James L. Smialek, Anita Garg, Richard B. Rogers, and Ronald D. Noebe

Glenn Research Center, Cleveland, Ohio

Oxide Scales Formed on NiTi and

NiPtTi Shape Memory Alloys

NASA/TM—2011-217096

September 2011
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James L. Smialek, Anita Garg, Richard B. Rogers, and Ronald D. Noebe

Glenn Research Center, Cleveland, Ohio

Oxide Scales Formed on NiTi and

NiPtTi Shape Memory Alloys

NASA/TM—2011-217096

September 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors thank R. Mattingly for assistance with XRD data acquisition and analysis.

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-217096 1 

Oxide Scales Formed on NiTi and NiPtTi Shape Memory Alloys 

James L. Smialek, Anita Garg, Richard B. Rogers, and Ronald D. Noebe 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Ni-49Ti and Ni-30Pt-50Ti (at.%) shape memory alloys were oxidized isothermally in air over the 

temperature range of 500 to 900 °C. The microstructure, composition, and phase content of the scales 

were studied by SEM, EDS, XRD, and metallography. Extensive plan view SEM/EDS identified various 

features of intact or spalled scale surfaces. The outer surface of the scale was a relatively pure TiO2 rutile 

structure, typified by a distinct highly striated and faceted crystal morphology. Crystal size increased 

significantly with temperature. Spalled regions exhibited some porosity and less distinct features. More 

detailed information was obtained by correlation of SEM/EDS studies of 700 C/100 hr cross-sections 

with XRD analyses of serial or taper-polishing of plan surfaces. Overall, multiple layers exhibited graded 

mixtures of NiO, TiO2, NiTiO3, Ni(Ti) or Pt(Ni,Ti) metal dispersoids, Ni3Ti or Pt3Ti depletion zones, and 

substrate, in that order. The NiTi alloy contained a 3 at.% Fe impurity that appeared in embedded 

localized Fe-Ti-rich oxides, while the NiPtTi alloy contained a 2 v/o dispersion of TiC that appeared in 

lower layers. The oxidation kinetics of both alloys (in a previous report) indicated parabolic growth and 

an activation energy (250 kJ/mole) near those reported in other Ti and NiTi studies. This is generally 

consistent with TiO2 existing as the primary scale constituent, as described here. 

Introduction 

Shape memory alloys (SMA) have been studied and developed for various medical and activating or 

switching devices that take advantage of a reversible martensitic transformation. While NiTi remains the 

most widely used composition, the phenomenon is characteristic of many B2 and other ordered intermetallic 

systems. One distinctive attribute of the NiTi-based nitinol system is the proximity of the transformation 

range to room temperature. More recently, however, systems with elevated temperature capability have been 

proposed. For example, ternary alloys based on the Ni-Pt-Ti system have demonstrated transformation 

temperatures in the range of 300 to 500 C (Ref. 1). This offers possible applications as turbine engine 

actuator devices in hot environments. Commonly used hot fabrication techniques (rolling, wire drawing) 

provide additional motivations to study oxidation behavior. Alternatively, SMA biomaterial implant devices 

often use pre-oxidized surfaces to facilitate tissue adhesion and bone growth to the TiO2 scale as well as 

limit the amount of toxic Ni that may dissolve in vivo fluids. Furthermore, Pt-modified alloys offer 

improved imaging in atomic number-sensitive CAT scans. Accordingly, the oxidation kinetics have been 

studied for a Ni-30Pt-50Ti SMA over the temperature range of 500 to 900 C in a prior companion study 

(Ref. 2) and compared to those of a commercial Ni-49Ti alloy. Generally, it was found that the kinetics of 

the NiPtTi alloy were lower than those of the binary by about a factor of 4, but with a similar temperature 

dependence or activation energy. The purpose of this paper is to augment these findings with corresponding 

microstructural and phase characterization of the scales using XRD and SEM/EDS, in both plan and 

transverse cross-section samples. 
NASA/TM—2011-217096 2 

Prior Literature 

NiTi alloy oxidation has been the focus of a number of studies, all finding essentially parabolic 

kinetics. The kinetic results are summarized in Table 1 (Ref. 2) for an overall temperature range of 450 to 

1000 C and for durations as short as 4 hr and as long as 100 hr. The results are grouped for those having 

high or low activation energies of 220 to 260 kJ/mole and 130 to 150 kJ/mole, respectively. By 

comparison, the activation energy of pure Ti is close to 220 kJ/mole and that of pure Ni varies over 

110 to 170 kJ/mole, possibly an effect of uncontrolled impurities. While it is tempting to attribute the 

lower range of NiTi data to some effect of NiO scales, this appears not to be the case as discussed below. 

At present, the reason for two groupings of activation energy is unresolved. 

TABLE 1.—COMPARATIVE ARRHENIUS PARAMETERS FOR Ni-X-Ti OXIDATION KINETICS 

In an aggregate picture of the scale make-up, multiple layers are seen to grow with TiO2 rutile as the 

outer and predominant scale layer. This gives way to a mixed layer of TiO2 and NiTiO3. Dispersed Ni(Ti) 

metal particles and porosity are often observed in the scale. Ti-depletion zones overlie the metal substrate 

structure, showing NixTi (x = 2,3,4) on top of a TiNi layer having a depressed Ms temperature due to Ti 

denudation. 

Specifically, Satow, Isano, and Honma, in perhaps the first reported oxidation study of NiTi shape 

memory alloys (1974) (Ref. 3), used a succession of 20 light polishing treatments for layer-by-layer XRD 

analysis of the scale. They found structures of outer TiO2, followed by a graded mixture with NiTiO3, a 

porous Ni(Ti) layer, Ni3Ti, and finally unaffected TiNi. Chan et al. found primarily TiO2 scales, with a 

depletion zone corresponding to “~Ni2Ti” (Ref. 4). Chu et al. (Ref. 5) found TiO2 external scales, with 

metallic Ni(Ti) particles dispersed in an intermediate scale layer, followed by a Ni3Ti depletion zone. 

Firstov et al. (Ref. 6) oxidized NiTi for 30 hr at various temperatures. After 30 min, XRD analysis 

revealed TiO at 300 to 500 C, TiO2 at 600 C, adding some NiO at 700 C and NiTiO3 at 800 C. The 

B19 martensitic room-temperature phase gave way to the B2 parent phase due to selective Ti oxidation. 
NASA/TM—2011-217096 3 

With increasing oxidation temperature, Ni3Ti depletion zones increased while a trace of elemental Ni, 

very near the gas surface, decreased. Nitinol wires were oxidized for 6 hr at 400 to 1000 °C and 

characterized (Ref. 7) as external TiO2 scales, followed by bands of Ni intermixed with TiO2, and 

underlying Ni3Ti, Ni4Ti depletion layers. Similarly, Vojtěch et al. oxidized nitinol wire for 4 hr at 430 to 

650 °C (Ref. 8), resulting in TiO2 scales, while Xu et al. (Ref. 9) oxidized NiTi for 4 hr at 450 to 750 C 

and found primarily TiO2 scale growth, with Ti-denuded internal oxidation and Ni3Ti depletion zone 

layers. Partial spallation occurred after 750 C oxidation, revealing somewhat of an open or porous 

interface and individual oxide crystallites. Tian et al. (Refs. 10 and 11) oxidized a high-temperature 

Ni-30Pd-50Ti at.% shape memory alloy at 900 C for 46 hr and identified a lenticular, oriented TiO2-rich 

outer scale, with an inner porous layer containing NiTiO3 and Ti4Pd2O. Below the scale, a 250 m Ti 

depletion zone in the substrate was found to cause a dramatic rise in hardness. These alloys oxidized 

somewhat slower than binary NiTi and pure Ti. Most recently Lin and Wu oxidized another ternary alloy, 

Ni10Cu50Ti, at 700 to 1000 °C for 6 hr (Ref. 12). They found highly-layered structures of external Cu2O, 

TiO2 + TiNiO3, TiO2+Ni(ss) and (Ni,Cu)3Ti depletion layers. 

The following, then, is an attempt to characterize the various scale phases and layers that form on the 

subject Ni-49Ti and Ni-30Pt-50Ti shape memory alloys that were reported in an earlier study of the 

500 to 900 C oxidation kinetics (Refs. 2 and 13). While XRD was used to identify phases, a wide variety 

of scale microstructural features were detailed by SEM/EDS. Most samples were studied in plan of the 

oxidized surface, but 700 C samples were characterized in more detail by cross-section microstructure 

and serial-section XRD. 

Materials and Experimental Procedure 

A conventional Nitinol shape memory alloy was obtained commercially from Johnson Matthey Inc., 

in the form of 0.125 in. (3.18 mm) rolled plate which was then surface ground to 0.100 in. (2.54 mm) and 

electro-discharge machining (EDM) sectioned into 3/4 by 3/4 in. (1.9 by 1.9 cm) coupons. Chemical 

analysis of the Ni-49Ti alloy (subsequently called Ni49Ti), performed by ICP, yielded Ni-49.3Ti-3.2Fe 

at.%. The Ni-30Pt-50Ti alloy (subsequently called Ni-30Pt-50Ti) was produced in our laboratory by 

vacuum induction melting in a graphite crucible, cast into a 1 in. diameter by 4 in. (2.54 cm diameter by 

10.2 cm) long cylinder, and forged to 0.42 in. (1.07 cm), under a 1093 C preheat. The plate was EDM 

sectioned into 0.100 in. (2.54 mm) thick by 0.6 by 0.6 in. (1.5 by 1.5 cm) coupons. The primary alloy 

phase at room temperature was the B19 ordered orthorhombic martensite, with an unintentional 2.1 vol% 

TiC dispersed particulate phase in the Ni-30Pt-50Ti material and an overall chemistry of 20.10 Ni, 

49.66 Ti, 29.1 Pt, 0.85 C, 0.2 O, 0.06 N, and 0.02 Fe atomic percent. Oxidation coupons were polished to 

a 600 grit finish with SiC emery paper and ultrasonically cleaned in detergent, then ethyl alcohol. 

The samples were oxidized in dried, bottled air, flowing at 100 cm3

/min., at 500, 600, 700, and 

800 C for 100 hr, and at 900 C for 10 hr in a Cahn 1000 thermogravimetric analysis (TGA) 

electrobalance using an alumina tube and resistance furnace. The detailed treatment of the TGA kinetic 

data was discussed in the companion paper (Ref. 2). 

The surface of oxidized coupons were characterized by x-ray diffraction (XRD). Select 700 °C 

samples were also examined by serial-sectioning the binary Ni49Ti alloy or by microfocus XRD by low 

angle (1) taper-section polishing of the ternary Ni-30Pt-50Ti alloy. Here the oxidized surface was 

carefully polished with 600 grit emery paper, finishing with 4000. Periodically the sample weight was 

recorded and the sample was submitted for XRD analyses. The relative amounts of phases were estimated 

from the diffracted intensities of each phase using a commercial software program. 

FEG-SEM/EDS scanning electron microscopy was performed on oxidized surfaces using a conductive 

carbon coating and a typical accelerating voltage of 15 kV. Scale feature chemistry was characterized by 

EDS using a windowless detector. The primary (and secondary satellite) peaks observed in this study follow 

in order of energy level as C K, (Ti L1, Ti L) O K, Ni L1, Ni L, (Pt M2) Pt M, (Pt M), Ti K, (Ti Kß), Fe 

K, (Fe Kß), Ni K, (Ni Kß), and (Pt L1, Pt L) over the energy range of about 0.2 to 10.0 kev. 
NASA/TM—2011-217096 4 

Results 

The as-received microstructure of the Ni-30Pt-50Ti alloy is shown in Figure 1, as a dispersion of TiC 

in a matrix of heavily twinned martensite. A cross-section of the 500 C oxidized sample reveals a thin, 

~0.2 m, Ti-O rich surface scale. The corresponding EDS results, Figure 2, support this chemistry, 

allowing for beam overlap at features smaller than the electron interaction volume, at ~1 m3

. (Here and 

afterwards, the abscissa refers to relative intensity in arbitrary units while the ordinate is the standard 

electron energy in keV). 
NASA/TM—2011-217096 5 

Spalling after cool-down was observed for some samples, shown by surface macrographs and weight 

change (Ref. 2). The relative amount of oxide weight loss on cool-down was compared to the final 

oxygen weight gain before cool-down. The relative amount of spalling was estimated, up to ~70 percent 

for Ni49Ti above 700 C and up to ~55 percent for Ni-30Pt-50Ti above 800 C, correcting for 

stoichiometry and assuming primarily TiO2 scales. Thus, it is recognized that surface characterizations 

discussed below will sometimes correspond to inner scale layers exposed due to spalling. 

Overview of XRD Phase Analysis 

A summary of the primary scale and metal phases identified by XRD of the oxidized surfaces is 

shown in Table 2. Representative patterns demonstrating these results are presented in Figures 3 and 4 for 

Ni49Ti and Ni-30Pt-50Ti after various exposures. The as-received room temperature metal phases were 

the cubic B2 NiTi “austenite” and the orthorhombic B19 Pt(Ni)Ti martensite structures. For both alloys, 

TiO2 rutile was the predominant surface scale. The anatase phase may have been present, but only in very 

small amounts at low temperatures (500 and 600 °C). The NiTiO3 nickel titanate phase appeared on both 

alloys and was more prevalent for the Ni-30Pt-50Ti alloy. NiO was not a major scale phase, but appeared 

with preferred orientation on the Ni-30Pt-50Ti alloy. 

TABLE 2.—XRD PHASE SUMMARY FOR OXIDIZED Ni(Pt)Ni SHAPE MEMORY ALLOYS 

a) Ni49Ti; b) Ni-30Pt-50Ti 

 
NASA/TM—2011-217096 6 

 
NASA/TM—2011-217096 7 

As already mentioned, cooling from oxidation at the higher temperatures produced massive spalling. 

Thus the true outer layer could not always be represented by a surface diffractometer scan. For Ni49Ti it 

was possible to collect some spalled material from the 700 and 800 °C scales for analyses; these 

confirmed essentially the same scale phases as the surface scans (Table 2). The metal phases identified 

after oxidation were B2 NiTi, L12 Ni3Ti, and A1 FCC Ni for the Ni49Ti alloy and B19 Pt(Ni)Ti, Pt3Ti, 

L10 NiPt, FCC Pt(Ni) and a D019 phase for the Ni-30Pt-50Ti alloy. 

Serial Polishing of Ni49Ti, Taper Section of Ni-30Pt-50Ti, 

and SEM Cross-Sections After 700 C Oxidation

Ni49Ti XRD Results 

The cross-sectional microstructure of a duplicate Ni49Ti sample oxidized at 700 °C for 100 hr is 

shown in Figure 5. The total scale and metal depletion zones are on the order of 30 and 17 μm, 

respectively. Multiple layers are clearly delineated. Some representative diffractometer scans highlighting 

the major phases are shown for the 700 °C samples in Figure 6. With no polishing (P0), TiO2, and NiTiO3

are seen as the major surface phases. After slight polishing (P2), the relative amount of NiTiO3 increases. 

Finally, near the substrate layer (P7), metallic Ni and Ni3Ti are revealed. More detailed data showing the 

gradation of phase mass content (estimated) with surface polishing is listed in Table 3 and Figure 7 for the

700 C samples. It should be noted in these analyses that the immediate underlying layers will contribute 

somewhat to the diffracted intensity because of significant penetration depths (many microns) in 

conventional XRD analyses. TiO2 rutile was the major oxide phase throughout the scale layers. Also, a 

small amount of NiTiO3 was mixed with TiO2 in the outer layers, reaching a maximum after about 
NASA/TM—2011-217096 8 

TABLE 3.—SEMI-QUANTITATIVE ESTIMATE OF PHASES IN POLISHED SERIAL SECTIONS 

Ni49Ti, OXIDIZED AT 700 °C, 100 HR 
NASA/TM—2011-217096 9 

2 mg/cm2

 scale removal, disappearing at 6 mg/cm2 removal. A small amount of Fe-rich oxide is 

identified, primarily at this maximum, arising from the Fe impurity in this alloy. Interestingly, a 

considerable amount of metallic Ni is seen to be increasing with depth throughout the scale. This Ni phase 

and the TiO2 scale finally diminish as the metallic Ni3Ti depletion zone is encountered. Most of the scale 

was removed during this sequence, as implied by the 9 mg/cm2

 mass removal approaching the estimated 

amount of 11 mg/cm2

 scale present. However, in this series, the underlying NiTi base alloy was never 

exposed. (Note: The final sequence of layer removal by polishing (dashed lines for P7) was graphed at an 

increased depth for clarity, corresponding to a tapered-edge scan). 

Cross-Sectional SEM/EDS of Ni49Ti 

For reference, the various features highlighted below are matched with corresponding XRD phase 

analyses of the serial sections in Table 4. The 15 kV EDS spectra corresponding to the outer layer in 

Figure 5 (point #1 and spectra #1) is a relatively featureless and pure TiO2, Figure 8. (Note the common 

overlap between the O K and Ti L1, Ti L lines. It is observed that these lines are not necessarily increased 

when there is a strong Ti K line. Therefore it assumed that these lines primarily reflect the contribution of 

O K). Carbon is commonly present in EDS spectra because of the conductive carbon coating on all 

samples, but primarily at 15 kV operating voltages. At 10 μm down in Figure 5, some porosity and ingrown 

Fe-rich oxide platelets or needles are noted (point #2 in Fig. 5), presumably associated with the 3 percent 

Fe impurity of this alloy. The corresponding EDS spectra (point #2 in Fig. 8) reveals strong Fe and O peaks 

and moderate Ti and Ni. The 15 m layer below the needles (#4 in Figs. 5 and 8) is again basically TiO2, but 

with a considerable dispersion of submicron Ni-rich particles (point #3 in Figs. 5 and 8), probably metallic. 

(This oxide spectra in Fig. 8 is distinguished by the low O peak). As with many other instances to follow, 

there is contribution to small particle (1 m) EDS analyses from the surrounding areas. 

 
NASA/TM—2011-217096 10 

TABLE 4.—SERIAL AND TAPER SECTION XRD WITH CORRESPONDING SEM IMAGE AND EDS SPECTRA 
NASA/TM—2011-217096 11 

Porosity is also increasing at inner regions of this layer, especially at the interface with the next lower 

layer. Here, the first connected metal phases appear, over a 4 μm thickness, as finger-like protrusions 

(#5 in Fig. 5), with porosity at the scale interface. The associated spectra in Figure 9 (#5) is similar to that 

of the Ni(Ti) particles (point #3 in Fig. 8), and the composition, corrected from EDS intensities, is near 

that of Ni3Ti, with 8 percent Fe and a substantial amount of oxygen (16 percent). These are connected to 

the dense depletion zone (#6 in Figs. 5 and 9), also presumably Ni3Ti, but with less Fe (1.5 percent) and 

oxygen (6 percent) than the protrusions. Finally, at a level ~10 m beneath this depletion zone, (#7 in 

Figs. 5 and 9), the composition of the substrate appears to be essentially the same as the interior of the 

sample, (#8 in Fig. 9). 

The correspondences between serial polishing XRD phase analysis and specific cross-sectional 

SEM/EDS features are listed in Table 4. In combination, they indicate primarily a duplex TiO2 scale 

throughout, with indistinguishable NiTiO3 in the outer layer, interfacial Fe3O4 needles above the inner 

layer, metallic Ni particles dispersed within the inner layer, followed by Ni3Ti fingers and depletion zone 

before reaching the NiTi substrate. 

Ni-30Pt-50Ti XRD Results 

A cross section of the Ni-30Pt-50Ti sample oxidized at 700 °C for 100 hr is shown in Figure 10. First, 

it appears that a 3-μm outer layer is missing by polishing pullout. The remaining scale can be described as 

about 5 μm of a three-phase zone and about 8 μm of an inner layer dispersed with numerous metal 

particles. This yields a total scale of about 16 μm. It is about half the scale formed on Ni49Ti at the 

corresponding conditions, as expected for the overall kp ratio of Ni-30Pt-50Ti to Ni49Ti of 1/4. 

Diffractometer scans of the layers in plan view were primarily resolved by a taper section, as one early 

polishing step had removed the entire scale at one corner of the coupon, precluding a controlled serial- 
NASA/TM—2011-217096 12 

sectioning study. Instead the microfocus capability of a special diffractometer was used to locate the beam 

within a series of distinct adjacent bands, referred to later as “A” to “E”, each a few mm wide, across the 

surface (not shown), and apply to different layers beneath the surface as revealed by taper polishing. 

Various polishing sequences (0, 1, 2, 3) are also identified by the numerical suffix, for example A0, A1, 

A2, A3, B2, B3, etc. for the original 5 bands, then F4 and F5 for polishing below the scale to the substrate 

as defined in Table 5. Some representative diffractometer scans highlighting the major phases are shown 

in Figure 11 for successive bands A0, D2, and F5, corresponding to increasing depth below the surface. 

Again, TiO2 was the primary outer scale phase, with noticeable NiTiO3 and NiO with preferred 

orientation. Deeper into the depletion zones, strong peaks for Pt3Ti were found with some Pt(Ni). Finally 

the unaffected substrate (F), deep into the sample, showed primarily a D019 ordered orthorhombic phase 

(cf. Ni3Sn and Ti3Al structures) and B19 Pt(Ni)Ti. 

Semi-quantitative XRD mass % results for scale phases of successive zones, through the 700 C scale 

on the NiPtTi alloy, are also listed in Table 5. By inspection, it is apparent that the unpolished and 1st 

polishing scans, A0 and A1, are nearly identical and the values of the five “A” to “E” bands identified in 

the 2nd and 3rd polishing series are fairly similar. The values for similar bands were thus averaged and 

presented in Figure 12(a) for primary scale phases and Figure 12(b) for primary metallic phases, to 

provide a semiquantative overview of the trend in surface phases with depth. TiO2 rutile was the 

predominant outer scale phase, at least through band “C”, with a noticeable concentration of NiTiO3. The 

latter phase was localized in the outer layer and decreased in the inner bands. A small amount of NiO was 

detected in the outermost layers. When presenting the metallic phases, Figure 12(b), it is apparent that 

some metallic FCC Pt(Ni) increases from 10 percent in the outer layer to about 

40 percent at band “C”. In band “D”, Pt3Ti takes over as a primary phase, with Pt(Ni) and Pt(Ni)Ti 

phases. Band “C” can thus be seen as a transition zone to the underlying metallic depletion zones. The last 

band, believed to be unaffected substrate (E3, F4, and F5 in Table 5), consists of B19 Pt(Ni)Ti, hexagonal 

phase, and the TiC dispersion. The hexagonal phase (c = 5.496Å, a = 4.500 Å) appears to belong to space 

group P63/mmc (194) with the Ni3Sn prototype structure (D019). It is isostructural with an Mn12Pt4N 

phase and has a good lattice parameter match, but little relation to this composition. To our knowledge, it 

has not been previously identified in the NiPtTi system. This phase will be referred to subsequently as 

“D019”. 
NASA/TM—2011-217096 13 
NASA/TM—2011-217096 14 

TABLE 5.—SEMI-QUANTITATIVE ESTIMATE OF PHASES IN TAPER SECTION BANDS 

Ni-30PT-50Ti, OXIDIZED AT 700 °C, 100 HR 

Cross-Sectional SEM of Ni-30Pt-50Ti 

For reference, the various other features highlighted below are matched with corresponding XRD 

phase analyses of the taper section in Table 4. The EDS spectra in Figures 13 to 15 correspond to various 

points labeled in the scale cross section of Figure 10. The spectra of Figure 13 indicate a small amount of 

NiO, more NiTiO3, and the remainder being TiO2 for points #1, 2, and 3 in Figure 10, i.e., the 

intermingled light, gray, and dark scale phases, respectively, with an enlarged example shown in the inset. 

Spectra from the next layer, Figure 14, indicate basically a dark TiO2 matrix (point #5), with bright, Ptrich dispersed submicron metal particles and coarser, elongated fingers (point #4). The high carbon peak 

here suggests some contribution from TiC particles, not readily distinguished. The Pt fingers are then 

ultimately connected to a continuous, 3-μm, nearly pure, Pt metal depletion zone (point #6). Spectra for 

the diffusion affected substrate, shown in Figure 15, shows two similar Pt(Ti)-rich gray (point #7) and 

light (point #8) metal regions, with dark Ti(Pt)-C dispersed particles (point #9). Finally, the interior of the 

sample, shown in Figure 16, displays a grain boundary necklace of a white phase with a light gray grain 

interior. These both produce similar Pt(Ti) EDS spectra, Figure 17, and very low Ni content in the white 

phase. The very dark particles reflect the dispersed TiC phase identified in the as-received material, but 

again with only a moderately enhanced carbon peak relative to the other spectra. There is also a dark gray 

Widmanstatten-type morphology that appears to be associated with the bright grain boundary phase. This 

gray acicular phase was not analyzed. 
NASA/TM—2011-217096 15 
NASA/TM—2011-217096 16 
NASA/TM—2011-217096 17 

Plan View of Surface Scales 

Ni49Ti 

Given this overview, we now proceed to the characterization of the various detailed features that 

present themselves at the surface in plan view. An overview of the surface structure for scales formed on 

Ni49Ti by oxidation at 600 to 900 °C is presented in Figure 18, showing the strong effect of oxidation 

temperature. Here angular, often striated and faceted crystals were easily discerned for the 600 to 800 C 

samples, with a grain size clearly increasing with temperature. These are hallmark characteristics of 

outward TiO2 rutile scale growth. For the sake of clarity, these are shown at higher magnification for 

600 C oxidation, Figure 19, along with a relatively clean Ti, O spectra. Unfortunately, at 900 °C, spalling 

was so prevalent that no area of intact scale surface could be located, and the exact scale-gas surface is 

not represented in Figure 18. 

Thus a feature common to the scales formed above 700 °C is the appearance of fracture ledges and 

plateaus due to complex spall patterns. An example of these is shown in Figures 20(a) and (b) for Ni-49Ti 

oxidized at 700 °C for 100 hr. Here three layers and two fracture ledges are revealed, with the top layer 

representing the gas surface. EDS spectra for the top fracture ledge indicates pure TiO2 (Fig. 21(a)). 

Within this ledge, some brighter particles yield a Ni-Ti-O rich chemistry, probably indicating NiTiO3

(Fig. 21(b)). Below this, the sharp, chard-like crystals yield a Ti-Fe-O rich chemistry (with some Ni), 

Figure 21(c), and probably correspond to the Fe-rich, spearhead-like crystals identified in cross-section 

Figure 5. EDS suggests this may be a FeTiO3 illmenite phase, however only Fe3O4 was detected by XRD 

in the serial sectioning study. Finally the lower layer yields a spectrum Figure 21(d) identical to the white 

particles in the ledge, namely a Ni-Ti-rich oxide (with some Fe), presumably NiTiO3 again. 

After 800 °C oxidation, some fracture ledge features, Figure 22, are characterized by EDS as pure 

TiO2 and as a Ti-Fe-Ni-rich oxide, somewhat suggestive of the Fe-rich needles discussed above. Another 

example of this combination is presented in Figure 23, for what appears to be a porous interface of a 

spalled layer. Finally, after 900 °C oxidation, some globular features revealed in fracture plateaus, 

Figure 24, appear to be dispersed metallic Ni particles (X) in a matrix of essentially pure TiO2 (Y), similar 

to the dispersed sublayer particles in the 700 °C cross-section, Figure 5. Another area, Figure 25, 

indicates a dispersion of Ti-Ni-O rich particles (X), consistent with NiTiO3, in a matrix of TiO2 (Y). 
NASA/TM—2011-217096 18 
NASA/TM—2011-217096 19 
NASA/TM—2011-217096 20 
NASA/TM—2011-217096 21 
NASA/TM—2011-217096 22 

Ni-30Pt-50Ti 

An overview for scales formed on Ni-30Pt-50Ti by oxidation at 600 to 900 °C is presented in 

Figure 26. Here colonies or outcroppings of the faceted and striated TiO2 grains are seen as isolated 

islands on a sea of finer grain, smoother material. The progressive increase in crystal size with 

temperature is apparent. (However, no unspalled areas showing these features could be located for the 

800 °C sample). 

The structure developed at 600 °C is shown in more detail in Figure 27, and, while both the large (X) 

and fine grained crystals (Y) are revealed as pure TiO2, a very fine localized dispersion of Pt(Ni)-rich 

spheroids (Z) was identified by the EDS spectra. At 700 °C, the large faceted crystals (X), Figure 28, are 

again TiO2, but some finer grain areas (Y) yield EDS spectra most consistent with NiO, as pointed out in 

Figures 4 and 12. At 800 °C, multiple levels were revealed, Figure 29, and a fractured plateau reveals a 

distinct dispersion of bright Pt-rich particles in a matrix of TiO2, Figure 30. This is similar to the 

dispersion of Pt-rich particles in the 700 °C cross-section surrounded by TiO2. Finally, at 900 °C, 

Figure 31, enormous, highly-striated TiO2 crystals are observed. Here even the fine grain external scale, 

Figure 31(b), appears highly crystallographic and faceted. Lower layers, exposed by spallation, show both 

cleaved, flat grains (Fig. 32(a)) and filamentary or needle-like features, clustered in parallel colonies 

(Fig. 32(b)). Both regions of the lower layers are peppered with a fine dispersion of bright particles, 

presumably Pt-rich metal again. All these regions produced EDS spectra consistent with TiO2 as the 

primary phase, Figure 33. 
NASA/TM—2011-217096 23 
NASA/TM—2011-217096 24 
NASA/TM—2011-217096 25 
NASA/TM—2011-217096 26 

Discussion 

The above confirms that a commercial NiTi and an experimental Ni(Pt)Ti shape memory alloy form 

primarily fast-growing TiO2 rutile scales over a wide temperature range and after relatively long-term 

exposure. Thus the report of activation energies (250 kJ/mole K) near that reported for other Ni-Ti studies 

exhibiting rutile as the primary scale phase is not unexpected. The TiO2 anatase phase may have been 

present as a trace for the NiTi alloy at low temperature. NiTiO3 formed as secondary oxides on both alloys, 

reaching a maximum on the order of 25 percent. This oxide phase was difficult to distinguish 

microstructurally in cross-section and often appeared as small dispersed particles in plan SEM. NiO was 

also somewhat elusive microstructually. As demonstrated by serial and taper-section XRD and by SEM in 

plan and cross-sections, it was present only as a minor phase 20 percent, appeared to predominate at the 

gas surface and only for the NiPtTi alloy. Proceeding inward, dispersed metallic Ni(Ti) and Pt(Ni) solid 

solution particles were observed on the NiTi and NiPtTi alloys, respectively. Finally, metallic depletion 

zones are encountered, first as fingerlike protrusions into the scale, then as a continuous layer of 

predominantly Ni3Ti or Pt3Ti, with some oxygen in solid solution. The NiPtTi substrate also appeared to 

have thermally-aged into a mixture of the B19 Pt(Ni)Ti martensitic phase and a new hexagonal structure of 

similar composition, perhaps a new martensitic variant or grain boundary-decomposition phase. The Fe3O4

and TiC were identified as artifacts from unintentional processing impurities, such as melting in graphite 

crucibles, and are believed to have been relatively innocuous to the primary oxidation process. 

Most of the scale features identified on the NiTi alloy have been observed to various degrees and under 

various conditions in the literature. The serial-sectioning XRD study by Satow et al. (Ref. 2) catalogued the 

transition from a pure rutile external scale, to a mixture with NiTiO3, to a Ni then Ni3Ti depletion zone, and 

finally the NiTi substrate. Others observed the anatase structure sporadically, primarily for lower 

temperature and shorter time exposures. Porosity was more prevalent in some studies, and spalling was 

occasionally observed. Dispersed metallic particles and depletion zones sometimes took the form of various 

NixTiy stoichiometry. A NiPdTi alloy exhibited some reduction in growth rate as did the NiPtTi here. 
NASA/TM—2011-217096 27 

It has been mentioned that the NiPtTi ternary alloy oxidized with parabolic kinetics about four times 

lower than that of the binary NiTi alloy. Since the oxide phases were quite similar on both alloys, the 

difference cannot be explained by modifications in the scale growth and diffusivity. However the 

existence of a continuous Pt3Ti depletion zone may serve to restrict Ti outward diffusion from the 

substrate, as compared to an Ni3Ti zone for the binary alloy. Associated with this feature, the dispersion 

of metallic particles in the scale appear as solvent remnants from the oxidation of Ti “solute” constituent. 

In the case of the NiTi binary, this phase appears as a fine dispersion in the inner layer of a duplex TiO2

scale, trending toward the Ni3Ti metallic fingers and the depeletion zone. For the NiPtTi alloy, a 13-m 

mixed cermet zone of Pt(Ni) + TiO2 inner scale trended toward Pt3Ti fingers and depletion zone. The 

noble inert Pt phase can thus be viewed as a partial diffusion barrier retarding the outward transport of Ti 

toward the oxidative interface (or retarding the inward transport of oxygen). 

One of the more unusual features of these scales is the presence of dispersed, unoxidized Ni particles 

embedded in a TiO2 matrix. These presumably were metallic remnants of the NiTi substrate after the Ti 

was completely depleted by selective oxidation. The fact that they were not oxidized perhaps indicates 

that the local pO2 is below that needed to oxidize Ni. Another possibility is that the growth of TiO2 is very 

rapid in comparison to NiO growth (about 10x on average kp increase (Ref. 2)). Thus, Ti is extracted out 

of the alloy, leaving Ni in a metastable metallic state, only gradually being converted to NiTiO3. The fact 

that relatively little NiTiO3 and metallic Ni are found in the scale suggests that the primary steady state 

growth mechanism is outward transport of Ti, leaving Ni-rich phases behind as depleted layers. However 

the concentration of NiTiO3 in the outer layers suggests an initial transient oxidation product and inward 

new growth by oxygen diffusion. Internal oxidation and breakup of the metallic diffusion zone found in 

more dilute alloys is similar to the finger structure of the depletion zones here and the isolation of 

individual particles in the scale. Since Ti is well known for its affinity for oxygen in solution, it is not 

surprising to find some manifestation of its internal oxidation tendencies here. 

It is also interesting to compare the above features to those observed for Ni-5,10,15Ti alloys (Ref. 14). 

Here more distinct layers were formed, with NiO at the gas surface, followed by an NiTiO3 layer, then 

internal fingers of TiO2. Ti-depletion zones were formed beneath the scale similar to the NiTi SMAs. The 

kinetics were analyzed to be somewhat between those of pure Ni and NiTi SMA alloys (Ref. 2). 

Given the rich, detailed structures described above, perhaps the most obvious structural feature is the 

most striking, that of striated, faceted external TiO2 crystals. These are seen to enlarge quite dramatically 

with oxidation temperature. Such external growth features suggest some component of outward Ti 

diffusion. A summary figure of the average crystal width is given in Figure 34, suggesting a type of 

Arrhenius relation based on a thermally-activated process. Note that the scale grains are approximately 

equal for both Ni49Ti and Ni-30Pt-50Ti alloys, even though the scale thickness growth rates were faster 

for the binary alloy. Also, note that the ternary alloy possessed a more prominent fine-grain base scale 

(dashed curve) that was approximately seven times smaller than the larger grain outcroppings. 

This figure also compares the grain size found for TiO2 scales on other NiTi and Ti alloys. Again, 

Arrhenius-type relations are suggested, but for considerably smaller sizes. This difference is primarily due 

to the long times used here (100 hr) compared to those in the literature (4 hr, Vojtêch et al. and Xu et al. 

(Refs. 8 and 9); 1 to 2 hr, Park (Ref. 15)). Vojtech et al. attempted to determine an activation energy by 

estimating scale growth kinetic constants, albeit from limited time data, assuming a t1/3 kinetic law. They 

determined that to be 45 kJ/mole, or about 1/3 that of the oxidation kinetics, as reflected in the shallow 

slope of their curve in Figure 34. No kinetic grain growth rates were determined in the present study. 
NASA/TM—2011-217096 28 

Summary 

Ni49Ti and Ni-30Pt-50Ti shape memory alloys have been isothermally-oxidized in air at 500 to 

900 C. The scales were characterized by SEM/EDS and XRD in plan, cross-section, and serial or tapered 

sections. TiO2 rutile phases dominated the scale make-up, with distinct striated and faceted external grain 

structure. The grain size was found to be highly temperature dependent and similar for each alloy, but 

more uniform on the binary alloy and duplex on the ternary. The NiPtTi alloy exhibited many features in 

common with the NiTi alloy, but at about 1/2 the overall scale thickness, as expected from the previous 

thermogravimetric kinetic study. NiTiO3 was also found, primarily for NiPtTi and in outer layers of the 

scale, but in amounts less than that expected for complete alloy oxidation. The excess Ni appeared to 

concentrate in dispersed metallic particles or in the Ni3Ti and (Pt,Ni)3Ti depletion zones. It is believed 

that the dispersed Pt(Ni,Ti) particulate and continuous (Pt,Ni)3Ti depletion zone contributes to a reduced 

oxidation rate by limiting outward diffusion of Ti compared to alloys without Pt. In addition, a previously 

unreported hexagonal phase with the Ni3Sn structure (D019) was observed in the Ni-30Pt-50Ti samples 

oxidized (aged) at 700 C. 

References 

1. R. Noebe, D. Gaydosh, S. Padula, A. Garg, T. Biles, and M. Nathal, “Properties and Potential of Two 

(Ni,Pt)Ti Alloys for Use as High-Temperature Actuator Materials,” in Smart Structures and Materials 

2005: Active Materials: Behavior and Mechanics, SPIE Conf. Proc. vol. 5761, (2005), pp. 364–375. 

2. J.L. Smialek, D.L. Humphrey, and R.D. Noebe, “Comparative Oxidation Kinetics of a High 

Temperature NiPtTi Shape Memory Alloy,” Oxidation of Metals, 74, No. 3 (2010), pp. 125–144. 

3. T. Satow, T. Isano, and T. Honma, “The High Temperature Oxidation of Intermetallic Compound 

NiTi,” Journal of the Japan Institute of Metals, 38 (1974) 242–246. 

4. C.M. Chan, S. Trigwell, T.W. Duerig, “Oxidation of an NiTi Alloy,” Surface and Interface Analysis, 

5, pp. 349–354 (1990). 

5. C.L. Chu, S.K. Wu, Y.C. Yen, “Oxidation behavior of equiatomic TiNi alloy in high temperature air 

environment,” Materials Science and Engineering, A 216, pp. 193–200 (1996). 

6. G.S. Firstov, et al., “Surface oxidation of NiTi shape memory alloy,” Biomaterials, 23, pp. 4863–

4871 (2002). 
NASA/TM—2011-217096 29 

7. L. Zhu, J.M. Fino, and A. Pelton, “Oxidation of Nitinol,” in SMST–2003, Proceedings of the 

International Conference on Shape Memory and Superplastic Technologies, eds. T.W. Duerig and 

A. Pelton, SMST–ASM International, Metals Park, OH (2003). 

8. D. Vojtěch, P. Novák, M. Novák, L. Jaska, T. Fabián, J. Maixner, and V. Machovič, “Cyclic and 

isothermal oxidation of nitinol wire at moderate temperatures,” Intermetallics, 16 (2008) 424–431. 

9. C.H. Xu, X.Q. Ma, S.Q. Shi, C.H. Woo, “Oxidation behavior of TiNi shape memory alloy at 450– 

750 C,” Materials Science and Engineering, A 371, pp. 45–50 (2004). 

10. Q. Tian and J. Wu, “Effect of the Rare Earth Element Ce on Oxidation Behavior of TiPdNi Alloys,” 

Materials Science Forum, 394–395, pp. 455–458 (2002). 

11. Q. Tian, J. Chen, Y. Chen, and J. Wu, “Oxidation behavior of TiNi-Pd shape memory alloys,” 

Z. Metallkde., 94, pp. 36–40 (2003). 

12. K-N. Lin and S-K Wu, “Oxidation Behavior of Ti50Ni40Cu10 Shape-Memory Alloy in 700– 

1000 C Air,” Oxidation of Metals, 71 (2009) 187–200. 

13. J.L. Smialek, D.L. Humphrey, and R.D. Noebe, “Oxidation Kinetics of a NiPtTi High Temperature 

Shape Memory Alloy,” NASA/TM—2007-214697, NASA, Washington, DC. 

14. C.L. Zeng, M.C. Li, G.Q. Liu, and W.T. Wu, “Air Oxidation of Ni-Ti Alloys at 650–850 C,” 

Oxidation of Metals, 58 (2002) 171-184. 

15. Y-J. Park, H-J. Song, I. Kim, H-S. Yang, “Surface characteristics and bioactivity of oxide film on 

titanium metal formed by thermal oxidation,” J. Mater. Med., 18 (2007) 565–575. 
NASA/TM—2011-217096 30 

Bibliography 

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Diffusion Mechanism for the Oxidation of Metals. Phil. Mag., vol. A45, pp. 823–833. 

DeCastro, J.A.; Melcher, K.J.; and Noebe, R.D. (2005): System-Level Design of a Shape Memory Alloy 

Actuator for Active Clearance Control in the High-Pressure Turbine. AIAA–2005–3988. 

Gulbransen, E.A.; and Andrew, K.F. (1954): The Kinetics of Oxidation of High Purity Nickel. J. 

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Kofstad, P. (1988): High Temperature Corrosion. Elsevier Applied Science, London, pp. 212 and 293. 

Lindquist, P.G.; and Wayman C.M. (1990): Shape Memory and Transformation Behavior of Martensitic 

Ti-Pd-Ni and Ti-Pt-Ni Alloys. Engineering Aspects of Shape-Memory Alloys, T.W. Duerig et al., 

eds., Butterworth-Heinemann, London, pp. 58–68. 

Noebe, R.D.; Biles, T.; and Padula, S.A. (2006): Chapter 7: NiTi-Based High-Temperature ShapeMemory Alloys: Properties, Prospects, and Potential Applications. Advanced Structural Materials: 

Properties, Design Optimization, and Applications, W.O. Soboyejo, ed., Taylor & Francis Group, 

Boca Raton, FL, pp. 141–180. 

Noebe, R., et al. (2006): Effect of Thermomechanical Processing on the Microstructure, Properties, and 

Work Behavior of a Ti50.5Ni29.5Pt20 High-Temperature Shape Memory Alloy. SMST 2006: 

Proceedings of the International Conference on Shape Memory and Superelastic Technologies, ASM 

International, Metals Park, OH, pp. 409–426. 

Otsuka, K.; and Wayman, C.M., eds. (1998): Shape Memory Materials. Cambridge University Press, 

Cambridge, U.K. 

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Progar, D.J.; and Lewis, B.W.: A Study of Oxidation Kinetics of Nickel Metal in Flowing Air and 

Oxygen-Nitrogen Mixtures. NASA TN–D–2224, 1964. 

Rios, O., et al. (2005): Characterization of Ternary NiTiPt High-Temperature Shape Memory Alloys. 

Smart Structures and Materials 2005: Active Materials: Behavior and Mechanics, SPIE Conference 

Proceedings, vol. 5761, pp. 376–387. 

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vol. 26, pp. 1763–1768. 
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14. ABSTRACT

Ni-49Ti and Ni-30Pt-50Ti (at.%) shape memory alloys were oxidized isothermally in air over the temperature range of 500 to 900 °C. The 

microstructure, composition, and phase content of the scales were studied by SEM, EDS, XRD, and metallography. Extensive plan view 

SEM/EDS identified various features of intact or spalled scale surfaces. The outer surface of the scale was a relatively pure TiO2 rutile 

structure, typified by a distinct highly striated and faceted crystal morphology. Crystal size increased significantly with temperature. Spalled 

regions exhibited some porosity and less distinct features. More detailed information was obtained by correlation of SEM/EDS studies of 

700 °C/100 hr cross-sections with XRD analyses of serial or taper-polishing of plan surfaces. Overall, multiple layers exhibited graded 

mixtures of NiO, TiO2, NiTiO3, Ni(Ti) or Pt(Ni,Ti) metal dispersoids, Ni3Ti or Pt3Ti depletion zones, and substrate, in that order. The NiTi 

alloy contained a 3 at.% Fe impurity that appeared in embedded localized Fe-Ti-rich oxides, while the NiPtTi alloy contained a 2 v/o 

dispersion of TiC that appeared in lower layers. The oxidation kinetics of both alloys (in a previous report) indicated parabolic growth and 

an activation energy (250 kJ/mole) near those reported in other Ti and NiTi studies. This is generally consistent with TiO2 existing as the 

primary scale constituent, as described here. 

15. SUBJECT TERMS

Nitinol alloys; Oxidation; Scale (corrosion) 

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