Residual stresses in fibre- and whisker-reinforced aluminium alloys during thermal cycling measured by X-ray diffraction

Residual stresses have been determined using X-ray diffraction in two different metal matrix composites, viz. a squeeze-cast Al-2%Mg matrix with 10, 20 or 26 vol.% Al₂O₃ fibres and an extruded AA 6061 alloy with 25 vol.% SiC whiskers. The composites have been studied after thermal cycling between 240 or 250 °C and room temperature succeeded in some cases by quenching to liquid nitrogen temperature. On the squeeze-cast composite, stresses were measured at room temperature and in situ at 240 °C. X-ray stress determinations were compared with the stress values calculated by a modified Eshelby model for equivalent inclusions. By the model, the stresses can be accurately predicted for both composite systems. Thermally induced plastic relaxation reduces the residual stresses. The degree of reduction depends on the reinforcement volume fraction, the difference in coefficient of thermal expansion between the phases and the magnitude of the temperature drop. At elevated temperature the stresses are less responsive to reiterated quenching and heating.     

Measurement of the evolution of internal strain and load partitioning in magnesium hybrid composites under compression load using in-situ synchrotron X-ray diffraction analysis

Energy dispersive synchrotron X-ray diffraction analysis has been applied to evaluate the evolution of average internal elastic lattice strains under compression load within the phases of magnesium hybrid composites, reinforced by silicon-carbide particles and Saffil® alumina short fibres. This allows for the calculation of phase stresses and thus the load partitioning. The mean elastic misfit stresses were calculated using an Eshelby type modelling. Considering the external load, prediction of the phase-specific stresses for elastic composite deformation was performed and the results were compared to the experimental data obtained.Matrix elastic lattice strains reveal high plastic anisotropy due to the activation of different deformation modes in the form of crystallographic slip and mechanical twinning. The formation of twins, verified by diffraction intensity shifts due to crystallographic reorientation, was found to affect the sharing of load between the participating phases. Consequently different regimes of composite deformation were specified. This comprises elastic regions characterized by linear strain and stress growth for all phases as well as plastic regions showing nonlinear distributions.     

Forming behavior and workability of 6061/B4CP composite during hot deformation

Compression tests of 6061/B₄C_P composite have been performed in the compression temperature range from 300 °C to 500 °C and the strain rate range from 0.001 s⁻¹ to 1 s⁻¹. The flow behavior and processing map have been investigated using the corrected data to elimination of effect of friction. The processing maps exhibited two deterministic domains, one was situated at the temperature between 300 °C and 400 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹ and the other was situated at the temperature between 425 °C and 500 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹.The estimated apparent activation energies of these two domains, were 129 kJ/mol and 149 kJ/mol, which suggested that the deformation mechanisms were controlled by cross-slip and lattice self-diffusion respectively. The optimum parameters of hot working for the experimental composite were 350 °C - 0.01 s⁻¹ and 500 °C - 0.01 s⁻¹. In order to exactly predict dangerous damaging mechanism under different deformation conditions exactly, Gegel’s criterion was applied to obtain processing map in the paper. The result showed that the processing map used Gegel’s criterion can be effectively to predict the material behavior of the experimental composite.     

Mechanical and fracture properties of an AZ91 Magnesium alloy reinforced by Si and SiC particles

A commercial AZ91 magnesium alloy (nominal composition Mg–9%Al; 1%Zn; 0.3%Mn, balance Mg in weight percent) reinforced with SiC particles and modified by the addition of Si has been used in this study. Formation of an “in situ” composite (Mg–Mg₂Si) results in strong bonding between Mg₂Si and the matrix interface. Samples were deformed in compression in the temperature interval from room temperature up to 300 °C. Stress relaxation tests were performed with the aim to reveal the thermally activated processes. Reinforcing effect of SiC and Mg₂Si particles decreases with increasing temperature. The estimated values of the activation volume as well as the activation enthalpy indicate that the main thermally activated process is connected with a rapid decrease of the internal stress. Fracture properties were studied in impact tests at various temperatures. A ductility enhancement was found at 200 °C and temperatures above 200 °C.