Establishing the microstructure-strengthening correlation in severely deformed surface of titanium

Surface nanostructuring of engineering materials can be utilised to enhance materials performance for various applications. The aim of this work was to investigate the evolution of microstructure and its correlation with strengthening mechanisms in nanocrystalline commercially pure titanium (cp-Ti) produced by surface mechanical attrition treatment (SMAT). The individual contributions of dislocation slip and twining as the deformation mechanisms during SMAT have been quantified using X-ray line profile analysis and corroborated with transmission electron microscopy and electron backscattered diffraction techniques. It is found that twining is operative only in the early stages of deformation. The absence of twin–twin intersections suggests that twining is not directly responsible for the initial refinement of grain size. Dislocation slip is the major deformation mode, which leads to the refinement of the microstructure by forming low-angle lamellar boundaries. Continuous dynamic recrystallisation is demonstrated to be the mechanism of nanocrystallisation in cp-Ti using detailed microscopic analysis. In contrast to previous studies, which have neglected the contribution of Taylor strengthening, it is observed that a combination of Hall–Petch and Taylor relationships can explain the strength only if separate set of parameters K (Hall–Petch constant) and α (geometrical factor in Taylor relationship) are used for the nanocrystalline surface and severely deformed sub-surface of cp-Ti. Taken together, this work provides new insights into the underlying mechanisms for engineering nanocrystalline materials.     

Formation mechanism of nanostructures in austenitic stainless steel during equal channel angular pressing

An ultra-low carbon austenitic stainless steel was successfully pressed from one to eight passes by equal channel angular pressing (ECAP) at room temperature. By using X-ray diffraction, optical microscopy and transmission electron microscopy, the microstructural evolution during ECAP was investigated to reveal the formation mechanism of strain-induced nanostructures. The refinement mechanism involved the formation of shear bands and deformation twins, followed by the fragmentation of twin lamellae, as well as successive martensite transformation from parent austenitic grains with sizes ranging from microns to nanometres through the processes γ(fcc)?→?ε(hcp)?→?α′(bcc). After pressing for eight passes, two types of nanocrystalline grains were achieved: (a) nanocrystalline austenite with a mean grain size of ～31?nm and (b) strain-induced nanocrystalline α′-martensite with a size of ～74?nm. The formation mechanisms are discussed in terms of microstructural subdivision via deformation twinning and martensite transformation.     

Grain refinement in coarse-grained 7475 Al alloy during severe hot forging

Grain refinement taking place in a coarse-grained 7475 Al alloy was studied in multidirectional compression at 490°C and at a strain rate of 3?×?10^?4?s^?1. The integrated flow curve displays significant work softening just after yielding and an apparent steady-state plastic flow at high strains. The structural changes are characterized by the development of deformation or microshear bands in coarse-grain interiors, followed by homogeneous evolution of new grains at high strains. The new grains are considered to be developed by a kind of continuous reaction through grain fragmentation that is similar to continuous dynamic recrystallization (cDRX). The mechanism of fine grain production and the factors controlling grain refinement during hot multidirectional deformation are discussed in detail.     

Modelling and simulation of dynamic recrystallisation based on multi-phase-field and dislocation-based crystal plasticity models

ABSTRACT

The mechanical properties of metals used as structural materials are significantly affected by hot (or warm) plastic working. Therefore, it is industrially important to predict the microscopic behaviour of materials in the deformation process during heat treatment. In this process, a number of nuclei are generated in the vicinity of grain boundaries owing to thermal fluctuation or the coalescence of subgrains, and dynamic recrystallisation (DRX) occurs along with the deformation. In this paper, we develop a DRX model by coupling a dislocation-based crystal plasticity model and a multi-phase-field (MPF) model through the dislocation density. Then, the temperature dependence of the hardening tendency in the recrystallisation process is introduced into the DRX model. A multiphysics simulation for pure Ni is conducted, and then the validity of the DRX model is investigated by comparing the numerical results of microstructure formation and the nominal stress–strain curve during DRX with experimental results. The obtained results indicate that in the process of DRX, nucleation and grain growth occur mainly around grain boundaries with high dislocation density. As deformation progresses, new dislocations pile up and subsequent nucleation occurs in the recrystallised grains. The influence of such microstructural evolution appears as oscillation in the stress–strain curve. From the stress–strain curves, the temperature dependence in DRX is observed mainly in terms of the yield stress, the hardening ratio, and the change in the hardening tendency after nucleation occurs.     

Surface nanocrystallization mechanism of a rare earth magnesium alloy induced by HVOF supersonic microparticles bombarding

A nanostructured surface layer with a thickness up to 60 μm was produced on a rare earth Mg-Gd-Y magnesium alloy using a new process named HVOF-SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding). The microstructural features of the treated surface at various depth of the deformed layer were characterized by optical microscopy (OM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with an aim to reveal the formation mechanism. Results showed that three steps during grain refinement process were found, i.e., twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets at the initial stage, stacking faults are generated and a number of dislocation slip systems are activated leading to the cross slips with increasing strain and strain rate, eventually high-density dislocation networks, dislocation cells and dislocation arrays are formed, which further subdivides the twin platelets and residual microbands into sub-microstructures. As a result, homogeneous nanostructure with a grain size of about 10-20 nm is formed through dynamic recrystallization in the topmost surface layer. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation with higher strain rate during the HVOF-SMB treatment in the rare earth Mg-Gd-Y alloy was proposed.     

Grain refinement and texture evolution during high precision machining of a Ni-based superalloy

Abstract

The grain refinement and texture evolution in the surface gradient microstructure of a Ni-based superalloy induced by high speed machining was studied in this research. The direct evidence of grain refinement induced by dislocation–twin interaction was revealed and the detailed grain refinement process was summarised as deformation twinning, dislocation-twin reaction, localied thinning of nanotwin lamellae and final fracture. The underlying dislocation–twin interaction mechanism was elucidated from the crystallographic perspective. Using electron backscatter diffraction and precession electron diffraction techniques, a multiscale texture analysis covering undeformed coarse grain region, ultrafine grain region and nanograin region was carried out. The texture evolution with decreasing depth to the machined surface was identified as cube in the bulk interior and a mixture of rotated cube {0?0?1}<1?1?0>, cube {1?0?0}<0?0?1>, copper {1?1?2}<1?1?1 > and Goss {1?1?0}<0?0?1> textures in the topmost 1.3-μm-thick nanograin layer. The intrinsic thermomechanical effects of high precision machining are responsible for crystallographic texture transformation.     

Nanostructures and deformation mechanisms in a cryogenically ball-milled Al-Mg alloy

An Al-7.6 at.% Mg alloy was ball milled in liquid N₂ for 8 h and its microstructures were investigated using transmission electron microscopy. Electron diffraction confirmed that the resulting powder is a supersaturated Al-Mg solid solution with an fcc structure. Three typical nanostructures with different grain-size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small-angle subboundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10-30 nm. In very small crystallites with dimensions less than 10 nm, twinning becomes an important deformation mechanism. The reasons for the different deformation mechanisms were discussed. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated in detail.     

A thermally activated dislocation-based constitutive flow model of nanostructured FCC metals involving microstructural evolution

Abstract

Due to their interface and nanoscale effects associated with structural peculiarities of nanostructured, face-centered-cubic (FCC) ultrafine-grained/nanocrystalline (UFG/NC) metals, in particular nanotwinned (NT) metals exhibit unexpected deformation behaviours fundamentally different from their coarse-grained (CG) counterparts. These internal boundaries, including grain boundaries and twin boundaries in UFG/NC metals, strongly interact with dislocations as deformation barriers to enhance the strength and strain rate sensitivity (SRS) of materials on the one hand, and play critical roles in their microstructural evolution as dislocation sources/sinks to sustain plastic deformation on the other. In this work, building on the findings of twin softening and (de)twinning-mediated grain growth/refinement in stretched free-standing NT–Ni foils, a constitutive model based on the thermally activated depinning process of dislocations residing in boundaries has been proposed to predict the steady-state grain size and simulate the plastic flow of NT–Ni, by considering the blocking effects of nanotwins on the absorption of dislocations emitted from boundaries. It is uncovered that the stress ratio (η_stress) of effective-to-internal stress can be taken as a signature to estimate the stability of microstructures during plastic deformation. This model not only reproduces well the plastic flow of the stretched NT–Ni foils as well as reported NT–Cu and the steady-state grain size, but also sheds light on the size-dependent SRS and failure of FCC UFG/NC metals. This theoretical framework offers the opportunity to tune the microstructures in the polycrystalline materials to synthesise high performance engineering materials with high strength and great ductility.     

纳米多晶铁的冲击相变研究

利用分子动力学方法研究了不同晶粒度的纳米多晶铁在冲击压缩下的结构相变过程,模拟结果表明:纳米多晶铁的冲击结构相变(由体心立方(bcc)结构 α 相到六角密排(hcp)结构 ε 相)发生的临界冲击应力在15 GPa左右.纳米多晶铁在经过弹性压缩变形后,晶界导致的塑性变形开始发生,然后大多数相变从晶界成核并最终发展为大规模相变.不同变形过程在应力和粒子速度剖面上能得到清晰的体现,并通过微观原子结构分析分辨.冲击压缩后的微观结构以晶界原子和以fcc结构原子充当孪晶界的hcp原子为主.晶粒度明显影响晶界变形及相变 关键词： 冲击相变 纳米多晶铁 冲击波 分子动力学     

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M₃C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350?°C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.     

Deformation-induced changes in single-phase Al-0.1Mg alloy

Al-0.1wt.%Mg has been chosen to explore the effects of deformation on the microstructure of nominally single-phase materials. For these materials, the rate of static grain growth is much higher compared to that of Zener pinned systems and dual-phase alloys, where growth is hindered due to the pinning force exerted by second-phase particles on grain boundaries. Therefore, deformation-induced microstructural changes in single-phase alloys occur without any restricting pinning pressure. This paper illustrates the complex effect of deformation on the microstructural changes mainly associated with dynamic recovery. The process affects the initial microstructure due to an intensive substructure development. Dynamic recrystallisation, associated with the formation of new grains, is considered to be a transient phenomenon that quantitatively influences the mean grain size formed during straining. This work also aims to estimate the stored energy of the deformed regions and texture components to explore their contribution to the migration of high-angle grain boundaries.     

The austenite microstructure evolution in a duplex stainless steel subjected to hot deformation

Abstract

The austenite microstructure evolution and softening processes have been studied in a 23Cr–6Ni–3Mo duplex stainless steel, comprising equal fractions of austenite and ferrite, deformed in uniaxial compression at 1000 °C using strain rates of 0.1 and 10 s^?1. The texture and microstructure evolution within austenite was similar in character for both the strain rate used. The observed large-scale subdivision of austenite grains/islands into complex-shaped deformation bands, typically separated by relatively wide transition regions, has been attributed to the complex strain fields within this phase. Organised, self-screening microband arrays were locally present within austenite and displayed a crystallographic character for a wide range of austenite orientations. The microband boundaries were aligned with the traces of {1?1?1} slip planes containing slip systems having high, although not necessarily the highest possible, Schmid factors. The slightly lower mean intercept length and higher mean misorientation obtained for the sub-boundaries at the higher strain rate can be ascribed to the expected more restricted dynamic recovery processes compared to the low strain rate case. Dynamic recrystallisation within austenite was extremely limited and mainly occurred via the strain-induced migration of the distorted original twin boundaries, followed by the formation of multiple twinning chains.     

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed.     

Mechanisms and Kinetics of Recrystallisation: A Two Dimensional Vertex Dynamics Simulation

The recrystallisation process in single phase materials is investigated using a vertex dynamics simulation. The situation is idealised (two dimensional and isotropic) to better understand the role of physical parameters (energy and mobility of subgrain and grain boundaries, shape and relative size of grains) on the competition between recrystallisation and recovery. Simulations show that subgrain growth can be very heterogeneous in the vicinity of grain boundaries, i.e. that recrystallisation develops following the bulging mechanism. Bulging is enhanced for low mobilities of subgrain boundaries, high relative desorientation and small relative sizes. The recrystallisation kinetics is well described by an Avrami law of low exponent.     

Microstructural evolution and grain subdivision mechanisms during severe plastic deformation of aluminum particles by ball milling

Electron backscattered diffraction technique was used to investigate the microstructure of aluminum particles deformed by high-energy ball milling. The lengths of different types of boundaries per area were calculated for different samples. The results show that the deformation mechanism and the rate of grain subdivision changed considerably as milling time increased. At the beginning of the milling, deformation banding subdivided grains and dynamic recovery formed a cellular structure of low angle boundaries. After further milling, particles were flattened; an increase in the aspect ratio of the original grains together with cold welding of the particles contributed to the formation of high angle grain boundaries (HAGBs). Lattice rotation progressively increased the misorientation of low and medium angle boundaries and transformed them to HAGBs, which resulted in formation of new small equiaxed grains by continuous dynamic recrystallization. This research shows subgrain rotation was the main mechanism for formation of new HAGBs.     

Grain refinement and strengthening of a Cu–0.1Cr–0.06Zr alloy subjected to equal channel angular pressing

Abstract

The grain refinement and mechanical properties of a Cu–0.1Cr–0.06Zr alloy subjected to equal channel angular pressing (ECAP) at a temperature of 673 K were examined. The microstructure evolution was characterised by the development of a large number of low-angle subboundaries at small strains. An increase in the true strain resulted in gradual transformation of low-angle subboundaries into high-angle grain boundaries that was assisted by the deformation micro-banding. The development of new ultra-fine grains was considered as a kind of continuous dynamic recrystallization, the kinetics of which was characterised by a sigmoid-type dependence on strain and could be expressed by a modified Johnson–Mehl–Avrami–Kolmogorov equation. ECAP led to significant strengthening of the alloy. The yield strength increased from 105 MPa in the initial state to 390 MPa after 8 ECAP passes. A modified Hall–Petch relationship was applied to analyse the contributions of grain refinement and dislocation density to the overall strengthening. In spite of significant strengthening, the electro-conductivity remained at a level of 80% IACS.     

Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.     

Mechanical properties and structural features of nanocrystalline titanium produced by cryorolling

A broad spectrum of physicomechanical properties of the VT1-0 nanocrystalline titanium produced by cryomechanical fragmentation of the grain structure using rolling at a temperature close to liquid-nitrogen temperature has been studied. It has been found that the mechanism of grain refinement is associated with grain fragmentation by twins. Exactly the twin nature of internal interfaces (crystallite boundaries) provides the thermal and structural stability of nanocrystalline titanium produced by cryomechanical grain fragmentation in the temperature range to ～500 K. It has been assumed that the observed decrease in the titanium density due to cryorolling is associated with a number of factors (high density of introduced dislocations, nanopore formation, and changes in titanium lattice parameters).