Deformation twinning in nanocrystalline materials

Elemental Mg and Mg-alloy (AZ91D) based composites reinforced with 15 vol.% silicon carbide (SiC) particulates (average particle size 15 μm and 150 μm) were synthesised by stir casting technique. Particle distribution, particle–matrix interfacial reaction, hardness and mechanical properties in the as cast as well as T4 heat-treated conditions were investigated. The composite materials show uniform distribution of SiC particulates. The average grain size decreases with the presence of SiC particulates and the grain size further decreases as the particle size decreases. The AZ91D alloy composite shows an increase in hardness and elastic modulus compared to monolithic alloys. The improvement in elastic modulus of composite containing 15 μm size SiC particles is significantly higher than the composite with 150 μm size particles. The ultimate tensile strength and ductility of composite materials were reduced compared to unreinforced alloy.

Processing and mechanical properties of SiC reinforced cast magnesium matrix composites by stir casting process

In situ TiB2 reinforced Al alloy composites

In the present work, the in-situ TiB2/A356 composites were successful synthesized through the salt–metal reaction route. The research was undertaken to study the mechanical properties of the A356 alloy reinforced with TiB2 particles at both as-cast state and T6 state. In two cases, the results showed the Young's modulus of composites was increased with increasing the TiB2 content up to 8.37% in volume fraction. The dependence of Young's modulus on the TiB2 particle content can be well rationalized by the Tsai-Halpin equation. Similarly, the yield strength of composites was improved with the increase of TiB2 particle content at both states. Using the modified shear lag model, the calculated yield strength can be well fitted with the experimental values at as-cast state, but not at T6 state. Further, a phenomenological model was proposed by accounting for TiB2 particulate strengthening mechanisms and microstructural features related coefficient in TiB2/A356 composites. The predictions of yield strength using this model are in good agreement with experimental data from both as-cast state and T6 state in this work and also reported in the literatures about in-situ TiB2/Al alloy composites.

Mechanical properties of in-situ TiB2/A356 composites

Wear behaviour of AA6061 aluminium alloy and its composites

The aging response of a SiC particulate reinforced powder metallurgy aluminum composite was examined as a function of particle size and volume fraction. The addition of SiC particles ranging in size from 24 to 142μm at 9 vol pct had no effect on the aging kinetics of the composites. Acceleration of the aging behavior or inhibition of the initial stage of the age-hardening process was observed at 18 and 27 vol pct. The accelerated aging kinetics were consistent with smaller particles creating larger thermal misfit dislocation densities. In addition, it was shown that different combinations of ceramic particle size and volume fraction lead to similar effects on the aging behavior. Loss of the initial hardening response was attributed to the suppression of Guinier-Preston (GP) zone formation due to the annihilation of excess vacancies at the thermal misfit dislocations.

The effect of particle size and volume fraction on the aging behavior of a liquid-phase sintered SiC/aluminum composite

The effect of changing the content of Ta on the gamma/gamma-prime carbide microstructure was investigated in two crystalline nickel-base superalloys: conventionally cast B-1900 + Hf, and both conventionally cast and directionally solidified MAR-M247. The changes occurring in the microstructure effects were similar in both alloys. The gamma-prime and carbide volume fractions increased linearly with Ta additions, while the gamma-prime phase compositions did not change. Bulk Ta additions increased the levels of Cr and Co (in addition to that of Ta) of the gamma phase, a result of the approximately constant partitioning ratios for these two elements. The addition of Ta led to a partial replacement of Hf in the MC carbides. In addition, Cr-rich M(23)C(6) carbides formed as a result of MC carbide decomposition during heat treatment.

The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 + Hf and MAR-M247

The plastic behavior of polycrystalline hafnium (Hf) was investigated over a range of strain rates under uniaxial compression. Hafnium exhibited considerable ductility and a moderately rate-sensitive plastic behavior. The stress-strain response consisted of initial yielding followed by parabolic hardening. Microstructural observations on quasistatically deformed specimens revealed that yielding occurred by dislocation activity and that hardening was dominated by twinning on {1012} planes and by slip/twin interactions. A considerable reduction in dislocation and twinning activity was observed in specimens deformed at high strain rates. Failure occurred by shear localization and void growth and coalescence within the shear bands. Measurement of the temperature rise during high strain rate deformation was also made. From these measurements, the fraction of work converted to heat as a function of strain was determined and found to decrease with increasing strain.

Plastic deformation of hafnium under uniaxial compression

The influence of γ-γ′ eutectic on the yield strength and ductility of conventionally cast MAR-M 247 was examined. The amount of the γ-γ′ eutectic present at the interdendritic-grain boundary regions was varied by heat treatment below and above the γ′ solvus temperature. The room temperature yield strength increased and the ductility decreased as the amount of γ-γ′ eutectic was reduced. These results are attributed to the fact that the γ-γ′ eutectic possesses a lower strength and greater ductility than the dendrite cores do. As a consequence, the ductile γ-γ′ eutectic hinderes crack propagation and causes a changes in the fracture morphology as the amount of γ-γ′ eutectic is reduced.

The influence of γ-γ′ eutectic on the mechanical properties of conventionally cast MAR-M247

The effect of interfacial structure on the mechanical properties of aluminum-ceramic composite materials fabricated by liquid phase sintering was studied. The composites were based on two matrix alloys (powder metallurgy alloys 201 and 601) reinforced with either Al2O3 or SiC particulate. Characterization of the interfacial regions demonstrated that the SiC-matrix interfaces were faceted whereas the Al2O3-matrix interfaces had an incomplete layer of a silicon-rich amorphous phase. Preferential attack of the particles during sintering is believed to cause the crystallographic facets to form on SiC. Locally high silicon concentrations near Al2O3 particles led to the formation of a glassy phase from the reduction of Al2O3. The difference in interfacial structure resulted in a higher particle-matrix bond strength and therefore improved composite mechanical properties in the SiC-reinforced materials compared with the Al2O3-reinforced materials.

B. J. Pletka

Papers

The effect of particle size and volume fraction on the aging behavior of a liquid-phase sintered SiC/aluminum composite

The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 + Hf and MAR-M247

Plastic deformation of hafnium under uniaxial compression

The influence of γ-γ′ eutectic on the mechanical properties of conventionally cast MAR-M247

The influence of interfacial structure on the mechanical properties of liquid-phase-sintered aluminum-ceramic composites