Plastic Deformation and Fracture Behavior of AI-Mg and AI-Cr Alloys

Abstract

<p>This research concerns a study of the deformation behavior and dislocation substructure characteristics resulting from plastic flow in single phase Al-Mg (0-4.11 at%Mg) and Al-Cr (0-0.36at%Cr) alloys. Tensile tests are performed at 298K, 78K, and 4.2K, and strain rate sensitivity tests are performed at 78K for the Ai-Mg alloys, and at 298K and 78K for the Al-Cr alloys. Resistivity measurements are carried out during tensile tests for all alloys deformed at 4.2K. The resulting structures are then studied using SEM and TEM microscopy. Solute strengthening is seen to occur in both systems, along with significant increases in strength and work hardening capacity in all alloys accompanying decreases in temperature. The limit for benefits from solute strengthening appears to lie close to the solubility limit for the Al-Mg system, but no clear limit is observed in the Al-Cr system. Resistivity data seems to indicate that a critical dislocation density is reached before fracture in all Al-Mg alloys studied, but that this critical density decreases with Cr content. Portevin Le-Chatelier (PLC) type instabilities are observed at room temperature in the Al-Mg alloys only, though both systems exhibit adiabatic shearing processes at 4.2K. A dislocation substructure resembling those observed in other Al-Mg alloys is observed, but the Al-Cr alloy dislocation substructure more closely resembles that observed in pure Al. Both substructures are seen to show greater dislocation density, distributed more homogenously over the structure as temperature decreases.</p>