Malcolm K. Stanford

Glenn Research Center, Cleveland, Ohio

Thermophysical Properties of 60-NITINOL

for Mechanical Component Applications

NASA/TM—2012-216056

December 2012
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Malcolm K. Stanford

Glenn Research Center, Cleveland, Ohio

Thermophysical Properties of 60-NITINOL

for Mechanical Component Applications

NASA/TM—2012-216056

December 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The technical assistance provided by Joy A. Buehler, Dereck F. Johnson, Terry R. McCue, Richard B. Rogers, Fransua Thomas,

Timothy J. Ubienski and Walter A. Wozniak is greatly appreciated. The author is grateful for the technical discussions with

Drs. Christopher DellaCorte and Ronald D. Noebe. This work was supported by the NASA Engineering & Safety Center.

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

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

This report contains preliminary ¿ ndings,

subject to revision as analysis proceeds.
NASA/TM—2012-216056 1

Thermophysical Properties of 60-NITINOL for

Mechanical Component Applications

Malcolm K. Stanford

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

The linear thermal expansion coefficient, specific heat

capacity, electrical resistivity and thermal conductivity of 60-

NITINOL were studied over a range of temperatures

representing the operating environment of an oil-lubricated

bearing. The behavior of this material appears to follow wellestablished theories applicable to either metal alloys, in

general, or to intermetallic compounds, more specifically and

the measured data were found to be comparable to those for

conventional bearing alloys.

Introduction

60-NITINOL is an ordered intermetallic material composed

of 60wt%Ni and 40wt%Ti. Due to its unique combination of

high hardness, low apparent elastic modulus and resistance to

aqueous corrosion, this material is of interest within the

aerospace community for use as a bearing material (Refs. 1

and 2). In spacecraft systems, bearings must withstand

transient loading from the vibration of launch and resist wearaccelerating corrosion from the operating environment as well

as from the corrosive ambient marine environment of many of

the U.S. spaceports. Unlike its extensively-studied cousin 66-

NITINOL, however, the thermophysical properties of 60-

NITINOL remain virtually unexplored. Knowledge of these

properties will enable better design and modeling of

components. The purpose of this investigation is to determine

selected thermophysical properties of 60-NITINOL and

compare them to those of conventional bearings to enable

better design and modeling of components.

Materials

Gas atomized 60-NITINOL (60wt% Ni–40wt% Ti) powder

was consolidated into an approximately 20 kg ingot by hot

isostatic pressing. The microstructure of the hot isostatically

pressed material is shown in Figure 1. The chemical

composition of the material was analyzed by inductivelycoupled plasma emission spectroscopy and the results are

listed in Table I.

Slightly oversized specimen blanks for were sectioned from

the ingot by wire electrical-discharge machining (wire EDM).

The specimen blanks were heat treated by furnace-cooling

(softening) and by water-quenching (hardening) heat

treatments as described in Table II. Photomicrographs of the

microstructures resulting from each heat treatment are shown

in Figure 2. Ni-Ti particles, not consolidated during hot

isostatic pressing, were observed throughout the material (see

Figure 2). The effect of these unconsolidated particles, thought

to be a result of oxidation of the Ni-Ti material during

atomization, has not been studied extensively in relation to

thermophysical properties. Although unconsolidated particles

were found at the fracture initiation sites in previous impact

testing of this material, the impact energy did not vary

significantly from that of material without unconsolidated

particles (Ref. 3). Although these features represent surface

defects within the material, their effect on the studied

thermophysical properties is currently unknown.

The crystalline phases present in the material after both heat

treatments were identified by x-ray diffraction using Cu K

radiation, as listed in Table III. Ni3Ti is a non-coherent phase

that ostensibly does not contribute to hardening of the material

while Ni4Ti3

 is a fine phase that can be precipitated through

aging to optimize hardness. Specimens for thermophysical

property testing were sectioned from the heat treated blanks by

wire EDM and then ground to final dimensions as listed in

Table IV.

Test Methods

The coefficient of thermal expansion, specific heat capacity,

thermal conductivity and electrical resistivity were measured

for 60-NITINOL after furnace-cooling and water-quenching

heat treatments. The temperature ranges were selected to

include the upper limit of temperature experienced by oillubricated components but certain atmospheres were selected

to eliminate the effect of oxidation on the property of interest.

Coefficient of Thermal Expansion

The linear coefficient of thermal expansion is determined

by measuring the relative change in length l of each specimen

with respect to temperature according to the relationship:

 = ( l/l)/(T – To).

where T is the test temperature and To is room temperature.

The test was performed in helium using a push-rod dilatometer
NASA/TM—2012-216056 2

Figure 1.—Scanning electron microscope photomicrograph of

the compacted material showing clusters of second phase

Ni3Ti within the grains and along the grain boundaries of the

parent NiTi phase.

TABLE I.—CHEMICAL COMPOSITION OF

COMPACTED 60-NITINOL POWDER

Composition 59.4wt%Ni–40.5wt%Ti

Impurities (ppm) O (580), Fe (100), Cu (90), Al (80), Cr (20), Co (5)

TABLE II.—HEAT TREATMENTS USED IN THIS STUDY

Designation Heat treatment

Water-quenched 2 h at 1000 C then quenched in room

temperature water (specimen agitated during

immersion)

Furnace-cooled 2 h at 1000 C then allowed to cool inside

closed furnace after it was powered-down

TABLE III.—CRYSTALLINE PHASE IDENTIFICATION

Water-quenched material Furnace-cooled material

Cubic NiTi Cubic NiTi

Hexagonal Ni3Ti Hexagonal Ni3Ti

Rhombohedral Ni4Ti3 Cubic NiTi2

Cubic NiTi2 Monoclinic NiTi

TABLE IV.—TEST SPECIMEN DIMENSIONS

Specimen type Nominal dimensions

Thermal and electrical conductivity 90- by 4.7- by 1-mm

Specific heat 5.8-mm diameter by 1.5-mm

Thermal expansion 50-mm diameter by 5-mm

according to a standard test procedure (Ref. 4). The expansion

of the specimen is compared to that of a platinum standard with

a heating rate of 2 °C per minute. This validity of this technique

as an evaluation tool assumes that the symmetry of the cubic B2

crystal structure will result in isotropic thermal expansion. For

calibration of the instrument, six standards with a range of

thermal expansion coefficients were obtained from the National

Institute of Standards (NIST) and measured against one another.

Figure 2.—Photomicrographs showing 60-NITINOL in (a) waterquenched and (b) furnace-cooled heat treatment conditions

(the green filter increases the resolution of the optical

photomicrographs). Furnace-cooling allows precipitation of the

second phase Ni3Ti within the parent phase of the material.

Specific Heat

A differential scanning calorimeter was used to measure

specific heat at constant pressure in argon according to a

standard test specification (Ref. 5). The procedure measures the

difference in power required to heat a 60-NITINOL specimen at

the same rate as a sapphire standard. With the known masses of

the specimen and the standard and the measured power

differential, the specific heat cp is calculated as a function of

temperature, following the well-known relationship

cp = q/m T

where q represents the heat supplied to the material of mass m

to produce a change in temperature T. The measured values

are directly traceable to NIST standards.
NASA/TM—2012-216056 3

Thermal Conductivity and Electrical Resistivity

Thermal and electrical conductivity were measured in

vacuum with a technique based on Kohlrausch’s Method

(Refs. 6 and 7). In this test method, thermocouples, which

serve as both temperature and voltage sensors, are affixed to

the test specimen at both ends and at the center. The specimen

is heated throughout its volume by Joule heating as a result of

the application of direct current. The product of thermal

conductivity and electrical resistivity are determined by

the relationship

 = (V3 – V1

)

2

/4[2T2 – (T1+ T3)]

where V3 – V1

 represents the voltage drop across the length of

the specimen (the ends are arbitrarily labeled positions 1 and

3), T2 is the temperature at the center (position 2) of the

specimen and T1+ T3 represents the sum of the temperatures at

both ends. The resistivity is simultaneously determined

using the expression

 = (V3 – V1) A/IL

where A is the cross-sectional area of the specimen, I is the

current and L is the length of the specimen between positions

1 and 3. The current heats the specimen and radiant heat is

supplied to the center of the specimen to minimize radial heat

loss while the ends are cooled with a liquid medium. The

current is varied to increase the resultant steady-state

temperature profile as the test parameters are recorded. The

measured values are directly compared to standards validated

by the NATO Science and Technology Organisation (formerly

the Advisory Group for Aerospace Research and

Development) and are accurate to within 5 percent.

Results and Discussion

Coefficient of Thermal Expansion

A plot of the thermal expansion coefficient of 60-NITINOL

from room temperature to 300 °C in both of the studied heat

treatments is shown in Figure 3. The thermal expansion

coefficient is slightly lower for the hardened material than for

the softened material due to the increased presence of

quenched-in defects, which restrict atomic mobility.

Specific Heat

The specific heat capacity is plotted in Figure 4. From 20 °C

to approximately 300 °C, the heat capacity of the waterquenched material is slightly lower than that of the furnacecooled material but the heat capacity for the water-quenched

material increases at a slightly higher rate with respect to

temperature. At 100 °C and above, the heat capacity of the

Figure 3.—Thermal conductivity of 60-NITINOL.

Figure 4.—Specific heat capacity of 60-NITINOL.

water-quenched material is higher than that of the furnacecooled material. The absence of any sharp deviations in the

curve indicates that there are no phase transformations

occurring over the reported temperature range.

Electrical Resistivity and Thermal Conductivity

Resistivity, plotted in Figure 5, shows the expected linear

temperature dependence predicted by the Bloch-Gruenessien

law (Ref. 8). It is noted that, while this behavior is not strictly

followed at very low or very high temperatures, it is

essentially linear for 60-NITINOL over the studied

temperature interval. Electricity is conducted through metallic

0

0.000002

0.000004

0.000006

0.000008

0.00001

0.000012

0.000014

0 50 100 150 200 250 300 350

Temperature, °C

FC

WQ

450

460

470

480

490

500

510

0 50 100 150 200 250 300 350

Temperature, °C

FC

WQ
NASA/TM—2012-216056 4

Figure 5.—Electrical resistivity of 60-NITINOL.

Figure 6.—Thermal conductivity of 60-NITINOL.

materials by the positive drift of electrons or, more precisely,

electron waves. Electrical resistivity tends to increase with

temperature due to the concomitant increase in lattice vibration,

which increases the probability of scattering these waves. As

predicted by Matthiessen’s rule, irregularities such as point, line

and surface defects also tend to increase resistivity due to

scattering (Ref. 9). Water-quenching is known to create antiphase boundaries that account for the increased resistivity in the

water-quenched material (Refs. 10 and 11). Nevertheless, in

both heat treatment conditions, the resistivity is in the range of

typical metallic alloys (~10–7 ·m).

Simply comparing the measured resistivity values at room

temperature and 300 °C in the two heat treatment conditions,

the increase in resistivity resulting from changing the heat

treatment condition from water-quenched to furnace-cooled is

slightly less than the increase in resistivity resulting from

increasing temperature over the range cited. This suggests that

a non-destructive test could be developed to indicate the heat

treatment condition of a component in the field exposed to

unexpected current or heat transients as a possible “health

monitoring” tool.

Thermal conductivity, plotted in Figure 6, is higher for the

furnace-cooled material and increases with temperature in

both heat treatment conditions. This behavior is similar to that

of electrical conductivity. Unlike electrical current, heat can

be conducted through a material by either electrons or

phonons, a phenomenon expressed by the Wiedemann-Franz

law (Ref. 12). For electrically conductive materials, the

abundance of conducting electrons makes electronic

conduction of heat more likely, which has been observed in

other B2-type materials (Ref. 13). Based on the similarity of

the electrical and thermal conductivity behavior in this

material, it is likely that heat is primarily conducted by

electrons in this material as well. Also, in contrast to the

resistivity results, the percent increase in conductivity due to

the difference in heat treatment condition appears to be

essentially the same as the increase due to increasing

temperature.

For ease of comparison, the average values of each of the

measured properties are listed in Table V along with

comparable values for several conventional bearing materials.

It should be noted that, while the tabulated data from the

literature is often reported as a single (averaged) value, it

typically varies over the reported temperature range.

Therefore, where warranted, a range of values is reported for

60-NITINOL.

The thermal expansion coefficient for 60-NITINOL is,

perhaps unsurprisingly, intermediate to that of Ni and Ti and

falls between that of 440C stainless steel and M50 tool steel.

One might expect, therefore, that there would be little issue

with thermal expansion mismatch if 60-NITINOL were mated

with a steel component over the studied temperature range.

The range of reported values for the specific heat capacity

of 60-NITINOL is comparable to that of M50 tool steel, 440C

and 304 stainless steels and is only slightly higher than that of

Stellite 6B and Hastelloy C. This is an indication that similar

furnace energy usage requirements could be expected to heat

treat 60-NITINOL as are found with steel heat treatment

furnaces, given the same heat treatment temperature. This

further indicates that it may be possible to use less aggressive

quenchants (e.g., oils or polymers) to achieve a sufficient

cooling rate for hardening heat treatments.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400

Temperature, C

WQ

FC

0

5

10

15

20

25

0 100 200 300 400

Temperature, C

FC

WQ
NASA/TM—2012-216056 5

TABLE V.—COMPARISON OF THERMOPHYSICAL PROPERTIES FOR SELECTED

MATERIALS AT ROOM TEMPERATURE (RT) EXCEPT WHERE SPECIFIED

Material Thermal expansion

coefficient,

10-6/°C

Specific heat capacity,

J/kg·K

Thermal conductivity,

W/m·K

Electrical resistivity,

·m

References

Cobalt 13.1 (RT-200 °C) 414 (RT-100 °C) 69 (RT-100 °C) 6.2 ×10–814,15

Nickel 13.9 (RT-200 °C) 440 (RT-100 °C) 92 (RT-100 °C) 8.0 ×10–814,16

Titanium 9.5 (RT-300 °C) 519 (RT-100 °C) 22 (RT-100 °C) 4.2 ×10–714,16

304 stainless steel 17 (RT-300 °C) 500 (0-100 °C) 15.1 (RT-100 °C) 7.2 ×10–714,16

Stellite 6B 13.4 (RT-400 °C) 421 14.7 9.1 ×10–714,17,18,19

Hastelloy C 11.3 (0-100 °C) 419 (RT-100 °C) 11.5 1.3 ×10–614,20

M50 bearing steel 12.1 (RT-260 °C) 462 (RT-100 °C) 25.9 (RT-100 °C) 1.8 ×10–714,21,22

60-NITINOL (water-quenched) 11.2 (RT-300 °C) 453-504 (RT-300 °C) 8.9-13.7 (50-300 °C) 1.0×10–6

 (50-300 C) 23,24

440C stainless steel 10.8 (0-315 °C) 460 (0-100 °C) 41.9 (RT-150 °C) 6.0× 10–714,16

Silicon nitride 3.0-3.5 (RT-1000 °C) 778 (127 °C) 22 1010 (RT-100 C) 25,26

The resistivity of 60-NITINOL is intermediate to that of Ni

and Ti, being slightly higher than that of Stellite 6B and

slightly lower than that of Hastelloy C. The thermal

conductivity of 60-NITINOL is comparable to that of

Hastelloy C and its upper limit is only slightly lower than the

value given for Stellite 6B. These data indicate that 60-

NITINOL would have the ability to dissipate electrical and

thermal transients in service or during machining at least as

well as the materials mentioned.

It is important to reiterate that, while “handbook” data such

as those listed in Table V can provide a general overview of

material properties, these properties typically vary with

respect to the service temperature envelope and the service

environment. Therefore, component designers should give

these issues requisite consideration.

Conclusions

Selected thermophysical properties of 60-NITINOL have

been studies and, based on this investigation, the following

can be said:

The thermal expansion coefficient, specific heat

capacity, electrical resistivity and thermal conductivity

of 60-NITINOL is comparable to a variety of

conventional engineering alloys used in bearings.

There is a significant difference in the thermophysical

properties of 60-NITINOL in the water-quenched and

furnace-cooled heat treatment conditions due to

quenched-in defects.

References

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Service, Springfield, VA.

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Bearings Made From Superelastic Ni-Ti Alloys for

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2011-217105, September 2011, National Technical

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