Influence of severe plastic deformation on the precipitation hardening of a FeSiTi steel

The combined strengthening effects of grain refinement and high precipitated volume fraction (~6 at.%) on the mechanical properties of FeSiTi alloy subjected to SPD processing prior to ageing treatment were investigated by atom probe tomography and scanning transmission electron microscopy. It was shown that the refinement of the microstructure affects the precipitation kinetics and the spatial distribution of the secondary hardening intermetallic phase, which was observed to nucleate heterogeneously on dislocations and sub-grain boundaries. It was revealed that alloys successively subjected to these two strengthening mechanisms exhibit a lower increase in mechanical strength than a simple estimation based on the summation of the two individual strengthening mechanisms.     

Investigation of texture and mechanical properties of copper processed by new route of equal channel angular pressing

The evolution of crystallographic texture and the mechanical properties of copper subjected to severe plastic deformation (SPD) using equal channel angular pressing (ECAP) were investigated. Samples were subjected to ECAP under two different processing routes: B₆₀ and B_C. As the cross sections of the samples were circular, a new route with a rotation angle of 60° in the same direction between consecutive passes was introduced. The material exhibited texture development similar to the simple shear texture in both routes and the most significant changes in texture strength in both processing routes took place after the second pass. Microstructure of ECAP processed samples were investigated using electron backscatter diffraction (EBSD) analysis. Comparison of the EBSD data with optical micrograph of the initial sample confirmed that ECAP process has led to a significant decrease in grain size. Significant increases in hardness and tensile strength were observed after the first pass of ECAP. Variations of tensile strength as a function of the number of passes were related to the dislocation densities and the average boundary spacing.     

A study on the mechanical properties of cryorolled Al–Mg–Si alloy

The mechanical properties of a precipitation hardenable Al–Mg–Si alloy subjected to cryorolling and ageing treatments are reported in this present work. The severe strain induced during cryorolling of Al–Mg–Si alloys in the solid solutionised state produces ultrafine microstructures with improved mechanical properties such as strength and hardness. The improved strength and hardness of cryorolled alloys are due to the grain size effect and higher dislocation density. The ageing treatment of cryorolled Al–Mg–Si alloys has improved its strength and ductility significantly due to the precipitation hardening and grain coarsening mechanisms, respectively. The reduction in dimple size of cryorolled Al–Mg–Si alloy upon failure confirms the grain refinement and strain hardening mechanism operating in the severely deformed samples.     

An evaluation of microstructure and microhardness in copper subjected to ultra-high strains

The microstructure and microhardness of copper subjected to large strains either using one or a combination of severe plastic deformation (SPD) processing techniques was evaluated. The individual SPD techniques used include equal-channel angular pressing (ECAP), high-pressure torsion (HPT), and chip formation during machining (M). Microstructural characterization using orientation imaging microscopy provided detailed information on the grain sizes and misorientation statistics after different processing routes. Vickers indentation analysis was used to evaluate the hardness of the deformed samples. The results show that excellent microstructures and properties are achieved when these three processes are used in combination, including grain sizes in the range of ~0.2–0.3 μm and hardness values up to >1,900 MPa.     

强塑性变形在铝合金中的研究进展

在过去20年中,强塑性变形技术作为制备超细晶金属及其合金的一种方法被广泛研究.主要介绍了强变形技术在铝合金中的研究进展,特别是对铝合金晶粒大小、晶界、晶体织构及第二相等微观组织参数,强度、塑性、疲劳、腐蚀及超塑性等力学性能的影响.     

Enhancement of mechanical properties of biocompatible Ti–45Nb alloy by hydrostatic extrusion

β-Type titanium alloys are promising materials for orthopaedic implants due to their relatively low Young’s modulus and excellent biocompatibility. However, their strength is lower than those of α- or α + β-type titanium alloys. Grain refinement by severe plastic deformation (SPD) techniques provides a unique opportunity to enhance mechanical properties to prolong the lifetime of orthopaedic implants without changing their chemical composition. In this study, β-type Ti–45Nb (wt%) biomedical alloy in the form of 30 mm rod was subjected to hydrostatic extrusion (HE) to refine the microstructure and improve its mechanical properties. HE processing was carried out at room temperature without intermediate annealing in a multi-step process, up to an accumulative true strain of 3.5. Significant microstructure refinement from a coarse-grained region to an ultrafine-grained one was observed by optical and transmission electron microscopy. Vickers hardness measurements (HV_0.2) demonstrated that the strength of the alloy increased from about 150 to 210 HV_0.2. Nevertheless, the measurements of Young’s modulus by nanoindentation showed no significant changes. This finding is substantiated by X-ray diffraction analyses which did not exhibit any phase transformation out of the bcc phase being present still before processing by HE. These results thus indicate that HE is a promising SPD method to obtain significant grain refinement and enhance strength of β-type Ti–45Nb alloy without changing its low Young’s modulus, being one prerequisite for biomedical application.     

Prediction models for the yield strength of particle-reinforced unimodal pure magnesium (Mg) metal matrix nanocomposites (MMNCs)

Particle-reinforced metal matrix nanocomposites (MMNCs) have been lauded for their potentially superior mechanical properties such as modulus, yield strength, and ultimate tensile strength. Though these materials have been synthesized using several modern solid- or liquid-phase processes, the relationships between material types, contents, processing conditions, and the resultant mechanical properties are not well understood. In this paper, we examine the yield strength of particle-reinforced MMNCs by considering individual strengthening mechanism candidates and yield strength prediction models. We first introduce several strengthening mechanisms that can account for increase in the yield strength in MMNC materials, and address the features of currently available yield strength superposition methods. We then apply these prediction models to the existing dataset of magnesium MMNCs. Through a series of quantitative analyses, it is demonstrated that grain refinement plays a significant role in determining the overall yield strength of most of the MMNCs developed to date. Also, it is found that the incorporation of the coefficient of thermal expansion mismatch and modulus mismatch strengthening mechanisms will considerably overestimate the experimental yield strength. Finally, it is shown that work-hardening during post-processing of MMNCs employed by many researchers is in part responsible for improvement to the yield strength of these materials.     

Phase decomposition behavior and its effects on mechanical properties of TiNbTa0.5ZrAl0.5 refractory high entropy alloy

The mechanical properties of refractory high entropy alloys(RHEAs) strongly depend on their phase structures. In this work, the phase stability of a BCC TiNbTa0.5ZrAl0.5 refractory high entropy alloy subjected to thermomechanical processing was evaluated, and the effects of phase decomposition on room/high temperature mechanical properties were quantitatively studied. It was found that, the thermomechanical processing at 800℃and 1200℃ leads to phase decomposition in the TiNbTa0.5ZrAl0.5 alloy. The phase decomposition is caused by the rapid rising of free energy of the primary BCC phase. The effect of the precipitates on room temperature strength is determined by the competition between the increasing in precipitation strengthening and the decreasing in solid solution strengthening. But at high temperatures(800-1200℃), the phase decomposition causes significant reduction in strength, mainly due to the grain boundary sliding and the decreasing in solid solution strengthening.     

Severe plastic deformation processes for thin samples

Among the known severe plastic deformation (SPD) techniques, one particular group can be defined as SPD processing of thin samples. Their distinctive feature is that one of the sample dimensions, namely the thickness, is much smaller than the other two dimensions. Examples include High Pressure Torsion and two recently developed techniques: the Cone–Cone Method and the High Pressure Tube Twisting. The mentioned group of SPD processes involve frictional forces acting on the large surfaces and a high hydrostatic pressure within the deformation zone. These techniques are particularly suited for microforming of metals. In this article, we outline the commonalities between these three techniques. The microstructure of copper samples deformed by all the three processes is presented and compared with those obtained by equal-channel angular pressing as a reference bulk forming SPD technique.     

Fatigue of Severely Deformed Metals

In this brief communication, we would like to review present data on fatigue performance of ultra‐fine grain materials fabricated by severe plastic deformation (SPD) and to discuss the possible mechanisms of their plastic deformation and degradation in light of currently available experimental data. The most prominent effect of SPD is often associated with significant grain refinement down to the nanoscopic scale. The other evident effect, which accompanies intensive plastic straining, is the dislocation accumulation up to limiting densities of 10¹⁶ m^–2. Since namely these two factors, the grain size and the dislocation density, govern the strengthening of polycrystalline materials, we shall primarily confine ourselves to their role in cyclic deformation of severely pre‐deformed metals.     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.     

Atomic-scale dissecting the formation mechanism of gradient nanostructured layer on Mg alloy processed by a novel high-speed machining technique

Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit supe-rior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics sim-ulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of～70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dis-locations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45 μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of deformation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.     

大塑性变形制备超细晶/纳米晶Al-Mg铝合金的研究进展

近年来,大塑性变形(SPD)制备具有先进结构和功能的超细晶和纳米晶Al-Mg铝合金的研究取得了很大进展。SPD后,合金的晶粒显著细化、位错密度提高及有非平衡晶界和晶界偏析形成,这些微观结构导致合金的强度、硬度大幅提高。然而,SPD合金的塑性普遍较低。综述了SPD制备的Al-Mg铝合金在结构和性能方面的一些最新研究成果。     

Effect of crystallographic texture on precipitation induced anisotropy in an aluminium magnesium silicon alloy

The effect of crystallographic texture on precipitation induced anisotropy in yield strength of an aluminium magnesium silicon alloy was investigated. Solutionized samples were subjected to unidirectional and multi-step cross rolling to yield distinct crystallographic textures in the Al–Mg–Si alloy. The rolled sheets were then subjected to annealing followed by second solutionizing treatment to provide sheets with similar grain size and dislocation content but distinct texture. Ageing experiments were carried out on these sheets at 443 K for different time intervals. It was observed that the evolution of anisotropy in yield strength of the age hardened alloy depends on texture. The difference in age hardening response brought about by varying initial texture controls the evolution of anisotropy in mechanical properties of the alloy. This was manifested in terms of transition from anisotropic to isotropic mechanical properties in the unidirectionally rolled samples after peak ageing. On the contrary, a transition from isotropic to anisotropic yield behaviour was observed for multi-step cross rolled samples. This is attributed to enhanced precipitation hardening in crystallographically softer orientations compared to crystallographically harder orientations.     

Strength and ductility in ultrafine-grained wrought aluminum alloys

Strength and ductility are two of the most important mechanical properties of engineering materials. In this work, a 6061 aluminum alloy was subjected to multi-directional forging (MF) and aging treatment. The samples possess high strength and high ductility after processing. The strength of samples was enhanced by dispersing ultrafine precipitate particles within the grains, reducing grain-size and increasing dislocation density after MF and aging. The ductility was improved due to reducing the forging stress during aging. Moreover, a mass of dispersing ultrafine precipitate particles widespread within the grains after aging, which helps to accumulate dislocations, increase the dislocation storage capability and resist dislocation slip that lead up to increasing work hardening, the ductility was also enhanced. A linear strengthening elastic–plastic model was developed by simplifying the stress–strain curves. On this basis, the strength and ductility of ultrafine-grained (UFG) materials were discussed. This also provides fundamental insight into the mechanisms that govern the strength and ductility of UFG materials.     

Recycling of titanium machining chips by severe plastic deformation consolidation

It has been demonstrated that severe plastic deformation (SPD) can be used to consolidate particles of a wide range of sizes from nano to micro into fully dense bulk material with good mechanical properties. SPD consolidation allows processing to be conducted at much lower temperatures and is therefore suitable for particles with highly metastable structures such as nanocrystalline. It is especially useful in the fabrication of multiphase materials including metal matrix nanocomposites. In this investigation, SPD consolidation was applied to recycle Ti machining chips. In particular, the as-received chips were consolidated by equal channel angular pressing at temperatures between 400 and 600 °C with the application of a back pressure from 50 to 200 MPa. Fully dense bulk Ti with fine grain sizes was produced, possessing strength comparable or higher than that of commercially pure wrought Ti. It is concluded that SPD consolidation is a promising method for recycling and value-adding of Ti chips.     

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys were investigated at 1089 and 1311 K in air. The specimens with serrated grain boundaries and those with normal straight grain boundaries were aged for 1080ksec at 1273 or 1323 K to cause the matrix precipitates of tungsten-rich b c c phase and M₆C carbide. The creep-rupture strength of both specimens were improved by the high-temperature ageing. The rupture strength at 1311 K was the highest in the specimens with serrated grain boundaries aged at 1273 K, while the specimens with straight grain boundaries aged at 1273 K of the highest matrix hardness had the highest rupture strength at 1089 K. The high-temperature ageing did not decrease the rupture ductility of specimens. The ruptured specimens with serrated grain boundaries exhibited a ductile grain-boundary fracture surface which consisted of dimple patterns and steps, regardless of whether high-temperature ageing was carried out. The fracture mode of the specimens with straight grain boundaries was changed from the brittle grainboundary fracture to the ductile one similar to that of the specimens with serrated grain boundaries by high-temperature ageing, since large grain-boundary precipitates which gave nucleation sites of dimples were formed during the ageing. The grain-boundary cracks initiated in the early stage of creep (transient creep regime) in both non-aged and aged specimens of L-605 alloys in creep at 1089 and 1311 K, although the time to crack initiation is shorter in the specimens with straight grain boundaries than in those with serrated grain boundaries. Thus, the period of crack growth and linkage occupied most of the rupture life. The strengthening mechanisms of the aged specimens were also discussed.     

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.     

The new trends in fabrication of bulk nanostructured materials by SPD processing

During the past decade, fabrication of bulk nanostructured metals and alloys using severe plastic deformation (SPD) has been evolving as a rapidly advancing direction of nanomaterials science and technology aimed at developing materials with new mechanical and functional properties for advanced applications. The principle of these developments is based on grain refinement down to the nanoscale level via various SPD techniques. This paper is focused on investigation and development of new SPD processing routes enabling fabrication of fully dense bulk nanostructured metals and alloys with a grain size of 40–50 nm and smaller, namely, SPD-consolidation of powders, including nanostructured ones, as well as SPD-induced nanocrystallization of amorphous alloys. We also consider microstructural features of SPD-processed materials that are responsible for enhancement of their properties.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.