Bauschinger effect in a modified Zr-2.5wt.%Nb pressure tube material

Abstract

Single-cycle tests, i.e. tension-unloading-compression (TUC) and compression-unloading-tension (CUT) tests, have been carried out along the axial direction of a modified Zr-2.5wt.%Nb pressure tube material to study the effect of direction of the applied stress and plastic prestrain on the Bauschinger effect. In both the tension-first and compression-first tests the material exhibits a very large Bauschinger effect. Two different methods were used to determine the back stress: one incorporated the effect of residual stresses and the other isolated the contribution of residual stresses. Calculation of back stresses using the two respective methods indicated that the large Bauschinger effect is attributable mainly to the residual stresses; these residual stresses are introduced during the preloading half-cycle as a result of the highly anisotropic mechanical properties of the zirconium grains. In the tension-first tests, as expected, the reverse (compressive) flow stress σ f c always decreases as the amount of prestrain is increased. However, for compression-first tests the reverse (tensile) flow stress σ f t only decreases initially with increasing prestrain; thereafter, for higher prestrains, σ f t is observed to increase with increasing prestrain. On the basis of research carried out by various workers on the deformation mechanisms operating in zirconium alloys, it is apparent that twinning must have occurred during the compression part of the cycle, resulting in grain reorientation such that the reoriented grain is more difficult to deform.