Texture control by {10-12} twinning to improve the formability of Mg alloys: A review

Texture control of wrought Mg alloys, particularly in rolled Mg alloy sheets, has been an important research topic for the past several decades because it has significant influence on stretch formability at room temperature. For Mg alloys, {10-12} twinning can be easily activated and causes a ˜86.3° lattice rotation. Thus, pre-twinning deformation is considered as an effective and low-cost method for texture control in wrought Mg and its alloys. Furthermore, it has been verified that texture control via pre-twinning deformation can remarkably improve stretch formability of rolled Mg alloy sheets. In this review, recent researches on texture control via twinning deformation and its influence on stretch formability will be critically reviewed. The main contents include the micro-mechanism and impact factors of control in twin-orientation, plastic processing techniques of pre-inducing twins and the application of pre-induced twins in improvement of stretch formability. Finally, further research directions on this field were proposed.     

Strong and ductile Mg alloys developed by dislocation engineering

Dislocation engineering concept has been successfully employed to tackle the strength-ductility trade-off in steels, resulting in the development of high-strength high-ductility deformed and partitioned (D&P) steel. The present perspective proposes to employ such dislocation engineering concept to develop strong and ductile magnesium (Mg) alloys. High density of?<?c?+?a?>?dislocations could be generated at appropriate temperature and retained in the Mg alloy after quenching to room temperature. Those?<?c?+?a?>?dislocations inherited from the warm deformation could provide?<?c?+?a?>?dislocation sources when the Mg alloy is deformed at room temperature, resulting in good ductility. The high dislocation density generated at warm deformation provides dislocation forest hardening, leading to improved yield strength of Mg alloy.     

Investigation on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys with various Zn/Mg ratios

The effects of Zn/Mg ratios on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys aged at 150℃ have been investigated by using tensile tests,optical metallography,scanning electron microscopy,transmission electron microscopy and atom probe tomography analyses.With increasing Zn/Mg ratios,the ageing process is significantly accelerated and the time to peak ageing is reduced.T'phase predomi-nates in alloys of lower Zn/Mg ratios while η'phase predominates in alloys with a Zn/Mg ratio over 2.86.Co-existence of T'phase and η'phase with a large number density is beneficial to the high strength of alloys.Such precipitates together with narrow precipitate free zones cause a brittle intergranular fracture.A strength model has been established to predict the co-strengthening effect of T'phase and η'phase in Al-Zn-Mg-Cu alloys,including the factors of the grain boundary,solid solution and precipitation.     

Texture and microstructure development during hot deformation of ME20 magnesium alloy: Experiments and simulations

The influence of deformation conditions and starting texture on the microstructure and texture evolution during hot deformation of a commercial rare earth (RE)-containing magnesium alloy sheet ME20 was investigated and compared with a conventional Mg sheet alloy AZ31. For all the investigated conditions, the two alloys revealed obvious distinctions in the flow behavior and the development of texture and microstructure, which was primarily attributed to the different chemistry of the two alloys. The presence of precipitates in the fine microstructure of the ME20 sheet considerably increased the recrystallization temperature and suppressed tensile twinning. This gave rise to an uncommon Mg texture development during deformation. Texture simulation using an advanced cluster-type Taylor approach with consideration of grain interaction was employed to correlate the unique texture development in the ME20 alloy with the activation scenarios of different deformation modes.     

高锰奥氏体低温钢力学性能及Hall-Petch关系的研究

高锰奥氏体低温钢作为一种经济型低温钢材料,具有非常巨大的发展空间.为进一步研究晶粒尺寸对高锰奥氏体低温钢的影响,本文对其进行了不同工艺的固溶处理,测试了不同晶粒尺寸实验钢的室温拉伸及低温冲击性能, 通过扫描电镜(SEM)、背散射电子衍射(EBSD)等方法进行表征并计算其Hall-Petch关系式.-196 ℃冲击实验结果表明,高锰奥氏体低温钢拥有良好而稳定的超低温冲击性能,且其超低温冲击性能不随晶粒尺寸的增加而发生变化.经计算实验钢屈服强度所对应的Hall-Petch常数K_y为7.27 MPa·mm^0.5,明显小于其他被研究的奥氏体高锰钢.进一步计算拉伸过程中不同真应变ε对应的Hall-Petch常数K(ε),发现在拉伸过程中,受TWIP效应产生的变形孪晶影响,K(ε)值随应变量的增加而增加;当真应变达到0.3时,组织内变形孪晶不再继续生成,加工硬化过程不再进行,K(ε)也达到极大值.     

Critical Assessment 9: Wrought magnesium alloys

This assessment is focussed on wrought magnesium alloys for lightweight applications, particularly in the transport sector. The challenges to their wider use are summarised, including poor low temperature formability, corrosion issues, dissimilar metal joining, and limited precipitation strengthening. The fundamental origins of these challenges, and current research to address them, are highlighted. Key developments such as the use of dilute rare earth additions to manipulate texture for improved formability are discussed. Opportunities to exploit the unique properties of wrought magnesium alloys where further research is required are identified.     

Effects of grain size on compressive behaviour in ultrafine grained pure Mg processed by equal channel angular pressing at room temperature

Ultrafine-grained pure magnesium with an average grain size of 0.8 μm was produced by refining coarse-grained (980 μm) ingot by multi-pass equal channel angular pressing (ECAP) at room temperature with the application of a back pressure. The compressive deformation behaviour at room temperature depended on grain size, with deformation twinning and associated work hardening observed in coarse-grained Mg, but absent in the ultrafine grained material as decreasing grain size raised the stress for twinning above that for dislocation slip. The ultrafine grained Mg showed good plasticity with prolonged constant stress after some initial strain hardening.     

Recent advances in biodegradation controls over Mg alloys for bone fracture management: A review

Magnesium (Mg) alloys possess comparable physical and mechanical properties to bone, making them an outstanding candidate of implant materials for bone fracture treatment. In addition to the excellent biocompatibility, and bioactivity, the engagement of Mg alloys is key for a number of biological functionalities in the human body. The unique biodegradation nature of Mg alloy implants implies that it may not require a secondary removal procedure when the expected supporting tasks accomplish, as they may simply and safely “disappear” over time. Nonetheless, the demonstrated drawback of potentially rapid degradation, is an issue that must be addressed appropriately for Mg implants and is consequently given unique attention in this review article. Herein, the critical criteria and the state-of-the-art strategies for controlling the degradation process of Mg alloys are reported. Furthermore, future developments of biodegradable Mg and its alloys systems with satisfactory specifications for clinical trials and deployment, are discussed. This review aims to provide information to materials scientists and clinical practitioners in the context of developing practical biodegradable Mg alloys.     

Effects of prior compression on ductility and yielding behaviour in extruded magnesium alloy AZ31

Abstract

In the present paper, the effects of precompression along extrusion direction (ED) on subsequent compression perpendicular to ED were investigated in an extruded magnesium alloy AZ31. The results showed that the yield stress under compression perpendicular to ED increased if there was precompression along ED. The evolution of deformation mechanism was responsible for increase in yield stress because plastic deformation was dominated by both basal slip and {10–12} twinning under compression perpendicular to ED in samples without prestrain, but basal slip was difficult to be activated and {10–12} twinning dominated deformation in samples with precompression. However, because basal slip had no obvious contribution to plastic deformation, the ductility decreased if there was precompression along ED.     

Hall-Petch relationship in two-phase TiAl alloys with fully lamellar microstructures

The dependences of the room temperature tensile properties of two-phase TiAl alloys with fully lamellar microstructures on colony size and the effects of alloying elements on the k value of Hall-Petch relationship were investigated. It is found that the ultimate tensile strength and the yield strength show Hall-Petch dependences on colony size, but the ductility does not obey a Hall-Petch dependence on colony size. The k value of Hall-Petch relationship increases when Ti-47Al (at.%) is alloyed with interstitial elements C or B, but the additions of substitutional elements Cr or Nb don’t lead to its apparent change.     

Atomic-scale dissecting the formation mechanism of gradient nanostructured layer on Mg alloy processed by a novel high-speed machining technique

Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit supe-rior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics sim-ulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of～70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dis-locations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45 μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of deformation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.     

The effect of basal dislocation on { 11(2)1 } twin boundary evolution in a Mg-Gd-Y-Zr alloy

This study aims to understand the features of { 11(2)1 } twin boundaries in a high strain rate compressed Mg-10Gd-3Y-0.4Zr using electron backscatter diffraction (EBSD) and transmission electron microscopy(TEM).Based on the Schmid factor calculations,the basal -dislocations are easily activated within the { 11(2)1 } twin and suppressed in the surrounding matrix.The consequent basal dislocation-twin interaction during deformation can lead to the misorentiation deviation of { 11(2)1 } twin boundaries,accompanying with the formation of a step at the twin boundary.Such a { 11(2)1 } twin boundary evolution mechanism provides a new vision of twinning behavior in Mg alloys.     

Effects of Sn,Ca additions on thermal conductivity of Mg matrix alloys

In the present work, the effects of Sn, Ca additions on thermal conductivity were investigated in as cast Mg–Sn–Ca alloys. The measured values of thermal conductivity of Mg–3Sn–xCa alloys obviously increased from 85.6 to 126.3?W?m^??1?K^??1 with the increasing Ca from 0 to 1.5?wt-%, and then decreased to 98.3?W?m^??1?K^??1 with the 2.5?wt-% Ca. In addition, the thermal conductivity of the Mg–Sn–Ca (Sn/Ca atomic ratio of 1) alloys decreased slightly from 154.2 to 132.1?W?m^??1?K^??1 with the increasing Sn, Ca. Meanwhile, the microstructures of the selected alloys were discussed in detail, suggesting that the solute atoms that caused lattice distortion had greater effect on thermal conductivity compared with the second phases formed in as cast Mg–Sn–Ca alloys.     

Strain localisation in solution heat treated Al–Zn–Mg alloys

Two Al–Zn–Mg alloys with recrystallised and fibrous grain morphologies are studied with regards to the microstructure after solution heat treatment, cold water quenching and immediate room temperature deformation. It was found that the dislocation movement was localised in narrow slip bands cutting through the dislocation tangles. This observation is related to dynamic strain ageing and to macroscopic shear bands frequently observed in these alloys.     

The influence of manganese addition on mechanical properties of commercial Al‐4Mg casting alloys

The aim of this study was to investigate the mechanical properties of Al‐Mg‐Si alloys aged to peak hardness with different dispersoid volume fraction. It was found that the tensile strength increases with dispersoid content, for alloys having similar ductility. The effect of an increasingly triaxial stress state on a fracture strain above mentioned alloys were measured using a series of notched tensile specimens whose notch root radius of curvature was changed. The alloy ductility was found to increase with dispersoid content and root radius and to decrease with increased stress triaxiality. The fracture toughness of these alloys was determined as a function of dispersoid content and notch root radius of curvature. It was observed that the fracture toughness increased as the dispersoid content and the notch root radius increased. scanning electron microscope analysis of the fracture surfaces revealed that fracture mechanism was transgranular fracture with dimples formation. It is argued that optimum mechanical properties in these alloys can be achieved at about 0.5 % Mn content.     

A tuning nano-precipitation approach for achieving enhanced strength and good ductility in Al alloys

A new strategy through controlling the aggregation states of alloying elements before cold-rolling is proposed to tune the nano-precipitation in Al alloys processed by combining deformation and aging. A considerable improvement in strength and good ductility is achieved in Al–Mg–Si–Cu alloy by our approach, which is applicable to many precipitation-hardened alloy systems and suitable for industrial application. A bi-modal distribution of precipitates, discrete lath-like Q″-type precipitate and continuous precipitate with the composition identical to Q′-type phase, occur in the annealed sample pre-treated by natural aging. A short-term artificial aging which induces a dispersion of nano-sized coherent particles, mainly GP zone of the monoclinic β″-type phase, before cold-rolling can lead to the formation of finely and homogeneously distributed Q″-type precipitates, which are in favor of an enhancement in strengthening potential for the final aging. Our finding suggests that the solutes aggregation state before deformation affects significantly the re-precipitation of solutes during the subsequent annealing.     

Effect of hydrogen in aluminium and aluminium alloys: A review

Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.     

EBSD Characterization of the Laser Remelted Surface Layers in a Commercially Pure Mg

Laser surface melting has been applied on a commercially pure Mg. The microstructure and texture modifications encountered in the surface layers were carefully investigated by using electron backscattered diffraction （EBSD） technique. Due to the melting followed by rapid solidification and cooling, a layer having graded microstructures and texture formed. At the bottom of the melted layer, the solidified Mg grains have an elongated shape with a 〈0001 〉 basal fibre texture nearly parallel to sample normal direction, while equiaxed grains were observed in the top melted layer having a much weaker basal fibre texture. Solidification twinning and deformation twinning were found in the vicinity of the melt/substrate interface where the Mg grains grow larger due to the heating. In addition, no epitaxial type grain growth was observed at the melt/substrate interface.     

Effect of Mg and Sr-modification on the mechanical properties of 319-type aluminum cast alloys subjected to artificial aging

Aluminum-based 319-type cast alloys are commonly used in the automotive industry to manufacture cylinder heads and engine blocks. These applications require good mechanical properties and in order to achieve them through precipitation hardening, artificial aging treatments are applied to the products. The standard artificial aging treatment for alloy 319, as defined by the T6 heat treatment temper, consists in solution heat-treating the product for 8 h at 495 °C, water quenching at 60 °C, and then artificially aging at 155 °C for 2–5 h.

The present paper reports on aging heat treatments that were performed on four Al–Si–Cu–Mg 319-type alloys: 319 base alloy, Sr-modified 319 alloy, 319 alloy containing 0.4 wt% Mg, and the Sr-modified 319 + 0.4 wt% Mg alloy. This investigation was carried out in order to examine the effect of Sr-modification and additions of Mg on the microhardness, tensile strength and impact properties of 319-type alloys over a range of aging temperatures and times (150–240 °C, for periods of 2–8 h).

The results show that the best combination of properties is found in the Sr-modified alloy containing 0.4 wt% Mg (i.e. alloy 319 + Mg + Sr). Also, the optimum artificial aging temperature changes when Mg is present in the alloy.