Malcolm K. Stanford, Fransua Thomas, and Christopher DellaCorte

Glenn Research Center, Cleveland, Ohio

Processing Issues for Preliminary Melts of the 

Intermetallic Compound 60-NITINOL

NASA/TM—2012-216044

November 2012
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Malcolm K. Stanford, Fransua Thomas, and Christopher DellaCorte

Glenn Research Center, Cleveland, Ohio

Processing Issues for Preliminary Melts of the 

Intermetallic Compound 60-NITINOL

NASA/TM—2012-216044

November 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

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7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

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

This report contains preliminary findings, 

subject to revision as analysis proceeds.
NASA/TM—2012-216044 1

Processing Issues for Preliminary Melts of the 

Intermetallic Compound 60-NITINOL

Malcolm K. Stanford, Fransua Thomas, and Christopher DellaCorte

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

The effect of various high temperature heat treatments and 

cooling rates on the hardness of cast 60-NITINOL (60wt%Ni40wt%Ti) was studied. The hardness ranged from 

approximately 33 HRC for annealed specimens to 63 HRC for 

water quenched specimens. Aging did not have a further effect 

on the hardness of the heat-treated and quenched material. The 

issue of material contamination and its possible effect on 

quench cracking during heat treatment above 1000 °C was 

explored. The Charpy impact energy of the material was found 

to be relatively low (ranging from 0.4 to 1.0 J) and comparable 

to that of cast magnesium. Selection of service environments 

and applications for this material based on these findings should 

consider the processing route by which it was produced.

Introduction

60-NITINOL is an intermetallic compound composed of 

60wt%Ni and 40wt%Ti. Since it was discovered that this 

material has high hardness, good dimensional stability, high 

specific strength, excellent corrosion resistance and 

exceptional tribological properties, it is being evaluated for 

advanced gears and bearings (Refs. 1 and 2). 60-NITINOL is 

an ordered intermetallic with the B2 crystal structure. In this 

arrangement, atoms of one type (i.e., Ni) are at the eight 

corners of a cubic cell and an atom of another type (i.e., Ti) 

sits at the center of the cube, as depicted schematically in 

Figure 1. Though composed of metallic atoms, the high degree 

of atomic ordering in this material causes it to behave more 

like a covalently bonded ceramic with intrinsic brittleness. 

Charpy impact testing was used as a screening test to evaluate 

the impact energy of this material as well as its mode of 

fracture after various heat treatments.

During preliminary research on this material, several of the 

prototype components ruptured during the water-quenching

step of the heat treatment process. Initially, it was thought that 

this quench cracking was initiated by high residual stresses 

that were concentrated at stress risers within the components 

such as fillets with high radii of curvature. A study to optimize 

the hardness of the material produced several more 

occurrences of this quench cracking phenomenon with 

specimens that only had relatively simple geometries. The 

crack typically propagated through the specimen resulting in 

two separate pieces. The purpose of this paper is to report the 

results of the heat treatment study and to present preliminary 

observations of samples affected by quench cracking. To 

better understand the behavior of the material, the impact 

energy was also studied.

Materials and Procedures

60-NITINOL is a Ni-rich version of the well-known shape 

memory alloy, 55-NITINOL (55wt%Ni-45wt%Ti). The 

studied compound is composed of 60wt% Ni and 40wt% Ti

and was fabricated by vacuum induction skull melting with a 

water-cooled copper crucible (Ref. 3). The crucible was 

coated with a zirconia-based lining to increase crucible 

durability. The cast material was subsequently hot isostatically 

pressed for 2 hr at 900 °C and 100 MPa to diminish porosity.

Heat treatment can be used to modify the strength and 

hardness of 60-NITINOL. The heat treatments used in this 

study are described in Table I. They were designed to 

determine the temperature necessary to produce maximum 

hardness by dissolving all of the second phase Ni3Ti to 

produce austenitic NiTi based on the nickel-titanium phase 

diagram (Ref. 4). High-temperature heat treatment followed 

by furnace cooling was designed to relieve residual stresses in 

the material but this technique typically provides the least 

strength and hardness due to the enhanced growth of the 

incoherent second phase Ni3Ti. The solution heat treatment is 

designed to dissolve the Ni3Ti into the parent NiTi phase and 

transform all of the material into austenitic NiTi. The 

austenitic NiTi is a homogeneous supersaturated solid solution 

that has high hardness. The aging heat treatment is designed to 

grow crystallographically coherent Ni4Ti3 precipitates that 

further increase the strength and hardness of the material to a 

condition that is desirable for bearing applications.

Heat treatment can, however, produce internal stresses that 

can lead to dimensional distortion and, in extreme cases, 

quench cracking, where thermally-induced stresses exceed the 

tensile strength of the material. The purpose of the heat 

treatment investigation was to determine the thermal 

processing necessary to maximize the hardness of the material. 

When quench cracking was observed, this issue was studied 

more systematically. The specimens used for the heat 

treatment investigation were approximately 12- by 12- by 

50-mm. A well-preserved heat treatment specimen that 

cracked during water quenching (WQ) was examined by fieldeffect scanning electron microscopy (SEM).

The setup of the Charpy impact test is shown schematically in 

Figure 2. The test measures the energy required to fracture a 

specimen by determining the difference between the initial 

height h and final height h′ when the hammer (or striker) on the 
NASA/TM—2012-216044 2

pendulum impacts the specimen. The fractured surface of the 

specimen can then be analyzed to determine the type of fracture 

the specimen underwent. Though fracture takes place in a 

fraction of a second, the mechanisms are assumed to be the 

same as if the specimen were subjected to a tensile test (Ref. 5). 

The impact energy of the specimen is, therefore, thought of as 

somewhat analogous to the area under the stress-strain curve (or 

toughness). The test articles used in this portion of the study 

were U-notched, 55 mm long by 10 mm wide by 10 mm high 

specimens as specified in ASTM E23-07aε1 (Ref 6). A 

schematic representation of the Charpy specimen is shown in 

Figure 3. The Charpy specimens were heat treated using the 

conditions listed in Table II (these heat treatments vary slightly 

from the heat treatments used to study hardness). The averages 

of three impact tests per heat treatment condition were reported. 

The fractured surfaces were examined by SEM.

Specimens for microstructural examination were sectioned 

with an abrasive cut-off saw using a resin bonded aluminum 

oxide blade. The wheel rotated at 3820 rpm with a traverse

speed of 0.1 mm/s. The specimens were prepared using standard

TABLE I.—HEAT TREATMENTS USED TO STUDY CAST 60-NITINOL HARDENABILITY

Designation Heat treatment Hardness,

HRC

Furnace-cooled

2 hr at 900 °C/furnace cool (FC)a

2 hr at 1000 °C/FC

2 hr at 1050 °C/FC

37 ± 1

33 ± 2

33 ± 2

Water-quenched (partially solution treated)

2 hr at 900 °C/WQb

2 hr at 1000 °C/WQ

2 h at 1050 °C/WQ

56 ± 2

63 ± 2

63 ± 1

Partially solution treated and aged

2 hr at 900 °C/WQ/1 hr at 400 °C/WQ

2 hr at 1000 °C/WQ/1 hr at 400 °C/WQ

2 hr at 1050 °C/WQ/1 hr at 400 °C/WQ

58 ± 1

61 ± 3

62 ± 1 a

Furnace cooling was done by shutting off the power to the furnace and allowing it to cool to room temperature. This took 

approximately 24 hr.

b

Water quenching was done in still room temperature water. The specimen was swirled about in the water.

TABLE II.—HEAT TREATMENTS AND RESULTANT ROCKWELL HARDNESS OF CAST-60NITINOL CHARPY SPECIMENS

Heat treatment Conditions Hardness,

HRC

Impact energy,

J

Furnace annealed 70 min at 1040 °C/furnace cool 32 ± 0 1.0

Solution treated 90 min at 980 °C/WQ 61 ± 0 0.4

Aged 90 min at 980 °C/WQ/60 min at 400 °C/WQ 63 ± 1 0.5

Figure 1.—60-NITINOL B2 unit cell showing (a) atomic 

arrangement and (b) relative size of atoms in the unit cell. 

(Image generated with Materials Studio modeling package 

from Accelrys Software, Inc.) Figure 2.—Charpy test of impact energy (Ref. 5).
NASA/TM—2012-216044 3

Figure 3.—Schematic of U-notched Charpy specimen used in 

this study (Ref. 6).

Figure 4.—Microstructure of the as-received cast 60-NITINOL.

metallographic procedures. About 1mm of the specimen was 

removed during the grinding and polishing procedure. The final

polish used colloidal silica on a vibratory polisher. These 

specimens were analyzed by optical microscopy, SEM and

energy-dispersive x-ray spectroscopy (EDS).

Results and Discussion

Table I lists the average Rockwell hardness of the cast 60-

NITINOL based on the heat treatment used. Heat treatment 

followed by furnace cooling produced the softest specimens. 

Also, the hardness of the furnace-cooled material tended to 

decrease with increasing heat treatment temperature. The 

hardness of the water-quenched specimens ranged from 56 to 

63 HRC, with the hardness increasing with increasing heattreatment temperature. Partial solution treatment (defined here 

as the high-temperature heat treatment and water-quenched 

condition) followed by aging resulted in hardness ranging 

from 58 to 62 HRC. Based on the statistical variation, the 

hardness obtained by the partial solution treatment and by 

partial solution treatment with aging is essentially identical for 

each of the studied heat-treatment temperatures.

The microstructure of the as-received material is shown in 

Figure 4. EDS was used to confirm that the dark grey parent 

phase is NiTi and the light grey second phase, decorating the 

grain boundaries and in a lath pattern within the grains, is 

Ni3Ti. A few pores are present in the microstructure, one of 

which is indicated on the photomicrograph (see Figure 4). 

Microstructures obtained from each heat treatment are shown 

in Figure 5. These photomicrographs show that the presence 

of Ni3Ti is decreased considerably with heat treatment above 

900 °C, but it is not eliminated, even after heat treatment at 

1050 °C (which is the reason these heat treatments are referred 

to as “partial” solution treatments). A higher temperature is 

required to completely dissolve the Ni3Ti. However, quench 

cracking was found to occur most often after heat treatments 

at 1000 °C and greater. Further work is needed to optimize the 

solution temperature without causing quench cracking. In 

addition, the amount of the (hardening) Ni4Ti3 precipitate

phase produced by aging has not yet been quantified. Current 

work is underway to optimize the precipitation of this 

hardening precipitate.

Quench Cracking

A quench cracked specimen that was recovered in good 

condition was examined. The mating fracture surfaces of this 

specimen were either convex or concave. A cross-section of 

the concave surface is shown in Figure 6. Examination of the 

concave fracture surface by SEM revealed classical chevron 

patterns indicating brittle fracture and the location of the 

fracture initiation site (see in Figure 7). The fracture origin for 

quench crack specimens was relatively rough and had areas 

that statically charged in the SEM. EDS was used to partially 

identify the bright areas, which were zirconia-rich, though no 

source of zirconium was known at the time. Later it was 

realized that these particles were probably contamination from 

the zirconia lining used to coat the induction crucible. Bulk 

chemical analysis also detected 20 ppm of zirconium along the 

fracture surface and at the fracture initiation site.

Charpy Impact Testing

The hardness of Charpy specimens after heat treatment is 

listed in Table II. As expected, the furnace-cooled material has 

the lowest hardness (32 HRC), while the quenched material is 

considerably harder (61 HRC), being approximately the same 

hardness as the partially solution treated and aged material 

(63 HRC). The hardness obtained following each heat 

treatment is essentially the same as that for the comparable 

1000 and 1050 °C heat treatments listed in Table I.

Table III shows the impact energy of the studied specimens 

compared to values for some common engineering alloys 

(Ref. 5) and for arc-melted 60-NITINOL (Ref. 7). The data for 

the studied specimens are comparable to those of cast partially 

solution treated or partially solution treated and aged 

magnesium, which is to say that their impact energy is very 

low. Inspection of the fracture surfaces revealed differences in 
NASA/TM—2012-216044 4

Figure 5.—Optical photomicrographs showing the microstructure of cast 60-NITINOL after furnace-cooling from 

(a) 900 °C, (b) 1000 °C, and (c) 1050 °C, and after partial solution treatment at (d) 900 °C, (e) 1000 °C and 

(f) 1050 °C and, similarly, after aging (g) to (i).
NASA/TM—2012-216044 5

Figure 5.—Concluded.

the fracture mechanisms between the furnace-cooled and the 

specimens. For furnace-cooled specimens, the fracture surface 

was dull and non-reflective to the naked eye. Examination by 

SEM revealed a blocky fracture surface with flat cleavage 

planes at 1000× magnification possibly indicating brittle, 

intergranular fracture (see Figure 8).

Figure 6.—Montage image of the cross-section of the fracture 

initiation site. Note the secondary cracks below the primary 

fracture (the vertical separation at the center of the image is 

merely an artifact of the software used to compile the 

montage).
NASA/TM—2012-216044 6

Figure 7.—(a) Quench crack fracture initiation site and 

(b) higher magnification view of the defect.

TABLE III.—IMPACT TEST RESULTS COMPARED TO 

OTHER ENGINEERED MATERIALS (REF. 5) AND TO 

EXTRUDED AND HARDENED 60-NITINOL (REF. 7)

Material Impact Energy,

J

L2 tool steel 26

410 stainless steel 34

Extruded 60-NITINOL

Hardened 60-NITINOL

6.8 (notched)

2.7 (notched)

Cast 60-NITINOL:

Furnace annealed

Solution treated

Aged

1.0 (notched)

0.4 (notched)

0.5 (notched)

AM100A cast magnesium 0.8

Figure 8.—Overall fracture surface of the furnace-cooled 

specimen and (b) to (c) higher magnification views of the 

fracture surface.
NASA/TM—2012-216044 7

Macroscopically, the fracture surfaces of the partially solution 

treated and the aged impact specimens were more reflective 

clear indications of beach markings that helped to identify the 

fracture origin. Closer examination of a partially solution treated 

specimen by SEM revealed the classical chevron pattern that 

indicated the direction of crack propagation from the origin (see

Figure 9). The fracture initiation sites for both of these

specimens had a grainy texture as with the quench-cracked

specimens with faceted particles embedded within the fracture 

initiation site. These locations acquired a static charge from 

exposure to the electron beam (see Figure 10). Based on the 

morphology of the contaminant particles, it was thought that the 

composition of the material was zirconia (ZrO2). This was 

confirmed by EDS (see Figure 10 and Figure 11). These faceted 

particles would tend to act as crack initiators. The maximum 

tensile stress developed at or near the center of the test 

specimens. If the angular zirconia particles were present, 

quench cracking occurred. It is believed that the thermallyinduced residual stresses have been magnified because of the 

added stress concentration of the zirconia particles.

Figure 9.—Partially solution-treated Charpy specimen fracture surface at (a) low 

and (b) higher magnification with arrows indicating fracture origin.
NASA/TM—2012-216044 8

Figure 10.—Scanning electron photomicrograph (a) and energy-dispersive x-ray spectra of fracture origin shown in Figure 9. The 

area at EDS-B (b) is the Ni-Ti parent material while EDS-C (c) indicates Zr and O (with some Ca).
NASA/TM—2012-216044 9

Figure 11.—(a) SEM and (b) EDS of faceted particle within fracture origin shown in 

Figure 10. With reduced electron beam voltage, it is clear that the primary constituents 

are Zr and O.

Given the inherent brittleness of this intermetallic material, 

its cleanliness is extremely important when used under 

conditions where tensile stresses can be generated. This would 

include impact loading and thermally-induced strain 

generation, as well as during basic tensile and flexural loading. 

A sharp, hard contaminant such as any of those discussed here 

tends to form a stress concentration that, when encountered by 

unavoidable tensile stresses, greatly reduces the stress needed 

for crack propagation. In a material with low ductility such as 

60-NITINOL, there is no mechanism for crack blunting, 

resulting in brittle fracture. 

It was determined that the source of the zirconia was an 

additive in the crucible liner. This lining has been eliminated 

from the 60-NITINOL casting process and new material is 

currently being evaluated. However, casting is a processing 

method that has intrinsic weaknesses. Porosity in cast material 

tends to be spherical and is thus resistant to elimination, even 

by hot isostatic pressing. Clearly, casting is not the best 

processing route for a brittle material that will be exposed to 

tensile stresses. However, in compression this material 

performs very well (Refs. 1 and 2). Component designers will 

need to take the processing route used to produce this material 

into consideration. Powder metallurgy processing for 60-

NITINOL is currently being developed and may provide better 

performance for applications subjected to multi-directional 

stresses and fatigue loads (Ref. 8).

Concluding Remarks

The hardness of cast 60-NITINOL after various heat 

treatments has been investigated. Issues of quench cracking 

and material contamination were also studied. This 

investigation has shown that 60-NITINOL can be hardened to 

approximately 63 HRC with relatively simple heat treatments. 

Further work will be required to explore heat treatment 

procedures that can yield a material with an adequate 
NASA/TM—2012-216044 10

hardness, based on the application envisioned, while avoiding 

quench cracking. In addition, further testing of the cast 

material, without the zirconia contaminant, is required to 

determine the Charpy impact energy. However, the cast form 

of this material may not be practical for some applications due 

to the intrinsically low toughness of castings. The results of 

this study provide guidance for the proper selection of

processing routes for 60-NITINOL based on the expected 

service environments.

References

1. C. DellaCorte, S.V. Pepper, R. Noebe, D.R. Hull, G.

Glennon, “Intermetallic Nickel-Titanium Alloys for OilLubricated Bearing Applications,” NASA/TM—2009-

215646, March 2009, National Technical Information 

Service, Springfield, VA.

2. S.V. Pepper, C. DellaCorte and G. Glennon, “Lubrication 

of Nitinol 60,” NASA/TM—2010-216331, June 2010, 

National Technical Information Service, Springfield, VA.

3. S. Reed, private communication, June 17, 2011.

4. H. Okamoto and T.B. Massalski, Desktop handbook: 

Phase Diagrams for Binary Alloys, Ed. H. Okamoto, 

ASM International, 2000.

5. J.F. Shakelford, Introduction to Materials Science for 

Engineers, Sixth Edition, 2005, Prentice-Hall, Upper 

Saddle River, NJ.

6. ASTM Designation E23-07aε1

, “Standard Test Methods 

for Notched Bar Impact Testing of Metallic Materials, 

2010, vol. 03.01, American Society for Testing and 

Materials, West Conshokocken, PA.

7. W.J. Buehler, “Intermetallic Compound Based Materials 

for Structural Applications,” in The Seventh Navy 

Science Symposium: Solution to Navy Problems through 

Advanced Technology, May 14, 15, 16, 1963, U.S. Naval 

Aviation Medical Center, Pensacola, FL, Vol. 1, Office of 

Naval Research, Arlington, VA, 16 May 1963.

8. C. DellaCorte, R. Noebe, M.K. Stanford and S.A. Padula,

“Resilient and Corrosion-Proof Rolling Element Bearings 

Made From Superelastic Ni-Ti Alloys for Aerospace 

Mechanism Applications,” NASA/TM—2011-217105, 

National Technical Information Service, Springfield, VA.