H. Hosoda et al., MARTENSITE-TRANSFORMATION TEMPERATURES AND MECHANICAL-PROPERTIES OF TERNARY NITI ALLOYS WITH OFFSTOICHIOMETRIC COMPOSITIONS, Intermetallics, 6(4), 1998, pp. 291-301
Effects of ternary additions, Co and Cr, on martensite transformation
temperatures and mechanical properties of offstoichiometric NiTi alloy
s were investigated. Martensite transformation start temperature (M-s)
and austenite transformation start temperature (A(s)) were determined
by means of differential thermal analysis (DTA) in the temperature ra
nge between 77 and 423 K. Mechanical properties of yield stress and wo
rk-hardening coefficient were investigated using B2-stabilized NiTi al
loys, whose alloy compositions were based on Ni-49 mol%Ti. It has been
shown that (1) M-s and A(s) considerably decrease with decreasing Ti
concentration on the Ti poor side of stoichiometry (the concentration
of Ti is less than 50 mol%); (2) M-s and A(s) change slightly when Ti
concentration is more than 52 mol%, where the alloys are in the two ph
ase region; and (3) M-s and A(s) decrease with increasing Cr and Co co
ntent with a constant Ti concentration. In the present analysis for ev
aluating the effect of ternary additions on M-s, we use M-s change by
adding 1 mol% of ternary elements. M-s changes by Cr addition are -65
K mol%(-1) on the Ti rich side and -46 K mol%(-1) on the Ti poor side.
M-s changes by Co addition are -15 K mol%(-1) on the Ti rich side and
-30 K mol%(-1) on the Ti poor side. Both Cr and Co atoms are sug gest
ed to locate Ni sites preferably if only M-s changes are considered. C
haracteristic stress-strain curves have indicated that the stress indu
ced martensitic transformation (SIMT) occurred at 77 K. Above room tem
perature, work hardening coefficient at a permanent strain of 1% was f
ound to be between 2 and 11 GPa. It should be emphasized that yield st
ress and work hardening coefficient increase with increasing test temp
erature between room temperature and about 650 K in most alloys. This
strength anomaly is not related to SIMT but to precipitation hardening
and/or anomalous dislocation motion, probably in a similar manner to
B2-type CoTi. (C) 1998 Elsevier Science Limited. All rights reserved.