The Influence of Hafnium Content, Cold Work, and Heat Treatment on the R-Phase Transformation of Niti Based Shape Memory
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t. Res. Soc. Symp. Proc. Vol. 459 0 1997 Materials Research Society
in vacuum. The ingots were at centerless ground rod having uniform Thewires rods were encased in steel, hot rolled 9000 C and then into colda drawn into awires. Thediameter. cold drawn were repeatedly vacuum annealed at 700°C to permit continued cold drawing to obtain different amounts 0 of internal stress (CW), which are listed in Table I. The annealed (700 C HT) Af is 121°C for the Ni49 TisoHf1 alloy and 132 0 C for the Ni49Ti48 Hf3 alloy. A low speed diamond blade saw was used to prepare differential scanning calorimetry (DSC) specimens from the cold worked wires. The weight of the DSC specimens was 10-±lmg. The DSC specimens were heat treated at selected temperatures (see Table I) for 1 hour in an argon atmosphere and furnace cooled in argon to room temperature. TA Instruments Model 2200 DSC test equipment was used to determine the A-4R and R--*A IT's of the DSC specimens. The cooling and heating rate during the DSC run was 100 C/min. Table I. SMA composition, CW and HT temperature used in this investigation Composition
Af
CW
HT Temperature
19.6%, 22.7%, 29.5%, 37.6%, 43.6%, 50.4%, N4Tl
I 6C60.3%, 66.8%, 71.5%, 74.1%
N4i5 fl11C16.0%, 24.9%, 28.2%, 36.1%, 46.7%, _____
Ni49Ti48Hf3
60.0%
__51.1%,
132°C
16.5%, 25.4%, 28.2%, 36.5%, 47.0%, 51.4%
300°C 350°C 4000 C 450oC
500oc
550°C 6000 C
RESULTS Typical DSC curves are shown in figure 1 for the Ni 49 Ti50 Hf1 alloy having 46.7% CW and 450°C HT. The complete cycle (A-R-)RM-*A) and partial cycle (A-*R---)A) with peak ITls are shown. In the complete cycle, A transforms to R on cooling and then to M on further cooling. On heating, M transforms to A. For the partial cycle, the cooling of the SMA is stopped before R transforms to M. Upon heating, R transforms back to A, however, at a temperature lower than the temperature that M transforms to A. In figures 2 and 3, the influence of 6 ___complete DSC cycle CW and HT temperature on the A.-R----------partial DSC cycle and R-*A transformations for NiTi based 4 34 0 A> opeecce alloys with 0, 1 and 3at% Hf is shown. 1l.1,4 C 43.29oC (A->R partial cycle) For the same thermal-mechanical 2 , processing conditions (CW and HT), the • A- R and R-• A ITs decrease with the Ni49 Ti 51 alloy with approximately 51% CW and 4500 C HT, the A-•R and • -2 R--*A TTs are 60 0 C and 65°C :• respectively. For the Ni49 TisoHfl alloy with the same CW and HT, the A*R and -4 R -*A TTs are 44°C and 50°C491CR-A respectively. For the Ni49 Ti 48 Hf 3 alloy -6 with the same CW and HT, 0 the A-*R and 0
R
.-
re A ~s 1 C
nd
0C
respectively. Figure 4 shows the8 relationship between Hf content and the I'T of the A--*R and R-->A transformations for the SMAs with approximately 51% CW and 450°C HT.
NiTiHfl (46.7%CW,450 0 CHT)
-150
-100
0 50 100 150 200 Temperature ("C) Figure 1. DSC scans for complete and partial cycles of Ni49Ti5 oHf1 SMA.
282
-50
63.97°C (M->A)
The results shown in figures 2 and 3 also show that for the three alloys investigated, the A-+R and R-4A tr
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