Showing papers on "Hardening (metallurgy) published in 1994"

Strain gradient plasticity: Theory and experiment

Abstract: Dislocation theory is used to invoke a strain gradient theory of rate independent plasticity. Hardening is assumed to result from the accumulation of both randomly stored and geometrically necessary dislocation. The density of the geometrically necessary dislocations scales with the gradient of plastic strain. A deformation theory of plasticity is introduced to represent in a phenomenological manner the relative roles of strain hardening and strain gradient hardening. The theory is a non-linear generalization of Cosserat couple stress theory. Tension and torsion experiments on thin copper wires confirm the presence of strain gradient hardening. The experiments are interpreted in the light of the new theory.

A nonproportionality parameter and a cyclic viscoplastic constitutive model taking into account amplitude dependences and memory effects of isotropic hardening.

Abstract: The present paper is concerned with the formultion of a viscoplastic constitutive model describing both cyclic hardening and cyclic softening under proportional and nonproportional loading conditions. First a nonproportionality parameter and the relevant internal structural tensor to describe the nonproportional hardening is discussed in detail. Internal variables and the related evolution equations to describe the amplitude dependence of cyclic hardening/softening are also examined. Then the history effects of cyclic hardening and softening are considered in the evolution equations of isotropic hardening variable. The proposed model is established by incorporating these equations into Chaboche model

Tensile behavior of a new single-crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures

Abstract: Tensile behavior of a new single-crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures. The investigation also examined the influence of γ′ precipitates (size and distribution) on the tensile behavior of the material. Tensile specimens were prepared from single-crystal CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. These specimens were given three different heat treatments to produce three different γ′ precipitate sizes and distributions. Tensile testing was carried out at both room and elevated temperatures. The results of the present investigation indicate that yield strength and ultimate tensile strength of this material initially increases with temperature, reaches a peak at around 800 °C, and then starts rapidly decreasing with rise in temperature. Both yield and tensile strength increased with increase in average γ′ precipitate size. Yield strength and temperature correlated very well by an Arrhenius type of relationship. Rate-controlling process for yielding at very high temperature (T ≥ 800 °C) was found to be the dislocation climb for all three differently heat-treated materials. Thermally activated hardening occurs below 800 °C whereas above 800 °C thermally activated softening occurs in this material.

Finite element simulations of shear localization in plate impact

Abstract: S hear band development in a tungsten heavy alloy (WHA) during pressure-shear plate impact is analysed numerically. The alloy has a microstructure of hard tungsten grains embedded in a soft alloy matrix. A two-dimensional, plane strain model of the alloy microstructure is used in the computations. For this model microstructure a fully coupled thermo-mechanical initial boundary value problem is formulated and solved, accounting for finite deformations, inertia, heat conduction, thermal softening, strain hardening and strain-rate hardening. Calculations are carried out for distributions of uniform grains and for micro-structures obtained from digitized micrographs of the actual alloy. The effects of variations in grain volume fraction and grain size are considered. Experiments and the numerical calculations show that the two phase alloy is more susceptible to shear banding than either of the constituent phases. While the onset of shear localization depends on the grain distribution and volume fraction, the shear band width is found to be set by heat conduction and is insensitive to the grain volume fraction and the grain morphology, The shear band width obtained from the calculations is in good agreement with what is observed in the experiments. Furthermore, the computed shapes of the deformed tungsten grains inside the band resemble closely the observed shapes of the deformed grains in the experiments.

Multiaxial Cyclic Plasticity Model for Clays

Abstract: A total stress‐based bounding surface plasticity model for clays is developed to accommodate multiaxial stress reversals. The model is constructed based on the idea of a vanishing elastic region undergoing pure translation inside a bounding surface, and an interpolation function for hardening modulus which varies with stress distance of the elastic region from the unloading point. Central to the development of the model are the general criteria for loading and unloading, which are phrased based upon the simple argument that with continued loading the hardening modulus should decrease monotonically with deformation. Combined with numerical integration of the elastoplastic constitutive equations in a form suitable for a robust computer implementation, the model is applied to cohesive soils undergoing undrained stress reversals and cyclic loading. With a suitable choice of the interpolation function for the hardening modulus, it is shown that existing one‐dimensional nonlinear laws for soils can be replicate...

Interfacial reactions and age hardening in AlMgSi metal matrix composites reinforced with SiC particles

Abstract: AlSiMg matrix composites have been fabricated by the compocasting technique using as-received and oxidized particles. Interfacial reactions occurring during fabrication and during subsequent remelting of the composites were characterized by detailed analytical transmission electron microscopy. The effects of these reactions on the age hardening response of the materials are also characterized and discussed in detail. The results show that it is possible to control the interfacial reactions either to totally suppress age hardening or to give rise to hardening in the case of a normally nonhardenable matrix alloy.

Consolidatable particulate material and well treatment method

Abstract: A well treating composition comprising: a gelled carrier; a particulate material; an epoxy resin system including at least one epoxy resin; and a finely-divided hardening agent which, when dissolved, is operable for hardening the epoxy resin. The finely-divided hardening agent is dispersed in the epoxy resin system. A method of treating a subterranean zone located in a well comprising the steps of: (a) preparing the above-described treating composition; (b) injecting the treating composition into the well such that the treating composition is placed in the subterranean zone; and (c) maintaining the finely-divided hardening agent under conditions such that, prior to injecting the treating composition into the well, the finely-divided hardening agent does not substantially dissolve.

An Analytical Approach to Elastic-Plastic Stress Analysis of Rolling Contact

Abstract: Based on a stress invariant hypothesis and a stress/strain relaxation procedure, an analytical approach is forwarded for approximate determination of residual stresses and strain accumulation in elastic-plastic stress analysis of rolling contact. For line rolling contact problems, the proposed method produces residual stress distributions in favorable agreement with the existing finite element findings. It constitutes a significant improvement over the Merwin-Johnson and the McDowell-Moyar methods established earlier. The proposed approach is employed to study combined rolling and sliding for selected materials, with special attention devoted to 1070 steel behavior. Normal load determines the subsurface residual stresses and the size of the subsurface plastic zone. On the other hand, the influence of tangential force penetrates to a depth of 0.3a, where a is the half width of the contact area, and has diminishing influence on the residual stresses beyond this thin layer. A two-surface plasticity model, commensurate with nonlinear kinematic hardening, is utilized in solution of incremental surface displacements with repeated rolling. It is demonstrated that a driven wheel undergoes greater plastic deformation than the driving wheel, suggesting that the driven wheel experiences enhanced fatigue damage. Furthermore, the calculated residual stresses are compared with the existing experimental data from the literature with exceptional agreements.

Yield behavior of metal powder assemblages

Abstract: W e present experimental data on the compaction of powder metals using two powder systems with different powder particle morphologies. The data have been collected using biaxial and triaxial compaction systems that load powders radially in deformation space. Our results indicate that several current models proposed for powder metal compaction do not represent actual constitutive behavior. Additionally, the powders tested demonstrate a strong dependence on powder morphology and a possible associated dependence on interparticle cohesion. This dependence on cohesion may necessitate the use of an additional state variable beyond those of relative density and particle hardening ordinarily used to represent powder yield behavior.

Magnetic hardening of FeSiBCuNb ribbons and wires during the first stage of crystallization to a nanophase structure

Abstract: The structural and magnetic properties of Fe73.5Si13.5B9Cu1Nb3 alloy wire and ribbon have been studied after thermal treatments up to temperatures at which partial (75%) devitrification to a nanocrystalline structure occurs. Nanocrystals are detected by x‐ray diffraction only after treatments around 500 °C, while differential scanning calorimetry studies suggest that substantial structural change is inititiated at a much lower temperature (about 400°). A clear magnetic hardening is observed in samples heated within the temperature range 400–480 °C. This phenomenon is also accompanied by an increase of the linear magnetostriction. These effects are discussed in terms of the local structural rearrangements produced during the first stages of the crystallization process. The study is also extended partly to FeSiB, FeSiBCu, and FeSiBNb alloys.

Influences of cell walls and grain boundaries on transient responses of an if steel to changes in strain path

Abstract: The effects of changes in strain path on plastic behaviour in sheets of an interstitial-free steel with two widely different grain sizes were investigated. The sheets were prestrained in rolling and, apart from supplementary tests, they were tested in uniaxial tension at 90° to the rolling direction. The results support the following conclusions. The magnitude of the increase in reloading yield stress and amplitude of the subsequent reduction in work hardening depend on the strength of dislocation walls generated in the prestrain rather than the grain size. The walls are more effective barriers to dislocation glide in freshly activated slip systems than to glide in the original slip systems operating in the prestrain. The primary cause of the subsequent reduction in hardening rate is disruption and partial dissolution of the original dislocation substructure. The final recovery in hardening rate is caused by generation of a new substructure compatible with the new deformation mode.

Precipitation hardening in ternary alloys of the AlScCu and AlScSi systems

Abstract: The processes of precipitation hardening in cast ternary alloys of the AlScCu and AlScSi systems were studied in the temperature range of aging from 100 to 450°C and at exposures to 200 h It was shown that the CuAl2 and ScAl3 phases were involved in the process of aging in ternary AlScCu alloys, and the Si and V (AlSiSc) phases, in ternary AlScSi alloys with excess silicon in a supersaturated solid solution The V phase was for the first time revealed as the hardening phase in aluminum alloys

Cyclic plasticity in type 316l austenitic stainless steel

Abstract: — The cyclic stress-strain response of two heats of austenitic stainless steel 3 16L was experimentally studied under a wide range of cyclic plastic strain amplitudes. Three domains on the cyclic stress- strain curve were found. In the middle domain the plateau behaviour corresponds to fatigue softening. The domain at low plastic strain amplitudes is characterized by stabilized behaviour and the domain of the highest plastic strain amplitudes by continuous hardening. Individual domains were correlated with observed dislocation structures. The existence of the plateau can be related to the localization of the cyclic strain into persistent slip bands. A Manson-Coffin plot of the material over a wide cyclic amplitude range is curved and can be approximated by a double logarithmic dependence.

Influence of matrix strength and damage accumulation on the mechanical response of a particulate metal matrix composite

Abstract: The starin hardening characteristics of a metal matrix, for various ageing treatments, has been found to influence the strength of a SiC particulate MMC. Experimental observations of this effect have been successfully compared with theoretical predictions using a self-consistent model based on Eshelby's equivalent inclusion method. The level of damage accumulation has been measured using modulus change experiments. The modulus is found to increase linearly with the level of plastic strain and is dependent on matrix strength. However, this effect does not fully account for the loss of strengthening exhibited by the composites at higher strains.

Transverse strength of metal matrix composites reinforced with strongly bonded continuous fibers in regular arrangements

Abstract: The composite limit flow stress for transverse loading of metal matrix composites reinforced with a regular array of uniform continuous fibers is calculated using the finite element method. The effects of volume fraction and matrix work hardening are investigated for fibers of circular cross section distributed in both sqyare and hexagonal arrangements. The hexagonal arrangement is seen to behave isotropically with respect to the limit stress, whereas the square arrangement of fibers results in a composite which is much stronger when loaded in the direction of nearest neighbors and weak when loaded at 45° to this direction. The interference of fibers with flow planes is seen to play an important role in the strengthening mechanism. The influence of matrix hardening as a strengthening mechanism in these composites increases with volume fraction due to increasing fiber interaction. The results for a power law hardening matrix are also applicable to the steady state creep for these composites. The influence of volume fraction on failure parameters in these composites is addressed. Large increases in the maximum values of hydrostatic tension, equivalent plastic stain, and tensile stress normal to the fiber-matrix interface are seen to accompany large increases in composite strength.

The Bauschinger effect in compression-tension of sheet metals

Abstract: It is important to know how sheet metals behave under complicated loading conditions, since yielding and hardening of a metal are actually dependent upon the stress state and deformation history. A ...

Low strain plasticity in a particulate metal matrix composite

Abstract: The strengthening that arises when reinforcing an aluminum alloy with approximately equiaxed shaped SiC particles has been measured, with careful attention paid to the elastic-plastic transition The results indicate that the majority of strengthening develops at low strains, due to the high initial strain hardening rate exhibited by these composite materials This can be interpreted in terms of stress partitioning to the second phase particles during the elastic/plastic transition and beyond The experimental results are compared with a self-consistent model based on Eshelby's equivalent inclusion method The model predictions are in good agreement with experimental data, once effects due to the inhomogeneous distribution of particles are incorporated

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

Abstract: The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.

Anisotropic plasticity model for foams and honeycombs

Abstract: A three‐dimensional plasticity model is developed for foams and honeycombs. The model accounts for the transverse isotropy of foams and the orthotropy of honeycombs by incorporating a shift on stress that provides a yield surface in the form of a sphere with the centroid shifted from the origin in the principal stress space. To model the extensive strains that occur at constant stress, an elastic‐perfectly plastic formulation is used until a critical plastic volumetric strain is reached. Then the initiation of lockup is represented as an isotropic hardening function of plastic volumetric strain. Both elastic and plastic constants are given as functions of the initial density of the cellular material. With lockup, the elasticity tensor transforms to that of the parent solid. To account for strain‐rate effects, the plateau stresses and the hardening parameter are taken as functions of volumetric strain rate. For each material, experimental data are given for various densities and illustrative plots are give...

Hardening and embrittlement of polyimides by ion implantation

Abstract: Nanoindentation tests performed with a sharp diamond pyramid of apical angle 35° provided very quantitative depth profiles of hardness in polyimides implanted with C, N, O, Ne or Si ions. In all cases the hardness increased steeply when the amount of deposited energy reached a value of the order of 20 eV/atom. For an amount of energy ten times higher the polymer was transformed into an amorphous hydrocarbon and the hardening factor saturated at a value of 13 to 20. But this carbonized layer was poorly adherent, as evidenced by reproducible discontinuities in the depth-versus-load curves, when the indentor tip reached the interface.

Laser surface modification of Ti-6Al-4V alloy

Abstract: Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 Μm melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 Μm. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450‡C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.

Cyclic stress-strain response of polycrystalline nickel

Abstract: Constant plastic strain amplitude fatigue experiments were conducted at room temperature on polycrystalline nickel (290 μm grain size) at plastic strain amplitudes from 2.5 × 10 −5 to 2.5 × 10 −3 . Cyclic deformation behavior was characterized by analyzing the cyclic hardening response, evaluating the evolution of the shape of the hysteresis loop, and optical and transmission electron microscopy observations. The results indicate that the cyclic stress-strain (CSS) curve has a pronounced bulge or region of reduced slope extending from a plastic strain amplitude of approximately 2 × 10 −4 to 8 × 10 −4 . This region of reduced slope is caused by the localization of plastic strain in persistent slip bands (PSBs). It is also shown that plastic strain amplitude has little influence on the friction stress of cyclically saturated nickel; however, the back stress increases with increasing plastic strain amplitude. A comparison of the cyclic deformation characteristics of polycrystalline nickel with those of copper indicates that these materials exhibit similar fundamental cyclic behavior on both a macroscopic and microscopic level.

Enhanced solder alloy performance by magnetic dispersions

Abstract: New Pb-free solder alloys with improved resistance to deformation and creep have been developed by dispersion hardening with essentially noncoarsening particles. Application of a magnetic field to molten solders containing fine ( >

Influence of grain size and texture on the cyclic stress-strain response of nickel

Abstract: The cyclic stress-strain behavior of fine-grain (24 μm) nickel was compared with previously published results on coarse-grain (290 μm) nickel to evaluate the effects of grain size on cyclic hardening. The fine-grain nickel exhibited a moderate 〈111〉〈100〉 fiber texture, while the coarse-grain material was essentially texture free. As a result, texture also influenced the cyclic responses. Fatigue experiments were conducted at plastic strain amplitudes ranging from 2.5 × 10 −5 to 2.5 × 10 −3 . At a plastic strain amplitude of 2.5 × 10 −5 , the cyclic responses of the two types of materials were nearly identical. Above that plastic strain amplitude the fine-grain-textured specimens exhibited higher saturation stresses. The difference became greaster plastic strain amplitude increased. The higher saturation stress in the fine-grain-textured material was primarily caused by a higher back stress. Both grain sizes exhibited cyclic stress-strain curve plateaux extending from plastic strain amplitudes of 2 × 10 −4 to 8 × 10 −4 . The results are discussed in terms of traditional monotonic Hall-Petch mechanisms and the cyclic stress-strain behavior of multiple-and single-slip-oriented single crystals.