Pseudoelasticity in High Strengthening Fcc Single Crystals
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The combination of the low stacking-fault y, energy and the high level of applied stresses due to dispersion hardening (alloy I, II) and solid solution hardening (alloy III) was supposed to provide the deformation mechanism changing from slip to twinning. PE will be associated with the twinning deformation. Moving forces for the reversible movement of twins in alloys I, II will be back long range stress fields arising at a by-passing of plastically non-deforming particles by twinning dislocations. The twinning deformation will be the new mechanism of removal of local deformation gradients forming near particles. Twinning dislocations will be "geometrically necessary" ones and will be able to the reversible movement in such gradient materials [8,10]. In crystals III twinning will lead to the interstitial atoms position changing from octahedral places to tetrahedral ones [ 10,11], RESULTS AND DISCUSSION The achievement of the high level of deforming stresses leads to the deformation mechanism
2 Fig. 1(a) The concentration dependence of solid solution hardening Ar&"= ('c 2(N)- AmmM)1/2 (where T..(N), "cM are critical shear stresses for crystals with and whithout nitrogen, respectively) for single crystals of alloy III; 1, 1'-T=77 K; 2, 2'-T=300 K; 3, 3'-T=573 K; (b) the temperature dependence of x,, at CN=0.7 wt.%; (c) the dependence of critical shear stresses for slip (,c ")and twinning (t~rt) on size of second phase particles; 1, 3 - tension (alloy 1); 2 - tension (alloy II); 4 - compression (alloy I); A=l/(L-D), where L-the particle spacing, D-the particle diameter in glide plane. Open triangles, circles and squares-%,, for slip, filled triangles, circles and squares-,tcr for twinning.
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changing from slip to twinning. At tension of crystals 1,11 the orientation dependence of deforming stresses was found (Fig. I c). [111], [ 123] crystals have smaller critical shear stresses -c than [001] ones and this difference increases with the growth of the deforming stress level. Metallographic, electron microscopic and X-rays investigations have shown, that at ,>Ž140 MPa in "soft" [111], [123] orientations deformation is result of twinning (Fig.2a), the parabolic stage with the high strainhardening coefficient e at T
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