We report mainly on resistance R(T) measurements between 4.2 and 1300 K on
NixTi100-x alloys, with 51 < x < 54.5at%, quenched from different annealing
temperatures T-A to room temperature. When quenched from the B2-phase stab
ility range (T-A = 1273 K), alloys with 51 < x < 54 show an increase in R(T
) with decreasing T below 300 K. Subsequent annealing at T-A = 653 K (1 h)
and quenching leads to a reduction of the R(T) anomaly below 320 K and the
occurence of a martensitic transition (MT) from the B2- to the R-phase, wit
h T-R = 310 K, rather independent of x. After annealing at 723 K (1 h) and
823 K (1 h), respectively, two-step MTs occur from B2 to R and subsequently
to B19', with M-S(B19') depending on x and T-A. After annealing at 923 K o
r higher T-A, MTs cannot be found anymore, and the R(T) behaviour is simila
r to that after quenching from 1273 K. Studies of R(T) at high T on samples
quenched from 1273 K reveal the occurence of mainly two annealing stages.
The first one at around 500 K marks structural changes inducing the martens
itic phases at lower temperatures. The second one at about 900 K marks the
formation of the B2-phase and the disappearance of other phases triggering
the MT. The R(T) results are compared with the thermal expansion a(T) and X
-ray investigations. The structural phase diagram of Ni-Ti around NiTi is d
iscussed. (C) 1999 Elsevier Science S.A. All rights reserved.