Showing papers on "Hardening (metallurgy) published in 1995"

An analysis of fully plastic Brinell indentation

Abstract: Indentation of a hard sphere into inelastic solids, Brinell indentation, is examined theoretically and numerically by aid of classical plastic flow theory. With the main interest focused on fully plastic behaviour at indentation the mechanical analysis is carried out for power-law hardening rigid-plastic materials where self-similarity features play a dominant role. It is explained in detail how the problem of a moving contact boundary may be reduced to a stationary one by an appropriate transformation of field variables. Within this setting classical empirical findings by Meyer (1908) and O'Neill (1944) are established on a rigorous theoretical ground. In particular, it is shown to advantage also for nonlinear materials how intermediate solutions for a flat die may by cumulative superposition generate solutions for a class of curved indenters. In the case of perfect plasticity it turns out in the present context that indentation hardness is independent of die profiles. For hardening solids when the material behaviour is history dependent, reduction to a stationary geometry is achieved also by expressing the accumulated strain by cumulative superposition. The intermediate flat die problem is then solved for a variety of hardening exponents by a finite element procedure designed to account for material incompressibility. With finite element computations as a basis desired solutions are obtained by straightforward numerical superposition procedures. Detailed results are then given for bulk quantities such as the mean contact pressure as well as relevant field variables. The influence of hardening characteristics on sinking-in and piling-up of indented surfaces and contact pressure distributions are discussed in the light of earlier findings based on deformation theory of plasticity and available discriminating experiments. Correlation is particularly sought with the celebrated universal hardness parameters proposed by Tabor (1951) and the existence of representative strain measures. Attention is also given to the elastic-plastic transition region of Brinell indentation in search for loading levels sufficiently high that the results tend to an asymptotic fully plastic state. A standard finite element technique employing contact elements for a moving boundary is used to analyse with tolerable accuracy the influence of elasticity and more elaborate hardening behaviour. Some relevant features are shown for a sequence of solutions from elastic Hertzian to fully plastic behaviour.

Radiation hardening revisited: Role of intracascade clustering

Abstract: Experimental observations related to the initiation of plastic deformation in metals and alloys irradiated with fission neutrons have been analyzed. The experimental results, showing irradiation-induced increase in the upper yield stress followed by a yield drop and plastic instability, cannot be explained in terms of conventional dispersed-barrier hardening because (a) the grown-in dislocations are not free, and (b) irradiation-induced defect clusters are not rigid indestructible Orowan obstacles. A new model called ‘cascade-induced source hardening’ is presented where glissile loops produced directly in cascades are envisaged to decorate the grown-in dislocations so that they cannot act as dislocation sources. The upper yield stress is related to the breakaway stress which is necessary to pull the dislocation away from the clusters/loops decorating it. The magnitude of the breakaway stress has been estimated and is found to be in good agreement with the measured increase in the initial yield stress in neutron irradiated copper.

Effects of cold work on precipitation in Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys

Abstract: A study has been made of the effects of cold work prior to aging on precipitation hardening in selected Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys. General aging characteristics have been determined by differential scanning calorimetry, and response to hardening has been correlated with microstructure using transmission electron microscopy (TEM), selected area electron dif-fraction (SAED), and quantitative stereology. Particular attention has been given to the phases Ω andT 1 that form on the {111 }α planes, although information on the precipitates θ′,S′ (orS), and δ′ is also reported. Although Ω andT 1, have similar morphologies and habit planes, their response to cold work prior to aging is different. Deformation promotesT 1 formation at the expense of the δ′ phase in Al-Cu-Li alloys and at the expense of δ′, θ′, andS′ in Al-Cu-Li-Mg-Ag alloys. On the other hand, in Al-Cu-Mg-Ag alloys, deformation assists precipitation of θ′ at the expense of Ω phase, and some decrease is recorded in the hardening response. Prior cold work is also found to reduce the response during natural aging in most alloys. These results are discussed in terms of the role of particular alloying additions.

Compression of aerogels

Abstract: When an aerogel is pressurized in a mercury porosimeter, the network is compressed, but no mercury enters the pores. Therefore, porosimetry cannot be used to measure the pore size distribution in an aerogel, but it does provide a measure of the bulk modulus of the network. For silica aerogels, the network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. In the plastic regime it is found that the bulk modulus has a power-law dependence on density with an exponent of ≈ 3.2. For the same gels, the linear elastic modulus (before compression) also obeys a power law, but the exponent is ⋍ 3.6, as found by several other groups. If a gel is compressed to a pressure, P1, that exceeds the yield stress, then returned to ambient pressure, the plastic deformation is irreversible; if that gel is then compressed to pressure P2 > P1, it behaves elastically up to ⋍ P1, then yields and follows the same power-law curve. Thus the location of the yield point of a previously compressed material indicates the maximum pressure to which the sample had been subjected; in particular, the compression curve can be used to estimate the capillary pressure exerted on a xerogel during drying.

Modelling of texture evolution for materials of hexagonal symmetry—II. application to zirconium and titanium α or near α alloys

Abstract: This work describes the evolution of texture and microstructure during cold rolling of different Ti and Zr alloys. These alloys accommodate deformation with prismatic glide and with “secondary” mechanisms (gliding and/or twinning) which are different according to the type of alloys and may vary with deformation degree. We have modelled texture evolution during cold rolling of two Ti and Zr alloys, using a Taylor theory. The choice or relevance of the model variant (FC = full Constrained, RC = Relax Constrained) are discussed. In order to account for the changes in the secondary systems during deformation, we have decided to work by steps, since there are no defined hardening laws accepted for each system. The results of the modelling, both for the Pole Figures (PF) and for the Orientation Density Function (ODF), agree well with experiment in the range from 0 to 80% reduction. Therefore, a good knowledge of the microstructure evolution and of the deformation mechanisms is required.

Effective plastic response of two-phase composites

Abstract: Finite element analyses of the overall stress-strain response of metal-matrix composites are carried out using axisymmetric and plane strain unit cell formulations. The metal matrix is characterized as an isotropically hardening elastic-plastic solid and the ceramic reinforcement is taken to be isotropic elastic. Perfect bonding between the matrix and the reinforcement is assumed. The focus is on the effects of reinforcement shape, size and spatial distribution. Under monotonic loading, the stress-carrying capacity in the plastic range increases in the following order for the reinforcement shapes considered: double-cone → sphere → truncated cylider → unit cylinder → whisker. The extent of the Bauschinger effect under reversed loading increases in the same order for particle reinforced composites. The effects of reinforcement size and distribution are analyzed by considering a plane strain model with two sizes of reinforcing particles. For certain distributions, it is found that the smaller family of particles plays virtually no role in affecting the stress-strain response. Thermal residual stresses are also considered and their effects are seen to persist far into the plastic range. The predicted plastic stress-strain behavior can be rationalized in terms of the evolution of matrix field quantities and, in particular, in terms of the effect of the constraint on plastic flow.

The effect of hydrogen on the yield and flow stress of an austenitic stainless steel

Abstract: Tensile tests on 310s stainless steel foils, with and without hydrogen, were conducted at temperatures from 77 to 295 K and strain rates from 10-3 to 10-6/s. Cathodic charging at elevated temperatures and at very low current densities was used to produce homogeneous solid solutions of hydrogen in this material. The yield stress and flow stress were found to increase with hydrogen content. Discontinuous yielding was observed at room temperature for specimens with hydrogen contents greater than 5 at. pct. The ductility, as measured by the strain to failure, was not critically dependent on hydrogen concentration at 77 and 295 K but was reduced at intermediate temperatures. The changes in mechanical behavior are discussed in terms of hydrogen-dislocation interactions.

Specific heat and thermal diffusivity of hardening concrete

Abstract: This paper focuses on the evolution of the thermal characteristics of early-age concrete as a function of the state of the hardening process. A literature review is presented, together with new test results obtained by means of improved test methods: For the determination of the specific heat of cement paste during hardening and of the thermal diffusivity of hardening concrete, test methods are developed based on existing methods. The specific heat is determined for hardening cement paste samples made with blastfurnace slag cement. It is concluded that the specific heat and thermal diffusivity decreases linearly with the degree of hydration.

A single-surface yield function for geomaterials

Abstract: The article outlines a seven-parametric yield function for geomaterials such as soils and rocks. Proceeding from a geometric representation in the principal stress space, the yield surface exhibits a closed shape, thus reflecting the sensitivity of the plastic response of this type of media to hydrostatic stresses. The yield function is able to describe the effects of primary yielding, as well as of isotropic and kinematic hardening. In addition the failure envelope contains an open cone when the number of material parameters is reduced from seven to five.

Influence of peak pressure and temperature on the structure/property response of shock- loaded Ta and Ta-10W

Abstract: The deformation behavior and substructure evolution of unalloyed-Ta and Ta-10W under quasistatic conditions have been compared to their respective responses when shock prestrained to 20 GPa at 25 °C as well as to unalloyed-Ta shocked to 7 GPa at 25 °C, 200 °C, and 400 °C. The reload yield behavior of shock-prestrained Ta and Ta-10W did not exhibit enhanced shock hardening when compared to their respective quasistatic stress-strain response at an equivalent strain level. In addition, the reload yield behavior of Ta shock prestrained to 7 GPa at 200 °C or 400 °C was found to exhibit increased hardening compared to the shock prestraining at 25 °C. The quasistatic substructure evolution and shock-hardening responses of Ta and Ta-10W were investigatedvia transmission electron microscopy (TEM). The dislocation substructures in both materials and at each strain rate condition and temperature were similar and consisted primarily of long, straight, ( α/2) 〈111〉 type screw dislocations. The propensity for long, straight screw dislocations, irrespective of the loading condition, supports the theory of strong Peierls stress control on defect generation and defect storage. The substructure evolution and mechanical behavior of Ta and Ta-10W are discussed in terms of defect storage mechanisms and compared to the mechanisms operative in face-centered cubic (fcc) metals.

Mechanical properties and microstructure of cast oxide-dispersion-strengthened aluminum

Abstract: Oxide-dispersion-strengthened aluminum containing 25 vol.%, 0.28 μm, alumina dispersoids was fabricated by pressure infiltration. The mechanical properties at room and elevated temperature are presented for both as-cast, coarse-grained materials and extruded, fine-grained materials. Although the room temperature yield strength is low (about 60 MPa), the 0.2% proof stress and ultimate tensile stress are much higher (about 200 MPa and 330 MPa respectively) as a result of the very high strain hardening rate. However, the initially high strain hardening rate decreases with strain. This behavior is explained by extending a model by Ashby for dilute dispersion-strengthened metals to the case of a matrix containing a large volume fraction of large particles, whereby the interaction of primary glide dislocations with secondary loops punched by dispersoids is considered.

Discontinuous precipitation of Cr2N in a high nitrogen, chromium-manganese austenitic stainless steel

Abstract: The main purpose of the present work is to study the effect of a high nitrogen content (1 wt% N), on the microstructural evolution of a Cr-Mn austenitic stainless steel aged over the [400–900 °C] temperature interval. Thermal treatments carried out between 700 and 900 °C lead to the decomposition of the nitrogen supersaturated austenitic matrix by discontinuous precipitation of Cr2N particles. The microstructural features of the reaction are described and analysed. In the present case, the cellular precipitation of Cr2N is a peculiar and complex phenomenon which involves two diffusion mechanisms: the diffusion of an interstitial element (nitrogen) and the diffusion of a substitutional one (chromium). The nucleation of the discontinuous precipitation arises from a reduction of the surface energy of the precipitates. Furthermore, the precipitation growth is a non-steady state process, because the reaction is governed at first by the intergranular diffusion of chromium, and then tends to be controlled by its bulk diffusion. Consequently, the features of this discontinuous precipitation do not fit in with the assumptions of usual theories, which have been established for binary substitutional systems that transform in steady state conditions. This discontinuous precipitation brings about a slight hardening. Then, the hardness of the aged samples can be described by an additive relationship between the hardness of the precipitation cells and that of the untransformed matrix. Beside the discontinuous precipitation of Cr2N, sigma phase forms with significant volume fractions.

Processing and hardness of electrodeposited Ni/Al2O3 nanocomposites

Abstract: Nanocomposite Ni/Al2O3 films have been produced by electrochemical deposition where 50 and 300 nm Al2O3 particles are dispersed in a nickel matrix. These films exhibit considerable enhancements in their hardness in comparison to pure nickel. The strengthening mechanism is explained in terms of an Orowan bowing hardening mechanism and, hence, related to the volume fraction of the reinforcing phase. These films may have application as strong coatings that retain many of the physical properties (e.g., optical, thermal, electrical) of the metal.

On the role of particle cracking in flow and fracture of metal matrix composites

Abstract: The flow response of particle-reinforced metal matrix composites is studied using finite element methods. Unit cells containing either intact or cracked particles in a power law hardening matrix are used to determine the corresponding asymptotic flow strengths. The effects of the hardening exponent and the elastic mismatch between the particles and the matrix on the flow response are examined. For comparison, the flow response of power law hardening solids containing penny-shaped cracks is also evaluated. The latter results are found to be in reasonable agreement with those corresponding to composites that contain low volume fractions of cracked particles. The asymptotic results are used to develop a one-dimensional constitutive law for composites which undergo progressive damage during tensile straining. This law is used to evaluate the strain at the onset of plastic instability. It is proposed that the instability strain be used as a measure of tensile ductility when the particle content is low and the particles are uniformly distributed through the matrix.

The effect of small additions of scandium on the properties of aluminium alloys

Abstract: Additions of up to 1 wt% scandium have been made to Al, Al-Mg, Al-Mg-Ag and Al-Zn-Mg alloys and the effects on age-hardening and mechanical properties studied. Scandium levels up to 1% could be retained in solution at solidification rates of about 300 K s−1. The precipitation of Al3Sc at ageing temperatures in the range 563–593 K (290–320 °C) gave significant additional hardening. The low solubility of Sc in the solid state makes it difficult to obtain optimum hardening from Sc and other precipitating elements because of difficulties in solution treatment. The effect of deformation prior to ageing and the temperature-dependent mechanical properties are described.

Transition theories of elastic-plastic deformation of metallic polycrystals

Abstract: A general approach to the problem of determination of elastoplastic behavior of metallic polycrystals at finite deformation is presented. The relation between moving dislocation density and global slip rate for grains is developed. Transition to grain response is obtained by introducing the hardening matrix. Field equations for heterogeneous elastoplastic metals are transformed into an integral equation, using Green functions technique. This allows to find the spin of the lattice related to texture formation.

The influence of heat treatment on the high-stress abrasion resistance and fracture toughness of alloy white cast irons

Abstract: The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated. Complementing an earlier study[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy. With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to vir-tually no change in wear performance and substantial falls in hardness. Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature. Tempering treatments in the range 400 °C to 600 °C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hard-ness and wear performance, but negligible changes in fracture toughness. Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer.

Experimental identification and mathematical modeling of viscoplastic material behavior

Abstract: Uniaxial torsion and biaxial torsion-tension experiments on thin-walled tubes were carried out to investigate the viscoplastic behavior of stainless steel XCrNi18.9. A series of monotonic tests under strain and stress control shows nonlinear rate dependence and suggests the existence of equilibrium states, which are asymptotically approached during relaxation and creep processes. Strain controlled cyclic experiments display various hardening and softening phenomena that depend on strain amplitude and mean strain. All experiments indicate that the equilibrium states within the material depend on the history of the input process, whereas the history-dependence of the relaxation and creep behavior appears less significant. From the experiments the design of a constitutive model of viscoplasticity is motivated: The basic assumption is a decomposition of the total stress into an equilibrium stress and a non-equilibrium overstress: At constant strain, the overstress relaxes to zero, where the relaxation time depends on the overstress in order to account for the nonlinear rate-dependence. The equilibrium stress is assumed to be a rate independent functional of the total strain history. Classical plasticity is utilized with a kinematic hardening rule of the Armstrong-Frederick type. In order to incorporate the amplitude-dependent hardening and softening behavior, a generalized arc length representation is applied [14]. The introduction of an additional kinematic hardening variable facilitates consideration of additional hardening effects resulting from the non-radiality of the input process. Apart from the common yield and loading criterion of classical plasticity, the proposed constitutive model does not contain any further distinction of different cases.

Influence of nitride (Cr2N) precipitation on the plastic flow behavior of high-nitrogen austenitic stainless steel

Abstract: Aging at 700 °C results in grain boundary nitride precipitation only, while aging at 900 °C results in grain boundary and cellular precipitation (40 vol%). These thermal treatments have a small but positive effect on YS, no effect on the UTS, but dramatically reduce the ability of the material to deform under localized plastic deformation (necking), leading to reduced tensile ductility. The plastic flow behavior of all annealed and aged high-nitrogen samples were modeled using the modified Ludwik relation (equation 2). Below the UTS, grain boundary nitride precipitation at 700 °C has no measurable effect on the plastic flow behavior of the material, and the modeling parameters n1, K1, n2, and K2 have values which do not deviate significantly from the behavior of the un-aged material. Cellular precipitation, caused by aging at 900 °C, does significantly affect the plastic flow behavior of the material at both low and high strains. Cellular precipitation causes both increased strengthening of the matrix in the low strain regime (0.001 < e < 0.03) and systematic decreases in the strain hardening exponent (n1) and strength coefficient (K1) with increased aging. The rate of strain hardening (dσde) measured from the σ-e plots is unaffected by isothermal aging and nitride formation.

Plastic behavior and deformation textures of poly(etherether ketone) under uniaxial tension and simple shear

Abstract: Samples of poly(etherether ketone) (PEEK) were subjected to large plastic deformations under uniaxial tension and simple shear by means of a new video-controlled testing method at constant true strain rate. The “equivalent” stress-strain curves obtained under the two loading modes are close at the yield point, but diverge drastically at large strains, with a rapidly increasing hardening in tension and a moderate hardening under simple shear. X-ray diffraction goniometry shows that these contrasting behaviors are associated with the different textures developed in the crystallite orientations. Under tension, the PEEK lamellae are progressively tilted in such a way that the chain axis becomes oriented parallel to the tensile axis; in the other mode, the final chain orientation is near to the shear axis. DSC analyses of deformed samples in both modes are carried out. The results show that the tension loading induces a fragmentation of the thin lamellae, while the shear mode generates less fragmentation. A quantitative model is presented that involves a composite approach: (i) the viscoplastic deformation of the crystalline lamellae, which is controlled by chain slip and transverse slip systems on planes parallel to the c axis, and (ii) the hyperelastic deformation of the amorphous phase, which depends on the affine unfolding of statistically distributed subchains. A discussion of the influence of the CRSS values on the stress-strain curves and textures is developed by means of this model.

Effects of reinforcement orientation on the tensile response of metal-matrix composites

Abstract: A three-dimensional unit cell model is used to analyze the effect of reinforcement orientation on the tensile response of particle- and whisker-reinforced metal-matrix composites. The metal matrix is characterized as an isotropically hardening elastic-viscoplastic solid and the ceramic reinforcement is taken to be isotropic elastic. Perfect bonding between the matrix and the reinforcement is assumed. The numerical results show a strong decrease in tensile stress level for small deviations of the whiskers from perfect alignment with the tensile axis. The tensile stress-strain response becomes rather insensitive to the precise value of the misalignment angle when the misalignment is sufficiently large. These trends are also seen in the experiments conducted in the present work on SiC whisker-reinforced aluminum alloys subject to tension at various angles to the whisker axis. For particle-reinforced composites, the computed overall tensile is much less sensitive to reinforcement alignment. The numerical simulations also provide results for the evolution of field quantities in the matrix and in the reinforcement. A strain-induced rotation of the reinforcement is predicted for the case of whiskers reinforcement, whereas nearly no rotation is predicted for the particles.

Effect of reinforcement size on low cycle fatigue behavior of SiC particle reinforced aluminum matrix composites

Abstract: From the examination of the particle size effect on the cyclic deformation in SiC/Al composites, the authors offer the following conclusions: SiCp/Al composite displayed continuous cyclic softening after initial hardening for any two or three cycles. In contrast, the unreinforced matrix showed initial cyclic hardening, cyclic stability and second hardening. Higher stress in response to a given strain amplitude were observed with the composite with fine (10 {micro}m) SiC particles in comparison with the composite with coarse (32 {micro}m) SiC particles. In addition, the evolution of cyclic softening became faster when the particle size increased. The low cycle fatigue endurance of the composites was generally lower than that of the unreinforced matrix. The decrease in particle size led to a higher fatigue resistance at lower strain amplitudes, but reduced the fatigue life at higher strain amplitudes. The particle size effect on cyclic response behavior can be explained when the difference in interparticle spacing and thus the difference in plastic deformation ability between the coarse and fine particle composites are taken into account. The particle size effect on fatigue-life behavior was explained in terms of the fracture modes.

Strain hardening of steels at large strain deformation. Part I: Relationship between strain hardening and microstructures of b.c.c. steels

Abstract: In the present work, the relation between strain hardening behaviour and microstructure of steels, especially for large deformations, was investigated with the help of the Kocks model. First, the different origins of flow stresses and the development of the dislocation density are discussed because of their importance to the Kocks model. For the measurement of stress strain curves at large deformations, a new method of tensile testing was developed. Steels with different but defined microstructures were produced. It was found that the strain hardening behaviour of Ti micro-alloyed steels and ferritic pearlitic steels depends significantly on the microstructure. Small precipitates or increasing volume fraction of hard second phase increase the strain hardening rate. A quantitative characterization of the microstructure of well-annealed steels can be given by the yield stress and the relation between strain hardening and microstructure can be replaced by the relation between strain hardening and yield stress.

Finite Element Analysis of Rolling Contact for Nonlinear Kinematic Hardening Bearing Steel

Abstract: A Mroz image point, two surface, nonlinear-kinematic-hardening-plastic (MNKP) representation of bearing steel is inserted into a finite element model of 2-dimensional, line contact for pure rolling. The calculations are compared with previous results for the same contact pressure derived for elastic-linear-kinematic-hardening-plastic (ELKP) behavior. The residual stress, deformation, and the connection between continuing cyclic deformation, etching bands, and cracks are analyzed. Unlike the ELKP constitutive properties, the MNKP behavior displays a distinct transient region which results in higher residual stresses.