ON SEQUENCES OF STABLE AND UNSTABLE REGIONS OF FLOW ALONG STRESS-STRAIN CURVES OF SOLID-SOLUTIONS - EXPERIMENTS ON CU-MN POLYCRYSTALS

The occurrence and behaviour of boundaries of load instabilities (Portevin-Le Chatelier effect) were investigated in dilute copper-rich Cu-Mn for the temperatures 20-degrees-C less than or similar to T less than or similar to 220-degrees-C and for the strain rates 1.2 x 10(-6) s-1 less-than-or-equal-to epsilon less-than-or-equal-to 4.9 x 10(-4) s-1. The measured stress-strain curves (with manganese concentration c(Mn), T and epsilon as parameters) yield the critical stresses sigma(i) and the strain rate sensitivity. In two T-epsilon intervals, named alpha and beta, two regions of stable and unstable deformation alternate along each deformation curve. The critical stresses depend sensitively on T and epsilon. Careful evaluations of thermal activation parameters using two different methods provide information about the underlying processes. For interval alpha we find a single activation energy, and for interval beta two different activation energies. Similarly, we determine a single value n almost-equal-to 1/3 for the exponent of epsilon-1 throughout interval alpha, whereas we find n almost-equal-to 1/3 for lower and n almost-equal-to 2/3 for higher stresses for interval beta. A theoretical model by Kubin and Estrin (Acta Metall. Mater., 38 (1990) 697; Phys. Status Solidi B, 172 (1992) 173) explains qualitatively the results for interval alpha. For interval beta we propose two different diffusion processes to act successively along the deformation curves. The n-values indicate that either pipe (n = 1/3) or bulk (n = 2/3) diffusion processes occur.