Showing papers on "Hardening (metallurgy) published in 1990"

A process model for age hardening of aluminium alloys—I. The model

Abstract: Process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys. A model for the isothermal ageing curve is developed. This is demonstrated for a number of alloys and the success of the approach is assessed. Applications and a new diagram, showing the variation of strength with temperature and time, are described in an accompanying paper.

An experimental and anaiytical investigation of the large strain compressive and tensile response of glassy polymers

Abstract: In this investigation, the plastic flow of polycarbonate (PC) was examined by obtaining true stress-strain data over a range of strain rates at room temperature through homogeneous, uniaxial, constant strain rate compression testing to strains as high as 125 percent. Uniaxial compressive loading conditions give rise to a planar molecular orientation process which results in the observed strain hardening in compression. Uniaxial tensile tests on PC were also conducted. The necked region of the tensile specimen is being cold drawn resulting in a uniaxial state of orientation. Therefore, the observed macroscopic strain hardening in uniaxial tension distinctly differs from that obtained In uniaxial compression, giving different stress-strain curves. The major differences experimentally obtained between the large strain response in compression and tension indicate a need for an orientation-based model of the strain hardening process. The experimental program also acts to uncouple the effects of strain softening and strain rate providing more accurate data for future modeling of the true strain softening process. A constitutive law which directly relates the strain hardening response to the state of molecular network stretch in the polymer is used to model and analyze the experiments. The model is found to simulate the observed rate dependent yield and post yield strain softening and hardening of the compressive data over the entire range of strain rates very well. The model is then utilized in a finite element analysis of the tensile tests on PC. Numerical results compared favorably with the experimental data including: load vs, contraction curves, natural draw ratio, and the axial stress-strain response of the cold drawing region.

Thermoplasticity of Saturated Soils and Shales: Constitutive Equations

Abstract: Plastic behavior of soils and shales due to heating and loading under constant elevated temperature is discussed in terms of a thermoplastic version of the critical state model. Rules for dependence of the yield surface on temperature in the elastic states and at yielding are proposed. The elastic domain is assumed to shrink during heating (thermal softening) and to expand during cooling, when the stress state is elastic. In a plastic state thermal softening occurs simultaneously with the plastic strain hardening. At a constant stress state, thermal softening may entirely be compensated by plastic strain hardening leading to thermal consolidation. Loading and unloading criteria are given to determine whether the soil response is thermoelastic or thermoplastic. As opposed to isothermal plasticity, stress rate excursions inside the current yield surface are admissable plastic processes, when temperature grows, even if strain hardening occurs. Also, outside stress rate excursions at the softening side may generate plastic strain, when cooling occurs. Thermally induced plastic strain rate non-associativity is discussed as well. Direct and inverse incremental strain-stress-temperature relationships are formulated. An analysis of the experimental results of thermomechanical testing of saturated clays is given in a companion paper.

Description of tantalum deformation behavior by dislocation mechanics based constitutive relations

Abstract: Dislocation mechanics based constitutive equation constants are determined for temperature, strain rate, work hardening, and polycrystal grain size influences on the deformation behavior of various tantalum materials. An analysis of the maximum load point strain provides a useful method of determining the work hardening constants. Different athermal stress constants and thermal activation related constants are obtained for certain groupings of the different tantalum materials. The variations are correlated with the annealing history of the materials and related to dislocation model parameters involved in the thermal activation strain rate analysis. Computed tantalum deformation results based on these constants are shown to agree with Gourdin’s reported expanding ring test measurements and with the deformed shape of a Taylor cylinder impact test specimen.

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

Abstract: The mechanical properties of multilayered Cu‐Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force‐displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch‐type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.

Mechanical properties and microstructural characterization of Al-0.5%Cu thin films

Abstract: Using a wafer curvature technique we have studied the variation of stress with tem-perature in Al-0.5%Cu thin films deposited on oxidized silicon wafers. Concurrently, the microstructural changes in the films induced by the thermal cycling inherent to this technique were studied with in-situ transmission electron microscopy heating experi-ments. On heating an as-sputtered film a stress drop occurs, corresponding to the onset of grain growth. The in-situ TEM experiments indicate that the extent of grain growth is significantly altered by the presence of compressive stresses in the film. During cool-ing, dislocation loops nucleate on {111} planes inclined to the film surface, although the grain size plays an important role in determining the extent to which this mechanism can account for the deformation. A native oxide can influence the stress levels in the film by pinning one end of the dislocation loops. Upon cooling below 200° C a rapid increase in stress occurs. Although this increase has been attributed to hardening due to the precipitation of excess copper, no evidence of precipitate-dislocation interactions were observed.

Electroplasticity—the effect of electricity on the mechanical properties of metals

Abstract: Investigations of the effects of an electric current on dislocation mobility and mechanical properties at low homologous temperatures (T < 0.5Tm) reveal a polarity effect and yield an electron wind force in some agreement with theory. An external directcurrent electric field has been reported to influence the creep rate of unalloyed metals at high homologous temperatures. During superplastic deformation of the 7475 Al alloy, such a field has been found to decrease the flow stress, reduce strain hardening, increase strain-rate hardening, reduce grain boundary cavitation and reduce grain growth. The effects of the field were polarity dependent and extended to the center of 1–2 mm thick specimens. No significant effect of the field on the flow stress occurred at low homologous temperatures. This suggests that the field influences atomic mobility through vacancy generation and/or migration. The occurrence of an uneven electron density at the interfaces between phases and at grain boundaries has been proposed as a factor, but this idea needs further consideration.

Plane-Strain Crack-Tip Fields for Pressure-Sensitive Dilatant Materials

Abstract: Abstraet--In this paper we present plane-stress crack-tip stress and strain fields for pressure-sensitive dilatant materials. A hydrostatic stress-dependent yield criterion and the normality flow rule are used to account for pressure-sensitive yielding and plastic dilatancy. The material hardening response is specified by a power-law relation. The plane-stress mode I singular fields are found in a separable form similar to the HRR fields (Hutchinson, J. Mech. Phys. Solids 16, 13-31 and 337-347, 1968; Rice and Rosengren, J. Mech. Phys. Solids 16, l-12, 1968). ‘Ihe angular variations of the fields depend on the material hardening exponent and the pressure sensitivity parameter. Our low-hardening solutions for d&rent degrees of pressure sensitivity agree well with the corresponding perfectly plastic solutions. An important aspect of the eRecta of pressure.-sensitive yielding and plastic dilatancy on crack-tip fields is the lowering of the opening stress and the hydrostatic stress directly ahead of the crack tip. This effect, similar to that under plane-strain conditions (Li and Pan, to appear in J. Appl. Mech. 1989), has implications in the material toughening observed in some ceramic and polymeric composites.

On the Calculations of the Stored Energy of Cold Work

Abstract: When a metal deforms plastically, most of the mechanical energy expended in the deformation process is converted into heat and the remainder is stored in the material. A method for the calculation of the stored energy from an experimentally determined load-displacement curve of an elastic-plastic structure is presented. The method is applied to the problem of simple tension of a polycrystalline metal and a simple technique for the calculation of the stored energy from the uniaxial stress-strain curve is presented. When a metal deforms plastically, most of the mechanical energy expended in the deformation process is converted into heat and the remainder is stored in the material. The stored energy is associated with residual stresses generated in the metal after unloading as well as with the creation of lattice imper­ fections. A summary of the experimental and theoretical de­ velopments concerning the stored energy of cold work has been given in two review articles by Titchener and Bever (1958) and Bever et al. (1973). The reported values of the ratio of the stored to the expended energy range from less than 1 to about 90 percent, with the great majority lying under 15 percent. A method for the calculation of the stored energy from an experimentally determined load-displacement curve of an elas­ tic-plastic structure is developed in this paper. The effects of hardening and initial stresses are discussed and several ex­ amples are presented. The method is applied to the problem of simple tension of a polycrystalline metal and a simple tech­ nique for the calculation of the stored energy from the uniaxial stress-strain curve of the metal is developed. The results in­ dicate that the stored energy increases with increasing hard­ ening capacity of the metal and that strain hardening and energy storage cease at the same stage of the deformation process. The paper closes with a comparison of the theoretical predictions with experimental observations. Standard notation is used throughout. Boldface symbols denote tensors, the order of which is indicated by the context. Double dots are used to indicate the following products

The Cyclic Stress-Strain Properties, Hysteresis Loop Shape, and Kinematic Hardening of Two High-Strength Bearing Steels

Abstract: Measurements of the shapes of the cyclic, stress-strain hysteresis loops obtained from AISI 1070 (HRC 60) and AISI 52100 (HRC 62) steels subjected to constant stress and constant plastic strain amplitude cycles in torsion are presented. The study examines plastic strain amplitudes in the range of 0.0002 ≤ Δep/2≤ 0.0015, which are similar to the strain amplitudes produced by rolling contact. The effects of a mean stress are also evaluated. The cyclic hardening of the two steels and other changes in the character of the loops during the cyclic life, 34 ≤N f ≤ 2156, are defined. A three-parameter, bilinear, elastic-linear-kinematic-hardening-plastic (ELKP) model is shown to describe the multivalued cyclic stress-strain relations of these steels. The principal material properties of the model, in addition to the elastic modulus, the kinematic yield strength, and the plastic modulus, are evaluated. The ELKP properties define the material’s resistance to cyclic plasticity, the loop shape and area (plastic energy dissipation), the conventional cyclic stress-strain curve, the endurance limit, and the rolling contact shakedown pressure. The implications for rolling contact are discussed.

Low-temperature and high-strain-rate deformation of nickel and nickel-carbon alloys and analysis of the constitutive behavior according to an internal state variable model

Abstract: Analysis of the strain-rate sensitivity of the yield stress and the strain-rate sensitivity of strain hardening in annealed and quasi-statically and dynamically prestrained Ni270, Ni-510 wt ppm (0.25 at.%) C, and Ni-1900 wt ppm (0.92 at.%) C is presented. Measurements in compression are analyzed according to an internal state variable model where for a given obstacle type the state variable represents the stress required for deformation at 0 K. State variables are defined for dislocations interacting with interstitial carbon atoms and for dislocations interacting with other dislocations. The application of a power law to combine the contributions of separate obstacles is investigated. Evolution of the mechanical threshold stress characterizing dislocation/dislocation interactions is analyzed using a modified Voce equation. The Stage II hardening rate is shown to be insensitive to the carbon concentration, while the hardening at large strains is strongly dependent on the carbon concentration. Dynamic strain aging is found to influence both the temperature and strain-rate dependent yield stress and the hardening rate in the nickel-carbon alloys. Deformation microstructures are characterized using transmission electron microscopy, and the measured dislocation cell size is correlated with the internal state variable characterizing dislocation/dislocation interactions.

Application of the Hall‐Petch Relation to Microhardness Measurements on Al, Cu, Al‐MD 105, and Al‐Cu Alloys

Abstract: The hardness of materials, H, is dependent on grain diameter, d, in a similar way as the flow stress in the Hall-Petch relation: H = Ho + KHd−1/2, where, Ho and KH are constants. The microhardness of 2S-Al (99.5% Al), pure Cu, Al-MD 105 (Al-1% Al2O3), and Duralumin (Al-3.94% Cu) is found to vary with grain size according to the Hall-Petch equation with reasonable accuracy. The grain boundary hardening KH of Al-MD 105 is found to be the highest although this material recrystallizes to larger grain sizes than those for the other materials; this is attributed to the resistance of boundaries to deformation arising mainly from the presence of hard, second phase alumina particles. However, the contribution of solution, precipitation, and dispersion hardening may be added to grain boundary hardening according to Hansen and Lilholt.

The interaction of cyclic hardening and ratchetting for AISI type 304 stainless steel at room temperature—I. Experiments

Abstract: Uniaxial zero-to-tension load controlled tests at stress rates differing by three orders of magnitude were performed on annealed AISI Type 304 stainless steel. The maximum stress was determined either as the stress reached in displacement control at one per cent strain (History I), or as the stress reached after a 1050 s relaxation test at one per cent strain (History II). The tests of History III were identical to those of History I, except that the specimens were subjected to cyclic hardening prior to the start of the ratchetting test. The ratchet strain accumulations in Histories I and II are significant and depend on stress rate. A stress rate decrease increases the accumulated ratchet strain. The final ratchet strain accumulation is independent of stress rate sequence. In History II, no ratchet strain was found at any stress rate. If the steel had behaved in a rate-independent fashion, no ratchet strain accumulation should have been found in any of the tests. They were performed with an MTS servohydraulic, computer controlled testing machine, a clip on-extensometer and digitized data acquisition.

Laser shock‐induced mechanical and microstructural modification of welded maraging steel

Abstract: The effect of laser‐induced high‐intensity stress waves on the hardness, fatigue resistance, and microstructure in the heat affected zone of welded 18 Ni(250) maraging steel was investigated. Laser‐shock processing increased the hardness and fatigue strength of the weldments. Some melting of the surface was involved during laser‐shock hardening which produced the reverted austenite phase. Microscopic analyses showed an increased dislocation density in the laser‐shocked area.

Aging kinetics of CF3 cast stainless steel in temperature range 300–400°C

Abstract: Castings to the ASME SA351 CF3 specification used in modern pressurised water reactor nuclear power stations are mainly austenitic, but contain up to 20% ferrite, and as a result are subject to loss of toughness in service at about 300°C. To show adequate end of life toughness, tests are being carried out on material with accelerated aging above 300°C. Data on the kinetics of embrittlement are required, and it must also be shown that the significant metallurgical changes are the same at both temperatures. The kinetics has been investigated using Charpy impact specimens aged at 300, 350, and 400°C and it has been related to the hardening of the ferrite and to the microstructural changes in this phase. The activation energies determined for the embrittlement, for the hardening of the ferrite, and for the underlying spinodal reaction in the ferrite are consistent with that for chemical diffusion in this system.MST/1187

A friction effect in low-load hardness testing of copper and aluminium

Abstract: Vickers hardness tests were conducted on samples of copper and aluminium in a cold rolled or annealed condition to determine the apparent hardness variation in the load range 15 g to 20 kg. The variation was greatest for the soft specimens. Lubrication with an extreme-pressure lubricant was effective in reducing the hardness values to a virtually constant level for each metal. It is therefore reaffirmed that the hardness variation is attributable to friction and that strain hardening propensity is important in governing the magnitude of the variation. Comparison of these findings with data previously reported for similar tests on iron suggests that the phenomenon is probably an indentation size effect.

Effects of changes in strain path on work-hardening in CP aluminium and an AlCuMg alloy

Abstract: Effects of abrupt changes in the direction of plastic deformation on work-hardening behaviour have been investigated in two-stage stretching of sheets of AA1050 and a heat-treated 2014 aluminium alloy aged at various temperatures up to 300°C The results show that reorganisation of dislocation distribution after a change in strain path can result in transient changes in work-hardening behaviour of two kinds Changes of the first kind, which tend to increase the hardening rate in early stages of the second mode of deformation, are associated with reorientation of internal stresses Changes in the second kind, which tend to cause transient reductions in hardening rate, are believed to be associated with partial dissolution of the original dislocation substructure The relative magnitudes and strain dependencies of these two kinds of change depend on the deformation sequence and on material variables The change in hardening rate of CP aluminium after a change in strain path is dominated by changes of the second kind which, after moderate prestrains, cause reductions in the limits of stable elongation In similar tests on overaged conditions of the 2014 alloy the overall changes in hardening rate are dominated by changes of the first kind, so that the limit of uniform elongation is increased by a change in strain path When dynamic ageing is active in the 2014 alloy changes of the second kind can be suppressed so that reductions in the hardening rate do not occur

Elastic-plastic and asymptotic fields of interface cracks

Abstract: In previous analyses [1, 2, 13], and full-field computational investigations, we found that the near tip plastic fields of cracks on a bimaterial interface do not have a separable form of the HRR type. Nevertheless they appear to be nearly separable in an annular region well within the plastic zone. Asymptotically, as the crack tip is approached, the material system responds like that of a plastically deforming solid bonded to a rigid substrate; in particular, the stress and strain fields in the more compliant (lower hardening) material behave like those of a material with identical plastic properties bonded to a rigid substrate. Furthermore, the asymptotic fields of the interface crack bear strong similarities to mixed mode HRR fields for the homogeneous medium characterized by the plastic properties appropriate to the more (plastically) compliant material. In this investigation, we elucidate the behaviour of the material system over two length scales which are physically relevant, namely, distances comparable to the dominant plastic zone and the crack tip opening displacement. The latter is approximately given by the plastic zone size times and the relevant yield strain. Over length scales comparable to the dominant plastic zone, the stress fields are governed by the characteristics of the weaker (lower yield strength) material. On the other hand, the near tip plastic fields are governed by the strain hardening characteristics of the more plastically compliant (lower hardening) material.

A case for Taylor hardening during primary and steady-state creep in aluminium and type 304 stainless steel

Abstract: Previous elevated-temperature experiments on 304 stainless steel clearly show that the density of dislocations within the subgrain interior influences the flow stress at a given strain rate and temperature. A re-evaluation shows that the hardening is consistent with the Taylor relation if a linear superposition of solute hardening (τ 0, or the stress necessary to cause dislocation motion in the absence of a dislocation substructure) and dislocation (αGbϱ 1/2) hardening is assumed. The same Taylor relation is applicable to steady-state structures of aluminium if the yield stress of annealed aluminium is assumed equal toτ 0. New tests on aluminium deforming under constant-strain-rate creep conditions show a monotonic increase in the dislocation density with strain. This and the constant-stress creep trends are shown to be possibly consistent with Taylor hardening.

Cyclic deformation behaviour of Cu-16at.%Al single crystals part II: Cyclic hardening and slip band behavior

Abstract: In an attempt to understand cyclic hardening and slip band behavior of planar slip alloys, studies have been carried out in strain control on Cu-16at.%Al single crystals oriented for easy glide. The gauge section was observed to become completely filled with slip bands by the repetition of strain bursts irrespective of strain amplitude. Slip in the old bands which carried the whole strain just before a strain burst seem to become inactive after a few successive strain bursts. Since the localized strain shifts around the gauge length because of the short active life of the slip bands, the overall deformation becomes rather homogeneous. This Luders-band-like behavior persists for a large fraction of the fatigue life, even after crack formation. A regular hill and valley surface morphology develops on the CuAl crystals with cycling. This morphology is related to strain burst behavior, the wavelength of the morphology increasing with strain amplitude because the strain bursts are larger in the early stages of life. The development of sharp extrusions commonly observed in copper is not observed in Cu-16at.%Al single crystals. Sharp extrusions are not expected to develop in this alloy because of its low stacking fault energy and its Luders-band-like slip behavior.

Inelastic Deformation Under Nonisothermal Loading

Abstract: The objective of this paper is to evaluate, both experimentally and analytically, the appropriate forms of the hardening evolution equations in unified constitutive models for conditions involving nonisothermal loading. Critical experiments were performed for a cast nickel-base superalloy by using variable temperature tensile, creep, and cyclic tests in the 538-982 C temperature range. These experimental results were compared with both isothermal data and predictions of the Bodner-Partom-Bodner, 1987 elastic-viscoplastic theory to assess the effects of thermal history on constitutive behavior. The results indicate that the hardening evolution equations based on isothermal data are applicable for nonisothermal loading of these precipitation strengthened alloys. Additional thermal history effect terms in the hardening evolution equations were not required beyond those accounting for the variation of material constants with temperature. Using material constants determined solely from isothermal data, the inelastic deformation behavior of B1900 + Hf subject to thermomechanical loading were adequately predicted by the Bodner-Partom model.

Cyclic deformation behavior of Cu16at.%Al single crystals Part III: Friction stress and back stress behavior

Abstract: To investigate the cyclic hardening behavior of planar slip alloys, the friction stress and the back stress acting on the dislocations were evaluated from analyses of fatigue hysteresis loops. In the initial stage of deformation the friction stress (12 MPa) is larger than the back stress (7 MPa). With accumulation of cycles the back stress increases and eventually leads the friction stress. The friction stress in the early stage of deformation, which is high relative to that of pure metals, is related to the elastic interaction of dislocations and segregated solute atoms. The cyclic hardening of this alloy is mostly caused by the increase of the back stress, which seems to be caused by the accumulation of multipoles and dislocation pile-ups frequently observed in this alloy. The equality of the back stress and the friction stress observed previously for copper breaks down for the behavior of multipoles in this alloy, because the dislocation structure is modified from that of copper to form kinked multipoles or cross-gridded multipoles, types which cannot occur in copper because of the ease of cross-slip in pure metal. The motion of these multipoles is discussed in relation to the observed friction stress and back stress behavior.