F.A. Mirza

A Unified Model for the Prediction of Yield Strength in Particulate-Reinforced Metal Matrix Nanocomposites

Lightweighting in the transportation industry is today recognized as one of the most important strategies to improve fuel efficiency and reduce anthropogenic climate-changing, environment-damaging, and human death-causing emissions. However, the structural applications of lightweight alloys are often limited by some inherent deficiencies such as low stiffness, high wear rate and inferior strength. These properties could be effectively enhanced by the addition of stronger and stiffer reinforcements, especially nano-sized particles, into metal matrix to form composites. In most cases three common strengthening mechanisms (load-bearing effect, mismatch of coefficients of thermal expansion, and Orowan strengthening) have been considered to predict the yield strength of metal matrix nanocomposites (MMNCs). This study was aimed at developing a unified model by taking into account the matrix grain size and porosity (which is unavoidable in the materials processing such as casting and powder metallurgy) in the prediction of the yield strength of MMNCs. The Zener pinning effect of grain boundaries by the nano-sized particles has also been integrated. The model was validated using the experimental data of magnesium- and titanium-based nanocomposites containing different types of nano-sized particles (namely, Al2O3, Y2O3, and carbon nanotubes). The predicted results were observed to be in good agreement with the experimental data reported in the literature.

Cyclic Deformation Behavior of a Rare-Earth Containing Extruded Magnesium Alloy: Effect of Heat Treatment

The present study was aimed at evaluating strain-controlled cyclic deformation behavior of a rare-earth (RE) element containing Mg-10Gd-3Y-0.5Zr (GW103K) alloy in different states (as-extruded, peak-aged (T5), and solution-treated and peak-aged (T6)). The addition of RE elements led to an effective grain refinement and weak texture in the as-extruded alloy. While heat treatment resulted in a grain growth modestly in the T5 state and significantly in the T6 state, a high density of nano-sized and bamboo-leaf/plate-shaped β′ (Mg7(Gd,Y)) precipitates was observed to distribute uniformly in the α-Mg matrix. The yield strength and ultimate tensile strength, as well as the maximum and minimum peak stresses during cyclic deformation in the T5 and T6 states were significantly higher than those in the as-extruded state. Unlike RE-free extruded Mg alloys, symmetrical hysteresis loops in tension and compression and cyclic stabilization were present in the GW103K alloy in different states. The fatigue life of this alloy in the three conditions, which could be well described by the Coffin–Manson law and Basquin’s equation, was equivalent within the experimental scatter and was longer than that of RE-free extruded Mg alloys. This was predominantly attributed to the presence of the relatively weak texture and the suppression of twinning activities stemming from the fine grain sizes and especially RE-containing β′ precipitates. Fatigue crack was observed to initiate from the specimen surface in all the three alloy states and the initiation site contained some cleavage-like facets after T6 heat treatment. Crack propagation was characterized mainly by the characteristic fatigue striations.

A modified Johnson-Cook constitutive relationship for a rare-earth containing magnesium alloy

Abstract Lightweight magnesium alloy has recently attracted a considerable interest in the automotive and aerospace industries to improve fuel efficiency and reduce CO 2 emissions via the weight reduction of vehicles. Rare-earth (RE) element addition can remarkably improve the mechanical properties of magnesium alloys through weakening crystallographic textures associated with the strong mechanical anisotropy and tension-compression yield asymmetry. While the addition of RE elements sheds some light on the alteration in the mechanical anisotropy, available information on the constitutive relationships used to describe the flow behavior of RE-containing magnesium alloys is limited. To establish such a constitutive relationship, uniaxial compressive deformation tests were first conducted on an extruded Mg-10Gd-3Y-0.5Zr (GW103K) magnesium alloy at the strain rates ranging from 1×10 −1 to 1×10 −4 s −1 at room temperature. A modified Johnson-Cook constitutive equation based on a recent strain hardening equation was proposed to predict the flow stresses of GW103K alloy. Comparisons between the predicted and experimental results showed that the modified Johnson-Cook constitutive equation was able to predict the flow stresses of the RE-containing magnesium alloy fairly accurately with a standard deviation of about 1.8%.

Cyclic Deformation of Rare-Earth Containing Magnesium Alloys

Cyclic deformation characteristics of a rare-earth (RE) element containing extruded Mg-10Gd-3Y-0.5Zr (GW103K) magnesium alloy were evaluated via strain-controlled low-cycle fatigue tests under varying strain amplitudes. Microstructural observations revealed that this alloy consisted of fine equiaxed grains and a large number of RE-containing precipitates. Unlike the RE-free extruded magnesium alloys, this alloy exhibited essentially cyclic stabilization and symmetrical hysteresis loop due to relatively weak crystallographic textures and reduced twinning-detwinning activities. The fatigue life of the present alloy was observed to be longer than that of the RE-free extruded magnesium alloys, which could also be described by the Coffin-Manson law and Basquins equation. Fatigue crack was observed to initiate from the specimen surface and crack propagation was basically characterized by fatigue striations.