Akira Nohara

The anomalous strength peak and the transition of slip direction in β-CuZn

Abstract The macroscopic slip directions of β-CuZn have been determined by studying the reorientation of single crystals with three different initial orientations during tensile deformation, between room temperature and 280°C, where the anomalous strength peak takes place. The macroscopic slip directions below and above T p (the temperature at which the peak in the curve of yield stress versus temperature occurs) were quite different from each other; below T p 〈111〉 slip was predominant, while above T p non- ≥ 111 ≥ slip was predominant. This result was in good agreement with TEM observation of the dislocation structures of β-CuZn deformed below and above T p. Thus, it is concluded that the anomalous strength peak and other peculiar phenomena observed in β-CuZn at high temperatures are to be attributed to a transition of the slip direction.

Anti-phase boundary energy in β-CuZn

Abstract The ⟨111⟩ superlattice dislocations in a B2 ordered β-CuZn alloy have been studied using weak-beam electron microscopy, and the anti-phase boundary (APB) energy γ has been determined without ambiguity from the separation of two super-partials for {110} and {112} planes. Values of 50 mJ m−2 and 37 mJ m−2 are obtained for γ110 and γ112, respectively. The operative slip systems in β-CuZn at low temperatures have been discussed on the basis of the direct measurement of γ110 and γ112.

Evidence for Suzuki effect in an Fe-Ni-Cr austenitic stainless steel

Abstract The widths of extended dislocations in a single crystal of a low carbon Fe-19wt% Cr-12wt%Ni stainless steel were measured after three different treatments. They include deformation at 523 K or 673 K (treatment l), deformation at room temperature (treatment 2) and annealing at 523 K after room-temperature deformation (treatment 3). In-situ thermal cycling experiments were also carried out. The room-temperature value of the width of extended dislocations was the smallest after treatment 2 and the largest after treatment 1. During the in-situ thermal cycling experiment the width decreased with increasing temperature in a reversible manner. These features were explained by assuming that the Suzuki segregation of solute atoms toward the stacking fault ribbon of an extended dislocation is enhanced significantly during deformation at high temperatures.

Effect of Size on the Strength of Metal Whiskers

Optical and electron microscopic observations have revealed that surface steps become noticeable on metal whiskers with larger diameters. Taking into account the stress concentration at the roots of these steps, a dislocation-loop nucleation model is proposed to explain the dependence of the yield strength on size as shown by both fcc and bcc metal whiskers. The proposed model explains the observed size-strength relation very well and is free from the difficulty of the divergence of the yield stress at zero size which is common in empirically-proposed relationships. The decrease in the stacking fault energy brought about by alloying is found to decrease the yield stress.

Direct Observations of Dislocation Motion in an Al-Mg Solid-Solution Alloy in Three Different Temperature Ranges by HVEM

Effect of deformation temperature on the behavior of individual dislocations in Al-3.3 at.% Mg solid-solution alloy single crystals was investigated by in situ observations of deformation in a high voltage electron microscope (HVEM). In the low temperature range (T\lesssim0.32Tm; 290 K), the moving dislocations assumed some curvature and their motion was jerky. In the high temperature range (T\gtrsim0.60Tm; 550 K), deformation was homogeneous and homogeneous collective motion of dislocations was observed; every dislocation observed exhibited smooth and viscous motion under stress. On the other hand, in the intermediate temperature range (0.32Tm<T<0.60Tm) where P-Leffect was expected, deformation was entirely inhomogeneous and slip bands were observed to run fast.

Variational Principle for High-Temperature Plasticity in Alloy-Type Solid Solutions

A variational principle is proposed for the energy dissipation rate in two deformation modes in alloy-type Al–Mg solid solutions at high temperatures. For constant-stress creep the principle leads to the steady-creep law and a partial differential equation for the net dislocation-multiplication rate in nonsteady state. The solution to this equation gives the time development of dislocation density, from which the time development of creep strain is obtained. Fitting a theoretical creep strain vs time curve to an experimental one allows us to determine the relaxation time of creep and provides a method of measuring the average energy of a dislocation . For constant-strain-rate deformation, on the other hand, the principle leads to the steady-deformation law and another partial differential equation for the net dislocation-multiplication rate in nonsteady state. On the basis of the solution to this equation, a stress vs strain relation is obtained.