关于焊接应力变形两个问题的进一步探讨

讨论了焊缝是否存在压缩塑性变形问题和拉达伊D应力应变原理图存在的问题。指出刚刚经历熔化—凝固过程的焊缝不存在压缩塑性变形 ,一直承受拉伸塑伸变形。拉伸塑性变形区的范围要比拉达伊D的原理图上给出的大得多 ,对拉达伊D的原理图进行全面修改 ,提出新的焊接热应力应变原理图。在新原理图中 ,三组曲线分别是升温—降温、压缩—拉伸和拉伸弹性—塑性变形的分界线。将等温线所处的温度明确为熔点 ,它包围的区域为熔池。取消弹性卸载区的提法 ,改为弹性变形区。整个焊接板由弹性变形区、压缩塑性变形区、拉伸塑性变形区和熔池组成。     

Internal stress and mechanical deformation of Al and Al/Ni multilayered nanowires

Isothermal molecular dynamics is used to study the correlation between the spatial distribution of internal stress and mechanical deformation of a 6.7-nm-diameter Al nanowire with <1 0 0> axis is subjected to an external uniaxial stress. The stress–strain relationship is asymmetrical. In the case of a tensile load, the internal stress distribution is found to result from the interplay between structure and morphology. As a general rule, yielding nucleates where the internal stress gradient is the highest. If the Al wire is interfaced with a harder material—Ni in this study—the highest gradients occur at the interface, where a characteristic interfacial stress pattern is induced. Remarkably, compressive and tensile yield strengths are found to be unaffected by the hard/soft interfaces. The structure of the stress–strain relationship is found to correlate with identified discrete plastic events. These may be complex, involving interactions between partial dislocations, stacking faults, surfaces and interfaces, internal stress localization and release.     

Effect of strain rate on compressive behavior of a Pd40Ni40P20 bulk metallic glass

Mechanical deformation of Pd₄₀Ni₄₀P₂₀ was characterized in compression over a wide strain rate range (3.3×10⁻⁵ to 2×10³ s⁻¹) at room temperature. The compression sample fractured with a shear plane inclined 42 degree with respect to the loading axis, in contrast to 56 degree for the case of tension. This suggests the yielding of the material deviates from the classical von Mises yield criterion, but follows the Mohr-Coulomb yield criterion. Fracture stress as well as strain was found to decrease with increasing applied strain rate. The compressive stress (1.74 GPa) was also found to be higher than the tensile fracture stress at a quasi-static strain rate. Close examination of the stress–strain curves revealed that localized shear might have occurred at a compressive stress of about 1.4 GPa, much lower than the “apparent” yield stress of 1.74 GPa. However, the stress of 1.4 GPa for shear band initiation is almost the same as the fracture stress measured at a dynamic strain rate of 5×10² s⁻¹. These results suggested that the fracture of a bulk metallic glass is sensitive to the applied loading rate.     

Mean stress effects on flow localization and failure in a bulk metallic glass

The effect of stress state on strain localization and subsequent failure of a bulk metallic glass alloy is examined. It is shown that failure is associated with a critical tensile mean stress of 0.95 GPa. This is in contrast with previous work utilizing superimposed compressive mean stresses, which found that failure resulted at a critical effective stress. Interestingly, the critical tensile mean stress measured in this study causes the same dilatation as a 274 K temperature increase, nearly to the glass transition temperature. The effect of mean stress on elastic variation of the average free volume is added to a strain localization model. This model describes the compressive mean stress behavior very well, and predicts a strong sensitivity to tensile mean stresses.     

Characteristics of the tensile flow behavior of Ti–46Al–9Nb sheet material – Analysis of thermally activated processes of plastic deformation

Flow behavior, strain hardening and activation parameters, i.e. activation volume, stress exponents and normalized free enthalpy of activation, of Ti–46Al–9Nb sheet with near-gamma microstructure have been investigated in tension tests between 700 and 1000 °C. The dependence of yield stress on temperature and strain rate, the course of the strain hardening curves and the values of activation parameters show that thermally activated dislocation mechanisms are mainly involved in the tensile deformation process of the investigated material. At constant temperature the value of the activation volume depends both on plastic strain and strain rate. The activation volume generally decreases with increasing strain. The decrease is particularly well observable for higher strain rates, thus indicating a growing role of thermally activated climb mechanisms governing the process of dynamic recovery. The activation volume calculated for a constant plastic strain (2% in case of this study) is a function of temperature and strain rate. At lower deformation rates, or alternatively at higher temperatures, the activation volume increases. Such behavior indicates a decrease in dislocation density due to the onset of dynamic recrystallization. The analysis of stress exponents and the obtained free enthalpy of activation confirm that different thermally activated processes are acting during deformation under the tensile test conditions studied.     

Twinning Behaviors During Thermomechanical Fatigue Cycling of a Nickel-Base Single-Crystal TMS-82 Superalloy

This paper provides further insight into the formation of deformation twins at different stages during the whole thermomechanical fatigue cycling in a nickel-base single-crystal TMS-82 superalloy. In general, it is found that twinning behaviors can always be associated with the applied stress orientation. The preferred twinning direction at the primary stage is 〈001〉-compression since the tangled dislocations which appear after the first plastic deformation provide an opportunity for twinning nucleation in compression. At the intermediate stage, the applied stress required for formation of twins in tension is much larger than that in compression; hence, twinning behaviors show distinct tension/compression asymmetry. A thick twin plate and a great many dislocations can be found after fatigue failure, and one can rationalize the reason for this twinning being associated with the TMF procedure. Twins at the tip of the crack in tension occur owing to the existence of compressive strain field.     

Evolution of lattice strain and phase transformation of β III Ti alloy during room temperature cyclic tension

An in situ high-energy X-ray diffraction cyclic tension test was carried out on a β III Ti alloy to study its micromechanical behavior and the stress-induced phase transformation. Pre-strained material showed a microscopic multi-stage re-loading behavior following the sequence of elastic deformation, stress-induced martensite (SIM) transformation, a second stage of elastic deformation followed by a final stage of SIM transformation. Based on the relationship of internal strains and diffraction intensities between the β phase and the SIM, it is concluded that after a small strain deformation, the austenite is divided into two different sets of grains with different properties. Those that previously experienced phase transformation have a lower critical stress for the SIM transformation due to residual martensite and dislocations, while the rest have a higher trigger stress and only transform to martensite after the stress is back to levels comparable to where transformation was seen in the previous cycle. The different properties within the same austenite grain family cause the multistage re-loading behavior. The reverse phase transformation during unloading was impeded by the combination of increased dislocation density in the austenite and the increased tensile strain in the martensite prior to unloading.     

Accommodation processes during deformation of nanocrystalline palladium

Atomistic simulations of uniaxial tensile and compressive straining of three-dimensional nanocrystalline palladium were performed at room temperature and different strain rates. Detailed analysis revealed that initial plastic deformation is due to grain boundary sliding accommodated by localized bending inside the grains and the formation of dislocation embryos. Intergranular cracking in the absence of dislocation activity was found at later stages of tensile straining. During compressive straining the sample shows a plastic response which is brought about mainly by intergranular accommodation processes. The contribution of extended partial dislocations emitted from the grain boundaries as well as full dislocations and twinning at later stages of deformation to the total strain was found to be insignificant.     

Steady state flow of the FeCoNiCrMn high entropy alloy at elevated temperatures

Steady state flow behavior of the FeCoNiCrMn high-entropy alloy at temperatures ranging from 1023 to 1123 K was systematically characterized. It was found that the stress exponent (i.e., the reciprocal of strain-rate sensitivity) was dependent on the applied strain rate, and specifically the stress exponent is high (∼5) in the high strain rate regime, but decreases with decreasing strain rate. Microstructural examinations of the samples before and after deformation were performed to understand the interplay of the microstructures with the corresponding properties. Based on the observations, it was proposed that, at high strain rates, the deformation of the current high-entropy alloy was controlled by dislocation climb and the rate limiting process was the diffusion of Ni. At low strain rates, however, the deformation appeared to be controlled by the viscous glide of dislocations. Moreover, at the slowest strain rate (i.e., the longest thermal exposure time), new phases evolved, which caused elemental redistribution and weakening of the material.     

Unconventional elastic properties, deformation behavior and fracture characteristics of newly developed rare earth bulk metallic glasses

Significant differences in the thermal, elastic and mechanical behavior of bulk metallic glasses (BMGs) based on rare earth (RE) elements (i.e., Pr-based, Ce-based and Nd-based) have been found when comparing them with archetypical Zr-based and Ti-based amorphous metallic alloys. Our results show that RE-BMG exhibits a large supercooled liquid region, low elastic constants and concomitant elastic softening, low hardness, complete lack of macroscopic plasticity and compressive fracture angles, ψ_C,F, larger than 45° (as opposed to polycrystalline materials, where ψ_C,F = 45°, and conventional BMGs, where ψ_C,F ≤ 45°). Most of these features stem from the rather low glass transition temperature displayed by these alloys, which is relatively close to room temperature. However, contrary to some previous studies, our observations reveal that the lack of plasticity of these materials cannot be simply rationalized in terms of their Poisson’s ratio but is also due to some tensile features (i.e., dilatational effects) accompanying compressive fracture behavior.     

Pseudoelastic behaviour of perforated NiTi shape memory plates under tension

This paper reports the tensile deformation behaviour of near-equiatomic NiTi plates with circular, elliptical and square holes. The investigation is done both experimentally and by mathematical modelling. It is found that the nominal stress–strain curve of such structures deviates from typical stress–strain variation of NiTi with flat stress plateaus, by exhibiting stress gradients over the forward and reverse transformations of the hole-affected areas. The hole-affected length is nearly triple the lateral dimension of the hole, and when 33% of the gauge length is covered by (circular) holes, the entire sample behaves like a functionally graded material. Such mechanical behaviour is advantageous for achieving better load–displacement controllability and wider stress window for shape memory actuation and sensing.     

The role of extrusion texture on strength and its anisotropy in a Mg-base alloy composed of the Long-Period-Structural-Order phase

A Mg–Y–Zn alloy composed almost completely of the Long-Period-Stacking-Ordered (LPSO) phase has been prepared by casting and extrusion to flat bar. An elongated microstructure is obtained with different grain orientations along the extrusion, transverse and normal directions, leading to a strong orientation dependence of strength. Differences of tensile and compressive stress are also noted. The importance of basal slip and other dislocation-dependent deformation mechanisms is discussed. The large tension-compression strength differences are considered to imply that twinning plays an important role in determining strength. The analysis of deformation mechanisms and strengthening is supported by metallographic studies of surface slip bands and dislocation distributions.     

Mechanical properties of intermetallic inclusions in Al 7075 alloys by micropillar compression

The constituent particles (also called inclusions) play an important role in the deformation behavior of Al 7075 alloys. The majority of inclusions in Al 7075 alloys can be classified as Fe-bearing and Si-bearing inclusions. Among them Al₇Cu₂Fe and Mg₂Si are predominant. In this study, the mechanical properties of these inclusions and Al 7075 matrix were studied using micropillar compression testing. Micropillars were fabricated by focused ion beam (FIB) milling and were tested using a nanoindenter equipped with a flat tip. For the first time, the stress–strain behavior of these intermetallic particles was obtained experimentally, resulting in the measurement of the compressive failure and yield strength. The stress–strain behavior obtained from pillar compression show that both inclusions possess higher strength than the Al 7075 matrix.     

Mechanical properties of highly porous Ti49.5Ni50.5 biomaterials

Ti_49.5Ni_50.5 shape memory alloy fibers were prepared by a melt overflow process. The martensitic transformation starting temperature of B2 → B19′ in the rapidly solidified fibers was 19 °C. Cylindrical billets of Ni-rich Ti–Ni alloy with 75% porosity were produced by a vacuum sintering technology using as-cast alloy fibers. The mechanical properties and shape memory properties of the highly porous Ti–Ni alloy is investigated using a compressive test. The plateau of the stress–strain curve was observed at about 7 MPa and resulted in 8% elongation associated with stress-induced B2 → B19′ transformation. Because of the high porosity of this specimen, the elastic modulus of about 0.95 GPa could be obtained. It was also found that a recovered strain was 5.9% on heating after the compressive deformation. This recovery of the length is ascribed to the shape memory effect which occurs during the martensitic transformation.     

Modelling the effects of tool-edge radius on residual stresses when orthogonal cutting AISI 316L

Tool-edge geometry has significant effects on the cutting process, as it affects cutting forces, stresses, temperatures, deformation zone, and surface integrity. An Arbitrary-Lagrangian–Eulerian (A.L.E.) finite element model is presented here to simulate the effects of cutting-edge radius on residual stresses (R.S.) when orthogonal dry cutting austenitic stainless steel AISI 316L with continuous chip formation. Four radii were simulated starting with a sharp edge, with a finite radius, and up to a value equal to the uncut chip thickness. Residual stress profiles started with surface tensile stresses then turned to be compressive at about 140 μm from the surface; the same trend was found experimentally. Larger edge radius induced higher R.S. in both the tensile and compressive regions, while it had almost no effect on the thickness of tensile layer and pushed the maximum compressive stresses deeper into the workpiece. A stagnation zone was clearly observed when using non-sharp tools and its size increased with edge radius. The distance between the stagnation-zone tip and the machined surface increased with edge radius, which explained the increase in material plastic deformation, and compressive R.S. when using larger edge radius. Workpiece temperatures increased with edge radius; this is attributed to the increase in friction heat generation as the contact area between the tool edge and workpiece increases. Consequently, higher tensile R.S. were induced in the near-surface layer. The low thermal conductivity of AISI 316L restricted the effect of friction heat to the near-surface layer; therefore, the thickness of tensile layer was not affected.     

Internal stress plasticity due to chemical stresses

Internal stress plasticity occurs when a small external stress biases internal mismatch strains produced by, e.g., phase transformation or thermal expansion mismatch. At small applied stresses, this deformation mechanism is characterized by a deformation rate which is proportional to the applied stress and is higher than for conventional creep mechanisms. In this work, we demonstrate the operation of internal stress plasticity due to internal chemical stresses produced by chemical composition gradients. We subject specimens of β-phase Ti-6Al-4V to cyclic charging/discharging with hydrogen (by cyclic exposure of specimens to gaseous H₂), under a small external tensile stress. As expected for internal stress plasticity, the average strain rate during chemical cycles at 1030°C is larger than for creep at constant composition (hydrogen-free or -saturated), and a linear stress dependence is observed at small applied stresses. Additionally, we present an analytical model which couples elastic and creep deformation with a transient diffusion problem, wherein the diffusant species induces swelling of the host lattice. Without the use of any adjustable parameters, the model accurately predicts both the observed strain evolution during hydrogen cycling of Ti-6Al-4V and the measured stress dependence of the deformation.     

应力状态对纯钛织构演化机制影响研究

采用拉伸、压缩的试验方法,结合Schmid因子计算和晶体塑性模拟计算研究了TA2纯钛在不同应力（拉应力、压应力）状态下织构的演化机制。结果表明：在拉伸变形过程中,较大的应变量也难以使织构发生显著变化,相对而言,压缩变形过程中织构变化较为显著。在不同应变路径下,变形初期启动的变形方式有一定的差异。在不同应变量下,随着变形程度的增加,发生基面滑移或锥面滑移或■拉伸孪生的晶粒数变多是导致形成不同织构的主要原因。     

Finite element analysis of controlling the TC4 thin plate weldment wave-like deformation by welding with impacting rotation

The new technology of welding with impacting rotation is put forward to decrease the wave-like deformation of the TC4 thin plate weldment.The thermal stress and strain are vital to understand the mechanism of controlling the wave-like deformation.In order to know the development of internal thermal stress and strain,finite element method is utilized for the stress and strain are difficult to be investigated by experimental methods during the welding process.Temperature field,thermal stress evolution and distortion of thin plate are compared with the test results such as weld thermal cycle,residual stress sectioning measurement,and the deflection of the thin plate respectively.By the finite element analysis and test results verification,the mechanism of the technology to control the wave-like deformation is brought forward,non-uniform thermal elastic strain between compressive plastic region and elastic extensive region is diminished by a certain amount of extensive plastic deformation by welding with impacting rotation process.     

单晶Al3Ti拉伸与剪切变形的分子动力学模拟

采用分子动力学方法研究单晶Al3Ti模型的拉伸和剪切力学性能。模拟Al3Ti在常温、恒定应变速率下的拉伸和剪切变形过程,讨论了温度和应变速率对体系拉伸和剪切性能的影响。结果表明,Al3Ti室温下很脆,弹性变形阶段结束后在短时间内体系产生的孔洞和位错迅速发展导致材料破坏。温度升高会导致Al3Ti的抗拉强度、杨氏模量、剪切强度和切变模量降低;应变速率增大能提高材料的拉伸和剪切强度,但不影响杨氏模量和切变模量大小。     

Dislocation-climb plasticity: Modelling and comparison with the mechanical properties of icosahedral AlPdMn

A model of plasticity controlled by the pure climb motion of dislocations is proposed and compared with the mechanical properties of icosahedral AlPdMn. This model takes into account the chemical stress due to an out-of-equilibrium average concentration of vacancies, and the difficult nucleation of jog-pairs on climbing dislocations. It accounts for several unexplained properties of AlPdMn, namely a high strain-hardening at yield, a steady-state flow stress twice higher than the elastic limit, and two-stage relaxation curves. It also explains values of the stress–strain rate sensitivity larger than expected a priori, and activation energies larger than the self-diffusion one. The model may also be applicable to high-temperature deformation of crystals.