Y. R. Mahajan

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.

Processing map for hot working of powder

The constitutive flow behavior of a metal matrix composite (MMC) with 2124 aluminum containing 20 vol pct silicon carbide particulates under hot-working conditions in the temperature range of 300 °C to 550 °C and strain-rate range of 0.001 to 1 s-1 has been studied using hot compression testing. Processing maps depicting the variation of the efficiency of power dissipation given by [2m/(m + 1)] (wherem is the strain-rate sensitivity of flow stress) with temperature and strain rate have been established for the MMC as well as for the matrix material. The maps have been interpreted on the basis of the Dynamic Materials Model (DMM). [3] The MMC exhibited a domain of superplasticity in the temperature range of 450 °C to 550 °C and at strain rates less than 0.1 s-1. At 500 °C and 1 s-1 strain rate, the MMC undergoes dynamic recrystallization (DRX), resulting in a reconstitution of microstructure. In comparison with the map for the matrix material, the DRX domain occurred at a strain rate higher by three orders of magnitude. At temperatures lower than 400 °C, the MMC exhibited dynamic recovery, while at 550 °C and 1 s-1, cracking occurred at the prior particle boundaries (representing surfaces of the initial powder particles). The optimum temperature and strain-rate combination for billet conditioning of the MMC is 500 °C and 1 s-1, while secondary metalworking may be done in the super- plasticity domain. The MMC undergoes microstructural instability at temperatures lower than 400 °C and strain rates higher than 0.1 s-1.

Strengthening of Al/20 v/o TiC composites by isothermal heat treatment

A chemical potential gradient exists across the reinforcement/matrix interface of metal-matrix composites which serves as the basis for either diffusion or chemical reaction between the two components at high temperatures. While poor control of this reaction may degrade mechanical properties, it may also be employed to improve them. Such improvements as an increase in elastic modulus in the case of Al-Mg/SiC(p) are attributable to improved load transfer, due either to an interactive diffusion process at the interface or the keying effect of interfacial Al4C3. A systematic study is presently undertaken of the effects of 600 C exposure on the microstructure and mechanical properties of Al/20 vol pct TiC; a reduction in tensile ductility is associated with an increase in strength and elastic modulus. 5 refs.

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.

EFFECT OF EXTRUSION PARAMETERS ON STRUCTURE AND PROPERTIES OF 2124 ALUMINUM ALLOY MATRIX COMPOSITES

Discontinuously reinforced aluminum matrix composites (DRA) have been attracting attention because of their amenability to undergo deformation processing by conventional metalworking techniques. Extrusion is used in processing of DRA composites for consolidation, redistribution of reinforcements, and shape forming. The important parameters that control the extrusion process are temperature and strain rate, which is a function of several equipment/extrusion parameters. Vacuum hot-pressed (VHP) 2124 Al/30 SiCp composite billets were extruded at different ram speeds (1, 10, 100 mm sec−1) and using different extrusion ratios (4:1, 10:1, and 20:1). The extruded samples were studied for their integrity, microstructure, and mechanical properties. The integrity of the extruded composite rod was very good at minimum extrusion speed of 1 mm sec−1, whereas 100 mm sec−1 extrusion speed resulted in extensive fir tree cracking. Extrusion of VHP billets, with necklace structure, resulted in elongated alternate stringers of matrix and SiCp in the extrusion direction. Matrix stringer width and aspect ratio were found to vary with extrusion ratio. Because of the microstructural refinement, both the strength and ductility of the metal matrix composites (MMCs) were improved. Microhardness of the matrix stringers was found to be a function (power relation) of their width, irrespective of the location and extrusion ratio.

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.

Studies on fusion zone fracture behaviour of electron beam welds of an α+β-titanium alloy

A study was undertaken to understand the fusion zone fracture behaviour of electron beam welded α+β-titanium alloy Ti-6.5 Al-3.3 Mo-1.8 Zr and 0.25 Si. The effect of base metal microstructure, the amount of heat input and post weld heat treatment cycle on the all-weld tensile properties and fracture behaviour was investigated in this work. In general, it was found that the tensile strength and ductility of α+β-base welds are higher than that of the β-base welds and the difference was attributed to the presence of wider fusion zone grains of β-base welds. The β-base weld tensile specimens always exhibited an intergranular fracture mode irrespective of the amount of heat input. The single pass low heat input α+β-base welds failed by ductile transgranular fracture mode, while high heat input single pass welds failed by a mixed mode (intergranular plus faceted) fracture. In general high heat input welds showed low ductility mainly on account of the strain localization effects at the grain boundary alpha phase. Post-weld heat treatments of α+β-base welds resulted in the improvement of tensile ductility and were associated with transgranular fracture due to the absence of strain localization effects at the grain boundary alpha phase.

Processing maps for hot-working of powder metallurgy 1100 Al-10 vol % SiC-particulate metal-matrix composite

The hot-working characteristics of the metal-matrix composite (MMC) Al-10 vol % SiC-particulate (SiCp) powder metallurgy compacts in as-sintered and in hot-extruded conditions were studied using hot compression testing. On the basis of the stress-strain data as a function of temperature and strain rate, processing maps depicting the variation in the efficiency of power dissipation, given by η = 2m/(m+1), where m is the strain rate sensitivity of flow stress, have been established and are interpreted on the basis of the dynamic materials model. The as-sintered MMC exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of about 30% at a temperature of about 500°C and a strain rate of 0.01 s−1. At temperatures below 350°C and in the strain rate range 0.001–0.01 s−1 the MMC exhibited dynamic recovery. The as-sintered MMC was extruded at 500°C using a ram speed of 3 mm s−1 and an extrusion ratio of 10∶1. A processing map was established on the extruded product, and this map showed that the DRX domain had shifted to lower temperature (450°C) and higher strain rate (1 s−1). The optimum temperature and strain rate combination for powder metallurgy billet conditioning are 500°C and 0.01 s−1, and the secondary metal-working on the extruded product may be done at a higher strain rate of 1 s−1 and a lower temperature of 425°C.

Effect of homogenization treatment on fatigue behaviour of 2124Al/20 vol % SiCp composite

The fatigue behaviour of 2124Al/20 vol % SiCp composite was studied in the as vacuum hotpressed condition, as well as after a homogenization treatment subsequent to vacuum hot pressing. It was found that there was a significant improvement in the tensile strengths, fatigue threshold stress intensity range, ΔKth, and cyclic fracture toughness, Kfc, as a result of the homogenization treatment. The improvement in the properties of the composite after homogenization is attributed to the dissolution of the coarse intermetallic precipitates present in the composite in the as-vacuum hot-pressed condition.