting lattice planes parallel to the loading axis and 

were selected to track the texture evolution. Regardless of the external stress applied, 

with increasing UCT, the ratios of the intensities of the 100 martensite reflections to the 

intensities of the 011 martensite reflections increase. This favorable alignment of 100 

martensite variants at the expense of 011 martensite variants relative to the loading 

direction is consistent with a (11-1) type I twinned structure [15]. The figure also shows 

that the magnitude of the applied stress has an impact on the orientation of variants 

formed during cycling under stress. Increases in the applied stress also tended to bias the 

system toward the alignment of 100 variants parallel to the loading direction, as is 

indicated by the larger 100/011 variant ratios. Note that the higher stresses and 

temperatures tend to converge to similar 100/011 ratios. This indicates similar underlying 

preferred martensite variant structures, i.e. the orientations of the martensite variants were 

similar. We note that the discussion presented here is based on representative single peak 

intensities in order to provide qualitative insight into the role of UCT and stress on the 

transformation and open-loop strains. A more complete and detailed analysis of the 

texture evolution that is consistent with the aforementioned observations from the 100 

and 011 martensite reflections is presented in a companion article [16]. 

Figure 8 shows slices through the transformation strain and open-loop strain surfaces 

(taken from Figure 6) superimposed with the ratios of the intensities of the 100 martensite 

reflections to the intensities of the 011 martensite reflections from the neutron diffraction 

experiments. By presenting the data in this manner, a number of observations can be 

made. First, an almost factor of two increase in the transformation strain was observed for 

the 100 MPa condition, merely by increasing the upper-cycle temperature used during 

thermal cycling. This increase resulted because of the texture evolution that occurred 

with increasing upper-cycle temperature, as is indicated by the change in the peak 

intensity ratios. Second, increasing the stress from 100 to 200 MPa also had an impact on 

the texture of the resulting martensite, which in-turn affected the amount of 

transformation strain observed. However, further increases in the applied stress did not 

lead to even greater increases in transformation strain but rather lead to a lowering of the 

NASA/TM—2011-217408 10
strain recovery capability of the material, as exemplified by the 300 MPa condition that 

had similar second cycle 100 to 011 martensite peak intensity ratio but lower recoverable 

strain capability than was observed for other conditions. Neutron diffraction data was not 

available for the 300 MPa/320 oC condition since the sample broke before the data could 

be generated, but it would be assumed that the ratios of the intensity of the 100 martensite 

reflection to the intensity of the 011 martensite reflection would be similar, i.e., at a level 

near 1.32-1.34. 

Figure 7: Section of normalized neutron diffraction spectra recorded at room temperature 

at different applied stresses of (a) 100 MPa, (b) 200 MPa and (c) 300 MPa showing the 

change in martensite texture, represented by ratios of the intensities of the 100 martensite 

reflections to the intensities of the 011 martensite reflections, with increasing upper-cycle 

temperature and stress. The spectra shown here are from diffracting lattice planes parallel 

to the loading axis. 

NASA/TM—2011-217408 11
Figure 8: Correlation between texture evolution within the martensite phase and 

corresponding transformation and open-loop strains. Numbers shown in the graph are 

ratios of the intensities of the 100 martensite reflections to the intensities of the 011 

martensite reflections from diffracting lattice planes parallel to the loading axis. 

This lowering of the transformation capability, in the presence of small to no measurable 

changes in the texture of martensite, is probably related to the corresponding increase in 

the open-loop character of the material. As can be seen in Figures 6 and 8, the open-loop 

character of NiTi at 100 and 200 MPa are very similar, thus the increase in martensite 

texture results directly in an increase in the amount of transformation strain observed. 

Upon increasing the applied stress to 300 MPa, a significant increase in the open-loop 

character was observed. This increase when coupled with the slowing of the texture 

evolution resulted in a lowering of the actuation potential. It is thought that the increase 

in open-loop strain response and subsequent lowering of transformation strain capability 

is related to increases in volume fraction of the retained martensite and small amounts of 

localized plasticity, both of which affect the amount of strain that can be recovered 

during subsequent thermal cycles. 

D.2 Evolution in Austenite 

While the texture evolution that occurs within the martensite plays a major role in the 

observed actuator specific properties, it is equally important to investigate if there is any 

change occurring within the austenite, as the nature of the martensite and austenite phases 

dictate the character of the strain changes that will be observed. As previously shown, the 

increase in strain developed in the martensite was directly linked to the texture evolution 

occurring due to the stress and temperature employed during thermal cycling. Thus, 

increased strain in the martensite is tied to the volume fraction and orientation of the 

variants present. This is analogous to the strain increases that have been predicted to 

occur with changes in martensite volume fraction for the stress-induced case by Oliver et 

al. [17]. The reason for the increases in the absolute strains observed in the austenite with 

thermal cycling (which in this case were on the order of 5-15% as demonstrated in Figure 

3) are not fully clear. While some of it can be explained as arising from retained 

martensite that co-exists with austenite, the role of other inelastic deformation 

mechanisms such as plasticity can also be a contributing factor. 

NASA/TM—2011-217408 12
The evolution of the austenite texture, when austenite co-exists with martensite, has 

previously been investigated by Bourke and Vaidyanathan [18-19]. To determine if 

retained martensite could contribute to the austenite texture observations, neutron 

diffraction information was collected at temperatures above the Af where the system 

would generally be presumed to be “fully” austenitic.

Figure 9 shows 111 and 110 austenite peak reflections from portions of normalized 

neutron diffraction spectra recorded at different UCTs and stress. The changes in the ratio 

of the peak intensities are indicative of evolving texture in the austenite that can arise due 

to its coexistence with retained martensite [18-19]. As this volume fraction of retained 

martensite changes with UCT and stress so does the ratio of the intensity of the 111 

austenite reflection to that of the 110 austenite reflection. Again, we note that the 

discussion presented here is based on representative austenite single peak intensities in 

order to provide qualitative insight into the role of UCT and stress on the transformation 

and open-loop strains. A more complete and detailed analysis of the texture evolution that 

is consistent with these observations from the 111 and 110 austenite reflections is 

presented in a companion article [16]. 

It is also important to note that a significant difference in the amount of peak broadening 

was observed between specimens cycled to 165 oC when compared to those cycled to 

higher temperatures. This is indicated in Figure 9 by the significant narrowing of the 111 

and 110 austenite peaks as the temperature is increased at the same applied stress. Figure 

10 shows the full width half maximum (FWHM) of 111 and 110 austenite peaks plotted 

as a function of UCT for various stresses. While this narrowing is expected and may 

normally be attributed to relaxation of intergranular mismatch stresses between grains 

with increasing temperature, resulting in a more homogenous strain distribution, we 

believe that the disappearance of retained martensite additionally contributes to this 

narrowing with increasing temperature by reducing mismatch stresses between the 

austenite and retained martensite. Other work on this same material [20] will show that 

performing multiple load-biased thermal cycles under a compressive stress of 150 MPa at 

130 oC results in a buildup of retained martensite with cycling, which leads to a 

corresponding increase in the strain broadening of the austenite peaks. 

In support of this idea, the diffraction data presented in Figure 11 for the 300 MPa/165 oC 

condition shows a 100 martensite peak providing direct evidence for the presence of 

retained martensite in the material. For this case, the volume fraction of retained 

martensite was determined to be roughly 3-4%. Thus the increased broadening at higher 

stresses is a result of the heterogeneous strain distribution arising from the higher applied 

external stresses themselves but also has contributions from the mismatch between 

retained martensite and austenite. It would be expected that the peak broadening observed 

for the 100 and 200 MPa conditions at 165 oC would also have contributions from the 

retained martensite albeit at levels less than that observed at 300 MPa, which would be 

below the detection limit of the neutron diffractometer. 

NASA/TM—2011-217408 13
Figure 9: Section of normalized neutron diffraction spectra recorded in the austenite state 

at the respective upper-cycle temperatures for (a-b) 100 MPa, (c-d) 200 MPa and (e-f) 

300 MPa, showing the change in internal strain and texture evolution, represented by 

peak intensity ratios of 111 austenite peak reflections to 110 austenite peak reflections, 

that develops with increasing upper-cycle temperature and stress. The spectra shown here 

are from diffracting lattice planes parallel to the loading axis. 

NASA/TM—2011-217408 14
Figure 10: Changes in the full-width, half-maximum values for the 111 and 100 austenite 

peak reflections showing decreased broadening with increasing temperature and at lower 

UCTs, decreased broadening at lower stresses. 

Figure 11: Neutron diffraction spectra taken in the austenite state at (a) 300 MPa 

showing significant strain broadening at 165 oC. (b) Magnified view of the spectra in (a) 

indicating the presence of retained martensite for the 300 MPa, 165 oC condition. The 

spectra shown here are from diffracting lattice planes parallel to the loading axis. 

Peak broadening can also arise as a result of increases in the heterogeneity of the internal 

strain distribution resulting from plastic deformation. However, if increases in the amount 

of plasticity present in the material were responsible for the broadening, increased 

broadening would be expected with increases in the upper-cycle temperature since higher 

temperatures should decrease the flow stress of the material, thereby causing greater 

plasticity. This is not the case, making retained martensite effects the more likely 

operative mechanism responsible for the observed changes in internal strain, at least for 

NASA/TM—2011-217408 15
the temperature and stresses investigated. In reaching this conclusion we note that 

changes in broadening due to temperature are not significant as noted by careful 

examination of comparable spectra at different temperatures. 

D.3 Overview of the Deformation Response 

The impact of the applied stress and upper-cycle temperature on the overall observed 

behavior will now be illustrated in more detail using specific results from the current set 

of experiments. Starting with the 100 MPa/165oC condition, the transformation strain is 

relatively low because of the relatively weak 100 martensite texture and possible 

presence of retained martensite. However, as the applied stress is increased to 200 MPa at 

165 oC, the texture evolves to a greater extent with a greater fraction of 100 variants 

aligned along the loading direction, thereby producing a greater amount of strain 

recovery. However, the amount of retained martensite can also be expected to increase. 

The results indicate that the change in the martensite texture at the higher stress 

overshadows the reduction in strain recovery that results from the higher levels of 

retained martensite at this stress. The result is a net transformation strain that is almost a 

factor of two larger for the 200 MPa condition when compared to 100 MPa at 165 oC. 

However, upon increasing the stress to 300 MPa, a different result is observed. In this 

case, the combination of a small increase in the strain potential due to the slight change in 

further evolution of the martensite texture with a greater amount of retained martensite, 

and hence lower amount of converted material, led to an overall transformation strain that 

is lower than was observed for the 200 MPa condition, albeit not by much at this 

temperature. 

As the upper-cycle temperature was increased to 230 oC, stronger 100 texture in the 

martensite was observed compared to 165 oC for all stress levels tested (Figure 8). This 

indicates that the higher upper-cycle temperature produces a condition that favors 

preferred 100 variant formation along the loading direction of the sample, thereby 

increasing the strain recovery potential of the material. Although going to higher 

temperature reduced the amount of retained martensite, some texturing of the austenite 

phase was also observed. As mentioned earlier, this texturing of the austenite phase is 

required to accommodate the highly textured, but very small volume fraction of retained 

martensite given the correspondence between austenite and martensite. When all of these 

factors are combined, the net result is a higher transformation strain than was observed at 

165 oC.

 

Similar arguments for the observed increases in transformation strain with increasing 

upper-cycle temperature can be made for temperatures up to 290 oC, where the 

transformation strain is maximized. Beyond this temperature, the levels of transformation 

strain were observed to decrease. As can be seen from the neutron diffraction 

information, the texture of the martensite phase reaches a maximum (for the peaks 

analyzed but also in a complete Rietveld analysis [16]) near the 290 oC condition. 

Beyond that, no additional strain potential would be expected for this material. 

NASA/TM—2011-217408 16
IV. Summary & Conclusions 

The effects of stress and upper cycle temperature on the load-biased, shape memory 

behavior observed in this study can be rationalized by their effects on two opposing 

features. First, transformation strain is increased by factors that promote the development 

of texture in the martensite. In this work, the preferred martensite texture observed 

consisted of 100 variants aligned along the loading direction, which corresponds to a (11-

1) type I twinned structure. In general, higher stresses and higher UCT tend to increase 

this observed martensite texture during the cooling portion of the thermal cycle, up to 

certain limits. In terms of applied stress, there appears to be a critical stress beyond 

which the observed martensite texture at cycle 2 appears to reach a maximum. This was 

about 200 MPa in the current set of experiments. Similarly, texture appears to reach a 

maximum at UCTs above about 230oC, which is roughly 110oC above the nominal Af. 

Second, transformation strain is limited by factors that promote retained martensite 

during the heating cycle, at temperatures above the nominal Af. Although these 

observations were drawn based on the behavior at cycle 2, it is believed that this 

information was sufficient to expose the key features and mechanisms responsible for 

strain evolution and changes in shape memory behavior early on during thermal cycling, 

where the largest changes are observed. It was noted that very small volume fractions of 

retained martensite, on the order of 3 vol. % or less, resulted in an austenite texture 

change and a corresponding decrease in transformation strain. In general, higher 

temperatures and lower stresses tended to produce less retained martensite upon heating 

while stresses above 200 MPa decreased the transformation strain because they result in 

increased retained martensite. Increases in UCT also eventually lead to a maximum in 

transformation strain, although the mechanisms responsible for the reductions in 

transformation strain above the maximum are not yet fully clear. 

Finally, the measured transformation temperatures were observed to vary significantly 

with changes in both applied stress and upper-cycle temperature. Curiously, the slope of 

the temperature vs. applied stress plot (dT/d) and the absolute value of the 

transformation temperature observed varied significantly, depending on the upper-cycle 

temperature used in the experiment. Reasons for the nature of this observation are not 

entirely clear and a detailed investigation is warranted. 

NASA/TM—2011-217408 17
References 

 

1. S.A. Padula II, D.J. Gaydosh, R.D. Noebe, G.S. Bigelow, A. Garg, D. Lagoudas, I. 

Karaman, and K.C. Atli: Behavior and Mechanics of Multifunctional and Composite 

Materials II, SPIE Conf. Proc., 2008, vol. 6929, pp. 692912-1 to 692912-11. 

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3. B. Ye, B.S. Majumdar, and I. Dutta: Acta Mater., 2009, vol. 57, pp. 2403-2417. 

4. J.H. Mabe, R. Ruggeri, and F.T. Calkins: Proceedings of the International Conference 

on Shape Memory and Superelastic Technologies (SMST), 2006, pp. 629-644. 

5. G.S. Bigelow, R.D. Noebe, S.A. Padula, and A. Garg: Proceedings of the International 

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131. 

6. R.D. Noebe, S.L. Draper, D.J. Gaydosh, A. Garg, B. Lerch, N. Penney, G.S. Bigelow, 

and S.A. Padula: Proceedings of the International Conference on Shape Memory and 

Superelastic Technologies (SMST), 2006, pp. 409-426. 

7. L.L. Toia, A. Coda, G. Vergani, L. Fumagalli, and F. Butera: Proceedings of the 

International Conference on Shape Memory and Superelastic Technologies (SMST), 

2006, pp. 499-506. 

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1545-1555. 

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10. A. Stebner, S. Padula II, R. Noebe, B. Lerch, and D. Quinn: J. Intell. Mater. Syst. 

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11. M.A.M. Bourke, D.C. Dunand, and E. Ustundag: Appl. Phys. A, 2002, vol. 74, S1707. 

12. R. Vaidyanathan, M.A.M. Bourke, and D.C. Dunand: Metall. Mater. Trans. A, 2001, 

vol. 32, pp.777-86. 

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13. S. Qiu, V.B. Krishnan, S.A. Padula II, R.D. Noebe, D.W. Brown, B. Clausen, and R. 

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1996, vol. 27, pp. 2820-36.
16. S. Qiu, S.A. Padula II, R.D. Noebe, D.W. Brown, and R. Vaidyanathan: Metall. Mater. 

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submitted) 

NASA/TM—2011-217408 19
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Effect of Upper-Cycle Temperature on the Load-Biased, Strain-Temperature Response of 

NiTi 

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14. ABSTRACT

Over the past decade, interest in shape memory alloy based actuators has increased as the primary benefits of these solid-state devices have 

become more apparent. However, much is still unknown about the characteristic behavior of these materials when used in actuator

applications. Recently we have shown that the maximum temperature reached during thermal cycling under isobaric conditions could 

significantly affect the observed mechanical response of NiTi (55 wt% Ni), especially the amount of transformation strain available for 

actuation and thus work output. The investigation we report here extends that original work to ascertain whether further increases in the 

upper-cycle temperature would produce additional changes in the work output of the material, which has a stress-free austenite finish 

temperature of 113 °C, and to determine the optimum cyclic conditions. Thus, isobaric, thermal-cycle experiments were conducted on the 

aforementioned alloy at various stresses from 50-300 MPa using upper-cycle temperatures of 165, 200, 230, 260, 290, 320 and 350 °C. The 

data indicated that the amount of applied stress influenced the transformation strain, as would be expected. However, the maximum 

temperature reached during the thermal excursion also plays an equally significant role in determining the transformation strain, with the 

maximum transformation strain observed during thermal cycling to 290 °C. In situ neutron diffraction at stress and temperature showed that 

the differences in transformation strain were mostly related to changes in martensite texture when cycling to different upper-cycle 

temperatures. Hence, understanding this effect is important to optimizing the operation of SMA-based actuators and could lead to new 

methods for processing and training shape memory alloys for optimal performance.

15. SUBJECT TERMS

Shape memory alloy; NiTi; Thermomechanical behavior; Actuator; Standards 

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