A.B. Pandey

Steady state creep behaviour of silicon carbide particulate reinforced aluminium composites

Tensile creep tests were carried out on 15SiC (vol.pct) particulate reinforced commercial pure aluminum (15%SiCp/Al) composite at 573 and 623 K. The steady state creep stage exists at the applied stresses under the condition of tension. The 15%SiCp/Al composite exhibits an apparent stress exponent of about 13 and an apparent activation energy of 253 kJ/mol. The creep data were normalized using a substructure invariant model with a stress exponent of 8 together with a threshold stress.

High-temperature creep of AlTiB2 particulate composites

Abstract Two Al20vol.%TiB2 composites produced by different processing routes, namely conventional powder metallurgy (PM) and XDTM (XDTM is a proprietary process of Martin Marietta Laboratories), were creep tested under compression in the temperature range 573–673 K to evaluate the steady state creep mechanisms. The steady state creep rate data of the conventional composite, covering alomost five orders of magnitude in creep rate, were rationalized in terms of the substructure invariant model, which predicts a stress exponent of eight and lattice diffusion control, together with the existence of a threshold stress. The higher values of the apparent stress exponent (8–14) and the apparent activation energy (339 kJ mol−1), observed for the XD composite, do not agree with the existing models for dislocation creep even after considering the threshold stress. This is possibly because of the anomalous creep behaviour of the XD composite observed at higher temperatures. A comparison of the steady sreep data for PM AlTiB2 and AlSiC composites shows that the SiC particulate provides better creep resistance than the TiB2 particulate. The XD AlTiB2 composite has better creep strength than the conventional AlTiB2 composite; however, its creep strength is inferior to the AlSiC composite. It has been suggested that an applied stress-independent load transfer mechanism is required to explain the origin of the threshold stress for steady state creep in discontinuously reinforced Al matrix composites.

Steady state creep behaviour of an AlAl2O3 alloy

Abstract The Al Al 2 O 3 alloy produced by powder metallurgy route has been tested under compression creep in the temperature range 573–723 K to evaluate the steady-state creep mechanisms. The steady state creep data covering almost five orders of magnitude in creep rate shows to distinct regimes of creep deformation at all the temperatures. In the high stress regime (region-II), the creep data shows high and variable apparent stress exponents, 25–30 and high apparent activation energy, 372 kJ/mol. However, in the low stress regime (region-I), the lower values of stress exponents, 7–11 and activation energy, 85 kJ/mol are observed. The TEM micrographs of the uncrept and crept specimens exhibit subgrains with Al 2 O 3 particles lying on the subgrain boundaries. The high stress regime data was examined in terms of pipe-diffusion controlled constant substructure creep model and thermally activated detachment controlled dislocation creep model. The detachment controlled dislocation model can describe the present data more successfully. The low stress regime data was analyzed according to the thermally activated detachment controlled diffusional creep model and pipe-diffusion controlled stress dependent substructure model. Thermally activated detachment controlled diffusional creep mechanism appears to be more appropriate for the present data.

Effect of isothermal heat treatment on the creep behaviour of an AlTiCp composite

Abstract The microstructure and steady state creep of a powder metallurgy processed Al-20 vol.% TiCp composite have been characterized in as-extruded and in heat-treated conditions (heat treated at 893 K for 24, 96 and 171 h). While as-extruded composite contains only TiC particles, the composite after heat treatment has Al3Ti and Al4C3 particles in addition to TiC. The steady state creep data of the composite show that the creep strength increases substantially after heat treatment. The increase in the creep strength is proportional to the duration of heat treatment up to a certain level and is attributed to the total increase in the volume fraction of particles. The apparent stress exponent and apparent activation energy for steady state creep of the as-extruded composite are very high, 16 and 259 kJ mol−1 respectively, like other aluminium matrix composites. The steady state creep data of heat-treated composite exhibit curvature indicating the possibility of a transition in the creep mechanism. The present results demonstrate that the creep strength of Al-TiC composite can be tailored to suit the requirement by choosing a proper heat treatment. A comparison of the creep strength of various aluminium matrix composites indicates that the Al-TiCp composite after heat treatment has the best creep strength.

Creep fracture in Al-SiC metal-matrix composites

Creep fracture behaviour of pure aluminium-matrix composites with 10–30 vol% SiC particulates at 623 K is reported. A comparison of tensile and compression creep data shows the existence of a “transition stress”. Above this transition stress no steady state creep is observed in tension. This transition stress is related to a transition from intergranular to transgranular fracture. The origin of transition stress is perhaps associated with the diffusional relaxation of stress concentration at the matrix/particle interface by lattice diffusion. The intergranular creep fracture of composites appears to be similar to that of unreinforced aluminium and it is power-law creep controlled. The transgranular creep fracture occurs by void nucleation and growth. The nucleation strain for voids is quite small and hence the tertiary stage starts before the end of the primary stage. The ductile fracture models overestimate the strain to fracture and do not predict the observed stress dependence of strain to fracture.

Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite

The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an Al-10 vol pct SiCp composite has been studied. The Al-4 wt pct Mg-10 vol pct SiCp composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol-1. These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol-1, at a constant stress, σ = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the Al-Mg-A12O3f composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the Al-Mg-SiCp, composite are compared with those of the Al-Mg-Al2O3f and 6061 Al-SiCp.w, composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the Al-SiCp composite.

An evaluation of steady state creep mechanism in an Al-Mg/26 Al2O3f composite

Abstract An analysis of the steady state creep data of Al-5Mg/Al 2 O 3f composite shows the substructure invariant creep mechanism to be operative. This is consistent with the dislocation creep mechanism map and a previous evaluation of steady state creep mechanism in 6061 Al/SiC composites. A temperature dependent threshold stress exists for the steady state creep mechanism.

On the anomalous creep behaviour of an XD Al-TiB2 composite

In recent years, discontinuously reinforced aluminum matrix composites have been identified as candidates for high temperature applications, where understanding of creep behavior of the material is important. In this study the authors report an anomalous creep behavior of an XD Al-TiB[sub 2] composite observed at higher temperatures. These anomalies include the presence of a sigmoidal primary creep stage, a temperature-dependent primary strain, and similar steady state creep strengths at 623 and 673 K. These observations differ from the normal creep behavior of Al-TiB[sub 2] composite produced by the conventional powder metallurgy route.

Microstructure of a creep tested Al-20 vol.% SiC composite

The authors conducted a transmission electron microscopy study of the Al-20 vol.% SiC composite. The purpose is to establish the state of microstructure before and after creep. For the substructure invariant model to be applicable, the microstructure should remain constant before and after creep. Conclusions are as follows: the grain size in Al-20 vol.% SiC composite remains constant during creep; the constant substructure and better correlation coefficient for n = 8 suggests the applicability of substructure invariant creep model for Al-20 vol.% SiC composite.