Impurity Effect on Wear Resistance of Ultrafine-Grained Copper

The wear properties of ultrafine-grained(UFG) Cu samples of different purities were investigated in comparison with the coarse-grained(CG) Cu.The UFG Cu samples,prepared by means of plastic deformation via quasistatic compression,exhibit an enhanced wear resistance relative to the CG Cu samples.For both the UFG and the CG Cu samples,wear volumes increase at higher purities.A steady state worn subsurface structure was formed in each sample after sliding for 60 min,consisting of a heavily deformed nanostructured mixing layer(NML) on top of a continuous dynamic recrystallization(DRX) layer.A pronounced correlation is identified that wear volume increases monotonically with an increasing grain size of the DRX layer.The impurity level of the Cu samples has an obvious influence on the DRX grain sizes,which in turn determines the wear resistance of the Cu samples.     

Dynamic observations of deformation in an ultrafine-grained Al–Mg alloy with bimodal grain structure

The tensile properties and deformation response of an ultrafine-grained (UFG) Al–Mg alloy with bimodal grain structure were investigated using a micro-straining unit and a strain mapping technique. Atomized Al 5083 powder was ball-milled in liquid N₂ to obtain a nanocrystalline (NC) structure, then blended with 50 wt.% unmilled coarse-grained (CG) powder, and consolidated to produce a bimodal grain structure. The blended powder was hot vacuum degassed to remove residual contaminants, consolidated by cold isostatic pressing (CIP), and then quasi-isostatic (QI) forged twice. The resultant material consisted of a UFG matrix and CG regions. The dynamic response during tensile deformation was observed using a light microscope, and the surface displacements were mapped and visualized using a digital image correlation (DIC) technique. The DIC results showed inhomogeneous strain between the UFG and CG regions after yielding, and the strain was localized primarily in the CG regions. Strain hardening in the CG regions accompanied the localization and was confirmed by variations in Vickers hardness.     

Microstructural change of ultrafine-grained aluminum during high-speed plastic deformation

Effect of strain rate on microstructural change in deformation of the ultrafine grained (UFG) aluminum produced by severe plastic deformation (SPD) was studied. Commercial purity 1100 aluminum sheets were highly strained up to an equivalent strain of 4.8 by the Accumulative Roll-Bonding (ARB) process at ambient temperature. The ARB-processed sheets were found to be filled with pancake-shaped ultrafine grains surrounded by high-angle grain boundaries. The ultrafine grains had a mean grain thickness of 200 nm and a mean grain length of 1100 nm. The ultrafine-grained aluminum sheets were deformed at various strain rates ranging from 2 to 6.0×10⁴ s⁻¹ by conventional rolling, ultra-high-speed rolling, and impact compression. High-speed plastic deformation generates a large amount of heat, inducing coarsening of the ultrafine grains during and after deformation. On the other hand, it was also suggested that high-speed plastic deformation is effective for grain-subdivision, in other words, ultra-grain refinement, if the effect of heat generation is extracted.     

Analyzing Deformation and Damage of VT1-0 Titanium in Different Structural States by Using Infrared Thermography

This study is devoted to the comparison of strength characteristics in the process of deformation and damage of axially strained VT1-0 titanium in the ultrafine-grained (UFG) and coarse-grained (CG) states. The temperature distributions on the surface of titanium specimens were recorded by means of IR thermography. The VT1-0 titanium in UFG state formed by applying severe plastic deformation is characterized by twice the yield stress and strength limit but half the deformation limit compared to CG titanium. The fracture of CG titanium is accompanied by local powerful generation of heat, while, in UFG titanium, the damage nuclei are less intense and more evenly distributed over the fracture cross-section. The titanium in UFG state, being deformed, utilizes structural channels of energy absorption more efficiently than in the CG state by involving the whole deformed volume in the fracturing process.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

Fatigue behavior of CoCrFeMnNi high-entropy alloy under fully reversed cyclic deformation

Bulk ultrafine-grained (UFG) CoCrFeMnNi high-entropy alloy (HEA) with fully recrystallized microstructure was processed by cold rolling and annealing treatment. The high-cycle fatigue behaviors of the UFG HEA and a coarse-grained (CG) counterpart were investigated under fully reversed cyclic deformation. The fatigue strength of the UFG HEA can be significantly enhanced by refining the grain size. However, no grain coarsening was observed in the UFG HEA during fatigue tests. Mechanisms for the superior mechanical properties of the UFG HEA were explored.     

Effect of grain refinement on strain hardening and fracture in austenitic stainless steel

The present study aims to investigate the effect of grain refinement on strain hardening behaviour and fracture surface characteristics in 316LN austenitic stainless steel (ASS). The ASSs with varying grain sizes were obtained through 90% cold rolled reduction and subsequently phase reversion annealing treatment. The results showed that the grain refinement from coarse-grained (CG) structure to ultrafine-grained (UFG) structure increased the yield strength whilst maintaining a reasonable ductility. The strain hardening curves in all the samples were divided into three stages. The fractures in all the samples were ductile fracture with dimples. The subtle differences in the strain hardening behaviour and fracture surface characteristics among the samples with various grain sizes from CG structure to UFG structure were influenced by the deformation mechanisms of austenite.     

Corrosion behaviour of Al 1050 severely deformed by rotary swaging

In this study, corrosion behaviour of ultrafine-grained (UFG) commercial pure aluminium Al 1050 processed by rotary swaging (RS) was examined using potentiodynamic polarization and weight loss immersion test in 3.5% NaCl solution. Corrosion behaviour of UFG Al 1050 was compared with that of coarse grained (CG) as-received material. The results showed that ultrafine grain refinement by RS led to marked improvement of the corrosion resistance. The improvement in corrosion resistance is profited from the denser and stable passive film due to more grain boundaries, larger fraction of non-equilibrium grain boundaries and residual stress of the UFG pure aluminium. The weight loss tests revealed low corrosion rate values of RS material compared to CG as-received material. Scanning electron microscopy (SEM) analysis revealed a higher number of rectangular shallow pits (more close to patches of general dissolution); larger pits size was observed in the as-received compared to RS materials.     

Nanoscale deformation of multiaxially forged ultrafine-grained Mg-2Zn-2Gd alloy with high strength-high ductility combination and comparison with the coarse-grained counterpart

Cold processing of magnesium(Mg) alloys is a challenge because Mg has a hexagonal close-packed(HCP)lattice with limited slip systems, which makes it difficult to plastically deform at low temperature. To address this challenge, a combination of annealing of as-cast alloy and multi-axial forging was adopted to obtain isotropic ultrafine-grained(UFG) structure in a lean Mg-2Zn-2Gd alloy with high strength(yield strength: ~227 MPa)-high ductility(% elongation: ~30%) combination. This combination of strength and ductility is excellent for the lean alloy, enabling an understanding of deformation processes in a formable high strength Mg-rare earth alloy. The nanoscale deformation behavior was studied via nanoindentation and electron microscopy, and the behavior was compared with its low strength(yield strength: ~46 MPa)-low ductility(% elongation: ~7%) coarse-grained(CG) counterpart. In the UFG alloy, extensive dislocation slip was an active deformation mechanism, while in the CG alloy, mechanical twinning occurred.The differences in the deformation mechanisms of UFG and CG alloys were reflected in the discrete burst in the load-displacement plots. The deformation of Mg-2Zn-2Gd alloys was significantly influenced by the grain structure, such that there was change in the deformation mechanism from dislocation slip(non-basal slip) to nanoscale twins in the CG structure. The high plasticity of UFG Mg alloy involved high dislocation activity and change in activation volume.     

Flow mechanisms in ultrafine-grained metals with an emphasis on superplasticity

An analysis was conducted to examine the flow behavior of ultrafine-grained (UFG) metals produced by severe plastic deformation (SPD) processing in equal-channel angular pressing. The results reveal two distinct types of behavior. At elevated temperatures, the analysis shows that superplastic flow is accurately described by the theoretical mechanism developed for coarse-grained metals so that flow in UFG materials may be interpreted using conventional flow mechanisms. By contrast, localized small-scale grain boundary sliding is observed during deformation at low temperatures and this is attributed to the movement of extrinsic dislocations in the non-equilibrium grain boundaries produced by SPD processing.     

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.     

Controllable bimodal structures in hypo-eutectoid Cu–Al alloy for both high strength and tensile ductility

To effectively demonstrate the dependence of ductility improvement on the scheme of introducing bimodal structure into nanostructured materials, a three-step processing was adopted in hypo-eutectoid Cu–Al alloys to obtain controllable bimodal structure of micrometer-grained pre-eutectoid phase embedded on ultrafine-grained (UFG) matrix with eutectoid composition: (1) pre-deformation heat-treatment was proposed to achieve controlled distribution of pre-eutectoid phase in the matrix with eutectoid composition, (2) both pre-eutectoid phase and eutectoid matrix were refined to submicrometer level by usage of high-pressure torsion (HPT), (3) annealed HPT-processed samples at selected temperature. All samples subjected to this novel processing route imparted a high strength, meanwhile obvious uniform plastic elongation in tensile deformation was also observed at those with bimodal structure.     

Effect of grain size on strain rate sensitivity of cryomilled Al–Mg alloy

Al–Mg alloy powder was cryomilled to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic forging. The consolidation resulted in a bulk material with ultrafine grains of about 250 nm, and the material exhibited enhanced strength compared to conventionally processed Al–Mg alloy. The hardness of as-cryomilled powder, the forged ultrafine-grained (UFG) material, and the conventional coarse-grained (CG) alloy were measured by nanoindentation using various loading rates, and the results were compared with strain rate sensitivity (SRS) from uniaxial compression tests. Negative SRS was observed in the cryomilled NC powder and the forged UFG material, while the conventional alloy was relatively insensitive to strain rate. The dependence on loading rate was stronger in the NC powders than in the UFG material.     

Cyclic deformation behavior and fatigue lives of ultrafine-grained Ti-6AL-4V ELI alloy for medical use

It was shown that introducing an ultrafine-grained (UFG) microstructure in pure metals as well as some alloys leads to strongly enhanced fatigue properties. The cyclic deformation behavior of UFG Ti-6Al-4V ELI (extra low interstitials) alloy is studied by both strain and stress controlled fatigue tests using plastic strain amplitudes between 3 × 10^?4 and 5 × 10^?3 and stress amplitudes ranging from 550 to 670 MPa. The UFG microstructures were obtained by equal channel angular pressing (ECAP) with different number of passes followed by a subsequent thermomechanical treatment (TMT). When compared to the conventional grain (CG) size counterpart, the UFG alloy exhibited a pronounced enhancement in the fatigue life in the S–N (Wöhler) diagram. It was also shown that additional UFG processing prior to TMT did not result in any further improvement of the fatigue resistance. Furthermore, microstructural investigations revealed a high cyclic stability of the UFG microstructure.     

Severe plastic deformation of copper and Al–Cu alloy using multiple channel-die compression

Severe plastic deformation studies of copper and Al–Cu alloy by multiple channel-die compression tests were investigated. The materials were tested under plane strain condition by maintaining a constant strain rate of 0.001/s. Extensive grain refinement was observed resulting in nano-sized grains after severe plastic deformation with concomitant increase in flow stress and hardness. The microstructural investigation of the severely deformed materials was investigated using optical microscope and scanning electron microscope. Shear band formation was identified as the failure mechanism in the two phase Al–Cu alloy. The results indicate difficulty in obtaining severe plastic deformation for alloys having two phase micro-structure.     

Recrystallization and Grain Growth in Ultrafine‐Grained Materials Produced by High Pressure Torsion

Ultrafine‐grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu–0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8 GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries.     

Effect of strain on “hardening by annealing and softening by deformation” phenomena in ultra-fine grained aluminum

Ultra-fine grained (UFG) metals fabricated by severe plastic deformation (SPD) sometimes exhibit peculiar mechanical properties. For example, the “hardening by annealing and softening by deformation” was reported in UFG aluminum, which was totally opposite to the behaviors of conventionally coarse-grained materials. In this study, the effect of SPD strain on the peculiar phenomena was investigated. The UFG aluminum was fabricated by various cycles of the accumulative roll-bonding (ARB) process with lubrication at ambient temperature. The specimen ARB-processed by ten cycles certainly showed the peculiar phenomena. On the other hand, the 6-cycle specimen did not show the phenomena but was softened by annealing and hardened by deformation normally. From the results of microstructural characterization, it was suggested that the difference in the change of the mechanical property during annealing and deformation between 6-cycle and 10-cycle specimens was caused by the difference in the grain size and/or the texture components, which depended on the SPD strain.     

ECAP变形制备超细晶金属材料变形行为的研究进展

综述了等径弯曲通道变形制备超细晶金属材料变形行为的研究进展,重点介绍了等径弯曲通道变形制备的超细晶金属材料变形行为的研究方法及其不同于常规材料的力学行为、应变速率敏感性和超塑性,分析了超细晶金属材料的主要变形机理,进一步指出了变形行为及变形机理微观层次研究中存在的问题及发展趋势。     

Grain size altering yielding mechanisms in ultrafine grained high-Mn austenitic steel:Advanced TEM investigations

The underlying mechanism of discontinuous yielding behavior in an ultrafine-grained(UFG)Fe-31Mn-3Al-3Si(wt.％)austenitic TWIP steel was investigated by the use of advanced TEM technique with taking the plastic deformation mechanisms and their correlation with grains size near the macroscopic yield point into account.Typical yield drop mechanisms such as the dislocation locking by the Cottrell atmo-sphere due to the presence of interstitial impurities cannot explain the origin of this phenomenon in the UFG high-Mn austenitic TWIP steel.Here,we experimentally revealed that the plastic deformation mechanisms in the early stage of deformation,around the macroscopic yield point,show an obvious association with grain size.More specifically,the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 μm.Our observation indicates that the grain size dependent deformation mechanisms transition is also deeply associated with the discontinuous yielding behavior as it could govern the changes in the grain interior dislocation density of mobile dislocations around the macroscopic yield point.     

高强度超细晶金属材料塑性行为及增塑研究进展

强度和塑性是金属结构材料最重要的力学性能指标,金属高性能化的关键是在高强度水平下保证良好的塑性,然而两者往往不能兼顾。在众多强化方法中,晶粒细化长期以来被认为是强化金属最理想的手段,在传统晶粒尺寸范围,细化晶粒既可以显著提高材料的强度,又能改善材料的塑韧性。因此,近几十年来超细晶/纳米晶金属得到了广泛研究和发展,出现了以大塑性变形(SPD)、先进形变热处理(ATMP)技术为代表的超细晶制备方法,所得晶粒可以细化到亚微米或纳米尺度,金属性能大大提高。然而,大量研究证实当晶粒细化到亚微米或纳米尺度时金属强度提高但塑性显著下降,与传统的细晶强化规律不符。对此,国内外学者进行了很多研究,试图阐明其机理、揭示晶粒超细化导致塑性降低的物理本质。此外,由于细化晶粒方法受到塑性的限制,新的高强度水平下增强塑性的方法成为钢铁材料高性能化的研究热点。针对塑性下降的事实,为了进一步提高超细晶金属材料性能,研究者开展了许多增强塑性的工作,获得了较好的效果,但仍存在一些不足。关于金属晶粒超细化导致塑性降低的普遍共性现象,目前广泛认可的理论主要有晶界捕获(吸收)位错的动态回复理论、位错运动湮灭理论、高初始位错密度以及位错源缺失机制等。前三者都主要关注超细晶金属材料低(无)加工硬化能力,并将其归结为延伸率降低所致。主要是因为低(无)加工硬化使材料在变形早期发生塑性失稳或局部变形从而表现出低塑性。超细晶金属增塑研究主要体现在增塑方法和机理方面,目前,增塑方法主要有(1)形成纳米孪晶;(2)获得粗晶-细晶双峰组织;(3)利用相变诱发塑性/孪生诱发塑性(TRIP/TWIP)效应;(4)引入铁素体软相;(5)利用纳米第二相粒子等。这些增塑方法的主要机理是利用组织结构的改变提高超细晶金属的加工硬化能力以维持良好的均匀塑性变形以及利用组织相变提高塑性。本文归纳了常用的超细晶金属制备方法,综述了超细晶金属材料塑性降低的研究进展,总结了超细晶金属增塑的研究结果,分析了目前研究中存在的不足,探讨了超细晶金属增强增塑的发展趋势,以期为超细晶金属塑性降低理论及增强增塑研究提供参考。