Key Engineering Materials Vols. 345-346

Abstract: The cyclic deformation behavior of 6061Al-T6 alloy was studied by uniaxial cyclic straining or stressing tests at room and high temperatures. The cyclic softening/hardening feature of the material and its effect on the uniaxial ratcheting occurred under asymmetrical cyclic stressing were discussed. The cyclic deformation behaviors of the material presented at various loading rates and with certain peak and/or valley strain or stress holds were addressed. It is concluded from the experimental results that 6061Al-T6 alloy presents slightly cyclic softening feature, and its cyclic deformation is time-dependent even at room temperature. The ratcheting strain produced at lower stressing rate and with longer hold-time is apparently larger than that at higher stressing rate and with fewer hold-time. Some significant conclusions useful to construct the constitutive model to describe such cyclic deformation behaviors are obtained.

Abstract: To evaluate the mechanical deformation phenomena of dental abutment screw after tightening and loosening, five species of abutments were selected (3i-three, Avana- two) respectively by two pieces. The abutment screws were repeatedly tightened and removed 20 times with a digital controller. Field emission scanning electron microscope (FE-SEM) was used to observe changes of each part caused by repeatedly tightening and loosening experiment. As the number of tightening and loosening was increased, the wear or distortion of hexed or squared slot that contacted with the driver tip was more severely progressed. Wear or distortion of hexed slot was more severe than that of squared slot and it was more remarkable in the titanium screw than in the gold screw. All the tested screws showed that the width in the crest of their screw thread decreased gradually as the test was proceeded.

Abstract: Mg-3%Zn-0.2%Zr based alloy sheets with various alloying elements additions were fabricated through thermo-mechanical process, and their microstructure and mechanical properties were investigated at room and elevated temperatures. CCV(conical cup value) and V-bend tests were also carried out to evaluate the formability of the fabricated alloy sheets. The experimental results showed that small amounts of Sn or Sr additions could improve the elongation at elevated temperatures, even though the room temperature tensile properties were slightly deteriorated by the Sr addition.

Abstract: In order to make clear the micro-yielding mechanisms of polycrystalline metals including twins, the movement of dislocations in the surface grains of Cu-6.8at%Al alloy and pure Mg polycrystals during the early stages of deformation was directly observed by using etch pit technique. The fresh dislocations multiply from the Frank-Read sources within the grains, and pile up against the twin and grain boundaries of two kinds of specimens. The pile-up dislocations on the primary and/or secondary slip planes are also confirmed in Cu-6.8at%Al alloys. Especially during the compressive loading for pure Mg, the occurrence of deformation twins is remarkable with an increase of strain rate, whereas the distribution of fresh dislocations tends to decrease in the surface grains. The present results suggest that the effect of twin boundaries on micro-yielding is almost equivalent for that of grain boundaries, which act as barriers to moving dislocations even in the pre-yield deformation.

Abstract: The effect of sub-grain on the yield stress of pure copper single crystals with the [253] orientation was investigated by using the etch pit technique. The single crystal plates were successfully prepared from the seed crystals, which were produced at the melting temperature of 1473
K by the Bridgeman method. The present investigation confirmed the Hall-Petch relation concerning the effect of sub-grain boundaries on the macroscopic yielding of pure copper. The result derived from the extrapolation of the relationship of critical resolved shear stress (CRSS) and the initial dislocation density and sub-grain size is in good agreement with the evaluation in high purity copper single crystals of low dislocation density.

Abstract: In this paper, the mechanical properties of several aluminum alloys are obtained experimentally at different temperatures, from room temperature to about 4000C by using the Gleeble 1500 thermomechanical system. Besides, the thermal softening characteristics, effects of heating rate and temperature holding time are discussed in details. It is found that the strength of LF6, a kind of antirust aluminum alloy, is not sensitive to the heating rate (0.1-1000K/s) and temperature holding time (0-1h). However, the mechanical behaviors of other alloys depend on these factors very obviously. At last, these phenomena are discussed in the viewpoint of the microstructures and techniques of ageing strengthening of these alloys.