Microstructure and mechanical properties of Cu and Cu-Zn alloys produced by equal channel angular pressing

Ultrafine-grained (UFG) Cu and Cu-Zn alloy were prepared using equal-channel angular pressing (ECAP) to investigate the effects of stacking fault energy (SFE) on microstructure evolution and mechanical properties. Combining with the previous researches, the grain refinement process of ECAP is divided into three stages based on the variation of tensile strength and plasticity. According to the influences of defects on strength and ductility during plastic deformation, the three stages are discussed in detail by considering the dislocation density, grain and twin boundaries. Besides, the impact of SFE on the strength and ductility of the UFG Cu-Zn alloys are evaluated, indicating that these two mechanical properties can be improved simultaneously in the whole ECAP process either through slightly or widely adjusting the SFE. This significant effect of SFE reflects in two aspects, one is in the microstructure evolution during ECAP processing and the other is in the subsequent tensile plastic deformation, both of which can be achieved through regulating the dislocation motion via changing the SFE.     

Three-dimensional numerical analysis and experimental investigation of grain refinement in multi-pass equal channel angular pressing for round-workpieces

Equal channel angular pressing (ECAP) has the capability of producing ultra fine-grained (UFG) materials bellow the dimension of 1 μm. At present, it is one of the most important methods to get bulk UFG materials. Multi-pass ECAP processes for round workpieces are investigated by using numerical simulations and experimental studies in this paper. The deformation mechanism of ECAP for grain refinement is obtained. Three processing routes A, B and C are simulated in order to study the influence of the processing routes to the deformation uniformity of the workpiece. The finite element (FE) analysis results of the multi-pass ECAP process show that the different processing routes result in the different deformation distributions. The grain in the workpiece is refined obviously after multi-pass pressing. The microstructures of the processed material are more different than that of the microstructure of the annealing initial equiaxed grains. The microstructure evolution of the workpiece can be changed via different processing routes. It is found that route B can get a high angle grain boundaries distribution in the workpiece than other routes. The results of the analysis show that the process of grain refinement can be described as a continuous dynamic recovery and recrystallization. The microstructure evolutions of the grain refinement mechanisms and micro-structural characteristics for different multi-pass ECAP processing routes are verified by using OM (optical model) and TEM (transmission electron microscope) analysis. In addition, the experimental microstructure results are also consistent with FE analysis results.     

Cryogenic equal channel angular pressing of commercially pure titanium: microstructure and properties

Cylindrical samples of CP Titanium (Grade 2) were deformed by one, two and three passes of equal channel angular pressing (ECAP) each at temperatures 77, 300 and 575 K, respectively. The microstructure of samples processed at 77 K shows retardation of recrystallisation, high density of dislocations and deformation twins, diffuse and obscure grain boundaries compare to microstructure of samples processed at room and high temperature, where recrystallised ultrafine equiaxed grains are observed. Mechanical properties for all structural states of Ti were studied by microhardness measurements at 300 K and uniaxial compression at temperatures 300, 170, 77 and 4.2 K. Higher levels of ECAP deformation (more passes of ECAP) lead to higher values of strength and hardness at all studied temperatures. Decrease of ECAP temperature leads to increase of strength characteristics in all cases. Influence of ECAP and compression temperatures on possible changes of deformation mechanism are discussed.     

Microstructure and mechanical properties of Mg–Zn–Ca alloy processed by equal channel angular pressing

An ultrafine-grained (UFG) Mg–5.12 wt.% Zn–0.32 wt.% Ca alloy with an average grain size of 0.7 μm was produced by subjecting the as-extruded alloy to equal channel angular pressing (ECAP) for 4 passes at 250 °C. The fine secondary phase restricted the dynamic recrystallized (DRXed) grain growth during the ECAP processing, resulting in a remarkable grain refinement. A new texture was formed in the ECAPed Mg alloy with the {0 0 0 2} plane inclined at an angle of 58° relative to the extrusion direction. The yield stress (YS) was decreased in the as-ECAPed alloy with finer grains, indicating that the texture softening effect was dominant over the strengthening from grain refinement. The ductility of the as-ECAPed alloy was increased to 18.2%. The grain refinement caused an obvious decrease in work hardening rate in the as-ECAPed alloy during tensile deformation at room temperature.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

Texture and microstructure evolution in ultrafine-grained AZ31 processed by EX-ECAP

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route B_c, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to the bimodal grain structure observed in this specimen. The grains larger than 10 μm show the orientation changes corresponding to the ECAP shear, which is characterised by the rotation of the basal poles by approximately 40° from the initial orientation. The fine grains with the average size of 1 μm maintain the initial orientation. The character of the bimodal grain structure and the distinct texture components between large and small grains remained unchanged up to 4 ECAP passes. Further ECAP pressing to 8 and 12 passes leads to a grain refinement through the whole sample volume and the orientation changes of all grains corresponding to the ECAP shear.     

A comparison of microstructures and mechanical properties in a Cu–Zr alloy processed using different SPD techniques

Experiments were conducted to evaluate the microstructures and mechanical properties of a Cu–0.1 % Zr alloy processed using two different techniques of severe plastic deformation: equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). The samples were processed at room temperature through ECAP for eight passes or through HPT for 10 turns. The results show HPT is more effective both in refining the grains and in producing a large fraction of grain boundaries having high angles of misorientation. Both procedures produce reasonably homogeneous hardness distributions but the average hardness values were higher after HPT. In tensile testing at 673 K, the highest strength and ductility was achieved after processing by HPT. This is attributed to the grain stability and high fraction of high-angle grain boundaries produced in HPT.     

Enhanced superplasticity of magnesium alloy AZ31 obtained through equal-channel angular pressing with back-pressure

Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity.     

In situ TEM investigation of microstructural behavior of superplastic Al–Mg–Sc alloy

Dynamic changes in microstructure of the superplastic ultrafine-grained Al–3Mg–0.2Sc (wt.%) alloy refined by equal-channel angular pressing (ECAP). were observed by in situ transmission electron microscopy at temperatures up to 300 °C (annealing and tensile deformation) in order to simulate the initial stages of superplastic testing. It was found that the microstructure changes significantly during the preheating before the superplastic deformation, which was accompanied by decreased microhardness. During the deformation at 300 °C, high dislocation activity as well as motion of low-angle grain boundaries was observed while high-angle grain boundaries did not move due to the presence of scandium in the alloy.     

Microstructure and mechanical properties of ultrafine grain ZK60 alloy processed by equal channel angular pressing

The microstructure evolution and tensile properties of ZK60 magnesium alloy after equal channel angular pressing (ECAP) have been investigated. The results show that the two-step ECAP process is more effective in grain refinement than the single-step ECAP process due to the lower deformation temperature, a mean grain size of ~0.8 μm was obtained after two-step ECAP process at 513 K for four passes and 453 K for four passes. The EBSD examination reveals that ZK60 alloy after two-step ECAP process exhibits a more homogeneous grain size and misorientation distribution than single-step ECAP process. Both alloys after ECAP process present similar strong {0002} texture. The tensile strength of two-step ECAP alloy has also been improved compared with the single-step ECAP alloy. The strengthening effect was mainly ascribed to grain refinement.     

等通道角变形对搅拌铸造SiC_P/AZ91复合材料显微组织与室温性能的影响

采用搅拌铸造法成功制备了SiC_P/Mg(AZ91)复合材料并对铸态复合材料进行了等通道角挤压变形(Equal channel angular pressing,ECAP)。结果表明,搅拌铸造态SiC_P/AZ91复合材料的基体组织致密,颗粒与基体结合良好,没有出现宏观团聚;SiC_P大部分聚集在晶界附近区域并呈"项链状"分布。ECAP变形可以有效地消除铸态SiC_P/AZ91复合材料中的SiC_P"项链状"分布,并且随着ECAP道次的增加,SiC_P分布更加均匀;在ECAP过程中,SiC_P发生了一定断裂但并不明显。SiC_P/AZ91复合材料基体晶粒随着变形道次的提高而逐渐细化。基体晶粒细化以及SiC_P分布均匀化是SiC_P/AZ91复合材料屈服强度和抗拉强度随着道次提升而逐渐增加的主要原因。     

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.     

Grain refinement in as cast Al–Mg–Mn alloy during high temperature equal channel angular pressing

Abstract

An as cast Al–Mg–Mn alloy with coarse equiaxed grain structure was processed by equal channel angular pressing (ECAP) at 350°C up to eight passes. Systematic studies were made on the microstructural evolution during ECAP by optical microscopy, electron backscattered diffraction and TEM. Equal channel angular pressing led to a considerable grain refinement, resulting in an average cell size of about 1 μm and a fraction of high angle boundaries of 75% after eight pressing passes. Deformation bands were not developed during the ECAP process, and a reasonably equiaxed substructure was obtained even after one pass. The main mechanism of grain refinement was attributed to the continuous dynamic recrystallisation based on the motion of deformation induced dislocations. Discontinuous recrystallisation at grain boundaries and triple junctions also contributed to the refinement, which played an important role especially at high strain of eight passes.     

Influence of Pressing Temperature on Microstructure Evolution and Mechanical Behavior of Ultrafine‐Grained Cu Processed by Equal‐Channel Angular Pressing

Pure Cu was processed by ECAP at five different temperatures from room temperature (RT) to 523 K. The influence of pressing temperature on microstructure evolution and tensile behavior was investigated in detail. The results show that as the ECAP temperature is increased the grain size and ductility both increase whereas the dislocation density and yield strength decrease. In the case of ECAP processing in the range of RT to 473 K the mechanism governing microstructural refinement is continuous dynamic recrystallization (CDRX), whereas at 523 K the mechanism changes to discontinuous dynamic recrystallization (DDRX). At higher ECAP temperatures, the kinetics of CDRX are retarded leading to a lower fraction of equiaxed grains/high‐angle grain boundaries and a higher fraction of dislocation cell structures. At 523 K, DDRX induces a high fraction of equiaxed grains with a very low dislocation density which appears responsible for the observed high tensile ductility. The sample processed at 523 K possessed a good combination of strength and ductility, suggesting that processing by ECAP at elevated temperatures may be a suitable alternative to RT ECAP processing followed by subsequent annealing.     

低温等通道转角挤压中定向凝固纯铜的组织及性能演变

采用光学显微镜(OM)和XRD技术对干冰冷却后的定向凝固纯铜(99.99%)经等通道转角挤压(ECAP)时的微观组织演变规律进行研究,并测试了ECAP后定向凝固纯铜的硬度及导电性能。结果表明,定向凝固纯铜在低温下经A和C路径变形后易于形成取向一致的纤维组织,并且保持(111)面的择优取向特征,而经Bc路径变形后,柱状晶破碎,形成均匀的等轴晶,且各晶面逐渐趋于随机取向;经过1道次变形后,各路径硬度大幅增加,约为原来的1.8倍,在随后的挤压中,硬度增加缓慢,经4道次ECAP后,Bc路径的硬度有所下降;在低应变下,晶粒取向的一致性使得导电率增加;随着应变的增加,晶格畸变使得电子发生散射,使导电率略有降低。     

Development of fine grained structures using severe plastic deformation

Abstract

Severe plastic deformation provides a processing tool for introducing fine grain sizes into polycrystalline materials. This paper describes the principles of equal channel angular pressing (ECAP) in which a material is pressed through a die where two channels form an L shaped configuration. The process of ECAP imposes a severe strain on the sample but there is no concomitant change in the cross-sectional dimensions so that repetitive pressings may be undertaken to achieve very high total strains. Several factors influence the nature of the microstructures attained in ECAP including the processing route by which the sample is rotated between consecutive pressings, the angle subtended by the two channels within the die, and the speed and temperature associated with the pressing. These various factors are described for a series of experiments conducted on samples of pure aluminium.     

Microstructures and properties of ultrafine-grained pure titanium processed by equal-channel angular pressing and cold deformation

Equal-channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) titanium as well as other metals and alloys. CP-Ti is usually processed at about 400 degrees C because it lacks sufficient ductility at lower temperature. The warm processing temperature limits the ability of the ECAP technique to improve the strength of CP-Ti. We have employed cold deformation following warm ECAP to further improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route Bc, with a clockwise rotation of 90 degrees between adjacent passes. The grain size obtained by ECAP alone is about 260 nm. The billets were further processed by cold deformation (cold rolling) to increase the crystalline defects such as dislocations. The strength of pure Ti was improved from 380 to around 1000 MPa by the two-step process. This article reports the microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold deformation.     

A modification on ECAP process by incorporating torsional deformation

In the present study, integration of equal channel angular pressing (ECAP), as a well known severe plastic deformation (SPD) technique, and torsion deformation, is studied by using three dimensional finite element analysis. This process is to be named as torsional-equal channel angular pressing (T-ECAP). In this modification a part of the exit channel in the ECAP die is rotating around its axis, to impose extra shear strains to the samples. To study deformation behavior in the T-ECAP process, three-dimensional finite element analysis (FEA) was carried out by using the elasto-plastic finite element analysis ABAQUS/Explicit Simulation. To investigate the validity of the simulation results, experimental studies were furthermore performed on commercially pure aluminum (AA 1050). Vickers hardness test was used to determine the distribution of hardness on both normal and longitudinal sections of the deformed samples with respect to the exit channel of the die. The hardness test results showed more uniform distribution of hardness in both sections of the T-ECAP processed samples regarding the ones produced by ECAP process. The load requirement comparison for performing both processes showed lower value for the T-ECAP with respect to the ECAP process. The simulation results for the strain values showed higher magnitude and more uniform distribution for the T-ECAP with respect to the ECAP process.     

Quantitative study of grain refinement in Al–Mg alloy processed by equal channel angular pressing at cryogenic temperature

Strain induced grain refinement of an Al–1 wt.% Mg alloy processed by equal channel angular pressing (ECAP) at cryogenic temperature is investigated quantitatively. The results show that both mean grain and subgrain sizes are reduced gradually with increasing ECAP pass. ECAP at cryogenic temperature increases the rate of grain refinement by promoting the fraction of high angle grain boundaries (HAGBs) and misorientation at each pass. The fraction of HAGBs and the misorientation of Al–1 wt.% Mg alloy during ECAP at cryogenic temperature increase continuously as a function of equivalent strain. Both {110} and {111} twins at ultrafine-grained size are observed firstly in Al–Mg alloy during ECAP. The analysis of grain boundaries and misorientation gradients demonstrates the grain refinement mechanism of continuous dynamic recrystallization.     

Achieving a Superplastic Forming Capability through Severe Plastic Deformation

Processing by severe plastic deformation (SPD) leads to very significant grain refinement in metallic alloys. Furthermore, if these ultrafine grains are reasonably stable at elevated temperatures, there is a potential for achieving high tensile ductilities, and superplastic elongations, in alloys that are generally not superplastic. In addition, the production of ultrafine grains leads to the occurrence of superplastic flow at strain rates that are significantly faster than in conventional alloys so that processing by SPD introduces the possibility of using these alloys for the rapid fabrication of complex parts through superplastic forming operations. This paper examines the development of superplasticity in various aluminum alloys processed by equal‐channel angular pressing (ECAP).