Pure magnesium exhibits poor ductility owing to pyramidal [Formula: see text] dislocation transformations to immobile structures, making this lowest-density structural metal unusable for many applications where it could enhance energy efficiency. We show why magnesium can be made ductile by specific dilute solute additions, which increase the [Formula: see text] cross-slip and multiplication rates to levels much faster than the deleterious [Formula: see text] transformation, enabling both favorable texture during processing and continued plastic straining during deformation. A quantitative theory establishes the conditions for ductility as a function of alloy composition in very good agreement with experiments on many existing magnesium alloys, and the solute-enhanced cross-slip mechanism is confirmed by transmission electron microscopy observations in magnesium-yttrium. The mechanistic theory can quickly screen for alloy compositions favoring conditions for high ductility and may help in the development of high-formability magnesium alloys.

http://science.sciencemag.org/content/sci/359/6374/447.full.pdf

Mechanistic origin and prediction of enhanced ductility in magnesium alloys.

Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s−1 was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary { 1 0 1 ¯ 2 } tensile twins and { 1 0 1 ¯ 1 } compression twins, and then { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg17Al12 phases at grain boundaries occurs together with the precipitation of discontinuous Mg17Al12 phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

The quenching and partitioning (Q&P) process is a novel heat treatment for the enhancement of the strength level of steels without a significant deterioration of ductility. In this work, a study of 40SiMnNiCr steel subjected to the one-step Q&P process is presented. The study results suggest that the strength level of the steel subject to one-step Q&P increases at first and subsequently decreases with the partitioning time because of the synergistic effect of the increase in the retained austenite fraction, the decrease in carbon supersaturation in martensite, the change in the dislocation density in martensite, and the formation of transition carbide. The presence of the transition carbide markedly increases the strength level of the one-step quenched and partitioned steel, with the ultimate tensile strength (UTS) over 2400 MPa and the ductility more than 10 pct during partitioning at 453 K (180 °C) for 180 seconds. Isothermal martensite transformation possibly occurred in this medium-carbon ferrous alloy during the one-step Q&P processing. Meanwhile, in the early stages of the low-temperature partitioning process, carbon partitioning from martensite to austenite plays a dominant role in the carbon redistribution competitions. In addition, the relationship between the microstructure and mechanical properties of the one-step quenched and partitioned steel is discussed.

Microstructure and Mechanical Properties of an Ultrahigh-Strength 40SiMnNiCr Steel during the One-Step Quenching and Partitioning Process

Fine grained (FG) and ultra-fine grained (UFG) materials processed by severe plastic deformation exhibit beneficial hardness and tensile properties. Constrained groove pressing (CGP) were employed for fabrication of FG and UFG sheet metals and accomplished into different types of metals and alloys, such as commercial pure aluminum, AA3003 aluminum alloy, commercial pure copper, nickel, titanium and low carbon steels. Tensile and hardness characteristics in the FG and UFG sheets have been assessed with the aim of evaluating the hardness−strength relationship frequently established for coarse-grained metals and alloys ( σ U T S / H V = 3.45 ). However, it was revealed that the FG and UFG materials do not obey widely used hardness−strength relationships in the conventional coarse grained structures. A new multiplicity factor less than 3, depending on the chemical composition of processed materials, is proposed in this study. This is attributed to different strain hardening response of the FG and UFG materials with slight work hardening before necking instability. In fine grained and ultra-fine grained structures failure does not occur in (or right after) the onset of necking point. That is, tensile deformation sustains significantly up to fracture point due to the role of superplasticity mechanisms.

Hardness−strength relationships in fine and ultra-fine grained metals processed through constrained groove pressing

Effect of heat treatment on strain hardening of ZK60 Mg alloy

This paper reports the results of an investigation of plates produced by the advanced thermomechanical processing (TMP) schedules, which were designed using the results of a laboratory study. There were two steel compositions that corresponded to X-80 with carbon contents 0.04 and 0.07 wt.%, respectively. The variation in microstructure, hardness, tensile properties and Charpy impact properties with TMP schedule were determined, and compared with the expected requirements for X-100 linepipe steel. The relationships between microstructure and mechanical properties were experimentally obtained and discussed.

Microstructure and mechanical properties of bainitic low carbon high strength plate steels

The plasticity of coarse and grain-refined Mg AZ80 alloys in the as-cast, γ-dissolved and homogenized states was investigated by specialized tensile testing at room temperature. Results indicate that microstructural parameters such as the activation volume and mean free path are important descriptors for these materials and capture the nature of the solute and second phase effect on strength and ductility. The as-cast alloys contain a microstructure consisting of α-Mg matrix, and divorced eutectic α-Mg/γ-Mg17Al12 phase with non-uniform Al solute content in the α-Mg. Dissolution of the majority of γ-phase occurs after annealing 5 h at 420 °C, and an almost uniform solid solution is obtained after 20 h at 420 °C. The yield strength is dependent upon the volume fraction of γ-phase and grain size. All alloys yield initially by basal slip and they exhibit different work hardening behaviour. The as-cast alloys show the fastest initial hardening and earliest saturation, and ultimately the lowest ductility. In contrast the solutionized alloys show a lower initial work hardening rate that is sustained, and enhanced ductility. The flow stress dependence of the strain rate sensitivity indicates that dynamical recovery processes associated with the dislocation–dislocation interactions, which develop in the as-cast alloys after small amount of deformation, lead to strain localizations and early failure. Results reveal that reducing the grain size and dissolving the γ-phase will enhance the ductility of AZ80 at room temperature.

I.A. Yakubtsov

Papers

Microstructure and mechanical properties of bainitic low carbon high strength plate steels

Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature