Nicole Stanford

Effect of Precipitate Shape and Habit on Mechanical Asymmetry in Magnesium Alloys

Asymmetric yield behavior in tension and compression is a common and usually undesirable feature of wrought magnesium alloys. To prevent yield asymmetry, it is necessary to favor slip over twinning, as it is the unidirectional nature of twinning combined with the strong textures produced in wrought magnesium alloys that produce yield asymmetry. In this article, the potential to use precipitates to strengthen selectively against twin growth is discussed. The effect of precipitates shape and habit on strengthening of slip and twinning is calculated using simple Orowan-based models. It is shown that basal plate precipitates, although being poor strengtheners against basal slip, are good strengtheners against twin growth. This is because they produce the maximum unrelaxed back-stress when they remain unsheared inside the twin. The predictions of the model have been validated against experiments for two alloys that form different precipitate types: AZ91 (basal plates). and Z5 (c-axis rods). Crystal plasticity modeling has been used to predict that an optimized distribution of basal plate precipitates is expected to strongly reduce yield asymmetry, even in strongly textured magnesium alloy.

{101¯2} Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.

Deformation and annealing of (011)[011̄] oriented Al single crystals

Abstract High purity Al single crystals of the (011)[01 1 ] orientation have been deformed in plane strain compression in a channel die. Deformation was carried out at a strain rate of 0.01 s −1 to true strains of 0.5 and 1.0, and at temperatures of 25, 200 and 300 °C. The as-deformed microstructure has been characterized using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). No recrystallization was detected after deformation, and the deformation texture analysis showed that the stability of the orientation decreased with increasing temperature, contrary to reports for other orientations. Annealing was carried out for various times at 300 °C. Nucleation of recrystallization exhibited periodicity, with distinct bands of recrystallized grains forming parallel to the transverse direction. This recrystallized microstructure has been examined using EBSD. A model is proposed to account for the origin of the periodicity of nucleation and the retention of rods or cylinders of unrecrystallized material after significant annealing times.

Observation of {1121} twinning in a Mg-based alloy

The magnesium alloy Mg–5%Y–2%Nd–2%RE–0.5Zr, known as WE54, was heat treated to produce different particle dispersions. Specimens were then compressed to a strain of 8%, and this resulted in prolific mechanical twinning. EBSD analysis revealed that twins were operative in this alloy, a twinning mode not reported before in magnesium alloys. Activation of this twinning mode is ascribed to the presence of alloying elements in solution. Removal of alloying elements from solution by precipitation treatments completely inhibited this twin mode.

Effect of Cooling Rate on Phase Transformations in a High-Strength Low-Alloy Steel Studied from the Liquid Phase

The phase transformation and precipitation in a high-strength low-alloy steel have been studied over a large range of cooling rates, and a continuous cooling transformation (CCT) diagram has been produced. These experiments are unique because the measurements were made from samples cooled directly from the melt, rather than in homogenized and re-heated billets. The purpose of this experimental design was to examine conditions pertinent to direct strip casting. At the highest cooling rates which simulate strip casting, the microstructure was fully bainitic with small regions of pearlite. At lower cooling rates, the fraction of polygonal ferrite increased and the pearlite regions became larger. The CCT diagram and the microstructural analysis showed that the precipitation of NbC is suppressed at high cooling rates, and is likely to be incomplete at intermediate cooling rates.

Complex precipitation phenomena in strip cast steels with high sulfur and copper contents

A series of three steel alloys with increasing Cu and S concentrations has been prepared by simulated direct strip casting. It was found that the rapid solidification that occurs during direct strip casting results in the formation of a high number density of fine MnS precipitates, while Cu was retained in solid solution above equilibrium concentration. Upon ageing the MnS particles were found to coarsen and increase in volume fraction, indicating that some S was retained in solid solution in the as-cast condition. Ageing also resulted in the precipitation of Cu-rich precipitates. A new method to determine precipitate composition from small-angle neutron scattering is presented. This methodology, in conjunction with atom-probe tomography, has been used to show that the composition of the Cu-rich precipitates depends on the alloys bulk Cu content.

Magnesium extrusion alloys: a review of developments and prospects

ABSTRACT Magnesium (Mg) alloys have received a significant interest in the past 20 years, owing to a nonlinearly increasing demand for lightweight structural materials. Magnesium extrusions alloys to date have had lower industrial uptake than their counterpart aluminium extrusion alloys, predominantly due to lower extrudability and formability, tension-compression yield asymmetry and no clear advantage in the specific strength. Any improvement in extrusion alloy properties requires a better understanding of the effects of alloy composition and processing conditions; and how these dictate the final alloy microstructure. This review sheds insightful information on the processing–microstructure–property relationships of extruded magnesium alloys. Historical and recent progress in magnesium extrusion alloys is critically reviewed, including the advances in extrudability, mechanical properties and microstructural characterisation. The challenges associated with the ‘gap’ in properties between the magnesium and aluminium extrusion alloys are identified, and prospects discussed regarding the development of high performance magnesium extrusion alloys.

Recrystallisation of magnesium alloys containing rare-earth elements

The static recrystallisation behaviour of two magnesium alloys after hot rolling have been examined. The alloys chosen for study were the conventional alloy AZ31, and an alloy containing the rare earth element Gadolinium. The recrystallisation kinetics were lower for the rare-earth alloy at low annealing temperatures, but at high annealing temperatures the kinetics were higher for the rare-earth alloy. It is suggested that this change in the comparative recrystallisation kinetics is a result of the improved mobility of the rare-earth solute at higher temperatures. This affects the recrystallisation kinetics through solute partitioning to the grain boundaries. The effect of this segregation on the recrystallisation texture is also discussed.

Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.

Atomic Scale Simulation of Deformation in Magnesium Single Crystals

The deformation behaviour of magnesium single crystals under plane strain conditions has been examined using molecular dynamics modelling. The simulations were based on an existing atomic potential for magnesium taken from the literature. A strain of 10% was applied at rates of 3x109s-1 and 3x107s-1. The simulations predicted the formation of mechanical twins that accommodated extension in the c-axis direction of the hexagonal unit cell. However, the predicted twin is not of the same kind found in magnesium, but is that commonly observed in titanium. It is believed that further analysis of the physical properties predicted by this interatomic potential will shed more light on the atomic processes controlling twinning in Magnesium alloys. It also highlights the need for improvements to the interatomic potential such that more accurate deformation behaviour can be attained.