Separation of macroscopic, elastic mismatch and thermal expansion misfit stresses in metal matrix composite quenched plates from neutron diffraction measurements

Neutron diffraction measurements of the strain profile in a quenched plate of an aluminium-dash;silicon carbide particle-reinforced metal matrix composite are reported. The results have been used to evaluate the efficacy of an analysis technique which allows distinction of the stiffness mismatch and shape misfit stresses between the matrix and reinforcement, as well as between these and any macrostress present. The analysis is presented for measurements made on a metal matrix composite plate which, as a consequence of quenching from elevated temperature, shows large variations in residual stress as a function of position through the plate thickness. The measurements illustrate the additional insight which can be obtained through the separation of the elastic mismatch and thermal misfit stresses. The stress components thus obtained show good agreement with calculated long-range residual and mismatch stresses.     

Multilayer thin films/substrate system with variable film thickness subjected to non-uniform misfit strains

Current methodologies used to infer thin-film stress from curvature measurements are strictly restricted to stress and curvature states that are assumed to remain uniform over the entire film/substrate system. These methodologies have recently been extended to a single thin film of non-uniform thickness deposited on a substrate and subjected to the non-uniform misfit strain. Such methodologies are extended to multilayer thin films of non-uniform thickness deposited on a substrate in the present study. Each thin film may have its own non-uniform misfit strain and non-uniform thickness. We obtain the film stresses and system curvatures in terms of the misfit strains and thickness in thin films. We derive the film stresses and interface shear stresses in terms of system curvatures and film thicknesses. They all feature a “non-local” dependence on curvatures, which make full-field measurement a necessity for the experimental inference of such stresses.     

Nitriding Response of Chromium-Alloyed Steels

Hardness-increasing, coherent, chromiuin-nitride particles of submicroscopic size develop from chromium initially dissolved in the ferritic matrix of FeCr and FeCrC alloys. Transformation of existing chromium-carbide particles leads to coarse, incoherent, chromium-nitride particles and precipitation of cementite at grain boundaries. Due to the long-range strain fields surrounding the coherent precipitates, the ferritic matrix can be supersaturated with nitrogen. The tendency of the case, as a whole, toward volume expansion leads to development of compressive residual surface macrostress. The misfit between secondphase particles and the matrix induces residual microstresses. At latter stages of nitriding discontinuous precipitation, loss of excess nitrogen leads to void development and the relaxation of compressive residual surface macrostress.     

The austenite to ferrite transformation of Fe–Ni under the influence of a uniaxially applied tensile stress

Differential dilatometry has been employed to study the austenite (γ)-ferrite (α) massive phase transformation of Fe-3.1 at.% Ni upon cooling under the influence of an applied constant uniaxial tensile stress. The applied stress level was chosen to be below the yield stress of the alloy. It was found that an extra length change in the specimens occurs during the transformation as a result of transformation-induced plasticity. The local plastic deformation induced by transformation plasticity results in an anisotropic volume change of the specimen in the longitudinal direction. A phase transformation model, involving site saturation, interface-controlled continuous growth and an appropriate impingement correction, has been employed to extract the velocity of the γ/α interface. The observed scatter in the interface velocity decreases with increasing applied stress due to relaxation of misfit deformation energy by transformation-induced inhomogeneous plastic deformation. The interface velocity remains almost constant during the whole transformation.     

镍基单晶高温合金定向粗化行为的取向依赖性Ⅱ有限元分析

本文对点阵错配力和施加外载后的内应力大小及分布进行了有限元计算。结果表明,基体通道的应力分布与γ＇颗粒的定向粗化有密切联系。定向粗化的驱动力是塑性变形不均匀分布导致的错配力各向异性松弛。     

Relation between residual stress and Barkhausen noise in a duplex steel

The relation between residual stress and Barkhausen noise (BN) was evaluated in a duplex stainless steel. The residual stresses had been created to the test piece by local heating. Rms value, position of the maximum of the BN burst, its full width half maximum, power spectrum and pulse height distribution were characterised. All the parameters depended to some extent on the residual stresses and none of them was totally independent on the other parameters. The best parameter for the residual stress evaluation was the rms value of the BN amplitude. The rms value varied according to the total stress (macrostress+homogeneous microstress) in the ferrite phase. The stress component perpendicular to the direction of the magnetization had also some influence. Based on the results it is possible to evaluate the residual stresses of the ferrite phase of duplex stainless steels by the BN method.     

Dislocation dynamics simulations of dislocation interactions and stresses in thin films

The dislocation interactions that stop threading dislocations (threads) during relaxation at increasing applied strains in single-crystal thin films are investigated using large-scale three-dimensional dislocation dynamics simulations. Threads were observed to stop via interactions with both threads and misfit dislocations (misfits). Both types of interactions were shown to depend on stress inhomogeneity. Low-stress regions enabled threads to stop in weak thread–misfit interactions even at high average film stresses. Threads were also concentrated in low-stress regions, which facilitated their interaction with other threads. Threads accumulated in thread–thread interactions, and stopped only temporarily in thread–misfit interactions. The mean free path for dislocation motion is shown to be accurately predicted from details of the inhomogeneous stress state arising from the applied strain and the misfit structure. These behaviors are analyzed to present a more complete picture of film strength, strain hardening and relaxation.     

Effect of Cu-Rich Phase Precipitation on the Microstructure and Mechanical Properties of CoCrNiCux Medium-Entropy Alloys Prepared via Laser Directed Energy Deposition

CoCrNiCux (x=0.16,0.33,0.75,and 1) without macro-segregation medium-entropy alloys (MEAs) was prepared using laser directed energy deposition (LDED).The microstructure and mechanical properties of CoCrNiCux alloys with increas-ing Cu content were investigated.The results indicate that a single matrix phase changes into a dual-phase structure and the tensile fracture behaviors convert from brittle to plastic pattern with increasing Cu content in CoCrNiCux alloys.In addi-tion,the tensile strength of CoCrNiCux alloys increased from 148 to 820 MPa,and the ductility increased from 1 to 11％with increasing Cu content.The nano-precipitated particles had a mean size of approximately 20 nm in the Cu-rich phase area,and a large number of neatly arranged misfit dislocations were observed at the interface between the two phases due to Cu-rich phase precipitation in the CoCrNiCu alloy.These misfit dislocations hinder the movement of dislocations during tensile deformation,as observed through transmission electron microscopy.This allows the CoCrNiCu alloy to reach the largest tensile strength and plasticity,and a new strengthening mechanism was achieved for the CoCrNiCu alloy.Moreover,twins were observed in the matrix phase after tensile fracture.Simultaneously,the dual-phase structure with different elastic moduli coordinated with each other during the deformation process,significantly improving the plasticity and strength of the CoCrNiCu alloy.     

Residual stress measurements in fibre reinforced titanium alloy composites

The residual stresses in two Ti/SiC unidirectional composite panels with thick cladding were measured using two experimental methods. The variation of in-plane residual stresses in the cladding of the materials was measured by using a crack compliance method, and the average fibre stresses were inferred from the results. The applicability of a simple bending theory for the calculation of the residual stresses from curvature measurements was confirmed by a finite element analysis. Using a matrix etching method, the longitudinal fibre strains were measured from the relaxation of fibres upon dissolving the matrix in a part of the composite. The stresses in the fibre and the matrix were then calculated using a concentric cylinder model. By combining the results of both methods, the out-of-plane stresses were also determined, so that the full stress state in the reinforced section of the material could be obtained.     

A unified model of environment-assisted cracking

Environment-assisted cracking is caused by enhanced crack-tip plastic deformation. Lattice resistance to shear slip is caused by the periodic fluctuation of the misfit energy across a slip plane. Lu et al. have shown that hydrogen reduces the misfit energy, slip resistance and resistance to dislocation motion. The 1s orbital of a hydrogen atom in a metal is not full, and the electron of the atom moves and reacts as a valence electron. The resistances to shear slip and dislocation generation at a crack tip are reduced by hydrogen electrons and hydrogen nuclei. Crack-tip plastic deformation is thus enhanced, and crack growth rate is increased. The model of reduced resistance to crack-tip dislocation generation and enhanced crack-tip plastic deformation can be extended to liquid metal-assisted cracking and stress corrosion cracking if the valence electron(s) of a liquid metal or chemical reduces the misfit energy and the slip resistance sufficiently.     

关于焊接残余应力形成机制的探讨

分析了关于焊接残余应力形成过程描述的传统观点的局限性和不足,指出材料力学的“截面法”不能用于分析横向残余应力分量。认为在不考虑材科相变的前提下,焊缝金属冷却时收缩受制也是导致焊接残余应力产生的重要原因。虽然残余压缩应变和残余收缩应变在导致焊接残余应力产生的作用方面是等价的,但其机理却有本质的不同,区别二者有利于研究和开发新的焊接残余应力调控技术。对固有应变理论进行了分析和补充,认为应统一到拉伸塑性应变上。用钢板上堆敷锡钎焊焊道的方法,在母材温升值低于塑性应变所需温差的条件下,由盲孔法测得了板上大范围分布的高值残余应力,且因焊道和母材的线膨胀系数差异较大,其应变测试值与环境温度有关。     

细观非均质性对镍基单晶双相合金塑性行为的影响

建立了考虑背应力演化和损伤演化的晶体塑性模型，利用所建立的晶体塑性模型研究了应变速率对镍基单晶合金基体相、强化相的影响。结果表明，镍基单晶基体相和强化相均表现出明显的率相关性。单晶双相合金的细观非均质性导致非均匀的微观应力和微观塑性滑移。塑性滑移变形首先发源于基体相。微观应力和塑性滑移的非均匀性可能是材料微观损伤积累和孔洞形核的根源。     

On the generation of microstrains during the plastic deformation of Waspaloy

Neutron diffraction has been used to characterize the development of microstrains during the plastic deformation of the polycrystalline nickel-base alloy, Waspaloy. Two types of experiment were performed: (i) in situ tensile testing on the diffractometer to determine the response parallel and perpendicular to the loading direction and (ii) measurement of the orientation dependence of the microstrain accumulation using a Eulerian cradle. Large residual microstrains are shown to develop. Along the loading direction, these are typically tensile in the γ′ phase and compressive in the γ phase; however, the values are sensitive to both the orientation of the diffracting crystallite and the amount of plastic deformation of the material. The behaviour is due to the differential deformation between grains in differing orientations (intergranular microstresses) and the two phases (interphase microstresses). It is shown that at low bulk plastic strains, intergranular microstresses develop rapidly, whilst at larger plastic strains the microstresses arising from interphase interactions become dominant. These effects have implications for the determination of residual stresses using diffraction-based techniques and these are discussed.     

Reliability of the plastic deformation behavior of a Zr-based bulk metallic glass

It is important to interpret the scattering of the plastic deformation behavior data for the structural-applications of bulk metallic glasses (BMGs), however, few studies have focused on statistical analysis of the variation and reliability of the plastic deformation behavior of BMGs. In this work, statistical analyses show unavoidable large variations in the maximum nominal strains of as-cast BMGs, although they exhibited greatly-enhanced average values of the maximum nominal strains with reduced sample sizes and in the presence of stress gradients. The large variations are attributed to the intrinsic variability of the atomic arrangements stemming from the solidification processes. Nevertheless, the investigations show enhanced cut-off nominal strains (safety threshold) in the specimens with stress gradients. The findings suggest that, despite large variations in the plastic deformation behavior, BMGs are still reliable in practical structural-applications where the materials always deform under more complex stress states.     

Thermal stresses in metal matrix composites studied by internal friction

Thermal stresses in metal matrix composites (MMC) can induce plastic deformation and damage accumulation in the region close to the reinforcements. In this connection, Al-4% Cu-based MMC reinforced with 10, 20 and 30% Al₂O₃ short fibres were characterized by internal friction (IF) measurements. As a function of the temperature, a maximum of damping has been observed in the composites at ca 150 K during cooling. This maximum is absent in the corresponding unreinforced AlCu alloy. The maximum is attributed to the generation and motion of dislocations produced by high thermal stresses at metal-fibre interfaces. The height of the maximum is found to be an index of the extent of relaxation of thermal stresses by the plastic deformation of the metal matrix. IF measurements have allowed us to identify whether plastic flow in the matrix or damage accumulation at matrix-fibre interfaces take place for different composites.     

Existence of two twinning-mediated plastic deformation modes in Au nanowhiskers

We have performed in situ scanning electron microscopy tensile experiments and molecular dynamics (MD) simulations on nominally defect-free single-crystalline Au nanowhiskers. The room temperature experiments reveal strengths on the order of the ideal strength and plastic strains of up to 12%, a direct result of deformation twinning that governs plastic flow. The in situ and post mortem electron microscopy observations can be divided into two broad classes of deformation morphologies that correlate with distinct stress–strain responses. MD simulations show that the mechanism of twin growth can change from layer-by-layer propagation to parallel and accelerated formation of coalescing nanotwins. The transition between mechanisms is caused by the bending moment resulting from the augmented stress state due to the initial twin and the boundary conditions when a twin grows beyond an embryonic state. These distinct manifestations of deformation twinning suggest that nanoscale material behavior can be tailored for high tensile ductility in addition to ultra-high strength.     

Elastic-plastic constitutive model for accurate springback prediction in hot press sheet forming

The present study investigates the effect of phase transformation plasticity on the springback of hot press formed parts. Employing an implicit finite element formulation to take phase transformations during rapid cooling from fully austenitic phase and their related thermo-mechanical behavior into account, two validations ?? (1) the loaded dilatometry problem which induces significant plastic deformation even at lower stress levels than the classical yield stress, and (2) the 2-D draw bending problem, which introduces high temperature forming and subsequent quenching ?? are presented and compared with known experimental data. The study showed that the magnitude of springback predicted by considering the phase transformation plasticity and the temperature change induced volumetric strain agreed well with date from experiments, while the results determined with the conventional elastic-plasticity theory in which only plastic deformation by external load was used, had considerable deviation for the springback profile after hot press forming. The negligible springback amount after hot press forming, which has been frequently reported in many experimental observations, is due to the stress relaxation by the addition of abnormal transformation plasticity.     

Cracking behaviour of PVD tungsten coatings deposited on steel substrates

Tungsten coatings have been deposited on steel substrates by magnetron sputtering. For the same processing conditions, the increase of the coating thickness enhances the (111) component of the crystallographic texture whereas the residual stress level decreases. Tensile and four-point bending tests, associated with an acoustic emission analysis, have been performed inside a SEM chamber in order to study the cracking mechanisms. When the residual stresses are taken into account, an intrinsic critical cracking stress and the associated energy release rate can be determined; the obtained values suggest an intergranular cracking mechanism. No debonding has been observed at the interface despite the large plastic deformation of the substrate at the crack tips. The observed strain localisation modes in the substrate near the interface have been shown to have a major influence on the limit crack density.     

On an elastically induced splitting instability

We show that a morphological instability driven by deviatoric applied stresses can generate elastically induced particle splitting during diffusional phase transformations. The splitting instability occurs when the elastic fields are above some critical value. For subcritical elastic fields, one observes a small perturbation of the particle shape consistent with splitting, but this perturbation is stabilized by surface tension. Both the onset of the splitting instability and the nonlinear evolution of the particle towards splitting depend on the precise form of the applied stress, the elastic constants of the precipitate and matrix, and the initial shape of the precipitate. We also investigate whether non-dilatational misfit strains can generate splitting instabilities in the absence of an applied stress.