Shear deformation and grain refinement during accumulative back extrusion of AZ31 magnesium alloy

The accumulative back extrusion (ABE), as a new-noble severe plastic deformation (SPD) technique, has been employed to clarify the microstructural evolutions of AZ31 magnesium alloy during severe deformation. The latter has been explored using a 3D finite element analysis along with the microstructural investigations. The distribution of ABE generated shear strain (SS) and its corresponding microstructures have been thoroughly studied. The results indicated that the restrictions of material flow during ABE processing had been led to the mechanical shear bands generation in the microstructure. In addition, the occurrence of continuous dynamic recrystallization (CDRX) within the bands has resulted in a local grain refinement in those areas. Consequently a bimodal structure including the fine recrystallized grains along with the elongated ones has been developed. The effect of deformation mode on the microstructural refinement has been also discussed through considering the developed SS history and the related microstructural refinement.     

Effects of Heat Treatment on Microstructure and Mechanical Properties of an ECAPed Al–Zn–Mg–Cu Alloy

Equal‐channel angular pressing (ECAP) has a considerable advantage in the preparation of bulk fine‐grained alloys. To investigate the effect of solid solution treatment (SST) on the microstructure and mechanic properties of an Al–Zn–Mg–Cu alloy after ECAP, a comparative study is conducted using experimental techniques. It is shown that ECAP processing introduces a strong grain refinement, while the SST induces precipitation of skeleton‐like second phases distributed discontinuously at the grain boundary and needle‐like second phases in the grain. In addition, SST can also improve significantly the fractions of both high angle grain boundaries and recrystallization. The {110}<001> texture is introduced and the polar density is reduced during SST. Microstructural evolution involves three typical characteristics, namely, shear bands, substructure, and precipitates. The corresponding mechanism of microstructure evolution is proposed, considering the effect of dislocations, precipitates, and grain boundaries. After SST, the improvement of strength and hardness is not obvious, but significant in plasticity by 33.3%. Different strengthening mechanisms are also examined during ECAP and subsequent SST.

     

Adiabatic shear bands and examples of their role in severe plastic deformation

The accommodation of severe plastic deformation in impact crater formation, ballistic rod flow and penetration in thick targets, shaped charge formation, and a variety of friction-stir welding and processing is shown to occur by microstructure refinement, particularly dynamic recrystallization in the development of localized or overlapping adiabatic shear bands. Optical metallography and transmission electron microscopy observations of solid-state flow in adiabatic shear zones are compared to illustrate these mechanisms which can often involve intermixed microstructural regimes composed of recovery, recrystallization, and grain growth phenomena.     

Promotion of texture weakening in magnesium by alloying and thermomechanical processing: (I) alloying

The recrystallization and texture evolution of four Mg–Zn–Ce sheets with a warm-rolled microstructure obtained through two stages that can be characterised as rough rolling and finish rolling were investigated at different stages of post-rolling annealing. On annealing, the same regions of the microstructure, located by hardness indentations, were examined and tracked by electron backscatter diffraction (EBSD). Furthermore, intragranular misorientation axes (IGMA) analysis was used to investigate the associated deformation mechanisms in the as-deformed material. By combining these two methods, the development of the recrystallization microstructure was investigated and important aspects, such as preferential nucleation sites, correlation between activated deformation mechanism and initial orientation of the recrystallized grains, were studied. The results showed that the Mg–1Zn–1Ce alloy, which had the highest Ce/Zn ratio, showed the weakest as-rolled texture and the most homogenous distribution of shear banding/twinning. The IGMA analysis also showed that in Mg–1Zn–1Ce, other types of dislocations rather than basal 〈a〉 were activated; in particular, prismatic 〈a〉 type was activated during deformation. Therefore, the weakening of recrystallization texture during rolling resulting from the addition of RE elements was linked with a change in dynamic recrystallization (DRX) behaviour. Since the Mg–1Zn–1Ce alloy corresponds to the highest level of Ce in solid solution, the observed texture weakening was possibly due to decreasing grain boundary mobility as a result of solute partitioning of RE elements to dislocations and grain boundaries.     

Microstructure and Mechanical Properties of Austenitic Stainless Steels after Dynamic and Post‐Dynamic Recrystallization Treatment

The effects of dynamic and post‐dynamic recrystallization (DRX and post‐DRX) on the microstructure and mechanical properties of austenitic stainless steels are critically reviewed. Particularly, the paper is focused on the grain refinement and strengthening by large strain deformation including severe plastic deformation conditions. The DRX and post‐DRX microstructures are considered with close relation to the operative recrystallization mechanisms. Specific emphasis is placed upon two recrystallization mechanisms, that is, discontinuous and continuous, and their dependence on the deformation/annealing conditions. The relationships between DRX microstructures and processing conditions are summarized and their effect on post‐DRX behavior is clarified. The structural strengthening mechanisms including the grain size and the dislocation density are elaborated.

     

Microstructure and mechanical properties of Mg-5Li-1Al sheets prepared by accumulative roll bonding

Ultrafine-grain and high-strength Mg-5Li-1Al sheets were prepared by accumulative roll bonding (ARB) process. Evolution of microstructure and mechanical properties of ARB-processed Mg-5Li-1Al sheets was investigated.Results show that, during ARB process, the evolution of deformation mechanism of t Mg-5Li-1Al alloy is as follows: twinning deformation, shear deformation, forming macro shear zone, and finally dynamic recrystallization (DRX). The grain refining mechanism changes from twin DRX to rotation DRX. With the increase in ARB cycles, strength of the Mg-5Li-1Al sheets is enhanced, whilst elongation varies slightly. With the increase in rolling cycles, anisotropy of mechanical properties decreases. It is conclusive that strain hardening and grain refinement dominate the strengthening mechanism of Mg-5Li-1Al alloy.     

Microstructure,mechanical properties and deformation characteristics of Al-Mg-Si alloys processed by a continuous expansion extrusion approach

In the present work,a double-pass continuous expansion extrusion forming (CEEF) process was proposed for an Al-Mg-Si alloy,in which the diameter of rods was gradually expanded.The microstruc-tural evolution,mechanical properties and deformation characteristics were investigated by utilizing microstructural observations,mechanical testing and a finite element method coupled with a cellular automata model.The results showed that the strength and ductility of the double-pass CEEF processed Al-Mg-Si alloys were improved synchronously,especially in artificially aged alloys.The grain size of the processed Al-Mg-Si alloy rods was refined remarkably by continuous dynamic recrystallization (CDRX)and geometric dynamic recrystallization (GDRX),and the homogeneity of microstructure was gradu-ally improved with increasing number of processing passes.The artificially aged alloy processed with double-pass CEEF and water quenching contained fine (sub)grains and high-density dislocations,which resulted in more needle-shaped β" precipitates and a larger precipitate aspect ratio than the as-received and air-cooled CEEF alloys owing to the different precipitation kinetics.The severe cumulate strain and microshear bands were found to accelerate CDRX and GDRX for grain refinement between adjacent posi-tions of the parabolic metal flow due to the special temperature characteristics and large shear straining during the CEEF process.     

Mg–3Al–1Zn alloy strips processed by electroplastic differential speed rolling

The effect of reduction per pass on the mechanical properties, microstructure and texture evolution of Mg–3Al–1Zn magnesium alloy strips processed by electroplastic differential speed rolling has been investigated. With increasing reduction per pass, the mechanical properties of the rolled strips increase. Both the shear effect and the current pulses play an important part in modifying the microstructure and texture by promoting dynamic recrystallisation. During dynamic recrystallisation, nucleus tends to nucleate in the vicinity of the shear bands. And as the progress of dynamic recrystallisation, shear bands are gradually consumed by the newly nucleated grains and become nearly indistinguishable. The athermal effect of current pulses plays an important role in promoting dynamic recrystallisation.     

Dynamic recrystallization behavior of AZ31 magnesium alloy processed by alternate forward extrusion

One of the important factors that affect the microstructure and properties of extruded products is recrystallization behavior. Alternate forward extrusion (AFE) is a new type of metal extrusion process with strong potential. In this paper, we carried out the AFE process experiments of as-cast AZ31 magnesium alloy and obtained extrusion bar whose microstructure and deformation mechanism were analyzed by means of optical microscopy, electron backscattered diffraction and transmission electron microscopy. The experimental results indicated that homogeneous fine-grained structure with mean grain size of 3.91 μm was obtained after AFE at 573 K. The dominant reason of grain refinement was considered the dynamic recrystallization (DRX) induced by strain localization and shear plastic deformation. In the 573-673 K range, the yield strength, tensile strength and elongation of the composite mechanical properties are reduced accordingly with the increase of the forming temperature. Shown as in relevant statistics, the proportion of the large-angle grain boundaries decreased significantly. The above results provide an important scientific basis of the scheme formulation and active control on microstructure and property for AZ31 magnesium alloy AFE process.     

Effect of recrystallization and texture on mechanical properties of extruded AA7003 alloy

The microstructure, texture and mechanical properties of AA7003 extrusion sheets processed with different parameters were investigated. Furthermore, the effects of the microstructure and texture on the mechanical properties were discussed. The grain morphology and the texture were analyzed by using a combination of scanning electron microscopy and electron backscatter diffraction. Moreover, the mechanical properties were measured by tensile tests and the tensile fracture morphology was also analyzed by scanning electron microscopy. The results showed that the maximum recrystallization extent occurred at the extrusion process conditions of T = 450 °C and v = 1 mm/s, and the increase in recrystallization extent had a negative effect on the tensile strength. Moreover, a relatively strong recrystallization cube orientation <100> existed when the extrusion temperature was 470 °C, showing a high tensile strength and elongation. Thus, it can be concluded that the cube texture is beneficial to the tensile strength and elongation in extruded AA7003 alloy.     

冷变形后不同回火温度对低碳贝氏体钢组织和力学性能的影响

通过在690℃高温回火后对15SiMn2Mo低碳贝氏体钢进行10％拉伸变形或不同变形量压缩变形,再进行不同温度回火,研究了冷变形（拉伸和压缩）和变形后不同温度回火对试验材料的组织和性能的影响。结果显示,随着回火温度增加,试验柯料的抗拉强度增加,300℃回火强度达到最大值,与热轧低温回火强度相当。超过300℃回火材料的强度下降,伸长率和断面收缩率增加。随着压缩变形量的提高,材料的硬度值升高,加工硬化效果显著,组织中出现铁素体形变带。压缩变形后随着回火温度的提高,材料组织发生回复与再结晶,形成细小等轴晶粒,组织细化,压缩变形量增加,细化效果增加。     

Microstructure evolution and mechanical properties of friction stir welded FeCrNiCoMn high-entropy alloy

To investigate the microstructural development and mechanical properties of friction stir welded high-entropy alloy, the stirring process and the air cooling period were separated for discussion. The texture component developed from A* {111}<112> to A {111}<110> in the stirring stage, and finally changed to B {112}<110> in the subsequent air cooling stage caused by the multiple mechanisms including discontinuous dynamic recrystallization, continuous dynamic recrystallization, static recovery and selected grain growth. This work also demonstrated that the static recovery and the selected grain growth during the air cooling stage remarkably deteriorated the microstructure and mechanical properties which is produced during the stirring stage, and it cannot be neglected when investigating the microstructure transformation and mechanical properties during the friction stir welding.     

Effects of initial texture on deformation behavior during cold rolling and static recrystallization during subsequent annealing of AZ31 alloy

This study demonstrates that the initial texture of Mg alloy significantly affects the microstructure developed during cold rolling and the recrystallization behavior during subsequent annealing. In a sample with a texture oriented toward the normal direction(ND sample), thick and large-sized shear bands are intensively formed during cold rolling and the deformation is strongly localized along these bands. In contrast,in a sample with a texture oriented toward the transverse direction(TD sample), many {10-12} twins are formed at the early stage of cold rolling, and then, numerous small-sized shear bands are formed in these twins. Results of nanoindentation tests reveal that the cold-rolled ND sample has internal strain energy that is substantially concentrated along the shear bands, whereas the cold-rolled TD sample has a large amount of internal strain energy that is homogeneously distributed throughout the material;this latter behavior is strongly related to the extensive {10-12} twinning and the resultant microstructural variations in the TD sample. During subsequent annealing, recrystallization occurs locally along the shear bands in the ND sample, which leads to the formation of a bimodal grain structure comprising fine recrystallized grains and coarse unrecrystallized grains. In contrast, during subsequent annealing of the TD sample, static recrystallization occurs homogeneously throughout the material, which results in the formation of a uniform grain structure that mostly comprises equiaxed recrystallized grains.     

Orientation splitting and its contribution to grain refinement during equal channel angular extrusion

The early stage mechanisms of grain refinement during ECAE of a single-phase aluminium alloy have been studied using the EBSD technique. It was found that, in addition to the formation of shear-plane cell bands and shear bands by “simple shear”, the development of deformation bands due to orientation splitting contributed significantly to the refinement of microstructure. “Regular” slab-like deformation bands and “irregular” transitional bands were observed after the first pass; both developed boundaries of high misorientations. In the second pass, moderate orientation splitting took place within the deformation bands, although new deformation bands were not detected. With increased strains, fine scale orientation splitting tended to occur in local bands, generating high densities of new high misorientation boundaries. The crystallographic features of the different types of orientation splitting are examined.     

Effect of initial orientation on the microstructure and mechanical properties of textured AZ31 Mg alloy during torsion and annealing

We studied the effect of crystallographic orientation and temperature on the microstructure and mechanical properties of extruded AZ31 magnesium alloy bar by torsion and subsequent annealing. The results show that the orientation between torsion axis (TA) and extrusion direction (ED) has a significant impact on the microstructure evolution. With TA parallel to ED, profuse extension twins appeared. After annealing, zones close to the surface were completely recrystallized and refined, while the center still had some extension twins left. With TA perpendicular to ED, extension twins were inhibited. It is speculated that, except extension twins, contraction twins is also acting as a major deformation mode. Upon annealing, this specimen was completely recrystallized, even at the center, which is considered as a result of more preferred nucleation sites for static recrystallization from contraction twins. As demonstrated, the temperature has little impact on the microstructure development when twisted at room temperature or liquid nitrogen temperature. Moreover, the compression tests show that the compression ductility and yield strength were improved simultaneously for both samples when compressed on the direction either along ED or perpendicular to ED, due to the combined effects of grain refinement and texture weakening.     

Effect of selective laser melting on microstructure and properties of AZ91D alloy

Selective laser melting technology is used to manufacture porous and solid AZ91D alloys. The effects of laser power and hatch spacing on the density, blowholes, microstructure and mechanical properties of AZ91D alloy are studied. The laser power and hatch spacing play a significant role in the density and blowholes of AZ91D specimens. The grains size of specimens increases from 1 μm–2 μm to 8 μm–10 μm from the bottom to the top in single molten pool. Compared with grain size of die‐casting alloy (30 μm), that of selective laser melted gets refinement. There is no significant change in microstructure in the bottom, middle and top of specimens. The micro‐hardness of AZ91D alloy, reaching up to 115.3 HV 0.1, is superior to that of die‐casting alloy (56 HV 0.1). The compression properties of porous and solid specimens reach the degree of die‐casting solid magnesium alloy. AZ91D alloy shows the potential in the application of medical biodegradable materials.     

低碳钢形变强化相变时铁素体织构类型的分析

利用背散射电子衍射取向成像技术分析了在热模拟单向压缩条件下Q235碳素钢形变强化相变时铁素体织构的类型。结果表明,在利用形变强化相变实现铁素体的超细化过程中会出现铁素体的相交织构和形变织构,在大应变条件下还会出现动态再结晶织构。在形变强化相变后细晶铁素体在整体上表现为以〈111〉方向为主的线织构。主要的相交织构在粗晶奥氏体内部形变带形核时产生并与〈111〉织构对应。形变织构是在形变时形成的铁素体受到继续变形所致,在形变强化相变过程中及完成后都会产生,对应〈111〉及〈100〉方向的线织构,随着形变的加大,〈100〉方向的织构增加得更快,形变温度的降低有利于形变织构的加强。在形变量很大且形变温度比较合适时(但不能过低)会发生铁素体的动态再结晶,它以连续的方式进行,导致形变织构的进一步加强,并使晶粒均匀细化。     

Microstructural evolution during hot shear deformation of an extruded fine-grained Mg–Gd–Y–Zr alloy

Mg–Gd–Y–Zr alloys are among recently developed Mg alloys having superior mechanical properties at elevated temperatures. Dynamic recrystallization (DRX) and rare earth-rich particles play important roles in enhancing the high-temperature strength of these alloys. Accordingly, the microstructural evolution of a fine-grained extruded Mg–5Gd–4Y–0.4Zr alloy was investigated after hot shear deformation in the temperature range of 350–450 °C using the shear punch testing (SPT) method. The results reveal the occurrence of partial dynamic recrystallization at the grain boundaries at 350 °C while the fraction of DRX grains increases with increasing deformation temperature. A fully recrystallized microstructure was achieved after SPT at 450 °C. The Gd-rich and Y-rich cuboid particles, having typical sizes in the range of ~50 nm to ~3 μm, show excellent stability and compatibility after hot shear deformation, and these particles enhance the high-temperature strength during hot deformation at elevated temperatures. The textural evolution, examined using electron backscattered diffraction, revealed a non-fibrous basal DRX texture after SPT which is different from the conventional deformation texture.     

Texture evolution in equal-channel angular extrusion

The focus of this article is texture development in metals of fcc, bcc, and hcp crystal structure processed by a severe plastic deformation (SPD) technique called equal-channel angular extrusion (ECAE) or equal-channel angular pressing (ECAP). The ECAE process involves very large plastic strains and is well known for its ability to refine the grain size of a polycrystalline metal to submicron or even nano-size lengthscales depending on the material. During this process, the texture also changes substantially. While the strength, microstructure and formability of ECAE-deformed metals have received much attention, texture evolution and its connection with these properties have not. In this article, we cover a multitude of factors that can influence texture evolution, such as applied strain path, die geometry, processing conditions, deformation inhomogeneities, accumulated strain, crystal structure, material plastic behavior, initial texture, dynamic recrystallization, substructure, and deformation twinning. We evaluate current constitutive models for texture evolution based on the physics they include and their agreement with measurements. Last, we discuss the influence of texture on post-processed mechanical response, plastic anisotropy, and grain refinement, properties which have made ECAE, as well as other SPD processes, attractive. It is our intent to make SPD researchers aware of the importance of texture development in SPD and provide the background, guidance, and methodologies necessary for incorporating texture analyses in their studies.     

铸态AZ81镁合金ECAP态组织与性能研究

采用自制的90°模具,经Bc路径在温度为300℃下研究对比了铸态及不同道次的等通道挤压(ECAP)态AZ81镁合金微观组织和力学性能.结果表明ECAP随着挤压道次的增加,AZ81镁合金显微组织和力学性能发生显著变化.当挤压到4道次,平均晶粒尺寸由原来铸态的145um细化为9.6um,拉伸断口韧窝明显增多;抗拉强度从180 MPa提高到306 MPa,延伸率和硬度分别达到15.8%和142HL.分析表明,AZ81镁合金在高温挤压过程中Mg17Al12相粒子被破碎,并部分溶入基体,$-Mg基体与%-Mg17Al12相互相阻碍其晶粒长大,获得细小晶粒组织.