Superplasticity of Mg-Zn-Y alloy containing quasicrystal phase processed by equal channel angular pressing

Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-4.3 wt.%Zn-0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼ 120 to ∼ 3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg-Zn-Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5 × 10^− 3 to 1.5 × 10^− 4 s^− 1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5 × 10^− 4 s^− 1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg-Zn-Y alloy in the temperature and strain-rate range investigated.     

Mechanical properties of a two-phase alloy Mg–8%Li–1%Al processed by equal channel angular pressing

The microstructural evolution and room temperature tensile properties of Mg–8%Li–1%Al alloy processed by equal channel angular pressing (ECAP) at 403 K were investigated. It was found that the strength could be improved pass by pass. The elongation-to-failure decreased dramatically after the first ECAP pass, but could be improved pass by pass during the subsequent ECAP procedure. The microstructure analysis gave the explanations for these phenomena.     

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.     

Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment

In order to examine the combined effect of plastic deformation and aging process, the Al 7075 alloy was subjected to equal channel angular pressing (ECAP) deformation by route B_C in various ECAP and aging conditions: pre-ECAP aging, post-ECAP aging and dynamic aging during ECAP at 393 K and 423 K. Followed by ECAP and aging treatment, Vickers microhardness and tensile test were performed and microstructural observations were undertaken using transmission electron microscopy (TEM) and X-ray diffractometer (XRD). TEM investigation showed that ultrafine-grained (UFG) materials with grain size less than 500 nm could be obtained after three or four passes of ECAP. Precipitates characterization revealed that maximum mechanical properties are achieved when the microstructure mainly consists of fine dispersion of small η^′ precipitates and minor quantities of GP zones. Dynamic aged specimens at 393 K and 423 K represented maximum and minimum mechanical properties, respectively, due to formation of fine η^′ precipitates plus GP zones and η^′ plus η precipitates, respectively. Dynamic aging during ECAP at 393 K appeared preferable to other procedures for attaining maximum mechanical properties as well as saving time and energy.     

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.     

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.     

Effect of equal channel angular pressing and heat treatment on the microstructure of Cu–Al–Be–B shape memory alloy

The combination of equal channel angular pressing (ECAP) and heat treatment was carried out to modify the microstructure of a Cu–Al–Be–B shape memory alloy. Microstructures of the alloy after ECAP and subsequent quenching were investigated by optical microscopy and X-ray diffraction (XRD). The alloy with 8 passes of ECAP at 743 K is characterized with ultra-fine grains (~ 2 μm), but with smaller fraction of martensites which implies the lower shape memory effect (SME). After reheated at 873 K and oil-quenched to room temperature, the grains become coarsen (~ 50 μm) but still finer than that of as-received (100–300 μm), and the fraction and order of martensites were increased simultaneously.     

Annealing twins in high purity aluminium processed by dynamic equal channel angular pressing

Annealing twins are synthesised in high purity aluminium processed by dynamic equal channel angular pressing during annealing. Annealing twins and recrystallised grains encircling the twins have specific crystallographic orientation relationships with the deformed matrix grains: approximately 35°–50°<110> for the twins and 30°–45°<100> for the recrystallised grains. Stored energy during dynamic pressing, and the crystallographic orientation between annealing twins and the deformed matrix, strongly affect twin growth during annealing.     

Design and mechanical property analysis of ultrafine grained gears from AA5083 previously processed by equal channel angular pressing and isothermal forging

In this present study, the isothermal forging of two different gears is carried out from material previously deformed by the severe plastic deformation (SPD) process known as Equal Channel Angular Pressing (ECAP). At present, there are only a few studies which use this material predeformed that exhibits improved mechanical properties as a result of the SPD process for use in subsequent processes or applications. The design and optimization of the die geometry required for the isothermal forging of gears are shown and both microhardness and microstructure are compared when these forged gears are obtained from annealed material (N0) and ECAP-processed material (N2). With this present research work, it is demonstrated that there is an improvement in forgeability and microhardness as well as a decrease in the grain size of the material predeformed by SPD.     

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.     

连续等通道转角挤压对Al-Ti-C合金组织与性能的影响EI北大核心CSCD

采用连续等通道转角挤压工艺,以连续的方式对Al-Ti-C合金进行多道次挤压,通过观察微观组织演化,探讨晶粒细化机理和力学性能变化。结果表明:连续等通道转角挤压工艺可有效细化Al-Ti-C合金微观组织,晶粒尺寸减小至1μm左右,形变诱导是变形过程中最主要的晶粒细化机制;高密度位错堆积引起Al基体和TiAl_(3)界面的裂纹以及TiAl_(3)内部的空洞产生,裂纹进一步扩展贯穿整个TiAl_(3)颗粒,最终导致第二相TiAl_(3)组织的细化,同时细小的第二相TiAl_(3)组织的钉扎机制和剪切机制促进了Al基体细化;连续等通道转角挤压1道次后,合金硬度提升最明显,与原始态相比提高59.2%;之后随挤压道次的增加,硬度提升的趋势变缓,合金塑性下降,韧性提高。     

Effect of initial microstructure on the processing of titanium using equal channel angular pressing

Equal channel angular pressing (ECAP) is a metal processing technique that is used to produce materials with ultrafine (<1 μm) grain sizes. In this work, the effect of the initial microstructure on ECAP of commercially pure titanium (CP Ti), a material used in many industrial applications, was investigated. To produce different initial microstructures, samples of CP Ti were exposed to different annealing conditions: no annealing (Material 1), annealed at 1033 K for 2 hr (Material 2), or annealed at 1173 K for 4 hr (Material 3). Each material was subjected to one pass of ECAP and the resulting microstructures were analyzed using XRD, SEM, and TEM, and compared to the microstructures before ECAP. It was found that each material developed a unique microstructure after one pass of ECAP, which was attributed to the varying microstructural characteristics before ECAP. Microhardness values before and after ECAP varied with each microstructure.     

Fatigue properties of ultrafine grained low carbon steel produced by equal channel angular pressing

Ultrafine grained low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating fatigue properties, including cyclic softening and crack growth rate. Emphasis was placed on investigating the effect of load ratio on the fatigue crack growth rates of ultrafine grained microstructure. The ECAPed steel exhibited cyclic softening. After the first cycle, the tension and compression peak stresses decreased gradually with the number of cycles. Fatigue crack growth resistance and the threshold of ECAPed ultrafine grained steel were lower than that of an as-received coarse grained steel. This was attributed to a less tortuous crack path. The ECAPed steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase in R ratio. The R ratio effect on growth rates and ΔK_th was basically indistinguishable at a lower load ratio (R>0.3) compared with other alloys, indicating that the contribution of the crack closure vanished. This was explained by the fact that finer grained materials produce a lower opening load P_op due to a relatively less serrated crack path. Consequently, K_min can reach K_op readily with a smaller increment of load ratio. The crack growth rate curve for the ECAPed ultrafine grained steel exhibited a linear extension to the lower growth rate regime than that for the coarse grained as-received steel. This behavior can be explained by a reverse crack tip plastic zone size (r_p) that is always larger than the grain size.     

On the effect of acute angles on deformation homogeneity in equal channel angular pressing

Finite element calculations for strain development and deformation homogeneities under equal channel angular pressing (ECAP) considering the channel angle, friction and the channel thickness show that general phenomena characteristic of ECAP still hold when the channel angle is acute ( = 75°), in contrast to the conclusion of the recent paper on acute channel angles [A.V. Nagasekhar, Y. Tick-Hon, S. Li, H.P. Seow, Mater. Sci. Eng. A410–A411 (2005) 269–272] that formation of corner gap was not a factor for the acute channel angles.     

Microstructure and mechanical properties of ODS Fe14Cr model alloy processed by equal channel angular pressing

Abstract

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y₂O₃ (wt-%) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.     

Texture and mechanical properties of single crystal copper during equal channel angular pressing by route C

Single crystal copper has excellent electrical and thermal conductivity, but the lower strength seriously limited its application. Traditional strengthening methods, such as alloying, will severely damage its conductivity. Severe plastic deformation is the most effective methods for increasing the metals strength and not reducing the conductivity. The microstructure and texture evolution of single crystal copper (99.999 %) during equal channel angular pressing by route C was investigated by scanning electron microscopy, X‐ray diffraction, electron backscatter diffraction and transmission electron microscopy, the mechanical properties and conductivity were tested, and the influence mechanism of texture and microstructure on mechanical properties and conductivity were analyzed. The results show that during equal channel angular pressing, the original <111> orientation gradually changed to <001>, accompany lots of low‐angle grain boundaries were formed. With strain increasing, the high‐angle grain boundaries increased gradually, and the deformation bands with <110> orientation was formed in the single crystal structure, which plays a positive role on the conductivity. After 5 passes, the tensile strength of single crystal copper increased from 168 MPa to 415 MPa by route A and 385 MPa by route C, and the elongation declined sharply from 63 % to 30 % and 27.9 %, respectively. After 16 passes, the hardness increased from 60.4 HV to 130.8 HV and the conductivity only slightly down.

     

等径角挤压过程中材料的流变行为研究

分析了等径角挤压过程中材料流变的原因和特征,认为每道次挤压材料内部发生的剪切变形量与时间的函数曲线呈近似正态分布,适当升高温度和增加背压能有效减小难变形区,降低通道与试样接触面之间的摩擦能减小材料内部的滞变区.通过实验证明,随着挤压道次的增加,材料内部的滞变区将减小,均匀化程度会逐步提高.