Electrical conductivity versus strength in a precipitation hardened alloy

There is a trade off between electrical conductivity and strength in precipitation hardened alloys. Nordheim's relation relates conductivity to dissolved solute. The Gibbs-Thomson equation relates dissolved solute to particle size. The Orowan theory equation relates particle size and spacing to yield strength. The inter-relations among these equations were determined in a Cu−0.74 wt% Ni−0.14 wt% P alloy. The methodology developed allows predictions of the optimum trade off between strength and conductivity for a particular electron device application of a precipitation hardened alloy in the coarsening regime.     

Asymmetrical mechanical behavior of a precipitation hardened beta titanium alloy

Precipitation hardened single crystals of a beta (bcc) Ti-40 at. pct V + 1.0 at. pct Si alloy have been deformed in compression at 77 K and 298 K. The dependence of the yield stress upon aging time at 843 K for solution treated crystals shows two maxima which are caused by silicide precipitates. The orientation dependence of the yield stress and of the active macroscopic slip plane have been determined as a function of aging time. The solution treated as well as aged crystals exhibit an asymmetry of both the yield stress and the plane of slip, the degree of asymmetry being larger at 77 K than at 298 K. The asymmetry of slip and yielding is not affected by the presence of precipitation hardening. The results indicate that the effect of the dislocation core structure on dislocation motion is independent of the presence of precipitates.

     

Asymmetrical mechanical behavior of a precipitation hardened beta titanium alloy

Precipitation hardened single crystals of a beta (bcc) Ti-40 at. pct V + 1.0 at. pct Si alloy have been deformed in compression at 77 K and 298 K. The dependence of the yield stress upon aging time at 843 K for solution treated crystals shows two maxima which are caused by silicide precipitates. The orientation dependence of the yield stress and of the active macroscopic slip plane have been determined as a function of aging time. The solution treated as well as aged crystals exhibit an asymmetry of both the yield stress and the plane of slip, the degree of asymmetry being larger at 77 K than at 298 K. The asymmetry of slip and yielding is not affected by the presence of precipitation hardening. The results indicate that the effect of the dislocation core structure on dislocation motion is independent of the presence of precipitates.     

The effect of cathodic polarization on the corrosion fatigue behavior of a precipitation hardened aluminum alloy

Fatigue experiments were conducted on polycrystalline and monocrystalline samples of a high purity Al, 5.5 wt pct Zn, 2.5 wt pct Mg, 1.5 wt pct Cu alloy in the peak-hardened heat treatment condition. These experiments were conducted in dry laboratory air and in 0.5N NaCl solutions at the corrosion potential and at applied potentials cathodic to the corrosion potential. It has been shown that saline solutions severely reduce the fatigue resistance of the alloy, resulting in considerable amounts of intergranular crack initiation and propagation under freely corroding conditions for polycrystalline samples. Applied cathodic potentials resulted in still larger decreases in fatigue resistance and, for poly crystals, increases in the degree of transgranular crack initiation and propagation. Increasing amounts of intergranular cracking were observed when applied cyclic stresses were reduced (longer test times). The characteristics of cracking, combined with results obtained on tensile tests of deformed and hydrogen charged samples, suggest that environmental cracking of these alloys is associated with a form of hydrogen embrittlement of the process zones of growing cracks. Further, it is suggested that stress corrosion cracking and corrosion fatigue of these alloys occurs by essentially the same mechanism, but that the often observed transgranular cracking under cyclic loading conditions occurs due to enhanced hydrogen transport and/or concentrations associated with mobile dislocations at growing crack tips.     

Effects of high temperature fatigue on microstructure and mechanical properties of a nickel-base precipitation hardened alloy

Effects of high temperature strain controlled push-pull fatigue on the microstructure and mechanical properties of a nickel-base precipitation hardened alloy were studied. The fatigue deformation alone at 700 °C did not impair the mechanical properties of this alloy; however, a hold period ranging from one minute to one hour at tension-peak decreased the tensile ductility and the fracture toughness significantly. This was mainly attributed to grain boundary cavitation. Continuous fatigue resulted in dislocation bands, whereas hold-time fatigue caused a coherency loss iny’ precipitates. Implications of these microstructural changes for the residual mechanical properties are discussed. Formerly with the Brookhaven National Laboratory, Upton, NY 11973. Formerly with the Brookhaven National Laboratory.     

A mathematical model describing carburization in multielement alloy systems

Previously, a finite difference model was set up describing the diffusion of carbon in high-temperature model alloys and its chemical reaction with one of the alloy components. The model described the formation of up to three different chromium carbides, two of which could coexist. This paper describes the further development of the model for application to commercial alloys. The model was extended to enable treatment of an arbitrary number of (a) metallic carbide-forming components in the alloys, (b) carbides which may form, (c) components out of which each carbide may be composed, and (d) carbides which may coexist. With this model, carbon concentration profiles and distribution profiles of precipitated carbides occurring during carburization of binary, ternary, and quaternary Ni-based alloys were calculated. Kinetic and thermodynamic data needed for the calculations were obtained by combining literacture data with experimental results and by fitting measured with calculated concentration profiles. The resulting calculated carbon profiles and carbide distributions were in good agreement with the experimental results. R. Supchulten, formerly with Kernforschungsanlage Jülich     

The role of alloy composition in the precipitation behaviour of high speed steels

The role of alloy composition in determining the microstructure and microchemistry of a series of related high speed steels has been investigated by a combination of analytical electron microscopy and atom-probe field ion microscopy. The four steels which were investigated (M2, ASP 23, ASP 30 and ASP 60) cover a large range of C, V and Co contents. Excepting the Co content, the composition of primary MC and M₆C carbides and as-hardened martensite was similar in all four alloys and the major effect of increasing the content of C and V was to increase the volume fraction of MC primary carbides. Precipitation of proeutectoid carbides (mainly MC and M₂C) occurred during hardening of all four steels and the extent of this was greatest in the highly alloyed ASP 60. Tempering at 560°C resulted in the precipitation of extremely fine dispersions of MC and M₂C secondary carbides with very mixed compositions in all four steels. It was found that, as well as hindering the formation of autotempered M₃C in the as-hardened martensite, additions of Co refined the secondary carbide dispersion and delayed overaging reactions. Overaging at 600°C resulted in the precipitation of M₃C, M₆C and M₂₃C₆ at the expense of the fine MC and M₂C secondary carbide dispersion.     

A constitutive model for the tensile deformation of a binary aluminum alloy at high fractions of solid

Tensile tests were performed on alloy Al-4.5 pct Cu with a solidification unit dubbed the direct chill surface simulator (DCSS) that simulates the primary cooling conditions encountered during the direct chill (DC) casting process of sheet ingots. The curves obtained showed a near linear increase of the load with strain up to the point where nonlinearity induced by hot tearing prevented further increase of the load. A constitutive model based on the Lahaie-Bouchard stress-strain theoretical model was generalized by assuming a nonsingular channel thickness distribution. This approach considers a fully lubricated arrangement that allows grain boundary sliding as the most important mechanism of accommodation. As the strain increases, the grain boundaries become more and more subjected to friction sliding and oppose a higher resistance than a fully lubricated sliding condition. This results in a gradual increase of stress with strain. The important variables of the model include solid fraction, channel thickness, geometric standard deviation, and creep law parameters. The model seems appropriate to correlate the tensile behavior of semisolid microstructures having up to 30 pct liquid phase distributed over a large spectrum of grain morphologies.     

On crack closure of precipitation hardened steels in aqueous solution

Fatigue crack propagation tests were carried out in air and in a 3.5 pct NaCl aqueous solution under cathodic potential of −0.85 V (Ag/AgCl) for aged-hardened high strength steel (Ni−Al−Cr−Mo−C steel). the emphasis in the study was placed on the crack closure behavior of age-hardened materials in air and in the NaCl aqueous solution. The degree of crack closure in air was dependent on the behavior of plastic deformation such as inhomogeneous or homogeneous slip under mixed modes I and II. The underaged material containing coherent precipitates with the matrix had a higher crack opening load in air, compared with the overaged steel containing incoherent precipitates with the matrix. The degrec of crack closure of the underaged material in the NaCl aqueous solution was lower than that in air and was similar to that of overaged materials in the NaCl aqueous solution. It was shown that the decreased crack closure level for the underaged material resulted from accelerated fatigue crack growth under mode I due to hydrogen embrittlement in the aqueous solution.     

基于球化机理的TA15钛合金热变形统一本构模型

应用晶界分离模型解释了片层α相的球化现象,阐述了TA15钛合金转变组织中次生片层α相的球化是其主要的流动软化机制.基于钛合金球化软化机理,建立了TA15钛合金的统一黏塑性本构模型.本构模型综合考虑了次生α相的球化、正则位错密度、等向硬化、塑性成形产生的温升、成形过程中的相变等物理变量.利用遗传算法确定了本构模型中的材料常数.本构模型能够较好地描述TA15钛合金热变形下的流动应力变化.     

影响烧结硬化合金钢齿轮特性之参数探讨

根据不同合金钢合金化的方式与添加元素的种类, 规划出4 组不同硬化能之烧结合金钢; 将这4 种硬化能不同之烧结合金钢粉末分别压制成密度为6.85 g/cm3 的齿轮胚体, 经RX型保护气氛、1 120 ℃烧结30 min后, 继以24℃/min与48℃/min的冷速率来控制冷却过程中合金钢的相变化行为, 再将烧结态之合金钢齿轮施以200℃、1~4 h之回火处理; 最终对合金钢齿轮进行硬度、齿轮精度、齿破裂负载等量测与金相观测。研究结果显示烧结合金钢的硬化程度随硬化能的增加或烧结淬冷速率的加快而提升, 而齿轮精度则随其相变后的马氏体含量的增加而降低。故针对烧结硬化合金钢齿轮的制造流程开发中, 烧结淬冷区的冷却速率须达48℃/min, 才能确保齿轮之硬度维持在HRC 30以上; 且硬化能倍数宜调整在15~25之间, 经1 h以上的回火, 即可获得具经济竞争力, 又具优异机械性能与齿轮精度组合的烧结硬化合金钢齿轮。     

Thermal stability of the structure of a heat-resistant cobalt alloy hardened with intermetallic γ'-phase precipitates

The thermal stability of the microstructure of a heat-resistant cobalt alloy, which consists of a γ solid solution strengthened with γ'-phase precipitates, has been studied. The temperature behavior of the dissolution of the hardening γ' phase and the kinetics of its coarsening at 700 and 800°C have been determined. It is found that, during prolonged annealing at 800°C, the γ' → β phase transformation occurs.     

A modified Johnson-Cook constitutive relationship for a rare-earth containing magnesium alloy

Lightweight magnesium alloy has recently attracted a considerable interest in the automotive and aerospace industries to improve fuel efficiency and reduce CO2 emissions via the weight reduction of vehicles.Rare-earth（RE） element addition can remarkably improve the mechanical properties of magnesium alloys through weakening crystallographic textures associated with the strong mechanical anisotropy and tension-compression yield asymmetry.While the addition of RE elements sheds some light on the alteration in the mechanical anisotropy,available information on the constitutive relationships used to describe the flow behavior of RE-containing magnesium alloys is limited.To establish such a constitutive relationship,uniaxial compressive deformation tests were first conducted on an extruded Mg-10Gd-3Y-0.5Zr（GW103K） magnesium alloy at the strain rates ranging from 1×10–1 to 1×10–4s–1 at room temperature.A modified Johnson-Cook constitutive equation based on a recent strain hardening equation was proposed to predict the flow stresses of GW103K alloy.Comparisons between the predicted and experimental results showed that the modified Johnson-Cook constitutive equation was able to predict the flow stresses of the RE-containing magnesium alloy fairly accurately with a standard deviation of about 1.8%.     

Microstructures at the fracture flank of precipitation hardened Al-1.79 w/o Cu alloys

The microstructures at the crack flanks of Al-1.79 w/o Cu alloys, which were aged to contain θ′ or θ precipitates, respectively, were studied by transmission electron microscopy. Convergent beam microdiffraction patterns and center dark field images revealed the existence of small, misoriented, and three-dimensionally connected volume elements. The locations of the precipitates at the crack edges suggest that interphase-interface decohesion was not the dominant mode of fracture initiation. It was concluded that dislocation cell walls are the preferred sites for microcrack initiation.     

Cyclic strain induced precipitation in a solution treated aluminium alloy

Cyclic mechanical tests were carried out on specimens of an Al-Mg-Si alloy, 6351, in the solution treated condition and the behaviour compared to that of an Al-Mg alloy, 5182. Both showed marked cyclic hardening typical of non precipitation hardened aluminium alloys. In the 5182 alloy, this hardening has been attributed to dislocation pinning by solute atoms. Differences in mechanical behaviour were observed which suggested that hardening of the 6351 alloy may be due to a cyclic strain induced precipitation hardening effect rather than solute atom pinning. Using a previously described weak beam technique, evidence was found for precipitate formation on dislocations in the cycled 6351 alloy. The evolution of the microstructure during cycling was different in the two alloys. In the 6351 alloy, the dislocation density reached a maximum and then decreased with further cycling even though hardening continued to occur. This was attributed to progressive deactivation of dislocation sources whilst normal annihilation reactions continued to occur among the mobile dislocations.