Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

Magnesium, the lightest structural metal, is approximately four times lighter than steel—the most widely used metal in industrial applications. Currently available Mg alloys, however, are impractically expensive for use in automotive structural components, as severe ductility problems require forming operations at elevated temperatures and an exclusion from critical safety components. With a strong impetus in research having sprung up during the last two decades, addition of rare-earth elements in small quantities emerged as a potential solution for simultaneously delivering the ductility and weight requirements for automotive applications. These improvements are arguably achieved by virtue of texture weakening and enhancement of non-basal slip. However, ways by which rare-earth elements modify texture remain very elusive, and no consensus on the driving mechanisms has been reached in the literature as of yet. We take a look back at different paradigms held for the action of rare-earth additions, and examine key facts that may reconcile controversies. We attempt to identify critical gaps and suggest venues to overcome them. These gaps, once filled, may promote Mg alloys to become a stronghold for lightweighting, which will exceptionally benefit our environment and wellbeing.

A review on the effect of rare-earth elements on texture evolution during processing of magnesium alloys

Uniaxial compression tests were performed on samples cut along the extrusion direction from AZ31 Mg alloy tubes. A stage of increasing work hardening rate was observed on representative true sigma-epsilon curves. Specimens compressed to various strain levels were examined by optical microscopy and electron backscattered diffraction (EBSD) techniques. The results indicate that the widespread formation of intersecting {10-12} extension twins is responsible for the increased strain hardening rate. (c) 2006 Elsevier B.V. All rights reserved.

Influence of {10-12} extension twinning on the flow behavior of AZ31 Mg alloy

Effect of composition on the texture and deformation behaviour of wrought Mg alloys

The deformation behavior, texture and microstructure evolution of six sample types of a commercial magnesium alloy AZ31 with different processing histories were investigated during plane strain compression at room temperature using a channel-die device. Although all the samples were deformed under the same conditions, i.e. temperature and strain rate, the initial state of the samples prior to deformation was responsible for the final texture and microstructure. Stress–strain curves showed a maximum ductility of 28% for the sample with a hot rolling history. EBSD analysis was carried out to give a better insight into the operating deformation mechanisms. Besides the expected { 1 0 1 ¯ 2 } -tensile twinning, { 1 0 1 ¯ 1 } -compression twinning and { 1 0 1 ¯ 1 } − { 1 0 1 ¯ 2 } -double twinning were also observed in some specimens and were correlated to microcrack formation, which caused an early shear failure.

Room temperature formability of a magnesium AZ31 alloy: Examining the role of texture on the deformation mechanisms

Plastic deformation of hexagonal metals such as magnesium is strongly affected by the hexagonal crystal structure, and thus, for an understanding of the deformation behaviour of magnesium polycrystals, texture must be taken into account. Plane-strain compression tests on pure magnesium as well as on magnesium alloy AZ31 were carried out at 100 and 200 °C. A special emphasis was laid upon the influence of different starting textures on the deformation characteristics and the development of texture and microstructure. The measured flow curves and the microstructure investigation reveal that plastic deformation of magnesium at these temperatures is generally inhomogeneous and dominated by the appearance of shear bands. However, if the initial texture is chosen such that the formation of a basal texture is slowed down or even suppressed, substantial ductility can be achieved at temperatures as low as 100 °C. The texture development due to crystallographic slip can be reasonably modelled by a relaxed constraints Taylor simulation and yields information on the activated slip systems.

Texture effects on plastic deformation of magnesium

Engineering approaches to model dynamic recrystallization (DRX) explicitly assume the existence of a critical strain for DRX. However, DRX is not set off at a certain “critical” amount of strain, rather is initiated by reaching critical conditions for microstructural instability. It was proposed recently that these critical conditions can be expressed in terms of a point of inflection on the θ(σ) curve, where σ is the flow stress and θ is the hardening rate. A theoretical framework is presented to predict these critical conditions on the basis of a dislocation work hardening model. Once tuned to a specific hardening curve the model allows to predict the onset of DRX for different temperatures and strain rates, and, in principle, for changing materials chemistry.

Prediction of the critical conditions for dynamic recrystallization in the austenitic steel 800H

Engineering approaches to model dynamic recrystallization (DRX) explicitly assume the existence of a critical strain for DRX. Since the strain is not a state variable of crystal plasticity, however, there is no critical strain, rather there are critical conditions for microstructural instability. It was proposed recently that these critical conditions can be expressed in terms of a point of inflection on the θ(σ) curve, where σ is the flow stress and θ is the hardening rate. A theoretical framework is presented to predict these critical conditions on the basis of a dislocation work hardening model. Once tuned to a specific hardening curve the model allows to predict the onset of DRX for different temperatures and strain rates, and, in principle, for changing materials chemistry.

Prediction of the critical conditions for dynamic recrystallization in metals

The microstructural changes at the interfaces in a NiAl composite reinforced with BN-coated continuous single crystal Al 2 O 3 fibers were studied by analytical electron microscopy and high-resolution electron microscopy (HREM) after annealing at 1000 °C for 100 h and 500 h. Both NiAl/BN and BN/Al 2 O 3 interfaces exhibited excellent stability during annealing, as did the AlN layer formed at NiAl/BN interface during processing. In the case of contamination which was introduced through the use of a glue during diffusion bonding of the composite, significant changes of the microstructure at both NiAl/BN and BN/Al 2 O 3 interfaces were observed. A continuous reaction layer formed at the BN/Al 2 O 3 interface during annealing. The reaction product was identified by electron diffraction as Al 18 B 4 O 33 . We propose to attribute this to the reaction: 9Al 2 O 3 +2B 2 O 3 →Al 18 B 4 O 33 . Many nanoparticles were observed in the BN matrix near the BN(AlN)/NiAl interface in the as-fabricated composite. Neither the AlN layer nor these nanoparticles grew during annealing. HREM images of the precipitates revealed a structure distinctly different from that of the BN matrix. B, C, N and Al were detected in these precipitates by EELS analysis.

M. Frommert

Papers

Texture effects on plastic deformation of magnesium

Prediction of the critical conditions for dynamic recrystallization in the austenitic steel 800H

Prediction of the critical conditions for dynamic recrystallization in metals

Interfacial thermal stability in BN-coated continuous Al2O3 fiber-reinforced NiAl composites

Influence of Texture on Deformation Behaviour of Magnesium Alloy AZ31