Engineering Considerations in the Application of NiTiHf and NiAI as Practical High-Temperature Shape Memory Alloys
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can be scaled up to practical, industrial heat sizes. To determine the workability of larger ingots, a series of heats was produced of the following compositions: Ni - 49.12 1/o Ti, Ni - 39 8/o Ti 12 /. Hf, Ni - 36 /oTi - 15 V/.Hf, and Ni - 36 /oTi - 15 a/o Hf- 0.1 a/. B. Each alloy was produced in the form of a triple electron beam melted 900 gm drop cast ingot, approximately 100 mm x 25 mm x 40 mm. Differential Scanning Calorimetry (DSC) of a fully annealed sample from each casting was used to determine austenite start (A,) temperatures of -48°C, 192°C, 247°C, and 248°C for the binary, 12 Hf, 15 Hf, and 15 Hf + B alloys, respectively. Samples approximately 15 mm x 15 mm x 15 mm were cut from the ingots and workability trials were conducted on each of the alloys from ambient temperature to 1093°C. A stroke rate of 6.35 mm per second was used and each sample was compressed to either failure, a maximum load of 22,680 kg, or a maximum of 50% reduction. The results are shown in Figure 1.
Figure 1.
Relative workability of NiTi and NiTiHf
Many smallcracks,
Fractured
alloys. Sample size: 15 mm x 15 mm x 15
mm, Stroke Rate: 6.35 mm per second,50%
1or2 mediumcracks
Manylargecracks
Many medium cracks, 1 or 2 large cracks
reduction.
1or2small cracksM3 Nc k N
M
Ni - 36Ti - 15Hf - 0.1 B As = 248°C, Af = 262'C Ni - 36Ti - 15Hf As = 247'C, Af = 260'C Ni - 39Ti - 12Hf As = 192'C, Af = 215'C Ni - 49Ti As = -48'C, Af = -25'C I
RT
538
I
I
I
593
649
704
I
760
I
816
871
I
I
I
I
927
982
1038
1093
Working Temperature (°C)
The results of the workability tests confirm that increasing Hf content has a negative effect on the workability. Further, B additions, which have been shown to improve the workability of NiTiPd alloys 1,appear to have little (if any) effect on NiTiHf workability. These trials also confirmed the work of AbuJudom, et al t, indicating that a suitable hot working temperature for these alloys is between 800 and 900"C. Based on these results, a 20 mm thick section from the bottom of the Ni - 36 V/.Ti - 15 O/oHf ingot was canned in mild steel and hot rolled at 850'C to a final thickness of 2.7 mm with 1.3 mm reduction per pass and anneals between every pass (see Figure 2). Additional sections were taken from farther up on the same ingot and were rolled using the same parameters. Unfortunately, the additional sections fractured readily. The cause of the poor fabricability of these sections is possibly related to both grain structure and intergranular precipitates. First, the section which was successfully rolled was taken from the bottom of the ingot (close to the chill) where the grains are expected to be finer. The other sections were taken from near the top of the ingot where the grains are coarser. Apparently a fine grain structure may be necessary for good fabricability. Second, metallography of the Ni - 36 a/o Ti - 15 u/. Hf casting revealed intergranular precipitates (see Figure 3). Scanning electron microscopy indicated that these precipitates are probably (Ti+Hfj2Ni. In another stud
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