Thermal fatigue characteristics of NiTiCu alloy coils

Materials Science and Engineering, A 136 ( 1991 ) L 1-L4

L1

Letter

Thermal fatigue characteristics of Ni-Ti-Cu alloy coils K. Tsuji, Y. Takegawa and K. Kojima Materials Research and Development Laboratory, MatsushitaElectric Works Ltd., 1048Kadoma, Osaka571 (Japan)

(ReceivedAugust 31, 1990; in revised form November 7, 1990)

Abstract Thermal fatigue resulting from heat cycling was studied using Ni-Ti-Cu alloy coils prepared by replacing part of the nickel with copper in a near-equiatomic Ni-Ti alloy at 6 and 9 at.%. Thermal fatigue is affected by the crystal structure of the martensite phase, the phase transformation mode, and working distortion. The most favorable thermal fatigue characteristics result when there is a one-step transformation from B2 to orthorhombic martensite and working distortion is introduced to the material interior. Ni-50Ti-9Cuat.% alloys which have been cold-worked and heat-treated at low temperatures give the best results.

1. Introduction Ni-Ti-Cu alloys, where part of the nickel or titanium is replaced with copper in a Ni-Ti alloy, exhibit the shape memory effect [1-2]. Previous studies show the relationship between copper addition and the phase transformation start temperature from B2 to martensite (Ms) temperature [2-5], mechanical characteristics [6-8[, and crystal structure [3, 4, 9, 10]. Knowledge of the thermal fatigue characteristics of shape memory alloys is fundamental for their practical application. Regarding this fatigue, tensile test results on Ni-50Ti-10Cuat.% are available [6], and results of studies on the phase transformation strain and on the fluctuation of phase transformation temperature in a wire shape are reported [8]. Both reports indicate 0921-5093/91/$3.50

that Ni-50Ti-10Cu alloys are more stable than Ni-Ti alloys. Reference 4 shows that the crystal structure of the martensite phase depends on copper concentration. It can be safely assumed that the dislocation density in the material varies with the heat treatment temperature after cold working, and thus the chemical composition and the heat treatment temperature are considered to have a significant effect on thermal fatigue. However, there are no systematic studies on the relationship between chemical composition and heat treatment temperature. The most likely application of Ni-Ti-Cu alloys as actuators, is as coil springs, but there are no quantitative data for thermal fatigue of this type of alloy in coil shape. In the present study, Ni-Ti-Cu alloy coils, prepared by replacing part of the nickel with copper in a near-equiatomic Ni-Ti alloy, were subjected to heat treatment at various temperatures after cold working. The effects of heat cycles on phase transformation temperature and output were then determined, thus revealing the thermal fatigue characteristics of the alloys. 2.

Experimentalprocedures

Ni-Ti-Cu alloy ingots with a specified chemical composition were obtained by normally arcmelting 99.9% electrolytic nickel, sponge titanium and copper in an argon atmosphere. The resulting ingots were shaped into bars via hot forging and swaging. These bars were then colddrawn so that 27% cold working rate was retained in wires 0.60 mm in diameter at the final drawing step. Table 1 shows the chemical composition analysed with inductively coupled TABLE 1 Chemical composition of Ni-Ti-Cu alloy coil (at.%) Ti

Cu

Ni

50.5

9.0 9.2

Remainder

49.5 49.7

6.1

Remainder Remainder

© Elsevier Sequoia/Printed in The Netherlands

L2 plasma (ICP). Three types of alloy composition were prepared. The copper contents of these alloys were 6 and 9 at.%, sufficiently below the content at which working characteristics, which are important when dealing with industrial materials, are favorable (10 at.%) [6, 7]. For copper at 9 at.% the titanium contents of the alloys were 49.5 and 50.5 at.%, deviating 0.5% from the equiatomic titanium content of 50 at.%. After forming coils with a diameter of 6.0 nun, heat treatment (723-823 K, 3600 s and nitrogen atmosphere) was carried out. Heat cycles were at constant displacement, and thermal fatigue characteristics were determined by comparing phase transformation characteristics before and after the heat cycles. Heat cycle conditions were such that the temperature of the alloy varied between two extremes, fluctuating broadly around the phase transformation temperature causing the most severe thermal fatigue. A shape memory alloy which was constrained to a given length was immersed in liquid for 600 s alternately at 273 and 373 K. Phase transformation characteristics were determined by changing the temperature of the shape memory alloy under constant displacement and measuring temperature-output characteristics.

Ni-Ti-Cu alloy. It can be seen from the graph that the output respectively increases and decreases at the boundaries As and Ms (the phase transformation temperatures). Based on these T-P characteristics, changes in phase transformation temperature and output before and after heat cycles were determined to evaluate thermal fatigue. Changes in phase transformation ternperature were obtained from As values. Output decrease was measured at 358 K after Af during heating, and normalized with the value before heating. Figure 2 shows the number of heat cycles imposed on Ni-49.7Ti-6.1Cuat.% constrained at 0.55% shear and heat-treated at various temperatures, the As value and the changes in output. It can be seen that thermal fatigue, which decreases output and causes a change in As, occurs at an early stage (before 300 cycles), with little subsequent change. Thermal fatigue characteristics of three types of alloy after 1000 cycles are plotted against heat treatment temperature in Fig. 3, and reveal the following. As is not largely changed by heat cycles

I Ni-49.TTi-6.1Cu(at~) ]

3. Results

Shear s t r a i n 0.55~

350 --

Figure 1 represents the temperature-output characteristics (T-P characteristics) of a typical

200

v

~ 330 --

150

723K

/

~

Ms

~"

~

773K

Af

"~ 100

1.0 so

~

o

Mf

723K

....

--~-__?__~_

.~ 0.5 --

0

I 280

I 300

I 320

I 340

I 360

Temperature (K)

Fig. l. Temperature-output characteristics of a typical NiT i - C u alloy Ti = 50 at.%. As, the phase transformation start temperature from martensite to B2; Af, the phase transformation finish temperature from martensite to B2; Mr, the phase transformation finish temperature from B2 to martensite,

~ ~ o z

823 K

I

500 Number o f c y c l e s

I

1000

Fig. 2. Thermal fatigue characteristics of N i - 4 9 . 7 T i 6.1Cuat.% alloys. • , 723 K heat treatment; ,,x 773 K heat treatment; o, 823 K heat treatment.

L3 10

[

A f t e r 1000 c y c l e s [ S h e a r s t r a i n 0.55~

1

F Ni-50"5Ti-9"0Cu(at%)

~ ~

|723k heat treatment RAFter 1000 cycles

--

•~ 5 m

~o

-

-~__L----a.

~

c~)

--

J

-5~

X

s 0.5 --10

- -

- -

o z

0

Shear s t r a i n

0.5 E

~)

-

~

-

(%)

Fig. 4. The relationship between load strain and thermal fatigue of Ni-50.5Ti-9.0Cuat.%.

(Output>

Z

0 [ ] ] [ v0o 750 800 B5o Heat treatment temperature (K) Fig. 3. Relationship between heat treatment temperature

and thermal fatigue. A, Ni-50.5Ti-9.0Cuat.%; B, Ni-49.5Ti9.2Cuat.%; C, Ni-49.7Ti-6.1Cuat.%.

up to a heat treatment temperature of 773 K in any chemical composition. With further elevation of treatment temperature, however, As decreases, Output reduction owing to heating is enhanced as heat treatment temperature increases. The reduction is greater with 6 at.% copper than with 9 at.% copper at all heat treatment temperatures. Comparison between titanium concentrations of 49.5 at.% and 50.5 at.% for 9 at.% copper shows that output reduction is greater with 49.5 at.% titanium up to 773 K, but that the relationship is reversed when the temperature is raised to 823 K. Changes in As show the same trend. When Ni-50.5Ti-9.0Cuat.% alloys are heat-treated at 723 K, As and output are almost never affected by heat cycles, but remain stable (load strain 0.55%). Figure 4 shows the relationship between load strain and thermal fatigue characteristics of Ni-50.5Ti-9.0Cuat.% after 1000 heat cycles. The graph reveals the following. As tends to elevate as load strain increases. However, the shift magnitude is very small at around + 1 K.

With the increase in load strain, output reduction is enhanced, but the rate is within 15% of the output reduction rate (strain 1.3% or less). This tendency increases when strain exceeds 1.3%. This material conforms to Hook's law up to a limit strain of 1.4% [1 1], which is almost equal to the strain at which such radical output reduction occurs. Output reduction owing to heat cycles usually results from plastic strain, which is caused by a slip in the crystal lattice. The 15% output reduction in Fig. 4 corresponds to a shearing strain of approximately 0.2%. 4.

Discussion

A differential scanning calorimetry (DSC) profile revealed that in Ni-50.5Ti-9.0Cu alloys the elevation ofheattreatmenttemperature removed working distortion. This, in turn, caused a slight peak at lower temperatures as a result of heat treatment at 773 K. Heat treatment at 823 K caused two clear peaks, indicating a two-step transformation from B2 to orthorhombic to monoclinic [12]. In Ni-49.5Ti-9.2Cu alloys, however, the increase in heat treatment temperature broadened the peak at the lower temperature side, showing a slight change. Heat treatment at 823 K did not form a second peak; it therefore remained a one-step transformation. Cold working inhibited the two-step transformation, permitting only the one-step transformation from B2

L4

to orthorhombic when heat treatment temperature was up to 823 K. Figure 3 reveals that Ni-50.5Ti-9.0Cuat.% and Ni-49.5Ti-9.2Cuat.% alloys differ in thermal fatigue dependency on heat treatment temperature. When comparing the data, it can be said that the difference in dependency of the two alloys on heat treatment temperature is attributed to the phase transformation mode of the alloys. The two-step transformation from B2 to orthorhombic to monoclinic on 50.5 at.% titanium alloys at a heat treatment temperature of 823 K rapidly increases thermal fatigue. In short, the orthorhombic-monoclinic phase transformation rapidly increases thermal fatigue. With regard to Ni-Ti binary alloys, a report [13] states that thermal fatigue increases more rapidly when the transformation is one step from B2 to rhombohedral (R) than when it is two-step from B2 to R to martensite (M). This is also true for Ni-Ti-Cu alloys, The effect of the amount of copper was as follows. With the material heat-treated at 723 K, as shown in Fig. 3, Ni-50.5Ti-9.0Cuat.% alloys underwent B2-orthorhombic transformation and Ni-49.7Ti-6.1Cuat.% alloys underwent B2-monoclinic transformation [12]. Since the cold working rate of these alloys was constant, the dislocation density of the material interior is considered to be constant. This implies that the difference in thermal fatigue characteristics is attributable to the crystal structure of the M phase during the above phase transformation; i.e. thermal fatigue is more favourable in B2-orthorhombic than in B2-monoclinic transformation. The shear modulus (shearing stress divided by shearing strain and related to the spring constant) of an orthorhombic system is approximately half that of a monoclinic system [11]. Supposedly, this fact also results in favorable fatigue characteristics. With regard to heat treatment temperature, the transformation mode of Ni-49.7Ti-6.1Cuat.% alloys is consistent. This means that the heat treatment temperature is elevated, and thermal fatigue increases when the working distortion inside the material is removed. The introduction of working distortion into the material interior via cold working, or the introduction of high-density dislocation, improves thermal fatigue characteristics,

5. Conclusion Thermal fatigue resulting from heat cycling was studied using Ni-TiTCu alloy coils prepared by replacing part of the nickel with copper in a near-equiatomic Ni-Ti alloy at 6 and 9 at.%. The thermal fatigue of Ni-Ti-Cu alloys is significantly affected by the crystal structure of the M phase, phase transformation mode, and material interior working distortion. For the M phase, the orthorhombic is more favorable, as is the one-step transformation from B2 to orthorhombic for the phase transformation mode. Two-step transformation from B2 to orthorhombic to monoclinic rapidly increases thermal fatigue. Favorable thermal fatigue characteristics result when working distortion is introduced to the material interior via cold working. In Ni-50Ti-9Cuat.% alloys, thermal fatigue characteristics are favorable when they are coldworked and heat-treated at low temperatures (e.g. 723 K). When such material is introduced into the Ni-50.5Ti-9.0Cuat.% alloys the fluctuation in phase transformation temperature after 1000 heat cycles is within + 1 K, and output reduction rate is 15% or less (shearing strain, 1.3% or less).

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(1990)2037.

13 H. Tamura, Y. Suzuki and T. Todoroki, Proc. Int. Conf. on Mar. Trans., 736 (1986).