Dynamic measurement of residual strains by x-ray diffraction in a metal matrix composite during rapid temperature cycling (aluminum)

This study was undertaken to characterize the temperature dependence of the residual stresses in an AA339 Al alloy reinforced with 23%(vol.) Kaowool short fibres. This material is being considered for service in a piston head by General Motors Corp. An experiment was designed to measure the residual stresses by x-ray diffraction dynamically during isothermal stress relaxations at 100{dollar}spcirc{dollar}C and 300{dollar}spcirc{dollar}C with rapid temperature ramping of 200{dollar}spcirc{dollar}C/min between the two states. This simulates the thermal cycling conditions between an idling engine (100{dollar}spcirc{dollar}C) and an accelerating one (300{dollar}spcirc{dollar}C).; A linear position sensitive detector was used to measure a peak profile in 5 seconds, and a novel high temperature stage was developed with a very low thermal mass and thermal expansion. The x-ray experiments were supplemented by an elastic model to examine the strain distribution due to a periodic array of cuboidal inclusions.; Stress relaxations at 100{dollar}spcirc{dollar}C after rapid cooling from 300{dollar}spcirc{dollar}C were observed in some of the runs. It was found that a slow rate thermal cycle (5{dollar}spcirc{dollar}C/min heating and cooling) eliminated the relaxations in cycles immediately following it. Several rapid cycles were required after which the relaxations reappeared. This behaviour can be attributed to the development of a fine dispersion of incoherent precipitates. These can be redissolved in a slow cycle, but are stable during rapid ones. High stresses developed during low temperature cooling unpin dislocations from their coherent precipitates and this unpinning reduces the elastic stresses when these particles become incoherent. (Abstract shortened by UMI.)...