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.

Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm3, making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers. Yet, the widespread use of magnesium is hampered by its intrinsic brittleness. While other metallic alloys have multiple dislocation slip systems, enabling their well-known ductility, the hexagonal lattice of magnesium offers insufficient modes of deformation, rendering it intrinsically brittle. We have developed a quantum-mechanically derived treasure map which screens solid solution combinations with electronic bonding, structure and volume descriptors for similarity to the ductile magnesium-rare earth alloys. Using this insight we synthesized a surprisingly simple, compositionally lean, low-cost and industry-compatible new alloy which is over 4 times more ductile and 40% stronger than pure magnesium. The alloy contains 1 wt.% aluminum and 0.1 wt.% calcium, two inexpensive elements which are compatible with downstream recycling constraints.

/pdf/a-rare-earth-free-magnesium-alloy-with-improved-intrinsic-475ddggi34.pdf

A rare-earth free magnesium alloy with improved intrinsic ductility

Improving the mechanical properties of extruded Mg–3Al–1Zn alloy by cold pre-forging

A remarkable improvement of the stretch formability of rolled Mg–3Al–1Zn alloy at room temperature was achieved by introducing initial {10–12} twins. The activation of detwinning and basal slip in twinned region was found to effectively accommodate a thickness-direction strain and this led to an enhancement of stretch formability. The improvement was obtained in proportion to the twin volume fraction and just the twin volume fraction of ∼24% enhanced the stretch formability by ∼65%, which can be achievable at 300 °C.

Enhanced stretch formability of rolled Mg–3Al–1Zn alloy at room temperature by initial {10–12} twins

In situ observation of collective grain-scale mechanics in Mg and Mg–rare earth alloys

Shear band formation has been reported to depend strongly on micro-structural factors, such as grain size, texture, alloying elements, and also on forming processes. The formation mechanism of shear bands has thus been investigated in the present work in relation to improve the forming limit of AZ31 Mg alloy. A series of compression tests have thus been carried out using strip-cast (SC) and hot-rolled (AR) AZ31 Mg alloys to compare their texture effects. The tests were performed under the strain rate of 10 −1 /s at room temperature (RT) and 300 °C to clarify temperature dependence of shear band formation. Shear bands in SC specimens were observed to develop mainly at the boundaries of cast structures by generating compression twins and dynamic recrystallization (DRX) induced by the activation of the and slips as well as twinning at elevated temperatures. AR specimens exhibited, on the other hand, well-developed basal-fiber texture to generate shear bands through fine grains developed by DRX along the geometrically maximum shear stress directions.

Shear band formation during hot compression of AZ31 Mg alloy sheets

Deformation behavior of AZ31 Mg alloy has been studied in relation to test temperature and grain size. Tensile specimens were prepared in the rolling direction of an as-rolled as well as a strip-cast sheet with 1 mm thickness. The as-rolled and strip-cast sheets had the mean grain sizes of 75 μm and 5 μm, respectively. Tensile tests were then carried out under the strain rate of 10 −2 /s at room temperature, 100 °C, and 200 °C. Biaxial tests were also performed to obtain flow curves in terms of stress and strain as well as to estimate the biaxial forming limit. Plastic deformation was found due to the usual dislocation slip and dynamic recrystallization (DRX) together with some twinning. A number of twins were observed especially in large grained specimens at low homologous temperatures. The strip-cast sheet exhibited many small grains generated by the DRX at higher temperature. These fine grains due to DRX appeared to lead into an enhanced formability, evidenced by a sudden formability increase of a strip-cast sheet at 200 °C. The double twin mode was observed under biaxial loading, contrary to extension twins observed under uniaxial loading.

Deformation behavior of as-rolled and strip-cast AZ31 magnesium alloy sheets

Deformation behavior of an AZ31 Mg alloy under a biaxial loading has been studied in relation to the initial texture developed during a prior hot-rolling process. Sheet specimens were thus prepared from a hot-rolled plate in the RD–TD planes (RTD) and ND–TD planes (NTD) to examine the effects of initial texture. Biaxial dome forming tests were then carried out under the punch speed of 0.1 mm/s at room temperature (RT), 100 °C, and 200 °C to obtain punch load vs. displacement curves during dome-forming tests. Different twin modes were observed under a biaxial loading at RT, viz. { 1 0 1 ¯ 1 } compression twins and { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twins in RTD specimens, while { 1 0 1 ¯ 1 } compression twins generated through the rotation of { 1 0 1 ¯ 2 } extension twins developed earlier in NTD specimens. Formability of NTD sheets was observed to increase more rapidly compared to that of RTD specimens as forming temperature was raised, which might be accounted for by reduced critical resolve shear stresses (CRSS) of prismatic slips due to their higher Schmid factor values.

Effect of initial texture on deformation behavior of AZ31 magnesium alloy sheets under biaxial loading

Study on the High-Temperature Deformation Behavior and Formability of Zr-Based Bulk Metallic Glass

The formability of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass has been investigated in the present study in relation to the heating rate. A series of extrusion tests after rapid heating has been performed in a laboratory scale together with sheet forming tests after slow heating for comparison purpose. The basic processing map based on dynamic materials model (DMM) and compression test data has been utilized to evaluate feasible forming conditions. The macroscopic formability, classified by fully formed, partially formed or a catastrophic fracture, is found to have a good correspondence with the iso-efficiency contour in the processing map. The region of high power dissipation efficiencies with η>0.8 is found to be broaden by avoiding crystallization events due to reduced exposure time in extrusion process with a faster heating rate.

Won Kyu Bang

Papers

Shear band formation during hot compression of AZ31 Mg alloy sheets

Deformation behavior of as-rolled and strip-cast AZ31 magnesium alloy sheets

Effect of initial texture on deformation behavior of AZ31 magnesium alloy sheets under biaxial loading

Study on the High-Temperature Deformation Behavior and Formability of Zr-Based Bulk Metallic Glass

The Effect of Heating Rate on the Formability of Bulk Metallic Glass in a Supercooled Liquid Region