Time-dependent deformation in an enhanced SiC/SiC composite

Time-dependent deformation in an enhanced SiC/SiC composite has been studied under constant load at high temperatures of 1200 °C, 1300 °C, and 1400 °C. Creep damage evolution was evaluated by a Young’s-modulus change of partial unloading and microscopic observation. The addition of the glassy phase in the matrix is very effective for protecting the composite from oxidation. The transient creep is dominant in creep life at all the temperatures. An empirical equation is proposed to describe creep behavior of the composite. It is found that creep activation energy increases with creep time at stresses lower than matrix cracking stress, but the activation energy remains constant at stresses higher than the matrix cracking stress. The creep strain rate of the composite is considered to be controlled by creep of fibers based on examining the time, strain, stress, and temperature dependencies of creep strain rates.     

Time-dependent deformation behavior of near-eutectic 60Sn-40Pb solder

The compressive creep and stress-strain behavior of the near-eutectic 60Sn-40Pb solder alloy has been investigated over the temperature range of −55 °C to 125 °C. The total primary creep strain is a strong function of stress and temperature: at lower temperatures and high applied stresses (i.e., near the power-law breakdown regime), it is quite large, while it is much smaller at higher temperatures and lower applied stresses. The compressive minimum creep rate as a function of stress and temperature is fit well by the Garofalo sinh equation. A discussion of the effective stress exponent, n _eff, in the context of the Garofalo sinh equation is presented to understand trends in the creep data. The values of n _eff, for the applied stress levels studied, are found to range from 3.09 to 5.00 at 125 °C, while they have a range of 10.75 to 15.79 at −55 °C. These trends are consistent with the interpretation of climb-dominated creep at higher temperatures and plasticity-dominated power law breakdown behavior at the lower temperatures. The microstructural observations suggest that, at elevated temperatures, deformation occurs by relative displacement of eutectic colonies in the solder microstructure accompanied by extensive grain coarsening in the colony boundaries. At lower temperatures (<0 °C), deformation occurs by cell displacement with very limited coarsening and, at high stresses, is dominated by plastic deformation. The application of the Garofalo sinh equation to other data sets for creep of eutectic Sn-Pb solder is also discussed.     

Plastic and viscous deformation of metals

Deformation characteristics of tensile specimens of several alloys, including electrolytic copper, α-brass, and 304 stainless steel, have been studied by application of stress and measurement of change of length in a soft tensile machine. By means of experiments in which the stress rate is reduced suddenly from a positive value to zero and the strain rate measured, both during loading and during creep, it is found that permanent deformation consists of two components, a plastic component for which the strain rate is a function of stress and stress rate, and a viscous component which is functionally dependent on stress and temperature. Plastic deformation is relatively more evident at increasing stress rate but declines in importance through the series copper, a-brass, and stainless steel. As a consequence, for a fixed strain rate during loading, the initial creep rate is low in copper and little creep occurs; in stainless steel, however, the initial creep rate is nearly equal to the loading strain rate and creep is pronounced. The theory is not fully developed but is based on a competition between thermal and mechanical release of dislocation segments from obstacles or sources. Release produces a strain increment which may be small or large depending on the relative values of stress and structural resistance. Plastic deformation occurs when the applied stress is close to the mechanical threshold, mechanical release is relatively easy, and the strain consists, at a given strain rate, of a few large strain increments per unit time. For viscous flow the relative stress is low, thermal release easy, and the strain rate is composed of many small strain increments in each unit of time.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S—B) twinning mechanism is conservative. The possible wider significance of the S—B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Lousiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S-B) twinning mechanism is conservative. The possible wider significance of the S-B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Structural features of tensile deformation in electrodeposited metals

Conclusions The characteristic feature of the structure of electrolytic copper and nickel is the presence in the grains of subgranular dislocation boundaries and twinning of growth origin. The subgranular dislocation boundaries are mostly found in the form of planar networks of dislocations having almost screw orientation. Other types of boundaries were also noticed: edge boundaries, dipole configurations, volumetric dislocation nodes, and ragged boundaries. The presence of thermal and stress fields causes disintegration of unstable subgranular dislocation boundaries of growth origin. The disintegration of the boundaries is related to significant restructuring of the dislocation structure and is clearly demonstrated by various methods of physical analysis.The stability of the structure formed during electrocrystallization and its evolution under load depend upon the stacking fault energy, the method of deposition and the subsequent thermomechanical treatment. Absence of unstable subgranular dislocation boundaries of growth origin in the structure, or their stabilization, is a necessary condition for obtaining galvanic coatings with good working properties.Translated from Poroshkovaya Metallurgiya, No. 6(342), pp. 90–97, June, 1991.     

The behavior of particles during plastic deformation of metals

The formation of voids in a metallic matrix around microscopic particles was investigated by observation of individual particles before and after the deformation process. Important factors influencing void formation are geometry and spacing of the particles, relative strength of particle and matrix, environmental pressure, and strength of the bond between particle and matrix. Three combinations of these parameters were investigated by deforming three steels with differing particle parameters. Deformation was performed by pulling specimens in tension and also by extruding split billets under hydrostatic pressure. The results were compared with a theoretical model from the literature. Formerly Graduate Assistant, Lehigh University, Bethlehem, Pa. 18015,     

Accelerated evolution of hydrogen from metals during plastic deformation

Plastic deformation accelerates the release of hydrogen from iron, Type 304L stainless steel, nickel, Inconel 718, and 5086 aluminum. The release rate is strain dependent: it increases rapidly when plastic deformation begins, reaches a maximum, and then decreases with additional strain with a final large release at fracture. The release rate is constant during L&#x00FC;der’s extension for iron, and fluctuates coincidentally with the serrated flow of 5086 aluminum. The release rate during deformation also depends on temperature and strain rate. The accelerated release rate during deformation is discussed in terms of hydrogen-dislocation interactions and is interpreted as being caused by the egress of of dislocations and their associated hydrogen atmospheres during plastic deformation.      

Accelerated evolution of hydrogen from metals during plastic deformation

Plastic deformation accelerates the release of hydrogen from iron, Type 304L stainless steel, nickel, Inconel 718, and 5086 aluminum. The release rate is strain dependent: it increases rapidly when plastic deformation begins, reaches a maximum, and then decreases with additional strain with a final large release at fracture. The release rate is constant during Lüder’s extension for iron, and fluctuates coincidentally with the serrated flow of 5086 aluminum. The release rate during deformation also depends on temperature and strain rate. The accelerated release rate during deformation is discussed in terms of hydrogen-dislocation interactions and is interpreted as being caused by the egress of of dislocations and their associated hydrogen atmospheres during plastic deformation.     

Formation of the limiting nanostructural state in metals during plastic deformation

The mechanisms that determine the formation kinetics of the limiting states in deformed solid nanostructures are considered. These states are characterized by the minimum average nanocrystallite size. The limiting states are shown to correspond to the point of dynamic equilibrium between the deformation-induced competing processes of nanograin fragmentation and growth.     

On the functions used to describe the plastic deformation of metals

The structures of the functions that can be used to describe the plastic deformation and creep deformation of metals are studied. Equations based on an additivity condition are obtained, and examples of the application of the solutions to these equations for determining stresses are given.     

Stretching limits in sheet metals: In-plane versus out-of-plane deformation

Limiting principal strains were measured by two different techniques of biaxial stretching of sheets, one of which permits free deformation in a flat plane, while the other causes constrained deformation in contact with a rigid or rubber punch. The latter method, which relates well with industrial experience, produces larger limiting strains under identical degrees of biaxiality. A possible explanation is based on the process of instability and strain localization. Siegfried S. Hecker, formerly with Res. Labs, General Motors Corporation, Warren, Michigan.     

Lubricant behavior in the deformation source during the pressure treatment of metals

The flow of viscous liquid (used to simulate lubricant) as it is compressed with shear by parallel plates (surfaces) may be determined by the method here proposed. This method provides information regarding the flow kinematics and the contact forces. Practical application of the results permits estimation of the surface quality of the metal when hot-rolled pipe passes through rolling mills of various types.     

A theoretical model for the elevated temperature deformation of dispersion hardened metals

A theoretical model is presented to describe the elevated temperature (above 1/2T_m @#@) deforma-tion of dispersion hardened metals such as TD-nickel and SAP alloys. The model is based on two proposals: 1) Both dislocation glide and climb are influenced by matrix stresses at small, incoherent, second phase dispersed particles produced by surface tension effects at the particle-matrix interface. 2) Two concurrent processes may contribute to the elevated temperature deformation of polycrystalline dispersion hardened metals, dislocation motion and diffusion controlled grain boundary sliding. The model may explain the origins of high apparent activation enthalpies and large stress sensitivities which have been observed in dispersion hardened metals. It may also provide guidelines for optimization of elevated temperature strength.     

The stacking fault energy dependence of the mechanisms of deformation in Fcc metals

The influence of stacking fault energy on the choice of deformation mechanisms is considered. Experimental evidence is reviewed to distinguish between the possible mechanisms and a limited analysis of crystal rotations is performed. It is shown that twinning, pencil glide, and the more usual octahedral slip are all mechanisms which could be operative under different conditions of stacking fault energy or temperature but that even if only octahedral slip occurs stacking fault energy may affect slip rotations by influencing the choice of operative slip systems. Using the simplifying assumption that deformation is homogeneous, and applying Bishop and Hill’s maximum work principle to derive possible combinations of slip systems, a set of criteria are developed to indicate the influence of stacking fault energy on the choice from among the available slip systems, and hence on slip rotations. An empirical method of relating stacking fault energy to texture is outlined and the experimental results obtained by this method are compared with results obtained by other methods. are compared with results obtained by other methods.