Deformation Mismatch Research Articles

Influence of cooling rate on deformation due to effective stress in a solidifying alloy

The relative volume rate at a local volume element, div V S′, that is used to evaluate the deformation in the two-phase zone of a solidifying alloy, is dependent on history of the effective stress. In a model of unidirectional solidification, the variation of the effective stress with x-direction or time at the fast cooling rate is larger than the one at the slow cooling rate. The different relative volume rates due to the inhomogeneous cooling rates produce deformation mismatch among the various volume elements in an ingot. It is suggested that accumulation of the deformation mismatch gives rise to thermal residual stain and stress, and hot-tearing tendency in the ingot. The effective stress may be residual in the solidified alloy if it is less than the strength of the alloy itself at the fast cooling rate, and may be relaxed by the deformation of the two-phase zone and play a role in forming the channel space filled with the unstable flow at the slow cooling rate.

Effect of Thermochemical Treatments on the Sliding Wear Mechanisms of Steels Under Boundary Lubrication

The effect of heat treatment, carburizing and plasma nitriding on the material-removal mechanisms and wear behavior of steels under boundary-lubricated conditions has been studied. A controlled procedure using mild abrasives under a light load was adopted in order to avoid long-term effects from severe wear in the running-in stage of pin-on-disc testing. Electron microscopy of the sliding surfaces showed that wear of the untreated steel took place by abrasive and adhesive mechanisms. Heat treatment by austenitizing, quenching and tempering (through-hardening) reduced the wear rate and carburizing produced a further reduction. The principal wear mechanisms in the both through-hardened and carburized steels was abrasive and delamination wear with adhesive wear being unimportant. Electron microscopy on cross-sections through the sliding surfaces of the carburized and through hardened steels showed extensive plasticity, cracks, and delaminated wear phenomena. The elimination of adhesive wear as a major wear mechanism is attributed to the influence of hardness on junction growth and the emergence of delamination wear to the effect of nano-crystalline carbides on fatigue life. Plasma nitriding resulted in an additional reduction in wear rate and the effective elimination of delamination wear. Electron microscopy on the plasma-nitrided steel revealed the presence of a dispersion of white layer regions, which raised the yield strength but also - resulted in micro-pitting due to a deformation mismatch with the matrix. Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 8–12, 1997

Evaluation of the adhesion strength in DLC film-coated systems using the film-cracking technique

A theoretical analysis of the adhesion dependence of film-cracking behavior resulting from the uniaxial tensile loading of a hard film on a ductile substrate is presented. An interface shear stress that develops due to the deformation mismatch of the film and substrate induces a normal tensile stress in the film; these stresses are analyzed as a function of external strain and crack spacing, using the shear lag theory. When the film tensile stress exceeds the film fracture strength σc, film cracking occurs at an early stage of uniaxial loading; but as the loading increases, the interface shear stress increases above the critical value τc and causes the interface failure. After that, no more film cracking occurs, since stress transfer into the film is impossible due to interface damage. Thus, the interface adhesion can be estimated in terms of the shear strength from the external strain εsat and crack spacing λsat measured at the time that film cracking stops. The test results for the diamond-like carbon (DLC)/Al system show an increase of the interface shear strength by a factor of 1.6 for 30-min Ar-plasma etching, compared with no etching. It was found that as the etching time increased, εsat increased and λat decreased, i.e. the interface shear strength increased. This strong dependence of εsat and λsat on interface adhesion indicates that these experimental parameters can be used as qualitative measures of the interface adhesion in a given system.

Computational modeling of metal matrix composite materials—IV. Thermal deformations

The mechanical behavior of particulate reinforced metal matrix composites, in particular an SiC reinforced Al-3 wt% Cu model system, was analyzed numerically using the computational micromechanics approach. In this, the fourth and final article in a series, the microscale effects of thermo-mechanical processing is investigated in detail. Two ideal processes are considered. The first represents a simple quench and the second combines a high temperature compression, to simulate rolling or extrusion, with a quench. These processes, through applied deformation and thermal expansion mismatch, produce inhomogenous, and localized, stress and plastic strain fields in the composite microstructures. The structure of these residual fields can be related to reinforcement volume fraction and microstructural morphology. For the simple quench, volume average plastic strains are almost proportional to reinforcement volume fraction and there is a negligible morphological dependence. Large magnitude residual stress and strain fields, as well as large geometry changes, result from the second process. The effects of these processes on subsequent deformation behavior of composites for a selection of morphologies is investigated. Apart from almost doubling yield strains, the simple quench has relatively minor microscale and macroscale effects. The second process has a considerable effect on yield strength and introduces differences between tensile and compressive behavior on both the macroscale and microscale. Plastic constraint is an important mechanism. The physical relevance of these particular processing simulations, and the implications for modeling of microscale failure are discussed.

Resistance Welding of Dissimilar Metals

This paper is concerned with the resistance welding of various dissimilar metals, primarily component parts such as ribbons, leads, and cases to one another. The study was made and is being continued to improve production welding in the manufacture of aluminum electrolytic, and tantalum capacitors. Resistance welding is primarily used by capacitor production because it is a fast, economical process, which lends itself readily to automation. The welds associated with the higher conductive, dissimilar metals are not fusion welds, but can be described as solid state bonds. There are some inherent defects in the solid state bond that do not normally affect the fusion bond. These are: (1) crystal lattice defects such as vacancies, (2) crystal deformation, and (3) atomic size mismatch. In spite of these inherent defects, excellent AQL's can be expected, provided certain requirements are met. The requirements include an overall program of weld design, equipment, schedules, process control, and component design, materials, and material control.