Ni₃(Al, Ti) 単結晶における高温降伏現象

抄録

A Ni₃(Al, Ti) single crystal with orientation near [001] showed a new type of yielding accompanied by a remarkable yield drop and a successive steady state deformation when it was deformed under a certain test condition at elevated temperatures. The characteristics of this high temperature yielding phenomenon (H.T.Y.) were studied by compression tests at various temperatures from 77 to 1273 K with strain rates varying from 1.4×10⁻² to 1.4×10⁻⁵ s⁻¹.
Results obtained are as follows: (1) {111} slip operates predominantly at higher temperatures beyond the maximum in flow stress under the conditions where the H.T.Y. occurs. (2) The state equation of this high temperature steady state deformation can be expressed as the same type as that of steady state creep. The value of the stress exponent is about 3 and the activation energy is nearly equal to that for diffusion. This suggests that {111} slip deformation is controlled by the viscous motion of dislocations. (3) In the H.T.Y., the stress drop was not so rapid and a Lüders band propagation was not observed. These characteristics are consistent with those of the Johnston type, so that the appreciable yield drop is considered to be due to the increase of mobile dislocation density during plastic deformation. (4) Using state equation obtained, the condition for the development of the H.T.Y. can be derived, by which H.T.Y. is expected only for single crystals having near ⟨100⟩ orientations. This prediction may well accounts for the experimental result.