Severe plastic deformation driven nanostructure and phase evolution in a Al0.5CoCrFeMnNi dual phase high entropy alloy

The effect of severe plastic deformation on microstructure and phase evolution was investigated in a dual phase Al_0.5CoCrFeMnNi high entropy alloy (HEA). For this purpose, the as-cast HEA was subjected to initial thermo-mechanical processing by warm-rolling and annealing. The annealed dual phase alloy showed FCC and B2 phases. The B2 phase was enriched with Ni and Al, while the converse held good for the FCC phase. These annealed HEA specimens were severely deformed by high pressure torsion (HPT) up to five complete rotations (R). The severely deformed HEA revealed nanostructured FCC grains containing nano-twinned regions and coarser B2 phase. The nanostructure formation in the softer FCC phase was attributed to greater strain partitioning and propensity for the formation of nano-twins. Although with increasing rotations, the hardness difference between the edge and centre region was reduced, the 5R HPT processed specimens showed inhomogeneity featured by intermittent hardness spikes. Upon annealing, recrystallized dual phase microstructure was confirmed in the 5R HPT processed specimen. Microstructural differences between centre and edge regions were revealed by way of large B2 clusters (5 μm-10 μm) at the centre region. Remarkably, annealing resulted in the formation of a (Fe,Cr) rich σ-phase. The formation of σ-phase resulted in much greater hardness inhomogeneity in the annealed material as compared to the 5R HPT processed material.     

Mechanics of large strain extrusion machining and application to deformation processing of magnesium alloys

An analysis of the mechanics of large strain extrusion machining (LSEM), a constrained chip formation process, is presented for deformation processing of bulk alloys. The deformation field is shown to be narrowly confined and controllable, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain/strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of Mg AZ31B, an alloy of commercial significance but noted for its poor workability, into sheet and foil forms. Noteworthy features of the processing are suppression of segmentation, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet/foil from the bulk in a single step of deformation without pre-heating. Guidelines for realizing specific sheet attributes, and scalability of LSEM for production are analyzed and discussed.     

Severe plastic deformation introduced by rotation shear

A newly invented method, called the rotation shear method, to achieve the high strain by the rotation shear is introduced in this paper. Equations for the calculation of the strain and the strain rate are presented. An experiment was performed with pure aluminum and the results showed that a shear strain up to 8.37 was achieved and the transmission electron microscope observation confirmed that an ultra-fine grained material with the size below 700 nm was obtained.     

Continuous and ultra-fine grained chip production with large strain machining

In this study, orthogonal cutting technique as a severe plastic deformation (SPD) method for producing chips with an ultra-fine grained microstructure and fair mechanical properties is investigated; further, it has been suggested that by controlling the cutting velocity and contact length between tool and material, it is possible to produce a severely deformed and continuous chip with unrestored microstructure even at high cutting velocities. Solution treated Al-6061 samples in plane strain condition were severely deformed through applying various cutting velocities (from 50 to 2230 mm/s) for three different rake angles (−5°, −10° and −20°) in fixed cutting parameters. Chip thickness, contact length, shear strain, and Vickers microhardness variations were examined for different samples and chip formation mechanism was discussed for different processing conditions. In addition, the microstructure of especial produced chips was studied using transmission electron microscopy (TEM). The results showed that during the dominance of seizure mechanism at the contact surface, microhardness and shear strain (as well as contact length) have inverse dependency upon the variation of the cutting velocity. The results are discussed by considering the heat-time effect contribution in the final microstructure and mechanical properties.     

大塑性变形制备超细晶铝锂合金的组织及力学性能研究进展

综述了大塑性变形工艺制备超细晶铝锂合金的显微组织及其力学性能,分析了大塑性变形过程中铝锂合金的组织演变及其影响因素。铝锂合金的强化机制主要是基于析出强化,结合大塑性变形得到的超细晶粒组织可以显著提高强度和塑性,并得到优异的超塑性。表明大塑性变形加工铝锂合金,尤其是等通道挤压制备的超细晶铝镁锂合金在超塑性工业具有广阔的发展前景。     

Predicting grain refinement by cold severe plastic deformation in alloys using volume averaged dislocation generation

Grain refinement during severe plastic deformation (SPD) is predicted using volume averaged number of dislocations generated. The model incorporates a new expansion of a model for hardening in the parabolic hardening regime, in which the work hardening depends on the effective dislocation-free path related to the presence of non-shearable particles and solute–solute nearest-neighbour interactions. These two mechanisms give rise to dislocation multiplication in the form of generation of geometrically necessary dislocations and dislocations induced by local bond energies. The model predicts the volume averaged number of dislocations generated and considers that they distribute to create cell walls and move to existing cell walls/grain boundaries, where they increase the grain boundary misorientation. The model predicts grain sizes of Al alloys subjected to SPD over two orders of magnitude. The model correctly predicts the considerable influence of Mg content and content of non-shearable particles on the grain refinement during SPD.     

Multiple plastic deformation mechanisms of NiTi shape memory alloy based on local canning compression at various temperatures

A Ni-rich NiTi shape memory alloy (SMA), which was in its austenitic state at ambient temperature, was subjected to plastic deformation by means of local canning compression at various temperatures ranging from room temperature to 800 °C. Depending on temperatures, NiTi SMA exhibited multiple plastic deformation mechanisms, such as dislocation slip, deformation twinning, grain boundary slide, grain rotation, dislocation climb and grain boundary migration. Amorphization, dynamic recovery and dynamic recrystallization of NiTi SMA were also observed at various temperatures. Mechanism of localized amorphization, in particular, was investigated based on dislocation slip and deformation twinning. Statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) were found to play an important role in the amorphization of the current NiTi SMA. There appeared a critical dislocation density below which NiTi SMA was unable to amorphize. Accordingly, at a fixed deformation strain, there should be a critical temperature above which amorphous phase would not occur in the NiTi SMA matrix. Furthermore, when NiTi SMA experienced plastic deformation at the critical temperature, amorphization and crystallization would occur simultaneously and compete with each other.     

H62铜合金和T2纯铜不同塑性应变状态微结构演变及其力学性能

对相同退火温度和相同尺寸的T2纯铜板和H62铜合金进行单向拉伸试验,分析两种材料在不同应变状态下对损伤度的影响。结果表明,T2纯铜和H62铜合金的断裂与损伤与其在拉伸作用微结构演变密切相关,微结构的演变对认识两种材料的断裂机理非常重要。两种材料的形状因子随着应变增大具有相似的变化趋势,但不同的是H62铜合金形状因子较大并且快速增大,而T2纯铜的形状因子较小并且缓慢增大。两种材料相对形状因子的变化非常相似,形状因子的相对增长趋势完全一致。H62铜合金比T2纯铜较早进入塑性变形阶段,并且比T2纯铜的塑性变形阶段较短。在突破一定阈值以后,比T2纯铜更快发生破坏变形。通过指数函数对两种材料的归一化形状因子曲线进行了拟合,建立了拟合方程,揭示材料宏观变形与微结构之间的关系。     

剧烈塑性变形珠光体钢丝退火组织与性能

研究了经剧烈塑性变形制备的珠光体扁钢丝在不同温度退火后的力学性能和组织.结果表明,在低于200℃退火时,由于应变时效的作用,强度和硬度有所增加；退火温度＞200℃时,强度和硬度不断降低.同时,在变形时引入的高应力和退火温度的作用下,片层状渗碳体在200℃以上退火时开始溶解,并随温度的升高,渗碳体溶解的速度加快,最终形成球化组织.     

Effects of cold severe plastic deformation and heating on dendritic and non-dendritic structures: A356 alloy

In this research the effects of cold deformation and heating to the semi-solid temperature on microstructure and mechanical properties of cast dendritic and non-dendritic structures of A356 alloy were investigated. To produce the non-dendritic samples, the semi-solid slurry was obtained by electromagnetic stirring and rheoforged and then the samples were heated to semi-solid temperature. In order to impose the deformation to the dendritic and non-dendritic samples, multidirectional forging process was used. Non-dendritic samples were deformed with applying one to three passes of the multidirectional forging and then were kept at the semi-solid range of temperature again. Microstructural and mechanical properties of the deformed samples after and before heating were investigated. The dendritic structure after one pass of the multidirectional forging was kept at the semi-solid temperature to reach a different non-dendritic structure by strain induced melt activation process. Micro and macro hardness tests were used to study the mechanical properties of these samples. For metallographic examinations, scanning electron microscopy and the optical microscope equipped with the Clemex image analyzer software were used. The circular diameter of the heated rheoforged samples to semi-solid temperature that deformed to one pass of the multidirectional forging and then kept at semi-solid state was smaller than that of ones deformed to two and three passes and kept at semi-solid state. From the results achieved for the strain induced melt activation sample, it was obvious that this is an effective process to produce non-dendritic structure with approximately the same sphericity and lower globule size compared with other studied samples.     

7050铝合金锻件显微组织均匀性的表层增塑累积变形调控（英文）

为调控大型飞机用铝合金结构件的组织均匀性,提出表层增塑累积变形的方法。采用电子背散射衍射、透射电子显微镜和X射线衍射技术研究7050铝合金锻件表层增塑累积变形后的显微组织。结果表明,7050铝合金锻件的显微组织演变对变形温度比应变速率更敏感。锻件的位错密度随着变形温度的降低和应变速率的增加而持续增加。通过表层增塑累积变形实现位错密度和形变储能的高效累积。此外,建立7050铝合金的静态再结晶模型,计算得到的静态再结晶体积分数与实验结果吻合较好,说明该模型能够准确地描述7050铝合金在表层增塑累积变形调控中的静态再结晶行为。     

Microstructure development of steel during severe plastic deformation

The microstructure development during plastic deformation was reviewed for iron and steel which were subjected to cold rolling or mechanical milling (MM) treatment, and the change in strengthening mechanism caused by the severe plastic deformation (SPD) was also discussed in terms of ultra grain refinement behavior. The microstructure of cold-rolled iron is characterized by a typical dislocation cell structure, where the strength can be explained by dislocation strengthening. It was confirmed that the increase in dislocation density by cold working is limited at 10¹⁶m⁻², which means the maximum hardness obtained by dislocation strengthening is HV3.7 GPa. However, the iron is abnormally work-hardened over the maximum dislocation strengthening by SPD of MM because of the ultra grain refinement caused by the SPD. In addition, impurity of carbon plays an important role in such grain refinement: the carbon addition leads to the formation of nano-crystallized structure in iron.     

Microstructural evolution and mechanical properties of niobium processed by equal channel angular extrusion up to 24 passes

We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300 nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress-strain curves exhibit elastic-nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress-strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate.     

Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation

Bulk components made of a Ni-base C-2000 alloy with a face-centered cubic crystal structure and a very low stacking fault energy have been severely plastically deformed at the surface region to attain a grain size gradient ranging from nanocrystalline at the surface to coarse grained in the bulk. The evolution of microstructural characteristics has been studied as a function of the processing time employing a variety of analytical techniques, including extensive conventional and high-resolution transmission electron microscopy analyses. The thickness of the nanocrystalline surface layer is found to increase with the processing time. Deformation twinning is ubiquitous and occurs at the earliest stage of deformation and the deepest region of the material where plastic deformation has taken place in the surface severe plastic deformation process. A grain-refinement mechanism led by deformation twins and complemented by dislocation activity has been put forth to explain the nanocrystallization of the coarse-grained material employed in this investigation.     

Production of ultrafine grain sizes in aluminium sheets by severe plastic deformation using the technique of groove pressing

Commercial purity aluminium was subjected to a severe plastic deformation technique called groove pressing at both room temperature and cryogenic temperatures. In both cases, submicron sized grain structures were obtained after deformation; there was no significant difference in microstructures obtained under room temperature and cryogenic temperature deformation conditions.     

High-pressure torsion, a new processing route for thermoelectrics of high ZTs by means of severe plastic deformation

High-pressure torsion (HPT) is a type of severe plastic deformation (SPD) that is highly suited to produce bulk ultrafine-grained and nanocrystalline materials, as it introduces many grain boundaries as well as dislocations and point defects. In this paper, HPT-mediated nanocrystallization was used to reduce the thermal conductivity and enhance the Seebeck coefficient of skutterudites. Both p- and n-type skutterudites have been processed by HPT with 4 and 5 GPa at temperatures up to 773 K, resulting in a strongly strengthened nanocrystalline structure, revealing oriented, lamellar-shaped crystallites with a size of ∼50 nm and an enhanced dislocation density. In comparison with ball-milled plus hot-pressed skutterudites, the HPT-processed samples show a reduction of the thermal conductivity up to 40%. This and the slightly higher Seebeck coefficient are the reasons why HPT proved to enhance the figure of merit (ZT) values up to a factor of 2, in spite of a markedly enhanced electrical resistivity.     

Incoloy825合金热变形为与组织演变

通过热压缩实验,研究了Incoloy825合金在变形量为60％,温度为950~1150℃和应变速率0.001~1s-1范围内热变形行为。基于Arrhenius方程和Zener-Hollomon参数模型,建立该合金的本构方程模型。采用金相显微镜（OM）和电子背散射衍射（EBSD）技术研究了合金的组织演变规律。结果表明,随着变形温度的升高或应变速率的降低,DRX的百分含量增加。热变形过程中DRX既包括晶界弓起形核机制的不连续动态再结晶（DDRX）也包括渐进式亚晶旋转形核机制的连续动态再结晶（CDRX）。随着变形温度的升高或应变速率的降低DDRX增强而CDRX减弱。此外随着温度的升高或应变速率的降低,低角度晶界逐渐向高角度晶界转化。同时随机分布的Σ3孪晶界趋于均匀化,且对动态再结晶起促进作用。     

一种椭圆截面螺旋等通道挤压制备超细晶材料的新工艺

近年来,对剧烈塑性变形法制备块体超细晶(UGF)材料的研究已成为材料科学领域的一大热点.基于剧烈塑性变形制备超细晶材料的机理研究,提出了一种新型成形技术——椭圆螺旋等通道挤压法(ECEA).本文系统地阐述了ECEA的基本原理、工艺特点和变形过程,给出了ECEA累积等效应变的解析解计算式.通过有限元模拟,分析了ECEA工...