Effect of Various SPD Techniques on Structure and Superplastic Deformation of Two Phase MgLiAl Alloy

MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal α and bcc β phases was cast under protecting atmosphere and hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grain refinement. Rods were subjected to 1–3 passes of Twist Channel Angular Pressing TCAP (with helical component), cyclic compression to total strain ε?=?5 using MAXStrain Gleeble equipment, both performed at temperature interval 160–200 °C and, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refinement of α phase from 30 μm down to about 2 μm and that of β phase from 12 to 5 μm. Maxstrain cycling 10?× up to ε?=?5 led to much finer grain size of 300 nm. KOBO method performed at RT caused average grain size refinement of α and β phases down to about 1 μm. Hardness of alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higher after MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density in the β region than in the α phase. Crystallographic relationship (001) α|| (110) β indicated parallel positioning of slip planes of both phases. Electron diffraction technique confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 °C showed superplastic forming after 1st and 3rd TCAP passes with better superplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.29 after 3rd TCAP pass for strain rate range 10^?5 to 5?×?10^?3 s^?1. Deformation by MAXStrain cycling caused much more effective grain refinement with fine microtwins in α phase. Superplastic deformation was also observed in alloy deformed by KOBO method, however the value of m?=?0.21 was obtained at lower temperature of deformation equal to 160 °C and deformation rate in the range 10^?5 to 5?×?10^?3. Tensile samples deformed superplastically showed grain growth and void formation caused by grain boundary slip. Summarizing, all methods applied resulted in sufficient grain refinement to obtain the effect of superplastic deformation for alloys of two phase α?+?β structure.     

Microstructure and texture evolution during equal channel angular extrusion of interstitial-free steel: Effects of die angle and processing route

Die angle (Φ) and processing route are two important factors that greatly impact microstructure and texture developments during equal channel angular extrusion (ECAE). In this study, the microstructure and texture evolution in interstitial-free steel during ECAE using a Φ = 120° die are investigated for up to four passes via three routes (A, B_C and C). Finite element simulations and polycrystal modeling using the Taylor and viscoplastic self-consistent models are conducted to help understand deformation behavior and texture formation. Transmission electron microscopy results reveal remarkable microstructural differences between various observation planes for each billet. The effects of Φ and processing route are discussed by comparing the current results with those for the same material processed via the same routes but using a Φ = 90° die. The efficiency of grain refinement is found to be only mildly dependent on Φ and processing route. The textures can be completely characterized by predefined ideal fibers, regardless of processing route and pass number. With the exception of even-numbered passes in route C, textures developed after the same route and pass number, but different Φ can be approximately related by a rotation about the axis normal to the flow plane and the rotation angle is half of the Φ difference.     

超声滚压剧烈塑性变形诱导金属材料结构演变的研究进展

首先,对表面完整性的基本概念和内涵进行了概述,同时简要介绍了超声实现滚压技术的基本原理及其优点。随后,对比分析了不同剧烈塑性变形方法的特点和局限性,引出了实现表面完整性的相关剧烈塑性变形协调机制。在此基础上,随后结合其他剧烈塑性变形强化工艺,重点总结了超声滚压剧烈塑性变形对金属材料表面微观结构演变的影响。具体探讨了剧烈塑性变形诱导晶粒细化机制、晶粒生长机制以及合金元素偏聚机制等,主要分别论述了不同层错能的面心立方、体心立方以及密排六方等不同金属晶体结构的晶粒细化机制(以位错滑移、变形孪晶为主导)、晶粒长大机制(以晶界迁移、晶粒旋转为主要)与合金元素偏聚机制(晶界偏聚、位错核心偏聚)等。最后,对以上内容进行了综合总结,并针对超声滚压技术研究中存在的问题给出进一步研究和发展的建议,从而为实现超声滚压金属材料的表面完整性的主动精准控制及提高其服役寿命与可靠性提供一定的参考。     

Evaluating the Plastic Anisotropy of AZ31 Using Microscopy Techniques

The tensile deformation mechanism of a rolled AZ31 alloy at 50°C, 150°C, and 250°C was investigated by a combination of in situ tensile testing, electron backscatter diffraction analysis, and ex situ atomic force microscopy analysis. With increasing temperature, there was a significant difference in the activity of the various deformation modes, along with a decrease in the plastic strain ratio. Extension twinning was only observed at 50°C, while at higher temperatures, a combination of basal and prismatic slip accounted for a large percentage of the observed deformation activity. Prismatic slip was prevalent at all testing temperatures and exhibited increased activity with increasing temperature. The activity of pyramidal 〈c + a〉 slip increased from 50°C to 150°C and then decreased at 250°C. Ex situ atomic force microscopy measurements suggested that the contribution from grain boundary sliding to the overall strains increased with increasing temperature. Overall, the in situ experiments combined with atomic force microscopy suggested that grain boundary sliding contributed more to the reduction in plastic strain ratio with increasing temperature than nonbasal slip activity.     

Effect of High-Temperature Severe Plastic Deformation on Microstructure and Mechanical Properties of IF Steel

Extensive research work has been carried out on interstitial-free steel to understand its response to deformation; particularly, the behavior during severe plastic deformation (SPD). However, most of these studies were mainly undertaken in the ferritic regime. The present investigation reports the initial results of our attempt to employ accumulative roll bonding (ARB), one of the variants of SPD, at a high temperature (950 °C). A considerable grain refinement has been observed, which may be attributed to the severity of deformation and recrystallisation at high temperatures. Nanoindentation tests have been performed at various stages of ARB process to understand the evolution of mechanical properties.     

Deformation mechanisms of pure Ti during equal channel angular pressing

The microstructural development of commercially pure titanium was investigated to elucidate the mechanisms of grain refinement and strain accommodation during equal channel angular pressing. The samples were processed at 623 K via route C, in which the sample was rotated 180° around its longitudinal axis between the passes. TEM micrographs of the sample undergoing the first pass revealed that the strain imposed by the pressing is accommodated mainly by {1011} deformation twinning. During the second pass, the deformation mechanism was changed to dislocation slip. TEM analysis indicated that the slip system consisted of alternating twin bands containing dislocations ofa slip on a prismatic plane and ofa+c slip on a pyramidal plane. Microstructural evolution in commercially pure titanium subjected to equal channel angular pressing was discussed based on the preferred orientation formed during the first pass and resolved shear stress for the slip systems.     

Structure and properties of aluminum alloy 1421 after equal-channel angular pressing and isothermal rolling

Sheets from the aluminum alloy 1421 with an ultrafine-grained (UFG) structure and a weak crystallographic texture were prepared by the method of equal-channel angular pressing (ECAP) through a die with channels of a rectangular cross section and by subsequent isothermal rolling. Both operations were carried out at a temperature of 325°C. It is shown that severe plastic deformation (SPD) leads to the formation of a completely recrystallized uniform microstructure with an average grain size of 1.6 µm in the alloy. At room temperature the alloy 1421 demonstrates high static strength (σ_u = 545 MPa, σ_0.2 = 370 MPa) in the absence of a significant anisotropy. At temperatures of hot deformation, the alloy showed ultrahigh elongations under superplasticity (SP) conditions. At a temperature of 450°C and initial deformation rate of 1.4 × 10^?2 s^?1 the maximum elongation at fracture was ～2700%. At static annealing at a temperature of SP deformation, the UFG structure formed in the process of SPD remains stable. The SP deformation is accompanied by an insignificant grain growth and pore formation.     

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.     

Superplastic deformation mechanisms of high Nb containing TiAl alloy with (α2 + γ) microstructure

In this paper, superplastic deformation behaviour of a high Nb containing TiAl alloy with fine (α₂ + γ) microstructure, Ti–43.5Al–8Nb–0.2W–0.2B (at.%), has been examined and studied by means of hot tension from 850 °C to 1050 °C under an initial strain rate of 10⁻⁴ s⁻¹. The mechanical behaviour and microstructure evolution have been characterized and analyzed. Besides, to gain insight into deformation mechanisms, the texture evolution during deformation at ordinary (non-superplastic) and superplastic conditions has been systematically studied. The results showed that, the alloy exhibited impressive superplastic elongation at 1000 °C with a strain-rate sensitivity exponent (m) of about 0.5 and an apparent activation energy (Q_app) value of about 390 kJ/mol. The microstructural characterization showed that, when the alloy was deformed at ordinary condition (850 °C), severe grain refinement occurred and the fraction of low-angle grain boundary notably increased. Meanwhile, the textures were characterized by <100> and <111> double-fiber components parallel to the tensile direction. All these observations suggested a dislocation slip and twinning mechanism. However, if deformed at the superplastic condition (1000 °C), it was found that the microstructure was fairly stable in terms of grain size, morphology and grain boundary characteristics during tension, but a continuous weakening of the initial <110> fiber texture (resulted from canned-forging) was observed. This was believed to be an indication of grain boundary sliding mechanism. Moreover, the deformation texture (<100> + <111>)—though is very weak—was simultaneously appeared. According to a detailed discussion on the deformation kinetics and microstructure evolution, it was believed that the slip/twinning-accommodated grain boundary sliding was responsible for superplastic deformation and the dislocation climb inside of γ grains was the rate-controlling step.     

Effect of rolling-assisted deformation on the formation of an ultrafine-grained structure in a two-phase titanium alloy subjected to severe plastic deformation

The effect of rolling in the temperature range 450–650°C on the fragmentation of the primary phase in a hot-rolled VT6 alloy rod preliminarily subjected to severe plastic deformation by equal-channel angular pressing at 700°C (scheme B _c, the angle between the channels is 135°, 12 passes) is studied. Rolling at 450°C without preliminary ECAP is shown not to cause α-phase fragmentation and to favor intense cold working of the alloy due to multiple slip. ECAP provides partial fragmentation of the initial structure of the α phase and changes the morphology of the retained β phase: it transforms from a continuous matrix phase into separated precipitates located between α particles. This transformation activates the fragmentation of the α phase during rolling at 550°C owing to the development of twinning and polygonization processes apart from multiple slip. Both a decrease (to 450°C) and an increase (to 625–650°C) in the rolling temperature as compared to 550°C lead to the formation of a less homogeneous and fragmented structure because of weakly developed recovery and intense cold working in the former case and because of the beginning of recrystallization and the suppression of twinning in the latter case. A relation between the structure that forms upon SPD followed by rolling and the set of its properties is found. A general scheme is proposed for the structural transformations that occur during ECAP followed by rolling at various temperatures.     

Influence of preliminary extrusion conditions on the superplastic properties of a magnesium alloy processed by ECAP

The microstructures and properties of a two-phase Mg–8% Li alloy were evaluated in three different conditions: after casting; after casting followed by extrusion at different temperatures and speeds; and after casting, extrusion and processing by ECAP for four passes at room temperature using a die with a channel angle of 135°. The results show extrusion introduces significant grain refinement and there is additional refinement in ECAP. In tensile testing, the elongations to failure increase with decreasing extrusion temperature, but are essentially independent of the extrusion speed. The ductilities are low in the cast condition, intermediate in the extruded condition and high after extrusion and ECAP. For the last condition, an exceptionally high elongation of ∼1780% was achieved at a testing temperature of 473 K. It is shown that it is advantageous to use a die with a channel angle of 135° because it permits pressing at room temperature where grain growth is limited.     

Microstructure evolution and deformation behavior of ultrafine-grained Al–Zn–Mg alloys with fine η′ precipitates

An aged Al–5Zn–1.6Mg alloy with fine η′ precipitates was grain refined to ～100 nm grain size by means of confined channel die pressing. Microstructure observations and mechanical tests were carried out to characterize the materials before and after various degrees of severe plastic deformation. Deformation processing enhanced the strength of the alloy, but limited its work hardenability. An analysis of deformation mechanisms revealed that plasticity proceeded by dislocation slip through ultrafine-grained cellular and subgrain arrangements. η′ precipitates strengthened the alloy by dispersion hardening, but retarded an increase in the strain rate sensitivity during grain refinement. The influence of η′ precipitates is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy.     

Mechanisms for initial grain refinement in OFHC copper during equal channel angular pressing

The stages of microstructural evolution during the first pass of equal channel angular pressing in polycrystalline oxygen-free, high conductivity (OFHC) copper are identified using transmission electron microscopy (TEM). Microstructural features are generated in the following order: randomly distributed dislocations, dislocation cell structures, elongated laminar substructures (ELSs), and if a transition in activated slip systems takes place, secondary ELSs and/or microbands. TEM analysis suggests that primary and secondary ELSs form along certain {1 1 1} slip planes via a self-organized gliding of dislocations. Prior to reaching the main shear plane (MSP), many ELS boundaries are nearly perpendicular to the MSP. After crossing it, they are most often nearly parallel to it (±15°). The initial grain orientation determines if such a transition in slip pattern occurs. Mechanisms for initial grain refinement are proposed and the final dimension of refined grains is found to be directly associated with some initial substructure characteristics prior to reaching the MSP.     

Effect of subgrain structure on the creep strength of alloy 1207

The effect of severe plastic deformation (SPD) on the creep resistance of the Al-6%Cu-0.48Mn-0.52Mg-0.3Sc-0.1Zr alloy has been examined in a temperature range of 125–180°C. It has been shown that SPD performed by the method of equal-channel angular pressing at 300°C to a true strain of ～1 leads to the formation of a well-defined subgrain structure, which is retained upon solution treatment before quenching because of the presence in the alloy of ultradisperse Al₃(Sc, Zr) particles with coherent boundaries. It was established that the creep strength at 125–150°C of the as-cast alloy and of the deformed material is approximately the same. At 180°C, the creep rate of the deformed aluminum alloy is almost an order of magnitude lower than that of the as-cast alloy. The reasons for the influence of the subgrain structure on the creep strength of the Al-Cu-Sc-Zr alloy are discussed.     

On the Issue of the Improvement of Magnesium Plasticity by Cold Severe Plastic Deformation

The paper presents the results of a study of the effect of the texture of a magnesium specimen on the mechanical properties of its plates and foils. Three cylindrical specimens with different columnar structure orientation were used for the experiments. The deformation was carried out in two steps at room temperature. First, long magnesium plates 1 mm thick (true deformation e ~ 3.9) were produced by lateral extrusion. These plates were then deformed by rolling to obtain foils, 120 μm thick (e ~ 6.0) and 10 μm thick (e ~ 7.5). It was seen that the sharpness of the basal texture and mechanical properties depend on the orientation of the initial samples. It has been shown that the deformation leads to a significant grain refinement: from several millimeters to several microns. This is due to the simultaneous development of slip, twinning, and dynamic recrystallization processes.     

Shape Memory Effects in TiNi-based Alloys Subjected to Electroplastic Rolling

One of the prospective methods for structure refinement is electroplastic rolling (EPR). The use of an electric current pulse during cold rolling enhances deformability (1.5-3 times for TiNi-based alloys). It was shown that EPR (e > 1) with post-deformation annealing at 450-500 °C leads to nanostructure formation with a grain size of 60-120 nm. Also, EPR leads to an increase in functional properties of TiNi-based alloys. So, the recovery coefficient was revealed as being better than the undeformed alloy (90-96% for Ti_49,2Ni_50,8 and 75-80% for Ti_50,0Ni_50,0). In the Ti_50,0Ni_50,0 subjected to EPR up to strain 3.6 and subsequent annealing at 450 °C for 1 h, the superelasticity effect is found.     

Texture,microstructure and mechanical properties of equiaxed ultrafine-grained Zr fabricated by accumulative roll bonding

The texture, microstructure and mechanical behavior of bulk ultrafine-grained (ufg) Zr fabricated by accumulative roll bonding (ARB) is investigated by electron backscatter diffraction, transmission electron microscopy and mechanical testing. A reasonably homogeneous and equiaxed ufg structure, with a large fraction of high angle boundaries (HABs, ∼70%), can be obtained in Zr after only two ARB cycles. The average grain size, counting only HABs (θ > 15°), is 400 nm. (Sub)grain size is equal to 320 nm. The yield stress and UTS values are nearly double those from conventionally processed Zr with only a slight loss of ductility. Optimum processing conditions include large thickness reductions per pass (ε ∼ 75%), which enhance grain refinement, and a rolling temperature (T ∼ 0.3T_m) at which a sufficient number of slip modes are activated, with an absence of significant grain growth. Grain refinement takes place by geometrical thinning and grain subdivision by the formation of geometrically necessary boundaries. The formation of equiaxed grains by geometric dynamic recrystallization is facilitated by enhanced diffusion due to adiabatic heating.     

Relationship among microstructure,mechanical properties and texture of TA32 titanium alloy sheets during hot tensile deformation

The relationship among microstructure, mechanical properties and texture of TA32 titanium alloy sheets during hot tensile deformation at 800 °C was investigated. In the test, the original sheet exhibited relatively low flow stress and sound plasticity, and increasing the heat treatment temperature resulted in an increased ultimate tensile strength (UTS) and a decreased elongation (EL). The deformation mechanism of TA32 alloy was dominated by high angle grain boundaries sliding and coordinated by dislocation motion. The coarsening of grains and the annihilation of dislocations in heat-treated specimens weakened the deformation ability of material, which led to the increase in flow stress. Based on the high-temperature creep equation, the quantitative relationship between microstructure and flow stress was established. The grain size exponent and α phase strength constant of TA32 alloy were calculated to be 1.57 and 549.58 MPa, respectively. The flow stress was accurately predicted by combining with the corresponding phase volume fraction and grain size. Besides, the deformation behavior of TA32 alloy was also dependent on the orientation of predominant α phase, and the main slip mode was the activation of prismatic 〈a〉 slip system. The decrease of near prism-oriented texture in heat-treated specimens resulted in the enhancement of strength of the material.     

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.     

Quasi-static Tensile and Compressive Behavior of Nanocrystalline Tantalum based on Miniature Specimen Testing—Part I: Materials Processing and Microstructure

Grain size reduction of metals into ultrafine-grained (UFG, grain size 100 nm < d < 1000 nm) and nanocrystalline (NC, d < 100 nm) regimes results in considerable increase in strength along with other changes in mechanical behavior such as vanishing strain hardening and limited ductility. Severe plastic deformation (SPD) has been among the favored technologies for the fabrication of UFG/NC metals. Primary past research efforts on SPD UFG/NC metals have been focused on easy-to-work metals, especially face-centered cubic metals such as copper, nickel, etc., and the limited efforts on body-centered cubic metals have mainly focused on high strain rate behavior where these metals are shown to deform via adiabatic shear bands. Except for the work on Fe, only a few papers can be found associated with UFG/NC refractory metals. In the first part of the present work (Part I), high-pressure torsion (HPT) is used to process UFG/NC tantalum, a typical refractory metal. The microstructure of the HPT disk as a function of radial location as well as orientation will be examined. In the subsequent part (Part II), the location-specific mechanical behavior will be presented and discussed. It is suggested that refractory metals such as Ta are ideal to employ SPD technology for microstructure refinement because of the extremely high melting point and relatively good workability.