Othmane Benafan, Ronald D. Noebe, Santo A. Padula II, Bradley A. Lerch, and Glen S. Bigelow

Glenn Research Center, Cleveland, Ohio

Darrell J. Gaydosh

Ohio Aerospace Institute, Brook Park, Ohio 

Anita Garg

University of Toledo, Toledo, Ohio

Ke An

Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Raj Vaidyanathan

University of Central Florida, Orlando, Florida 

Macroscopic and Microstructural Aspects of the 

Transformation Behavior in a Polycrystalline 

NiTi Shape Memory Alloy

NASA/TM—2013-217878

June 2013
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientific and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Officer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of significant 

scientific and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 443–757–5803

• Phone the NASA STI Information Desk at

443–757–5802

• Write to:

 STI Information Desk

 NASA Center for AeroSpace Information

 7115 Standard Drive

 Hanover, MD 21076–1320
Othmane Benafan, Ronald D. Noebe, Santo A. Padula II, Bradley A. Lerch, and Glen S. Bigelow

Glenn Research Center, Cleveland, Ohio

Darrell J. Gaydosh

Ohio Aerospace Institute, Brook Park, Ohio 

Anita Garg

University of Toledo, Toledo, Ohio

Ke An

Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Raj Vaidyanathan

University of Central Florida, Orlando, Florida 

Macroscopic and Microstructural Aspects of the 

Transformation Behavior in a Polycrystalline 

NiTi Shape Memory Alloy

NASA/TM—2013-217878

June 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

Funding from the NASA Fundamental Aeronautics Program, Aeronautical Sciences and Fixed Wing Projects is gratefully 

acknowledged. The authors thank D.E. Nicholson and H.D. Skorpenske for technical support and helpful discussions. This work 

has benefited from the use of the Spallation Neutron Source at Oak Ridge National Laboratory, which is funded by the Division 

of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC05-00OR22725 

with UT-Battelle, LLC. 

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

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

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. 
NASA/TM—2013-217878 1

Macroscopic and Microstructural Aspects of the Transformation 

Behavior in a Polycrystalline NiTi Shape Memory Alloy

Othmane Benafan, Ronald D. Noebe, Santo A. Padula II, Bradley A. Lerch, and Glen S. Bigelow

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Darrell J. Gaydosh

Ohio Aerospace Institute

Brook Park, Ohio 44142

Anita Garg

University of Toledo

Toledo, Ohio 43606

Ke An

Oak Ridge National Laboratory

Oak Ridge, Tennessee 37831

Raj Vaidyanathan

University of Central Florida

Orlando, Florida 32816

Abstract

The mechanical and microstructural behavior of a polycrystalline Ni49.9Ti50.1 (at.%) shape memory 

alloy was investigated as a function of temperature around the transformation regime. The bulk 

macroscopic responses, measured using ex situ tensile deformation and impulse excitation tests, were 

compared to the microstructural evolution captured using in situ neutron diffraction. The onset stress for 

inelastic deformation and dynamic Young’s modulus were found to decrease with temperature, in the 

martensite regime, reaching a significant minimum at approximately 80 °C followed by an increase in 

both properties, attributed to the martensite to austenite transformation. The initial decrease in material 

compliance during heating affected the ease with which martensite reorientation and detwinning could 

occur, ultimately impacting the stress for inelastic deformation prior to the start of the reverse 

transformation.

1.0 Introduction

The integration of shape memory alloys (SMAs) in high work output, solid-state actuators has been a 

topic of major interest in recent years. The functional properties of SMAs have been shown to favorably 

exceed those of conventional actuators leading to new solutions and capabilities for a number of potential 

applications (Refs. 1 and 2). However, the transition from functional properties to functional devices in 

large scale commercial applications has been hindered by concerns about dimensional/thermal stability 

and the long term durability of the SMAs. These concerns stem from the limited knowledge base for these 

unique materials and the governing transformation and deformation processes. Thus, an understanding of 

these key issues will further the maturation and implementation of SMAs in actuator applications.

Among many classes of SMAs, near equiatomic NiTi has been the material of choice for most 

research and development tasks. Near equiatomic NiTi exhibits large actuation strains (exceeding 4%) 

associated with the phase transformation between the higher temperature B2 austenite and the lower 
NASA/TM—2013-217878 2

temperature B19' martensite phases. Recently, the deformation mechanisms in these two phases have 

been studied in detail using in situ neutron diffraction techniques (Refs. 3 to 5), more clearly identifying 

and mapping the various deformation processes and regimes. However, more work is required to fully 

understand the material behavior during the temperature excursion in an actuation cycle.

Thus, the objective of this study was to investigate the influence of temperature on both the deformation 

and transformation behavior of bulk polycrystalline NiTi through the transformation regime. Combined 

macroscopic and atomic-scale experiments were performed for that purpose. Macroscopic experiments 

consisted of (i) uniaxial tensile loading at different temperatures to identify the apparent onset of inelastic 

deformation, and (ii) impulse excitation of vibration tests to determine the temperature dependence of the 

dynamic Young’s modulus. Since NiTi undergoes a diffusionless phase change during the martensitic 

transformation, most conventional characterization tools are of limited utility in determining microstructural 

details over this range. However, through the unique capabilities of in situ neutron diffraction, it is possible 

to determine the effect of stress and temperature on the mechanical response of the alloy while 

simultaneously following changes in the alloy’s internal state and microstructure.

2.0 Material and Experimental Procedures 

The material used was a binary Ni49.9Ti50.1 (at.%) alloy produced by Special Metals, New Hartford, 

New York (now SAES Smart Materials). Various physical and thermomechanical properties of this 

particular alloy are available in the literature (Refs. 4 to 13). Test specimens were machined from the hotrolled/hot-drawn and hot-straightened 10 mm diameter rod (as-received condition). Differential scanning 

calorimetry (DSC) measurements were performed on the as-received material using a standard 

heating/cooling rate of 10 °C/min, and the martensite start (Ms), martensite finish (Mf), austenite start (As) 

and austenite finish (Af) temperatures were found to be 75, 47, 89 and 113 °C ± 2 °C, respectively.

Ex situ thermomechanical tests were performed on an 810 MTS servohydraulic load frame. Prior to 

mechanical loading, the specimens were thermally cycled in the load frame under no-load between room 

temperature and 200 °C at a rate of 20 °C/min while measuring strain. In addition to determining the 

stress-free transformation temperatures, the thermal cycling process was used to relieve any residual 

stresses in the samples due to processing or machining and to ensure a self-accommodated martensite 

starting structure. Adopting the ASTM Standard F2082–06 (Ref. 14), Ms, Mf, As and Af temperatures were 

measured using the tangent intercept method and were found to be 71, 55, 92, and 105 ± 2 °C, 

respectively, consistent with the temperatures determined by DSC. The cylindrical dogbone specimens 

(5 mm gage diameter) were subsequently deformed in uniaxial tension to 18% in strain control at a rate of

1×10-4 sec-1 at temperatures between 30 and 230 °C (Figure 1(a)).

Dynamic Young’s modulus testing (Figure 2) was performed using the impulse excitation of 

vibration method as per ASTM Standard E1876 (Ref. 15). Rods of circular cross section (φ = 4.95 × l = 

50.2 mm) and rectangular bars with two different geometries (4.9×4.9×50 mm and 3×4×50 mm) were 

heated in a furnace in an argon atmosphere at rates of 200 and 765 °C/hr (Figure 2). Tests were conducted 

between room temperature and 200 °C with data sampled every 2 °C.

In situ neutron diffraction experiments were performed on the engineering materials diffractometer 

(VULCAN) at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory (ORNL). A detailed 

description of the instrument can be found elsewhere (Ref. 16 and 17). In short, the incident neutron beam 

impinges on the sample with the loading axis positioned at 45o relative to the beam, with diffraction data 

collected simultaneously by the two scintillation detectors positioned on either side of the incident beam 

path at ±90° at a distance of 2 m from the sample. The in situ experimental setup is shown in Figure 3. An 

incident beam size of 3×4 mm and a chopper speed of 20 Hz at 1 MW nominal source power were used 

for the diffraction measurements. Cylindrical dogbone samples (5 mm gage diameter) were thermally 

cycled between room temperature and 230 °C while maintaining zero stress. Neutron data were collected 

continuously and in intervals while heating and cooling at a rate of 200 °C/hr. Neutron diffraction data 

was analyzed using the event mode data reduction software VDRIVE (Ref. 18). 
NASA/TM—2013-217878 3

Figure 1.—(a) Tensile stress-strain responses of Ni49.9Ti50.1 samples deformed isothermally to 18% strain at 

temperatures between 30 and 200 °C. (b) 0.2% offset yield stress behavior of Ni49.9Ti50.1 determined in tension as a 

function of temperature. 

Figure 2.—Dynamic Young’s modulus for Ni49.9Ti50.1 as a function of temperature for (a) 4.9×4.9×50 mm rectangular 

bar using a heating/cooling rate of 200 °C/hr, (b) 3×4×50 mm rectangular bar using a heating rate of 765 °C/hr, (c) 

φ = 4.95 × l = 50.2 mm rod of circular cross section using a heating rate of 200 °C/hr, (d) 5×4×50 mm rectangular bar 

using a heating rate of 200 °C/hr, and (e) and (f) modulus data above 200 °C. (Different symbols correspond to multiple 

samples that were tested under the same condition). 

0

200

400

600

800

0 4 8 12 16

180 oC

150 oC

160 oC

120 oC

100 oC

80 oC

90 oC

70 oC

60 oC

50 oC

40 oC

30 oC

stress (MPa)

strain (%)

(a)

100

200

300

400

0 40 80 120 160 200

0.2% offset-stress (MPa)

temperature (oC)

(b)

50

60

70

80

90

0 40 80 120 160 200

(a) cooling

T = 200 oC /h .

L =50

h =4

w = 3

heating 

50

60

70

80

90

0 40 80 120 160 200

heating 

(b)

T = 765 oC /h .

L =50

w = 4.9

w

50

60

70

80

90

0 40 80 120 160 200

heating 

(c)

T = 200 oC /h .

L =50.2

d = 4.95

50

60

70

80

90

0 40 80 120 160 200

heating 

(d)

T = 200 oC /h .

L = 50

h = 4

5

50

60

70

80

90

100

0 200 400 600 800 1000

heating 

(f)

T = 200 oC /h .

L =50.2

d = 4.95

50

60

70

80

90

100

0 200 400 600 800 1000

heating 

(e)

T = 200 oC /h .

L = 50

h = 4

5

temperature (oC ) temperature (oC )

temperature (oC )

temperature (oC ) temperature (oC )

temperature (oC )

Young's modulus (GPa)

Young's modulus (GPa)

Young's modulus (GPa)

Young's modulus (GPa) Young's modulus (GPa)

Young's modulus (GPa)

Testing above 200 oC
NASA/TM—2013-217878 4

Figure 3.—VULCAN experimental setup for in situ neutron diffraction measurement. 

1. Incident beam, 2. beam-stop, 3. collimator, 4. Detector shielding box, 5. axialtorsional load frame, 6. induction heater, 7. NiTi sample mounted between water 

cooled grips, parallel induction coils and high temperature extensometer (viewed 

from the upstream of neutron beam).

3.0 Results and Discussion 

The macroscopic stress-strain response of the Ni49.9Ti50.1 alloy is shown in Figure 1(a) for samples 

deformed isothermally to 18% strain over a wide range of temperatures. The 18% strain level was 

sufficient to reveal multiple deformation regimes at each temperature as previously determined for this 

material by Benafan (Refs. 3 and 5) and Stebner (Ref. 4). In the martensite phase, the following 

deformation regimes were observed with increasing stress: elastic deformation plus martensite 

reorientation, transformation twinning processes, deformation twinning and slip (Refs. 3 and 5). Above 

As, deformation consisted of elastic, stress-induced martensite, and plastic deformation of austenite by 

dislocation mediated processes and twinning (Ref. 3). Unlike the elastic response of the austenite phase,

which is linear up to about 350 MPa, the apparent martensitic elastic region showed non-linear behavior 

due to the simultaneous operation, at very low stresses, of martensite reorientation/detwinning 

mechanisms (Ref. 7). As a result, the macroscopic measurements from extensometry furnished deflated 

values for Young’s moduli, as discussed later in this paper. With increasing strain, the martensite tensile 

curves exhibited a stress plateau or region of low hardening rate that was dependent on temperature. This 

behavior was discernible in samples tested between room temperature and 90 °C where the material is 

martensitic (T < As), and gradually transitioned into a more conventional hardening behavior at 

temperatures above Af.

3

4

2

1 5

6

7
NASA/TM—2013-217878 5

The onset of the macroscopic non-linear deformation was measured using a 0.2% strain offset 

criterion and the results as a function of temperature are summarized in Figure 1(b). Additional tensile 

tests (not shown in Figure 1(a)) were performed between room temperature and 200 °C to better define 

the statistical fit in Figure 1(b). Although the initial B19' response exhibits nonlinear behavior, a 0.2% 

offset criteria was still used over a region of best linear fit between 0 and 150 MPa. The onset of inelastic 

deformation (generally referred to as ‘yield’) in the martensite phase is dominated by reorientation and 

detwinning mechanisms and was found to decrease with increasing test temperature, reaching an averaged 

minimum value of 140 MPa between 65 and 80 °C (Figure 1(b)). The onset stress then sharply increased 

in the two-phase region and reached near saturation (with a still slightly positive slope) at 350 MPa near 

130 °C. Inelastic deformation over this temperature range (~90 to 130 °C), which includes the B19'→ B2 

phase transition, is attributed to the nearly concurrent operation of stress-induced martensite and plastic 

deformation (Ref. 3).

A similar trend with temperature was also obtained in the dynamic Young’s modulus data obtained 

from the impulse excitation of vibration tests as illustrated in Figure 2. This data was generated using 

different sample geometries and temperature rates, but always yielded similar results. Consistently, the 

average dynamic modulus of martensite at room temperature was about 70 GPa, but decreased with

increasing temperature with an average minimum value of 60 GPa at ~80 °C. The modulus then increased 

through the two-phase region reaching an average value of 78 GPa at 130 °C. The strong temperature 

dependence in the two-phase region is due to the effective modulus of the constantly changing volume 

fractions of martensite and austenite with increasing temperature. Note that the modulus in the austenite 

phase (> 130 °C) did not saturate at a unique value, consistent with the yield behavior shown in Figure 

1(b), but continued to gradually increase with temperature. These results suggest that martensite is still 

present at some level in the material at temperatures above 130 °C, even though the Af determined by 

DSC was only 113 °C. Also included in Figure 2(a) is the dynamic modulus data measured during 

cooling. In addition to the hysteresis associated with the forward transformation, the minimum modulus 

obtained during cooling was identical to that during heating and averaged 60 GPa.

Apparent elastic moduli were also determined from the initial slopes of the tensile stress-strain curves 

for each phase. Using this technique, the modulus was 50 GPa for the martensite phase at room 

temperature and was 74 GPa for austenite at 165 °C. Both values are lower than the dynamic moduli, 

which were 70 and 83 GPa, respectively. The differences stem from the early onset of inelastic 

deformation as the tensile load was applied during mechanical testing (i.e., reorientation and detwinning 

in the martensite phase and the presence of stress-induced martensite in the austenite phase), resulting in 

deflated modulus values, particularly at lower temperatures (Refs. 7 and 19). However, it is important to 

note that even the modulus values determined by the dynamic technique resulted in deflated values 

compared to the “true” values for this material. The “true” elastic modulus of martensite is higher than 

that of the austenite as was reported in previous studies, which is not revealed by the current macroscopic 

methods. It was reported that the averaged, “true” B19' Young’s modulus of this alloy was 134 GPa 

(Ref. 7), measured using in situ neutron diffraction, 101 GPa from instrumented indentation (Ref. 19), 

and 122.3 GPa determined using ab initio calculations (Ref. 20). The higher martensite modulus was also 

reported for a single crystal, superelastic NiTi determined through repeated unloading experiments in 

compression (Ref. 21). Consequently, the macroscopic moduli for the studied NiTi alloy investigated in 

this study are considered the apparent moduli since inelastic deformation mechanisms, namely martensite 

reorientation/detwinning, can reduce the “true” elastic modulus value.

In situ neutron diffraction was used to track detailed microstructural changes during the 

transformation process. The strain-temperature responses measured ex situ at NASA Glenn Research 

Center and in situ at VULCAN during the neutron diffraction experiments are compared in Figure 4. The 

data correspond to a no-load thermal cycle and are essentially identical, confirming the consistency in 

temperature measurement between the two facilities. From Rietveld refinement of the neutron data, the 

B19' martensite exists in the self-accommodated state with best-fit lattice parameters of a=2.903 Å, 

b=4.64 Å, c=4.12 Å, α=β=90° and γ=97.63° that transforms to B2 austenite with a best-fit lattice 

parameter of a=3.03Å. The output of the refined B19' and B2 neutron spectra under no load at RT and 

450 °C, respectively are shown in Figure 5.
NASA/TM—2013-217878 6

Figure 4.—Macroscopic strain-temperature response from ex situ

and in situ no-load thermal cycle of Ni49.9Ti50.1.

Figure 5.—Rietveld refinement for NiTi with diffracting lattice planes parallel to the loading axis with no applied load.

(a) Monoclinic martensite at 30 °C and (b) cubic austenite at 230 °C. The measured data are indicated by crossmarks and the calculated profile is indicated by the solid line. The tick-marks below the profile pattern indicate the 

reflections. The lower curve is the difference between the measurement and refinement.

Neutron spectra were collected continuously during temperature cycling and for visualization 

purposes the data are displayed as contour plots in Figure 6(a) for the heating cycle and in Figure 6(b) for 

the cooling cycle. These spectra are from lattice planes with their plane normals parallel to the sample 

axis. The dashed lines across the contour plots indicate the start and end of the phase transformation as 

obtained by the tangent intercept method from the macroscopic strain-temperature data in Figure 4, which 

are also similar to the DSC values. The diffraction patterns acquired during heating (Figure 6(a)),

however, clearly showed the austenite phase starting to form at ~75 °C, which is much lower than the 

macroscopically determined As of 92 °C or DSC measured As of 86 °C. This lower As

neutron of ~75 °C 

explains the minima reached between 65 and 80 °C in both the yielding characteristics (Figure 1(b)) and 

in the dynamic modulus data (Figure 2), which occur below the As temperature as determined by the bulk 

methods discussed above. Furthermore, the spectra also revealed that even for no-load cycling, the (1�10)M

martensite reflections, although weak, were retained in the microstructure after the Af and did not 

completely transform even at 230 °C. In contrast, the phase transformation start and finish temperatures 

determined from Figure 6(b) on cooling were nearly identical to the bulk measured Ms and Mf. 

The broader transformation range during heating can be attributed to the elastic (Ref. 7) and thermal 

(Ref. 8) anisotropy that arises from the symmetry of monoclinic martensite. The highly anisotropic and 

low symmetry martensite in polycrystalline form gives rise to internal constraints between variants, which 

facilitate the early transformation of some variants and significantly hinder others. As a result, a broader 

0 40 80 120 160 200

strain (%)

temperature (oC)

0.1%

in situ

ex situ

heating

cooling

1.0 2.0 3.0 

lattice plane spacing (Å)

normalized intensity (arbitrary units) 

(100)

(011)

(111)

(102)

_

(110)

(111)

_

(121)

_

data and Rietveld fit

difference curve

peak positions

(a)

1.0 2.0 3.0 

lattice plane spacing (Å)

normalized intensity (arbitrary units) 

(100)

(110)

(111)

data and 

Rietveld fit

difference curve

peak positions

(200)

(210)

(211)

(b)
NASA/TM—2013-217878 7

Figure 6.—Normalized neutron diffraction contour plots as a function of dspacing and temperature during (a) heating and (b) cooling. Subscripts ‘A’ 

and ‘M’ refer to austenite and martensite, respectively.

transformation range is detected microscopically, especially skewed to higher temperatures, as all 

martensite variants under an imposed internal stress have to transform to a single unique austenite 

structure. In contrast, during cooling the higher symmetry austenite phase is relatively isotropic in nature 

resulting in a well-defined (macroscopically and microscopically) and narrow transformation range. The 

lack of internal constraints in this case allows the austenite structure to transform to multiple variants of 

martensite to form a self-accommodated martensite structure with matching macroscopic and microscopic 

Ms and Mf transformation temperatures. It should be noted that there was no evidence of retained austenite 

during the forward transformation on cooling.

To further support this hypothesis, peak intensities were measured at discrete temperatures. The 

normalized integrated intensities associated with specific martensite (open symbols) and austenite (solid 

symbols) reflections as a function of temperature through the transformation are shown in Figure 7(a) to 

(b). Single peaks were fit to determine these intensities for various representative reflections of each 

phase with plane normals parallel to the sample axis. Just above room temperature and well below the 

macroscopically determined As, the intensities vary with temperature, indicative of the preferential 

transformation of selected martensite variants associated with changes in the material’s compliance (as 

shown in Figure 2). This preferential transformation is indicated by the increase in (100)M and (030)M

orientations at the expense of (002)M and (011)M.

d-spacing (Å)

1 1.5 2 2.5 3

0

1

0

1 intensity (a.u.)

(a)

(b)

temperature (oC)

30heating cooling

230

intensity (a.u.)

30

As

Af

Ms

Mf

(100)A

(110)A

(111)A

(210)A(221)A

(211)A

(011)

M

(100)

M

(110)

M

(111)

M

(110)

M

(111)

M

(102)

M

(030)

M

(202)

M

(320)A

retained B19' 

As

neutron
NASA/TM—2013-217878 8

Figure 7.—Normalized intensities from lattice planes with plane normals parallel to the sample axis as a function of 

temperature during (a) heating and (b) cooling. The shaded vertical bands indicate the start and finish of the 

transformation as determined from the macroscopic strain-temperature measurements. 

The significant intensity evolution of the martensite phase above 75 °C is mainly due to the changes 

in the martensite/austenite volume fractions and not due to additional texturing of the martensite from 

variant reorientation. The internal constraints associated with the anisotropy and symmetry force some 

variants to transform to austenite as early as 75 °C. As the temperature approaches Af, most of the B19' 

intensities decrease, approaching zero between As and Af. Simultaneously, the B2 intensities increase, 

starting around the same temperature range where the slopes for the yield and dynamic modulus data 

change sign. No significant evolution in texture was observed for the austenite phase, but traces of 

retained (1�10)M martensite were still detectable above Af. On cooling, most of the intensity changes took 

place in the transformation band and no significant intensity changes were observed below Mf. These 

discrete intensity measurements further confirm the broader transformation band on heating with 

differences between the macroscopic and microscopic As and Af; and the narrower transformation region 

with similar Ms and Mf obtained from both macro and micro measurements.

The neutron diffraction data explains the minima reached in the yield data (Figure 1(b)) and in the 

dynamic modulus data (Figure 2), since the actual As

neutron for this material takes place at ~75 °C. The As

was overestimated at ~90 °C based on the strain-temperature data and the DSC measurements.

Furthermore, it is clear that martensite is retained in the microstructure at temperatures well above the 

bulk Af. The bulk transformation temperatures were not consistent with those determined 

microstructurally during the reverse transformation on heating but matched the measurements during the 

forward transformation on cooling, for the same reasons presented previously. 

4.0 Summary and Conclusions

The effect of temperature on the transformation behavior of polycrystalline Ni49.9Ti50.1 (at.%) was 

investigated. Based on the combined macroscopic and microstructural data presented, the following 

conclusions were drawn; 

1) The yield or onset stress for inelastic deformation of NiTi was determined in a temperature range 

from 30 to 200 °C. Three major deformation regimes were identified as a function of temperature: i) from 

30 to ~75 °C, where the material is fully martensitic; yield stress decreased gradually with temperature 

and was dominated by reorientation and detwinning of the martensite, ii) from 80 to 130 °C, which 

traverses the martensite-to-austenite transformation region; yield increased sharply with temperature and 

was controlled by the formation of stress-induced martensite (that occurred almost concurrently with 

plastic deformation of the austenite phase), and iii) at temperatures above 130 °C, which is above the 

macroscopic Af temperature (though trace amounts of retained martensite are still present); yield stress 

increased very gradually with increasing temperature to at least 200 °C, presumably due to the continued 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

40 80 120 160 200

011

100

110

111

002

102

030

100

110

111

210

211

221

temperature (oC)

normalized intensity (I/I

o

)

(a)

B19' martensite B2 austenite

heating As A

f

As

neutron 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

40 80 120 160 200

temperature (oC)

normalized intensity (I/I

o

)

cooling

(b)

Ms

M

f
NASA/TM—2013-217878 9

presence of trace amounts of retained martensite and its effect on deformation behavior, though 

significant scatter was observed in the mechanical test data. 

2) The dynamic Young’s modulus determined from the impulse excitation of vibration method was 

found to decrease with temperature reaching a minimum of 60 GPa at ~80 °C. This change in compliance 

was responsible for the corresponding decrease in yield stress over this same temperature range by 

making it easier for martensite reorientation/detwinning to occur. This was followed by an increase in 

modulus with temperature to 80 GPa at temperatures beyond 130 °C, which was a reflection of the 

constantly changing martensite to austenite volume fractions with increasing temperature. 

3) Transformation temperatures during the reverse transformation measured from strain-temperature 

and DSC data were found to differ from the actual onset of transformation as seen in neutron spectra and 

supported by the yield and dynamic modulus data, which suggested an actual austenite start temperature 

of ~75 °C. This was attributed to the anisotropy that arises from the symmetry of monoclinic martensite 

and the associated internal constraints between variants. A small amount of retained martensite was found 

in the microstructure at temperature as high as 230 °C. In contrast, the transformation temperatures during 

the forward transformation determined by neutron diffraction were consistent with those determined by 

DSC and strain-temperature data since the elastic anisotropy of the high symmetry austenite is negligible.

Furthermore, no evidence of retained austenite was found in the current study.

References 

1. S. Hirose, K. Ikuta, Y. Umetani, Adv. Robot. 3 (1989) 3-16.

2. C. Mavroidis, Res. Nondestr. Eval. 14 (2002) 1-32.

3. O. Benafan, Ph.D. dissertation, University of Central Florida, Orlando, 2012.

4. A.P. Stebner, Ph.D. dissertation, Northwestern University, Evanston, 2012.

5. O. Benafan, S.A. Padula II, R.D. Noebe, T.A. Sisneros, R. Vaidyanathan, J. Appl. Phys. 112 (2012) 

093510.

6. S. Manchiraju, D. Gaydosh, O. Benafan, R. Noebe, R. Vaidyanathan, P.M. Anderson, Acta Mater. 

59 (2011) 5238-5249.

7. S. Qiu, B. Clausen, S.A. Padula II, R.D. Noebe, R. Vaidyanathan, Acta Mater. 59 (2011) 5055-5066.

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

Appl. Phys. Lett. 95 (2009) 141906.

9. S. Padula II, S. Qiu, D. Gaydosh, R. Noebe, G. Bigelow, A. Garg, R. Vaidyanathan, Metall. Mater. 

Trans. A 43 (2012) 4610-4621.

10. O. Benafan, Deformation and Transformation Processes in Polycrystalline NiTi And NiTiHf High 

Temperature Shape Memory Alloys, in: vol Ph.D. dissertation, University of Central Florida, 

Orlando, 2012.

11. O. Benafan, R.D. Noebe, S.A. Padula II, D.J. Gaydosh, B.A. Lerch, A. Garg, G.S. Bigelow, K. An, 

R. Vaidyanathan, Scr. Mater. 68 (2013) 571–574.

12. A.P. Stebner, D.W. Brown, L.C. Brinson, Acta Mater. 61 (2013) 1944-1956.

13. O. Benafan, S.A. Padula II, R.D. Noebe, D.W. Brown, B. Clausen, R. Vaidyanathan, Acta Mater. 

http://dx.doi.org/10.1016/j.actamat.2013.02.040 (2013).

14. ASTM Standard F2082–06, “Standard Test Method for Determination of Transformation 

Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery,” 2010, ASTM 

International: West Conshohocken, PA, DOI: 10.1520/F2082-06, www.astm.org.

15. M.R. Daymond, M.A.M. Bourke, R.B.V. Dreele, B. Clausen, T. Lorentzen, J. Appl. Phys. 82 (1997) 

1554-1562.

16. K. An, H. Skorpenske, A. Stoica, D. Ma, X.-L. Wang, E. Cakmak, Metall. Mater. Trans. A 42 

(2011) 95-99.

17. X.-L. Wang, T.M. Holden, A.D. Stoica, K. An, H.D. Skorpenske, G.Q. Rennich, E.B. Iverson, A.B. 

Jones, Mater. Sci. Forum 652 (2012) 105-110.
NASA/TM—2013-217878 10

18. K. An, X.-L. Wang, A.D. Stoica, Vulcan Data Reduction and Interactive Visualization Software 

(VDRIVE), http://www.ornl.gov/~k6e/VDRIVE/ (2010).

19. S. Rajagopalan, A.L. Little, M.A.M. Bourke, R. Vaidyanathan, Appl. Phys. Lett. 86 (2005) 081901.

20. M.F.-X. Wagner, W. Windl, Acta Mater. 56 (2008) 6232-6245.

21. H. Sehitoglu, X.Y. Zhang, Y.I. Chumlyakov, I. Karaman, K. Gall, and , H.J. Maier (Eds.), 

Observations on Stress-Induced Transformations in NiTi alloys, Hong Kong, 2002, p. 103-109.

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-06-2013 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Macroscopic and Microstructural Aspects of the Transformation Behavior in a Polycrystalline 

NiTi Shape Memory Alloy 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Benafan, Othmane; Noebe, Ronald, D.; Padula, Santo, A., II; Lerch, Bradley, A.; Bigelow, 

Glen, S.; Gaydosh, Darrell, J.; Garg, Anita; An, Ke; Vaidyanathan, Raj 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 794072.02.03.03.01 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-18681 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2013-217878 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Categories: 37 and 32 

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

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The mechanical and microstructural behavior of a polycrystalline Ni49.9Ti50.1 (at.%) shape memory alloy was investigated as a function of 

temperature around the transformation regime. The bulk macroscopic responses, measured using ex situ tensile deformation and impulse 

excitation tests, were compared to the microstructural evolution captured using in situ neutron diffraction. The onset stress for inelastic 

deformation and dynamic Young’s modulus were found to decrease with temperature, in the martensite regime, reaching a significant 

minimum at approximately 80 °C followed by an increase in both properties, attributed to the martensite to austenite transformation. The 

initial decrease in material compliance during heating affected the ease with which martensite reorientation and detwinning could occur, 

ultimately impacting the stress for inelastic deformation prior to the start of the reverse transformation. 

15. SUBJECT TERMS

Mechanical engineering; Metals and metallic materials 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

18 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18