Recent deformation experiments on semiconductors have shown the occurrence of a break in the variation of the critical resolved shear stress of the crystal as a function of temperature. These and many other examples in the literature evidence a critical temperature at which a transition occurs in the deformation mechanism of the crystal. In this paper, the occurrence of a similar transition in two polytypes of SiC is reported and correlated to the microstructure of the deformed crystals investigated by transmission electron microscopy, which shows evidence for partial dislocations carrying the deformation at high stresses and low temperatures. Based on these results and data in the literature, the explanation is generalized to other semiconductors and a possible relationship to their brittle-ductile transition is proposed. a)
The temperature dependence of the strength of Ir 3 Nb with the L1 2 structure has been investigated using compression tests between ¡ 196 and 12008C. Below room temperature, the strength decreased with increasing temperature. An anomalous temperature dependence of the strength was observed between room temperature and 8008C; the strength increased with increasing temperature. Above 8008C, the strength again decreased with increasing temperature. The Burgers vectors of the dislocations in samples deformed at room temperature, 6008C and 12008C were investigated using both dark-® eld weak-beam imaging techniques and the equithickness fringe method with a transmission electron microscope. The ah 110i -type superdislocations dissociated into … a=3 † h 211i -type super partial dislocations bounding a superlattice intrinsic stacking fault in the samples deformed at 600 and 12008C. Dissociation of the antiphase boundary bounding dislocation was not observed at any of the deformation temperatures.
Recently, newly developed bake-hardenable (BH) steel sheets strengthened by copper sulfide (CuS) have been successfully employed in commercial production lines that supply automotive outer panels. The metallurgical concepts governing fabrication of these new BH steel sheets require keeping carbon content as low as 0.0015 wt.% without any additional amount of titanium and/or niobium for solute carbon scavenging. The role of CuS precipitates has turned out to raise the yield strength acting as a barrier against dislocation movement. In this paper, we studied the effects of chemical compositions and manufacturing process variables on the microstructure and mechanical properties of newly developed BH steel sheets. We found that the control of carbon and nitrogen showed a good balance between bake-hardenability (BH) and yield point elongation (YP-El). We identified the crystallographic relationship between the nano-size CuS precipitates and the ferrite matrix of (001)sulfide//(001)α-Fe and [001]sulfide//[001]α-Fe. We also found that the BH and YP-El were affected by the formation of aluminium nitride (AlN) and the annealing temperature.
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