The peculiar temperature and orientation dependence of L1₂-type Co₇₄Ni₃Ti₂₃ single crystals

Abstract

The flow behaviour of L1₂-type Co₇₄Ni₃Ti₂₃ single crystals has been investigated in compression tests as a function of orientation, test temperature and strain rate. The measured critical resolved shear stress (CRSS) shows a minimum around 473 K (denoted as T _b) and a maximum around 1000 K (denoted at T _p). A negative temperature dependence of the CRSS is observed below T _b and above T _p, and a positive temperature dependence of the CRSS is observed between T _b and T _p. Slip trace analysis shows that the (111) primary octahedral slip plane is operative below T _p and both (111) and (001) slip planes are activated around and above T _p. The CRSS is almost strain-rate independent for (111) slip and has a positive dependence for mixed (111) and (001) slip.

A model involving a thermally activated process of superdislocation core transition from sessile segments to glissile segments is proposed to explain the temperature and orientation dependence of the CRSS at low temperatures. The orientation dependence of the CRSS on N-value (the Schmid factor ratio between slip systems (010)[01] and (111)[01]) and Q-value (the Schmid factor ratio between slip systems (111)[11] and (111)[01]) indicated that a cross-slip mechanism is responsible for the anomalous increase of the CRSS with increasing temperature between T _b and T _p. The CRSS for (111)[01] slip is then expressed as [tacute]u = [tacute]u_G + [tacute]u_n + [tacute]u_p, where [tacute]u_G is a temperature-insensitive stress component and represents the CRSS required to move the glissile segment of the superdislocation, and [tacute]u_n and [tacute]u_p are stress components attributed to two thermally activated processes at low and high temperatures, respectively. Very good agreement is obtained between the calculated and measured CRSS-temperature curves.