Showing papers on "Hardening (metallurgy) published in 1993"

Micromechanical modeling of large plastic deformation and texture evolution in semi-crystalline polymers

Abstract: A micromechanically-based composite model is proposed to study large plastic deformation and texture evolution in semi-crystalline polymers. The microstructure of many semi-crystalline polymers consists of co-existing crystalline and amorphous phases locally associated with each other in a fine plate-like morphological structure. An aggregate of two-phase composite inclusions is used to model these materials. Special consideration is given to molecular chain inextensibility within the crystalline phase. The introduction of a back stress tensor in the constitutive model of the amorphous phase accounts for hardening due to deformation-induced molecular alignment. Interface compatibility and traction equilibrium are enforced within each composite inclusion. A Sachs-like model and two newly-developed self-consistent-like hybrid models are proposed to relate volume-average deformation and stress within the two-phase composite inclusion to the remote (macroscopic) fields. Applications of these composite models arc made to predict stress strain behavior and texture evolution in initially isolropic high density polyethylene (HOPE) under different modes of straining.

Precipitation hardening in 350 grade maraging steel

Abstract: Evolution of microstructure in a 350 grade commercial maraging steel has been examined. In the earlier stages of aging, the strengthening phases are formed by the heterogeneous precipitation, and these phases have been identified as intermetallic compounds of the Ni3 (Ti, Mo) and Fe2Mo types. The kinetics of precipitation are studied in terms of the activation energy by carrying out isothermal hardness measurements of aged material. The mechanical properties in the peak-aged and overaged conditions were evaluated and the flow behavior examined. The overaging behavior of the steel has been studied and the formation of austenite of different morphologies identified. The crystallography of the austenite has been examined in detail. From the microstructural examination of peak-aged and deformed samples, it could be inferred that the dislocation-precipitate interaction is by precipitate shearing. Increased work hardening of the material in the overaged condition was suggestive of looping of precipitates by dislocations.

A new mechanism of work hardening in the late stages of large strain plastic flow in F.C.C. and diamond cubic crystals

Abstract: A new mechanism of work hardening is proposed to explain the athermal hardening in Stage IV of f.c.c. and diamond cubic crystals. The mechanism is related to a cellular dislocation microstructure in which during Stage III, hardening by dislocation accumulation and recovery by various mechanisms occurs primarily in the cell walls. Hardening of the cells is through the build-up of long range misfit stresses that result when the primary dislocation flux cuts trough the geometrically required dislocation density of the cell walls that is associated with the lattice misorientations between cells. Experiments show that these misorientations increase monotonically with increasing strain. There is no recovery in the cells. At the end of Stage III, hardening in the cell walls saturates, but the hardening due to misfit stresses in the cells continues unabated, giving rise to the rate independent hardening of Stage IV. Eventually this hardening is also terminated in Stage V when the misfit stresses inside cells reach a critical level that triggers rate dependent stress relaxation in the cells by secondary glide processes. The new mechanism makes successful predictions for Stage IV processes, including: hardening rate, plastic resistance levels, the gradual increase in hardening rate with plastic resistance, the residual lattice strains on unloading that can be measured with X-ray peak distortions and broadening, and for the Baushinger effect.

Plastic flow of crystals

Abstract: Publisher Summary This chapter provides an overview of plastic flow of crystals. Plastic flow under multiple slip, particularly in metallic crystals, has been widely studied by materials scientists and applied mechanicians since the early 1900s. Finite strain crystal plasticity is a rigorous nonlinear continuum theory. With few exceptions, subtleties of this theory such as multislip hardening and non-Schmid effects have not been tested sufficiently in critical experiments. In particular, through the interplay between small secondary slips and hardening, there is now compelling evidence that important effects largely have been ignored. Furthermore, because intermetallic compounds are currently of growing interest in high-temperature applications, non-Schmid effects, which are also important in BCC metals and alloys, should also be considered more seriously. In this chapter, recent studies of multiple-slip interactions and hardening are brought together within a time-independent theory, and their influence on strain localization is explored. It discusses in detail about yield behavior including non-Schmid effects. Flow behavior including non-Schmid effects is elaborated. Concepts of hardening behavior and strain localization are also explained in the chapter.

Solid-solution hardening

Abstract: Recent advances in the understanding of solid-solution hardening (SSH) of crystalline materials, as well as some basic early papers are briefly reviewed. This survey shows that models of SSH based on the concept of a frictional drag on dislocations migrating through fields of point-like obstacles, whether randomly dispersed or clustered, do not encompass the principal features of SSH, e.g. the temperature dependence of the yield stress, the stress and temperature dependences of the activation volume, and the phenomenon of stress equivalence. However, a model based on the nucleation of slip, involving the breakaway of dislocation segments from several pinning points, formulated in closed form, is shown to account satisfactorily for the principal observations.

A Review of the Stages of Work Hardening

Abstract: Stages of work hardening are reviewed with emphasis on links between each stage. Simple quantitative descriptions are given for each stage. Similarities between stage I, easy glide, and stage IV, large strain hardening, are pointed out both in terms of magnitude of the hardening rate and of the underlying mechanism of dislocation debris accumulation. Stage II is described as an athermal hardening stage that occurs when statistical variations in the dislocation ``forest`` lead to geometrical storage of dislocations. The steadily decreasing hardening rate observed in stage III is characterized by the increasing rate of loss of dislocation density due to dynamic recovery. Stage III appears to have an asymptote to a saturation stress which is determined by the characteristics of the dislocation tangles, or cell walls. The imperfect nature of the dynamic recovery process, however, leads to the accumulation of dislocation debris and this, by analogy with stage 1, causes the apparent saturation stress to rise, thus causing stage IV.

Phase chemistry and precipitation reactions in maraging steels: Part I. Introduction and study of Co–containing C–300 steel

Abstract: This article introduces a series of studies of phase transformations in maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Hardness measurements, transmission electron microscopy (TEM), and thermochemical calculations were also used. The composition and morphology of precipitates in the commercial-grade C-300 steel were compared for different aging times at 510 °C to investigate the aging sequence. Both Ni3Ti and Fe7Mo6 were found to contribute to age hardening. The decomposition starts with the formation of small Mo-enriched Ni3Ti particles at very short aging times. The Fe7Mo6 phase forms at a later stage of aging. The matrix concentrations of both Ti and Mo were measured and were found to be low after standard aging conditions. The observation of the Fe7Mo6 μ phase is supported by thermochemical calculations. Austenite reversion has been found at the aging temperature, and its composition approaches the predicted equilibrium composition after 8 hours of aging.

Plastic deformatioon in AlCuFe icosahedral phase

Abstract: Plastic deformation of icosahedral AlCuFe has been studied by compression tests for temperatures ranging from room temperature to 750°C. Whereas at room temperature no ducibility is observed, the samples can be homogeneously deformed up to 130% at high temperatures with no final hardening stage. SEM observations reveal, at the ultimate stage of deformation, a vein pattern similar to the observed in amorphous metals but without slip bands or shear bands. Stress relaxation measurements shows to different regimes at low and high temperatures with a transition at 660°C. It is concluded from these experiments that dislocations are probably not responsible for the plastic deformation of icosahedral quasicrystals.

Computational modeling of metal matrix composite materials. I: Isothermal deformation patterns in ideal microstructures

Abstract: The mechanical behavior of particulate reinforced metal composites, in particular an SiC reinforced Al-3 wt% Cu model system, was analyzed numerically. The Computational micromechanics approach was taken, i.e. a detailed representation of microstructure in which the material was characterized by a finite deformation, thermo-elastic-viscoplastic crystallographic theory. Individual matrix grains and reinforcing particles were represented, in the context of two dimenssional repeating unit cell models. The performance of the microstructure under variation in microstructural parameters such as (1) reinforcement volume fraction, (2) morphology and (3) matrix strain hardening properties was investigated, as was the effect of change in loading state. In this, the first in a series of four articles, the isothermal microstructural deformation behavior is examined in detail. Localization of plastic deformation is seen to be a natural part of the deformation process and evolves according to patterns, which develop from the onset of yield and are determined for the most part by the positions of the reinforcing particles. This is in contrast to the microscale behavior of single phase polycrystals where deformation patterns only emerge at larger overall strains. Localization intensity depends strongly on reinforcement volume fraction and morphology and less significantly on matrix hardening properties. Results for tensile and compressive loading histories are compared showing differences that depend on particle position and finite geometry changes during deformation.

Higher-order analysis of crack tip fields in elastic power-law hardening materials

Abstract: A HIGHER-ORDER asymptotic analysis of a stationary crack in an elastic power-law hardening material has been carried out for plane strain, Mode 1. The extent to which elasticity affects the near-tip fields is determined by the strain hardening exponent n. Five terms in the asymptotic series for the stresses have been derived for n = 3. However, only three amplitudes can be independently prescribed. These are K1, K2 and K5 corresponding to amplitudes of the first-, second- and fifth-order terms. Four terms in the asymptotic series have been obtained for n = 5, 7 and 10; in these cases, the independent amplitudes are K1, K2 and K4. It is found that appropriate choices of K2 and K4 can reproduce near-tip fields representative of a broad range of crack tip constraints in moderate and low hardening materials. Indeed, fields characterized by distinctly different stress triaxiality levels (established by finite element analysis) have been matched by the asymptotic series. The zone of dominance of the asymptotic series extends over distances of about 10 crack openings ahead of the crack tip encompassing length scales that are microstructurally significant. Furthermore, the higher-order terms collectively describe a spatially uniform hydrostatic stress field (of adjustable magnitude) ahead of the crack. Our results lend support to a suggestion that J and a measure of near-tip stress triaxiality can describe the full range of near-tip states.

Complete theoretical analysis for higher order asymptotic terms and the HRR zone at a crack tip for Mode I and Mode II loading of a hardening material

Abstract: A complete development for the first two terms of the crack tip fields for both Mode I and Mode II loading of a hardening material in either plane stress or plane strain is performed, including the elastic deformation in the analysis. It is shown that the determination of the order of the second term depends on bothn and whether plane stress or plane strain is considered. In addition, regions of HRR dominance at a crack tip for the field variables are estimated. Comparison of the analytic predictions with finite element results indicates that the analytic results for the zone of HRR dominance are in agreement with numerical predictions.

Concrete in Hot Environments

Abstract: SOMMAIRE : 1.Portland Cement. - 2.Setting and Hardening. - 3.Mineral admixtures and Blended Cements. - 4.Workability. - 5.Early volume changes and cracking. - 6.Concrete strength. - 7.Drying shrinkage. - 8.Creep. - 9.Durability of concrete. - 10.Corrosion of reinforcement.

Plasticity-Damage Model for Concrete Under Cyclic Multiaxial Loading

Abstract: A model that combines plasticity and damage mechanics is developed to assess both multiaxial monotonic and cyclic behavior of concrete. The model adopts a bounding surface concept and combines plastic deformation with the deformation due to damage. Plastic strain components are calculated by using the plastic modulus that is a function of the distance from the current stress point to the bounding surface along the deviatoric stress direction Sij. Similarly, damage growth rate is obtained by the hardening modulus, which is a function of the distance just defined. The hardening behavior of concrete is assumed herein to be controlled by both damage and plasticity, while the strain‐softening regime is controlled by damage processes only. The simultaneous use of the plasticity surface and the damage surface, leads to a constitutive model that accounts for the essential features of concrete such as pressure sensitivity, shear compaction‐dilatancy, stiffness degradation, and softening behavior.

A Monte Carlo study of the influence of dynamic recovery on dynamic recrystallization

Abstract: A Monte Carlo simulation technique has been used to study dynamic recrystallization in a polycrystalline matrix. The model employed, maps the microstructure onto a discrete, two dimensional lattice with use of a magnetic analog, the Q-state Potts model. The response of the material to hot deformation is simulated by adding recrystallization nuclei and stored energy continuously with time. The simulations presented in this work use an energy storage rate procedure that models stage III hardening in metals and thereby incorporates both work hardening and dynamic recovery. This model reproduces many features found recently by Rollett et al. in a more simple model that did not include an explicit dynamic softening mechanism. It is found, that the type of work hardening law ssumed results in relationships between transient and steady state crystallization parameters which more closely approach those observed in physical experiments. In this view, the roles of the controlling mechanisms of dynamic recrystallization in real polycrystalline materials are discussed.

On the mechanism of bake-hardening.

Abstract: The bake-hardening behaviour of a deep drawing steel sheet was investigated. Aging experiments after different prestrains were carried out in the temperature range from 50 to 180 °C. The change of mechanical properties, especially the increase in yield stress, was measured by tensile tests. It was found that the increase in the yield stress by simulated baking treatment occurs in two successive steps. It is assumed that the first rise is based on the Cottrell-effect and the further strengthening in the second step is caused by the precipitation of coherent carbides. A kinetic interpolation-model of the bake-hardening effect is introduced and this allows a description of the experimental data and a calculation of the bake-hardening behaviour

Metallurgical study of low-temperature plasma carbon diffusion treatments for stainless steels

Abstract: We recently reported a novel low-temperature carbon diffusion technique forsurface hardening of stainless steels. The treatment was shown to provide benefits in terms of abrasive wear resistance. There is also evidence to suggest that by performing diffusion treatments at low temperatures ( i.e. below 400°C), these benefits can be achieved without compromising corrosion resistance. Here a variety of surface analysis and depth profiling techniques have been used to determine the physical and mechanical properties of carbon-rich layers produced on a range of stainless steel substrate materials. X-ray diffraction (XRD) was employed to determine the crystallographic structure, whilst wavelength dispersive X-ray analysis (WDX) and glow discharge optical spectroscopy (GDOS) gave information on the concentration and distribution of the diffused species within the treated layers. A variety of carbide-based structures was detected, including the expected M 23 C 6 and, more surprisingly, M 3 C. Optical and electron microscopy techniques were used to provide information on layer morphology. The surfaces produced by the low-temperature carbon-diffusion process generally exhibit a distinct diffusion layer of between 1 and 20 μm, depending on the material and the treatment conditions. Austenitic stainless steels appear to give the best response to treatment, however other types of stainless steel can be treated, particularly if the microstructure contains above 5% retained austenite. Here we discuss the changes in mechanical and metallurgical properties provided by this technique and its potential value for treatment of both austenitic and other stainless steel substrate materials.

Effect of grain size and annealing texture on the cyclic response and the substructure evolution of polycrystalline copper

Abstract: In attempting to interpret the mechanical response of polycrystalline copper, for which the results in the literature show marked scatter, the effects of microstructure on the cyclic behavior and the substructure evolution of copper polycrystals have been investigated. The microstructure is described by a complex factor—grain size and texture combined. It is found that there is a very significant effect of microstructure in the cyclic response of copper at low and intermediate strain amplitudes, where dislocation structures which localize deformation are expected to be present. In general, the cyclic response of coarse-grained copper shows a much more pronounced cyclic hardening and higher saturation stresses than those for fine-grained copper. This behavior is associated with a well defined hard 〈111〉−〈100〉 fiber texture, inherited in the coarse-grained material after annealing at relatively high temperatures. The multiple slip associated with the 〈111〉−〈001〉 oriented grains homogenizes the deformation very early, resulting in strong cyclic hardening, and a faster substructure evolution into cell structure.

The influence of Cr addition on the ordered microstructure and deformation and fracture behaviour of a Fe28%Al intermetallic

Abstract: Samples of Fe28%Al and Fe28%Al4% Cr were recrystallized, and heat treated at 500°C to obtain B2 and DO3 ordering. While the material containing Cr was well ordered, and other material showed a two-phase ordered plus disordered microstructure with the degree of ordering, the domain size and the fraction of disordered phase depending sensitively on the experimental conditions. The material containing Cr was relatively soft, showed low work hardening and was fairly ductile: these properties are obtained by the ready movement of superdislocations. The other material was stronger and generally less ductile: these properties depended on whether single dislocationswere present leaving APB trails and leading to low work hardening rates, or whether looseky-coupled superdislocations moved through a two-phase microstructure at a lower stress and with higher work hardening rates. This study emphasizes the need for complete microstructural characterization in order to understand the mechanical behaviour of such materials.