Quantification of the microstructures of high purity nickel subjected to dynamic plastic deformation

A quantitative microstructural analysis is presented for pure polycrystalline nickel processed by means of dynamic plastic deformation at high strain rates (10²-10³ s⁻¹) to strains from 0.3 to 2.9. This analysis covers a number of structural parameters, such as the spacing between and the misorientation angle across dislocation boundaries and high angle boundaries. These boundaries subdivide the structure on a finer and finer scale towards saturation at the highest strain. The structural evolution follows a hierarchical pattern from the formation of cells and cell blocks to a characteristic lamellar structure, which is similar to that observed in metals deformed at a low strain rate by conventional deformation processes. However, at a constant strain the increase in strain rate increases the dislocation density and reduces the distance between deformation-induced dislocation boundaries and high angle boundaries. Shear bands and twins have not been observed. In order to underpin the structural analysis, the mechanical properties as a function of strain have been determined by tensile and hardness tests. The flow stress is 850 MPa, showing that high strain rate deformation has potential as a method to produce strong nanostructured metals by imposing only a moderate strain.     

Strengthening mechanisms in nanostructured high-purity aluminium deformed to high strain and annealed

Samples of pure aluminium (99.99%) have been produced by accumulative roll-bonding to a large strain followed by a heat treatment, where a two-step annealing process has been used to produce samples with large variations in structural parameters such as boundary spacing, misorientation angle and dislocation density. These parameters have been quantified by a structural analysis applying transmission electron microscopy and electron backscatter diffraction, and the mechanical properties have been determined by tensile testing at room temperature. Strength–structure relationships have been analysed based on the operation of two strengthening mechanisms—grain boundary and dislocation strengthening—and good agreement with experiments has been found for the deformed sample. However, for samples where the density of dislocation sources has been reduced significantly by annealing, an additional strengthening mechanism, so-called dislocation source-limited hardening, may operate as a higher stress is required to activate alternative dislocation sources.     

<110>取向银单晶宏/微观裂化及界面失配角分布

以低层错能110取向单晶银为研究对象,采用EBSD和TEM等技术,系统分析了冷拔变形过程中的宏观裂化、微观裂化和界面失配角分布的变化规律与内在机制。研究结果表明,随应变量的增加,冷拔银单晶的宏观裂化不断加剧,变形带数量增加,宽度和间距减小。当应变量大于0.94时,形成了与冷拔方向平行的纤维状组织。与层错能相近的合金相比,纯金属单晶银的交滑移和攀移的被抑制程度降低,除了变形孪晶,在低层错能的单晶银中还出现大量随机捕捉位错界面和几何必须位错界面。界面失配角分析结果表明,低应变下,变形以位错滑移为主;中等应变下,滑移和孪生相互竞争;高应变下,孪生为主要变形机制。     

Strain Hardening During Hot Compression Through Planar Dislocation and Twin-Like Structure in a Low-Density High-Mn Steel

In this work, we study the flow hardening behavior and microstructural evolution in a low-density Fe-21Mn-0.1C-2.0Al-2.5Si (wt.%) transformation-twinning-induced plasticity steel during hot compression. The substructures were examined by transmission electron microscopy and electron backscatter diffraction methods. The alloy exhibits a combination of strain hardening (at low and medium true compression strains, <0.2) followed by a strain softening at 800 °C. We explain the flow hardening behavior in terms of substructure refinement due to planar dislocation, as well as twin-like structure. The transmission electron microscopy results suggest that short-range ordering triggers slip planarity after straining to 0.05 at 800 °C. Twin-like structure with misorientation angle higher than 20° is formed during further plastic deformation.     

Deformation behavior of Mg–Zn–Y alloys reinforced by icosahedral quasicrystalline particles

New Mg-rich Mg–Zn–Y alloys, reinforced by quasicrystalline particles, have been developed by thermomechanical processes. The deformation behavior of these alloys at room and elevated temperatures has been investigated. Yield strength of these alloys, which increases with an increase in the volume fraction of quasicrystalline particles, is relatively high due to their strengthening effect. The variation of the flow stress in the alloys is characterized by linking the microstructural evolution during deformation at high temperatures. The flow softening is related to dynamic recrystallization developed under the dislocation climb controlled creep; the flow hardening is related to grain growth that occurs under the grain boundary diffusion controlled creep. Quasicrystalline particles in the Mg–Zn–Y alloys resist coarsening due to their low interfacial energy, thereby forming of stable quasicrystalline particle/matrix interface and also prohibit against microstructural evolution of the α-Mg matrix during deformation at temperatures up to near the eutectic temperature. Stability of both quasicrystalline particles and matrix microstructure in the Mg–Zn–Y alloys provides large elongation to failure with no void formation at the quasicrystalline particle/matrix interface.     

Hierarchical structures in cold-drawn pearlitic steel wire

The microstructure and crystallography of drawn pearlitic steel wires have been quantified by a number of electron microscopy techniques including scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction and nanobeam diffraction, with focus on the change in the structure and crystallography when a randomly oriented cementite structure in a patented wire during wire drawing is transformed into a lamellar structure parallel to the drawing axis. Changes in the interlamellar spacing and in the misorientation angle along and across the ferrite lamellae show significant through-diameter variations in wires drawn to large strains ? 1.5. The structural evolution is hierarchical as the structural variations have their cause in a different macroscopic orientation of the cementite in the initial (patented) structure with respect to the wire axis. The through-diameter variations subdivide the lamellar structure into two distinctly different types: one (called A_A) has a smaller interlamellar spacing and smaller dislocation density than the other (called A_BC). During drawing, the thickness of the ferrite and cementite lamellae are reduced to 20 and 2 nm, respectively, and high-angle boundaries form in the A_BC structure parallel to the cementite lamellae. The structural and crystallographic analyses suggest that boundary strengthening and dislocation strengthening are important mechanisms in the cold-drawn wire. However, differences in structural parameters between the A_A and the A_BC structure may affect the relative contributions of the two mechanisms to the total flow stress.     

Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Predictive modeling of grain refinement during multi-pass cold rolling

Recently, grain refinement and grain misorientation have been experimentally studied for various materials with ultra-fine grained microstructures, which are achieved by the multi-pass cold rolling process. In this paper, a numerical framework is developed to model the evolution of grain size and grain misorientation based on a dislocation density-based material model. Novel finite element models embedded with the dislocation density-based material subroutine are developed to model the plastic deformation and microstructural evolution during the multi-pass cold rolling process. The multi-pass cold rolling processes of commercially pure titanium (CP Ti) and aluminum (AA 1200) are simulated in order to assess the validity of the numerical solution through comparison with experiments. The dislocation density-based material models are developed for CP Ti and AA 1200, which reproduce the observed material constitutive mechanical behavior under various strains, strain rates and temperatures occurring in the cold rolling process. It is shown that the developed model captures the essential features of the material mechanical behaviors and predicts a minimum grain size of below 100 nm after five-pass cold rolling of CP Ti with equivalent strains up to 2.07 and the average incidental dislocation boundary (IDB) misorientation angle increased to 4.6° after six-pass cold rolling of AA 1200 with equivalent strains accumulated to 5.77.     

A model of continuous dynamic recrystallization

During continuous dynamic recrystallization (CDRX), which occurs in high stacking fault energy metals, new grains are formed by the progressive increase of low-angle boundary misorientations. A simple CDRX model is proposed, that deals with a set of “crystallites” delimited partly by low-angle (LABs) and partly by high-angle (HABs) boundaries. Strain hardening and dynamic recovery are described by the Laasraoui-Jonas equation, which has been modified to take HAB migration into account. Recovery occurs in two ways, viz. by the condensation of dislocations into new LABs, and the absorption of dislocations in the pre-existing boundaries, which leads to the transformation of LABs into HABs. The predictions of the model are presented for both the transient and steady states, including stress-strain curves at various temperatures and strain rates, as well as the associated evolutions of the microstructural parameters such as crystallite sizes, dislocation densities and LAB misorientation distributions. The effect of HAB migration on these parameters is emphasized.     

5A90Al-Li合金超塑性变形过程中的显微组织及微观织构演变

通过采用电子背散射衍射技术研究具有扁平状组织的5A90 Al-Li合金板材在变形条件475℃、8×10^-4S^-1时的显微组织演变。结果表明,在进行超塑性变形前,其显微组织表现为扁平状组织,具有大量的小角度晶界,且初始板材具有明显的黄铜织构{110}（112）。在变形过程中发生了晶粒的长大、晶粒形貌的改变、晶粒取向差的增大以及织构的弱化,而且大角度晶界的含量在流动应力到达最大值后开始逐渐增加。同时,对不同阶段的相关变形机制进行了讨论。超塑性变形初始阶段的主要变形机制为位错运动,随着变形的进行而开始发生动态再结晶,晶粒旋转作为晶界滑移的主要协调机制。在超塑性拉伸的最后大应变阶段,晶界滑移是主要的变形机制。     

Scaling of the spacing of deformation induced dislocation boundaries

Methods for determining the distribution of spacings between near-planar dislocation boundaries are discussed. Subsequently, distributions of spacings for these boundaries have been determined for both single crystal and polycrystal samples. Misorientations across the extended planar dislocation boundaries were also measured for the single crystal samples. The probability density distribution for spacings exhibits a scaling behavior of nearly identical form to that seen for the probability density distribution of misorientation angles across these boundaries. The scaling behavior of the spacing distributions persists over a very wide strain range (ε_vM=0.2–ε_vM=4.5) for different materials and deformation conditions. The scaling behavior of the boundary spacings can be accounted for by simple geometric models allowing for either formation or coalescence of these walls.     

Microstructures and compression properties of copper processed by high-pressure torsion

通过高压扭转对铜试样施加不同程度的变形, 研究了样品扭转面(ND面)和纵截面(TD面)上微观组织特征. 对ND面, 在较小的剪应变下, 原始晶粒形貌模糊, 晶粒内部形成等轴状的位错胞及亚晶结构; 随变形量的增大, 亚晶 间取向差及亚晶内部的位错密度增大, 最后形成亚微米尺度的等轴晶粒. 对TD面, 变形初期原始晶粒被拉长, 晶粒内 部为位错墙分割成的层状结构, 层内为拉长的位错胞; 随变形程度的增大, 拉长晶粒的宽度减小, 与剪切方向的夹 角减小, 晶内层状组织间距减小, 并逐渐演化成拉长的亚晶组织; 进一步增大变形, 晶粒拉长痕迹消失, 变形组织 与ND面相似, 为等轴状亚微米晶粒. 压缩实验表明, 经16圈扭转后, 整个试样上的压缩性能基本均匀, σ0.2达到385 MPa, 应变率敏感性指数增大至0.021.     

Characterization of twin-like structure in a ferrite-based lightweight steel

The present study examined cold to warm compressive deformation behavior of a ferrite- based lightweight steel through characterization of the banded structures. Compression tests were carried out at 25 to 500 °C at a strain rate of 0.01 s^-1 up to true strain of 0.6. Analysis of the microstructural evolution using electron back scatter diffraction indicated that the twin-like bands in the large ferrite grains occurred with the {112}[111] system at a 60° misorientation. Density of the twin-like bands is increased by raising the deformation temperature. EBSD results showed that the primary and secondary twins occurred in the [-11-1] and [1-1-1] directions. In addition, the strain at 500 °C distorted the twin-like bands and resulted in wavy boundaries. The strain hardening behavior was also examined using the Crussard-Jaoul (C-J) model and the n-values were calculated for each stage of imposing strain. The results showed high dislocation density in the adjacent of twin-like boundaries intersections which resulted in the n-value increment.     

Evolution of microstructural parameters and flow stresses toward limits in nickel deformed to ultra-high strains

A quantitative analysis of microstructure and strength as a function of strain is presented for polycrystalline nickel (99.5%) deformed by high-pressure torsion in the strain range 1–300 (ε_VM, von Mises strain). Typical lamellar structures consisting of extended boundaries and short interconnecting boundaries have been found, with additional features at large strains which are equiaxed regions, small equiaxed subgrains and deformation twins. The evolution of microstructure and microstructural parameters falls in stages: (i) the first stage at ε_VM = 1–12; (ii) a transition stage at ε_VM = 12–34; and (iii) a saturation stage at ε_VM ⩾ 34. A scaling analysis of spacing between boundaries shows a universal behavior up to ε_VM = 300, indicating that the predominant deformation mechanism is dislocation glide whereas twin formation is of minor importance. A clear link is observed between the evolution in structure and flow stress, which can guide the development of strong metals with a structural scale extending below 50–100 nm.     

Effect of Heat Treatments on Microstructures and Tensile Properties of Cu–3 wt%Ag–0.5 wt%Zr Alloy

The microstructures and tensile properties of Cu–3 wt%Ag–0.5 wt%Zr alloy sheets under different aging treatments are investigated in this research. As one kind of precipitate, Ag nanoparticles with coherent orientation relationship with matrix precipitate. However, after the peak-age point, most of Ag nanoparticles grow into short rod shape with the interface translating to semi-coherent, which leads to the lower strength of over-aging sample. The yield strength is estimated by considering solid solute, grain boundary and precipitation strengthening mechanisms. The result shows that the Ag precipitates provide the main strengthening role. Then a constitutive equation representing the evolution of dislocation density with plastic strain is built by considering work-hardening behavior coming from shearable and non-shearable precipitates which is mainly the particles containing Zr. The flow stress contributed by shearable particle hardening is higher than that of non-shearable one. Due to the coarsening of grain boundary precipitates and low rate of damage accumulation of these non-shearable particles, the micro-cracks nucleate easily at grain boundary which leads to intergranular fracture.     

The influence of boron-doping on the effectiveness of grain boundary hardening in Ni3Al

The influence of boron-doping on the effectiveness of grain boundary hardening in Ni₃Al has been investigated by measuring microhardness profiles across grain boundaries of binary and boron-doped Ni₃Al bicrystals. It was found that although boron gives rise to significant solution strengthening in Ni₃Al, the effectiveness of grain boundary hardening in Ni₃Al is lessened by the addition of boron. Furthermore, the contribution of grain boundary hardening to the overall strength decreases as the segregation extent of boron at the grain boundary increases. A theoretical model of grain boundary hardening considering the various effects of boron-doping has been developed. Application of the model can deconvolute the individual effects of boron-doping on solution hardening, distribution of microcavities along grain boundaries and the interaction of dislocations on different slip systems. Analyzing the experimental results with the model suggests that boron-doping can (i) improve the transfer efficiency of shear stress across a grain boundary by reducing the amount of microcavities along the grain boundary; (ii) suppress the hardening effect from the interaction of dislocations moving on different slip systems and (iii) cause a significant solution hardening effect.     

Electroplasticity—the effect of electricity on the mechanical properties of metals

Investigations of the effects of an electric current on dislocation mobility and mechanical properties at low homologous temperatures (T < 0.5T_m) reveal a polarity effect and yield an electron wind force in some agreement with theory. An external directcurrent electric field has been reported to influence the creep rate of unalloyed metals at high homologous temperatures. During superplastic deformation of the 7475 Al alloy, such a field has been found to decrease the flow stress, reduce strain hardening, increase strain-rate hardening, reduce grain boundary cavitation and reduce grain growth. The effects of the field were polarity dependent and extended to the center of 1–2 mm thick specimens. No significant effect of the field on the flow stress occurred at low homologous temperatures. This suggests that the field influences atomic mobility through vacancy generation and/or migration. The occurrence of an uneven electron density at the interfaces between phases and at grain boundaries has been proposed as a factor, but this idea needs further consideration.     

工业纯钛位错界面的高速形变响应

以工业纯钛为密排六方金属的模型材料。通过多道次冷轧工艺制备具有不同位错界面类型的工业纯钛板材。利用分离式霍普金森压杆(SHPB)实现高速形变,采用透射电子显微分析技术观察位错界面结构的变化,从而分析出不同类型位错界面的高速形变响应。结果表明:在应变速率为1000 s~(-1)时,初始位错界面成为高速形变过程中位错滑移的主要障碍。几何必须位错界面间距为0.5μm的板材冲击后会出现与原始界面交截的新生位错界面,初始几何必须位错界面(GNB)间距为0.3μm以下的工业纯钛在高速形变后会出现位错团结构;初始位错界面在0.1μm或以下,局部剪切的组织模式只是初始位错界面的扭折和位错塞积,在高度局域化的组织中,基体扭折位错界面并未产生,但有位错塞积和亚晶结构。     

A crystal plasticity based work-hardening/softening model for b.c.c. metals under changing strain paths

Metal forming processes typically involve changes of strain paths, which are accompanied by transients in softening or hardening behaviour. The physical cause of these transients in stress–strain responses can be attributed to the evolution of the underlying microstructural details. A crystal plasticity based model is presented to capture the complex hardening/softening transients observed in deformation of b.c.c. polycrystals at low homologous temperatures. For each crystallite, the microstructure, i.e. the cell block boundaries and cell structure, is modelled with three dislocation densities. The cell block boundaries are treated as geometrical obstacles to slip on non-coplanar slip systems. This model is implemented in a Full Constraints Taylor model to obtain the response of a polycrystal from the response of the constituent single crystals. It was found that several important features observed in the experimental stress–strain curves of b.c.c. polycrystals during complex strain paths could be reproduced.     

Coupled quantitative simulation of microstructural evolution and plastic flow during dynamic recrystallization

A new modelling approach that couples fundamental metallurgical principles of dynamical recrystallization (DRX) with the cellular automaton (CA) method has been developed to simulate the microstructural evolution and the plastic flow behaviour during thermomechanical processing with DRX. It provides an essential link for multiscale modelling to bridge mesostructural dislocation activities with microstructural grain boundary dynamics, allowing accurate predictions of microstructure, plastic flow behaviour, and property attributes. Variations of dislocation density and growth kinetics of each dynamically recrystallizing grain (R-grain) were determined by metallurgical relationships of DRX, and the flow stress was evaluated from the average dislocation density of the matrix and all the R-grains. The growth direction and the shape of each R-grain were simulated using the CA method. The predictions of microstructural evolution and the flow behaviour at various hot working conditions agree well with the experimental results for an oxygen free high conductivity (OFHC) copper. It is identified that the oscillation of the flow stress–strain curve not only depends on thermomechanical processing parameters (strain rate and temperature) but also the initial microstructure. The mean size of R-grains is only a function of the Zener–Hollomon parameter. However, the percentage of DRX is not only related with the Zener–Hollomon parameter, but also influenced by the nucleation rate and the initial microstructure.