Norio Furushiro

Strain rate dependence of internal and effective stresses in superplastic deformation

Abstract Flow stresses at a wide range of the strain rate have been analyzed by evaluation of an internal stress and an effective stress in the flow stress for the superplastic Zn-Al eutectoid. A sigmoidal relationship between the maximum stress on the stress-strain curve and the strain rate has been observed in the double logarithmic plot, so that deformation behavior is devided into three regions I, II (superplastic region) and III, with an increase in the strain rate. It has been found also that the rate of the internal stress σi to the flow stress σ, increased with a decrease in the strain rate ϵ. The rate, σ i σ was unity in region I. The strain rate sensitivity exponent of the internal stress σi or the effective stress σe(= σ − σi), mσi or mσe, (σ = Kϵm, K: constant) was evaluated to be 0.2 or 0.8 in region II, respectively. The activation energy, the grain-size exponent of the flow stresses and the contribution of grain boundary sliding to the total strain have been estimated systematically for each deformation region. It has been considered that the internal stress is a back stress to dislocation motion, while the effective stress induces the dislocation movement which results in the grain boundary sliding or in the dislocation creep in region II or III, respectively. Basing on results obtained, the deformation mechanism in each region and the transition between neighboring regions will be discussed.

Low temperature superplasticity and intragranular misorientation in severely rolled sheets of Al-4.5%Mg-0.7%Mn based alloys

Severely rolled sheets of Al-4.5%Mg-0.7%Mn based alloys have been prepared to achieve grain refinement leading to low temperature superplasticity. Rolling is one of straining technique which has a high possibility of industrial application. The alloys used were T1: base one adding 0.08%Zr and 0.05Ti for suppression of grain growth, F1: 0.80%Fe addition and F2: 1.92%Fe addition to increase nucleation sites of recrystallization. For comparison, M1: Al-4.2%Mg was also prepared. All of samples were 99.8% cold rolled to thin sheet 0.1mm thick. The maximum elongation of 150%, which was markedly large in consideration of specimen thickness of 0.1mm, was obtained at 533K and 1.4×10-3s-1 for T1 sample while the elongations for F1 and F2 showed less than 100%. On the other hand, M1 exhibited larger elongation of 190% at 533K and 1.4×10-3s-1. Further, difference in superplastic deformation of these alloys was investigated by using SEM/ EBSP analysis of intragranular misorientation, which reflects strain or stored energy generated during deformation with dislocation glide, to discuss the deformation mechanism.

High Temperature Deformation and Crystallographic Orientation Distribution for an Al-Mg-Mn Alloy Sheet Worked by Continuous Cyclic Bending

High temperature deformation and crystallographic orientation distribution of the Al-MgMn sheet worked by the continuous cyclic bending (CCB) and the subsequent annealing have been investigated. The sheet consists of a coarse-grained surface and fine-grained center layers. The elongation to failure has a peak value at 713K at initial strain rates of 5.6×10 -4 s -1 and 5.6×10 -3 s -1 in both of as-received sample (0P) with fine grains and CCBent and annealed one (20P_A) with the coarse and the fine grains in spite of different microstructures. The m value decreases for 20P_A and increases for 0P with increasing temperature. However, the increase of the m value is not correspondent to the change in the elongation. The deformation mechanism is discussed with relation to activation energy. The SEM micrographs of the original surfaces of tensile specimens deformed to failure reveal that at the relatively high temperatures many cracks are formed inside the coarse grains. The change in crystallographic orientation distribution is investigated during tensile testing in consideration of the deformation mechanism.

PHASE DECOMPOSITION PROCESS OF A RAPIDLY SOLIDIFIED Al-3 wt% Hf-0.3 wt% Si ALLOY DURING AGING

The phase decomposition sequence of a solid solutioned Al-3wt%Hf-0.3wt%Si alloy during aging at 723 K has been investigated mainly by an electron diffraction technique. As a result, it is suggested that a possible structure of the H phase, which was found as a new transition phase of Al 3 Hf from the Ll 2 (metastable) to the DO 22 (equilibrium) phases, is considered to be P mmm in space groups, as shown in Fig. 2 in the text. On the basis of this structure, a reasonable sequence for the decomposition process of this alloy is discussed with proposed mechanisms involving atomic shears in every two planes of 1/2[110](110)Ll 2 and 1/2[010](101) H for the transitions from the the Ll 2 and the H structures to the H and the DO 22 structures, respectively.