Rolling contact deformation,etching effects,and failure of high-strength bearing steel

This paper examines the connections between the continuing cyclic plastic deformation, the etching effects, and the fatigue life of a high-strength bearing steel under rolling contact. Etching effects, called the “dark etching regions” and the “white etching bands,” are observed after several million cycles. The inclinations of the white etching bands vary between 20 to 30 deg and 70 to 80 deg to the rolling direction, depending on the loading conditions and geometry of the rolling elements. The principal axes of stress and plastic strain rotate continuously as the rolling element translates over a fixed point below the running surface. At the same time, the cyclic plastic activity varies. A finite element model is used to calculate the inclinations and amounts of cyclic plastic strain as the roller translates over the running surface. The calculations are performed for both elastic linear kinematic-hardening plastic (ELKP) and elastic perfectlyplastic (EPP) material behaviors. Inclinations of concentrated plastic strain activity combined with low hydrostatic pressure are identified. There is a good correlation between the inclinations of the white etching bands and the inclinations of concentrated plastic activity calculated for the ELKP material behavior. No such correlation is obtained for the EPP behavior. Strain concentrations are intensified by the hydrostatic pressure dependence of the kinematic yield strength. While an equal amount of plastic strain activity occurs in the conjugate directions, no etching bands are observed at these inclinations. The reasons for this are not clear. The shakedown limit obtained for the two models is essentially the same. The fatigue lives under rolling contact are compared with the lives obtained in simple cyclic torsion experiments with the same cyclic plastic strain amplitudes. The rotation of the principal shear direction and the high hydrostatic pressure attending rolling contact may be responsible for the seven orders-of-magnitude longer contact lives.     

High-temperature deformation of commercial-purity aluminum

The stress-strain behavior of aluminum 99.5 pct (2–9) purity deformed under hot-working conditions has been found to be satisfactorily described by combining the exponential saturation equation earlier proposed by Voce and a latter model advanced by Kocks. Voce’s equation describes the strain dependence of the flow stress, whereas the temperature and strain rate dependencies of both the initial flow stress and the saturation or steady-state stress are introduced by means of Kocks’ model, which leads to the definition of a different temperature-compensated strain rate parameter. The basic principles of the dynamic materials model (DMM) advanced by Gegel and co-workers has been reassessed, leading to a different proposition in relation to the calculation of both the power dissipator co-content (J) and the power dissipation efficiency (η), which makes use of the constitutive equation previously developed. Such concepts are later applied to the analysis of a typical industrial hot-rolling process conducted on commercial-purity aluminum. From the microstructural point of view, hot rolling of commercial-purity aluminum has been found to be conducted under conditions of relatively low power dissipation efficiency (η ≈ 0.20 to 0.25), which is likely to be associated with the predominance of dynamic recovery as the main dislocation rearrangement mechanism.     

The deformation of aluminum foams

The compressive flow behavior of Al, Al-7 pct Mg and 7075 Al alloy foams has been determined in structures whose void fraction varies from 0.80 to 0.95 of the total volume. In all cases, a greater than linear increase in flow strength with increase in density was exhibited, indicating that bending stresses within the foam structure are an important feature of the collapse mode. The flow strength did not follow proportionately changes in bulk flow strength in comparisons of either alloy or of heat-treatment conditions. Ancillary tensile and metallographic observations show that this lack of correlation arises because the different foams collapse by different modes with localized fracture becoming dominant in the higher strength 7075 alloy. The energy absorbing efficiency was found to be independent of foam density for all the materials. However, the efficiency was found to be a strong function of the alloy and heat treatment increasing from about 30 pct in Al, to 43 pct in Al-7 pct Mg and to 50 pct in the solution heat treated and aged 7075 alloy. The increase in efficiency occurs because of an increase in the propensity to fracture in the higher strength alloys which introduces the potential for a propagating constant-stress collapse process.     

Cyclic deformation of pure aluminum bicrystals

The dynamic behavior of 〈112〉-tilt grain boundaries in aluminium bicrystals under the influence of cyclic stresses at elevated temperatures is reviewed. Bicrystals, containing low- and high-angle grain boundaries within a wide range of misorientation angles, were deformed at several combinations of stress, temperature, and number of cycles. The grain-boundary (GB) displacement and the deformed structure of bicrystals were framed using standard optical microscopy. The grain orientations were measured using the electron backscatter diffraction (EBSD) technique with a scanning electron microscope (SEM), before and after the deformation. There is distinct evidence of a sharp transition angle between low- and high-angle grain boundaries, with respect to the ability of the boundaries to move under the given parameters. The experimental observations lead to the conclusion that a difference in the dislocation structure in two grains causes the driving force for GB migration.     

表面致密化齿轮的滚动接触疲劳

此文是PM2TEC2005会议的优秀技术论文,对粉末冶金变速器齿轮的开发具有重要意义。作者是加拿大Stackpole公司汽车齿轮事业部的技术领导人。     

表面致密化齿轮的滚动接触疲劳

表面致密化给粉末冶金技术提供了制造高强度制品(如汽车变速器齿轮等)的机遇.对这样的应用来说,寿命设计的一个主要技术要求是,确定齿轮齿的滚动接触疲劳的允许工作应力.本文指出,滚动接触疲劳性能是由材料剪切疲劳强度与施加的表面下剪切应力分布的相互作用决定的.适当选取表面致密化层深与热处理渗碳层深度,可使粉末冶金齿轮的滚动接触疲劳耐久性至少达到相当于锻钢的水平.如所指出的那样,一些滚动接触疲劳试验方法,可能低估了表面致密化粉末冶金材料的表面耐久性.本文就表面致密化粉末冶金材料的失效方式、显微结构以及汽车变速器齿轮形状等问题,综述了滚动接触疲劳试验方法.推荐了开发的滚动接触疲劳试验方法,使之产生能代表实际齿轮应用中存在的试验应力分布.     

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6 × 10_-5 s_-1 to 5.6 × 10_-2 s_-1 and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

The large strain deformation of some aluminum alloys

The stress-strain behavior of both non-heat-treatable and heat-treatable aluminum alloys has been examined over a large strain range by a variety of deformation modes. In superpurity aluminum deformed in torsion, the work hardening rate approaches zero at strains of four to five, while a definite saturation in the flow stress is observed at much lower strains in the precipitation hardened alloys. In the non-heat-treatable alloys, a saturation in the flow stress is not approached at even very large strains. Nevertheless, the stress-strain behavior of all the alloys can be reasonably represented by a saturation type stress-strain equation. The deformation behavior of the alloys can be qualitatively understood in terms of the dislocation accumulation processes and slip morphology in the different alloys. Finally, it is shown that alloys deformed on a commercial rolling mill exhibit equivalent stress-strain behavior to that found in these laboratory deformation studies.     

High-pressure,laser-driven deformation of an aluminum alloy

Recent development of a laser-based experimental platform allows loading materials to high pressures in the solid state while controlling both strain rate and peak pressure. The drive utilizes momentum transfer from a plasma generated by the introduction of a strong shock in a reservoir of low-Z material. This study looks at the response of a commercial aluminum alloy (6061-T6) subjected to pressures of 18 and 40 GPa at strain rates of 10⁷/s and 5 × 10⁷/s, respectively. It was found that the depth of the crater formed on the sample surface is a good indicator of the general yield behavior of the material and that a relatively simple strength model prevails under the loading conditions considered here. Metallographic examination of recovered samples showed no evidence of shear-band formation or significant melting due to plasma-surface interactions. Crystal plasticity-based calculations were used to assess the effects of material texture. Lack of shear-band formation during the laser-based drive is rationalized by considering the strain gradient as compared to grain size and texture. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

Structure and room-temperature deformation of alumina fiber-reinforced aluminum

Plastic deformation under uniaxial longitudinal tension and compression is investigated for pure aluminum reinforced with a high volume fraction of parallel alumina fibers. The matrix substructure is also examined in transmission electron microscopy. The aim is to study thein situ room-temperature mechanical behavior, particularly the work-hardening rate, of pure aluminum when reinforced with a high volume fraction of chemically inert ceramic reinforcement. The matrix substructure prior to deformation, composed of cells about2 μm in diameter, is similar to that of highly deformed unreinforced aluminum. Measured compressive composite elastic moduli agree with rule of mixture predictions; however, no elastic regime is found during tensile loading. As tensile deformation proceeds above a strain around 0.05 pct, a constant rate of work hardening is reached, in which the matrix contribution is negligible within experimental error. Upon unloading from tensile straining, Bauschinger yielding begins before the composite reaches zero load, as predicted by the rule of mixtures. The matrix substructure after load reversal retains a2- μm cell size but with greater irregularity in the dislocation configurations. Using the rule of mixtures,in situ stress-strain curves are derived for the reinforced aluminum matrix and described by a modified Voce law. Formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Molecular-dynamics study of mechanical deformation in nano-crystalline aluminum

We report on molecular-dynamics (MD) simulations of tensile loading of nano-crystalline Al modeled by an embedded-atom method (EAM) potential. Usage of two different sample preparation methods of the nano-crystalline material allows us to compare mechanical properties for different sample qualities. A Voronoi-constructed polycrystal exhibits nearly no pores and has different mechanical properties compared to a material that is sintered under pressure and temperature from spherical nanoparticles, resulting in a lower-density sample. We found an inverse Hall-Petch relation for the flow stress for grain sizes smaller than 10 nm. Intergranular fracture was observed for the larger Al grain sizes, but not for nano-crystalline Cu. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6×10⁻⁵ s⁻¹ to 5.6×10⁻² s⁻¹ and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

Gel electrode imaging of metal fatigue: Part ii. deformation in 1100 Aluminum

During the early stages of metal fatigue, the accumulation of damage produces microcracks in the surface oxide film. This process was first measured quantitatively by the exoelectron method. This paper describes a new and much simpler electrochemical technique, which can also detect microcracks in surface oxide films and provide quantitative information on the distribution and severity of fatigue damage. In these experiments the specimens are coated initially with thin (14 nm) anodic oxide films. After fatigue cycling, a semisolid electrolyte is placed in contact with the specimen, and the flow of current to the microcracks in the anodic oxide is measured. The distribution of fatigue deformation which accumulates prior to the appearance of a fatigue crack is easily measured, and in this regard the sensitivity of the technique is shown to exceed that of a scanning electron microscope. Fatigue deformation in 1100 aluminum is detected as early as 1 pct of the fatigue life, and the electrochemical current flow increases systematically with fatigue cycling as the density of microcracks in the oxide increases. The charge flow can therefore be used to predict the remaining fatigue life.