Showing papers on "Hardening (metallurgy) published in 1977"

Finite deformation analysis of crack-tip opening in elastic-plastic materials and implications for fracture

Abstract: A nalyses of the stress and strain fields around smoothly-blunting crack tips in both non-hardening and hardening elastic-plastic materials, under contained plane-strain yielding and subject to mode I opening loads, have been carried out by use of a finite element method suitably formulated to admit large geometry changes. The results include the crack-tip shape and near-tip deformation field, and the crack-tip opening displacement has been related to a parameter of the applied load, the J -integral. The hydrostatic stresses near the crack tip are limited due to the lack of constraint on the blunted tip, limiting achievable stress levels except in a very small region around the crack tip in power-law hardening materials. The J -integral is found to be path-independent except very close to the crack tip in the region affected by the blunted tip. Models for fracture are discussed in the light of these results including one based on the growth of voids. The rate of void-growth near the tip in hardening materials seems to be little different from the rate in non-hardening ones when measured in terms of crack-tip opening displacement, which leads to a prediction of higher toughness in hardening materials. It is suggested that improvement of this model would follow from better understanding of void-void and void-crack coalescence and void nucleation, and some criteria and models for these effects are discussed. The implications of the finite element results for fracture criteria based on critical stress or strain, or both, is discussed with respect to transition of fracture mode and the angle of initial crack-growth. Localization of flow is discussed as a possible fracture model and as a model for void-crack coalescence.

Plastic deformation and failure in granular media

Abstract: Pre-failure plastic deformation is modelled as a process controlled by simultaneous plane strain hardening and plane strain softening associated with no-extension directions and maximum stress obliquity planes. The equality of rates of strain hardening and softening defines failure. Principal axes of stress and strain increment may or may not coincide depending on the constraints on a deforming soil element. The model defines the orientations of thin rupture layers which may form at failure and suggests the possible form of a hardening law and yield surface. Data in agreement with the model are presented from seven different shear apparatuses and a range of plane strain model tests. The shear apparatuses include those with entirely rigid boundaries, a mixture of rigid and flexible boundaries, and entirely flexible boundaries. The full variation of the intermediate principal stress is covered. La deformation plastique avant rupture est definie comme un processus regi simultanement par un durcissement de de...

Low Cycle Fatigue Behavior of Inconel 718 at 298 K and 823 K

Abstract: The cyclic stress-strain response and the low cycle fatigue life of conventionally heat treated Inconel 718 were studied. Fully reversed strain-controlled tests were performed at room temperature and at 823 K. Optical and electron microscopy were used to study the development of deformation and cracking during cycling. A power-law relationship between life-time and plastic strain amplitude was obtained. A substantial decrease in fatigue life occurred as the temperature was increased from 298 to 823 K and as the cycling frequency was lowered from 3 cyclesJmin to 0.3 cyclesJmin at 823 K. At 298 K, for all the strain amplitudes investigated, an initial rapid hardening was followed by softening, while at 823 K only softening occurred. Electron microscopy showed that the precipitates were sheared in the course of cyclic straining and that plastic deformation proceeded by the propagation of planar bands. These bands were identified as twins. Twinning was found to be more abundant at elevated temperatures than at room temperature, especially at lower frequencies. Cracking was generally initiated along the interfaces between these twin bands and the matrix but, at elevated temperatures and low strain rates, intercrystalline cracking took place, as well. The influence of particles shearing and twinning on the cyclic stress-strain response of the material are discussed. The importance of planar deformation and twinning on intergranular cracking is emphasized.

The influence of nitrogen and grain size on yield strength in Type AISI 316L austenitic stainless steel

Abstract: Effects of nitrogen additions (0.05–0.18%) as well as of grain size on the tensile yield strength (0.2% proof stress) in annealed conditions were investigated for an austenitic stainless steel Type AISI 316L in the temperature range 20–600°C. It was found that the total strengthening contribution from N is composed of two parts, one grain-size independent and the other strongly grain-size dependent. The former is markedly thermal in this temperature interval and should be attributed to a true solid-solution hardening. The latter appears as a pronounced a thermal increase in the grain-size coefficient in the Petch-Hall relation with increasing N content. Thus, in the temperature range 20–600°C, the total strengthening effect from a given addition of N is grain-size dependent, and in fact increases strongly with decreasing grain size. An important consequence of this remarkable behaviour of N is that a comparatively high yield strength, even at elevated temperatures, is achievable through a combinat...

A numerical analysis of the tensile test for sheet metals

Abstract: The strain hardening, strain-rate hardening, and plastic anisotropy properties of metal sheets are normally determined in a tensile test during the nearly uniform deformation prior to the maximum load. Beyond this point, strain nonuniformity leading to a neck is poorly understood in terms of interaction of these material properties with changes in strain-rate and stress-state within the neck, and the resulting load-extension plot. Satisfactory modeling of this problem has been achieved by using a rigid/plastic constitutive law including strain hardening and strain-rate hardening. Progressive cessation of deformation starting from elements in the specimen fillet region toward the center is demonstrated. This effect is shown to generate a strain peak (neck) at the gage length center. The predicted load-extension plots and strain distributions in the neck agree well with experiments conducted on a number of test materials. This work provides a quantitative measure of the influence of various material parameters on tensile ductility and identifies the proper constitutive law for input into mathematical models of more complex forming operations.

Finite-Element Analysis of Crack Growth Under Monotonic and Cyclic Loading

Abstract: An elastic-plastic (incremental) finite-element analysis, in conjunction with a crack-growth criterion, was used to study crack-growth behavior under monotonic and cyclic loading. The crack-growth criterion was based on crack-tip strain. Whenever the crack-tip strain equals or exceeds a critical strain value, the crack grows. The effects of element-mesh size, critical strain, strain hardening, and specimen type (tension or bending) on crack growth under monotonic loading were investigated. Crack growth under cyclic loading (constant amplitude and simple variable amplitude) were also studied. A combined hardening theory, which incorporates features of both isotropic and kinematic hardening under cyclic loading, was also developed for smooth yield surfaces and was used in the analysis.

A Simple Mathematical Theory of Finite Distortional Latent Hardening in Single Crystals

Abstract: A simple (one-parameter) hardening law is proposed which accounts for the perpetuation of finite single slip, beyond the symmetry line, in the tensile test of f.c.c. crystals and reduces to Taylor's rule at infinitesimal strain. This new law emerges as the simplest case of a general mathematical theory of finite deformation of elastic-plastic crystals. The fully anisotropic finite-distortional hardening of latent slip systems predicted by the simple theory is in qualitative agreement with experiment.

Effects of heat treatment on structure and properties of 12%Cr steels

Abstract: Studies have been made of the effects ofaustenitizing and tempering heat treatments on the microstructures and mechanical properties of three 12% Cr steels. The effects of heat treatment on the constitution are shown to be in agreement with previously published data. In addition, the grain-refining actions of niobium carbonitride particles and delta ferrite have been demonstrated. Investigations of the tempering characteristics of the steels have shown that molybdenum, vanadium, and niobium accentuate secondary hardening and retard overaging primarily by inhibiting the annealing out of dislocations. Detailed descriptions of the tempering sequences have been given. Structure-property relationships have been investigated and the prior austenite grain size has been related to the Charpy impact transition temperature. Furthermore, the separate contribution of the strength of the material to the impact transition temperature has been established, together with the effect of the strength on the impact ...

Yield Strength and Dislocation Mobility of KCl?KBr Solid Solution Single Crystals

Abstract: The concentration and strain rate dependence of the critical shear stress in KCl–KBr solid solutions was measured by compression tests. These data were compared with the stress dependence of the dislocation velocity measured by an etch pit technique. Edge dislocation moved slower than screw dislocation at the same stress level. The critical stress was determined by the edge dislocation mobility, and the mobile dislocation density at yielding was about 1.2×1011 m-2 for the three kinds of composition tested. An analytical model of the hardening mechanism is proposed on the basis of the elastic interaction (size misfit) between an edge dislocation and solute atoms. When compared with experimental results, this model gives satisfactory results not only for KCl–KBr but also for NaCl–NaBr, and NaBr–KBr solid solutions.

The cyclic stress-strain response of polycrystalline, pseudoelastic Cu-14.5 wt pct Al-3 wt pct Ni alloy

Abstract: Limited results on the fatigue of pseudo-elastic material indicate that, as a class, these materials should have truly outstanding fatigue properties. To check this, the mechanisms of cyclic deformation and fracture have been studied in Cu−Al−Ni chosen because its transformation behavior is well understood. Since this alloy is notoriously brittle, pulsating compression fatigue tests were carried out in polycrystalline material. The details of the stress-induced martensite behavior were studied byin situ video observations. The alloy was found to undergo cyclic hardening and failure eventually occurred by multiple nucleation of cracks at grain boundaries, by a mechanism similar in principle to that which occurs in regular metals cycled at high plastic strains. The Coffin-Manson law was obeyed.

Correlation of yield strength with internal coherency strains for age-hardened Cu−Ni−Fe alloys

Abstract: The maximum yield strengths for a given aging temperature were measured for age-hardened Cu−Ni−Fe alloys. The yield strengths were found to be proportional to the difference in cubic lattice parameters of unstressed precipitating phases and independent of other factors such as precipitate particle size and precipitate volume fraction. The yield strength dependence on lattice parameter differences alone indicated coherency stresses controlled the yield strengths. An analysis of the yield strength based only on internal coherency strains and stresses subsequently led to the derivation of an equation for the yield strength,i.e., $${_{}^{} }$$ where $$m$$ is the Taylor factor for converting from single crystal shear stress to polycrystalline tensile stress results,C ijare single crystal elastic stiffness constants and Δa is the difference in, anda 0 the average of the cubic lattice parameters of the precipitating phases. The equation indicates the yield strength is dependent only on the internal coherency strains and independent of particle size and precipitate volume fraction, as observed. The correlation of the experimentally measured yield strengths with the equation was quite good.

Mechanical Properties of Solid Solutions

Abstract: Dislocation theory is used to predict the yield stress of a single-phase alloy from the characteristics of its solute atoms and its microstructure. For fcc dilute solutions, the hardening laws of Fleischer-Friedel and of Labusch are reviewed. Particular attention is paid to the problems of superposition of different hardening mechanisms and of the effects of multiple solute additions. The temperature-independent “plateau” of the yield stress is explained as a dynamic phenomenon. Bcc metals solution harden at intermediate temperatures, most likely by the influence of solute atoms on kink motion. The solution softening observed at low temperatures in this structure is less clearly understood. For polycrystals, a new theory by Suzuki for the Hall-Petch relation is discussed. Work hardening of solid solutions does not differ much from that of the pure metals except for stage III and beyond, where dynamic recovery by cross slip (and by climb) is influenced by stacking-fault energy (SFE) and thus by solute concentration. The SFE is also one of the parameters determining the distribution of slip on various planes and this, in turn, largely influences the fatigue resistance of solid solutions.

Multi-component intrinsic solid solution softening and hardening

Abstract: A formal treatment of multi-component intrinsic solid solution softening and hardening is presented. The proposed equivalence model agrees with extensive literature data on ternary solid solution softening and hardening in molybdenum at different temperatures. This agreement lends further support to the hypothesis that solid solution softening in Group VIA metals is an intrinsic phenomenon dominated by the electronic structure.

Low expansion superalloy

Abstract: of the Disclosure Nickel-iron and nickel-iron-cobalt alloys contain chromium and gamma-prime hardening elements in proportions balanced according to special compositional relationships providing desired thermal expansion, inflection temperature, strength and ductility characteristics, particularly including notch strength needed in machinery and structures subjected in use to varying temperatures and thermal gradients where operating temperatures become elevated above 500°F

Hardening of Gold-Based Alloys

Abstract: The relationships between composition, heat treatment, hardness, and microstructure have been investigated for a number of commercial dental casting alloys and for several series of experimental gold-silver-copper compositions. The hardnesses and typical microstructures found are reported. The results indicate that at least two distinct hardening mechanisms must occur in these alloys. The ternary alloys outside the two-phasr region of the gold-silver-copper system harden by ordering. The commercial alloys and those within the two-phase region may harden in part by ordering, but some other mechanism in required to explain the observed hardening. This mechanism is not due to the observed grain boundary precipitates. It may be associated with the intragranular needlelike structures seen, but reasons exist to question the sufficiency of that structure as an explantation of the hardening.