Showing papers on "Deformation (engineering) published in 1992"

Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition

Abstract: Nanocrystalline fcc metals have been synthesized by mechanical attrition. The crystal refinement and the development of the microstructure have been investigated in detail by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The deformation process causes a decrease of the grain size of the fcc metals to 6–22 nm for the different elements. The final grain size scales with the melting point and the bulk modulus of the respective metal: the higher the melting point and the bulk modulus, the smaller the final grain size of the powder. Thus, the ultimate grain size achievable by this technique is determined by the competition between the heavy mechanical deformation introduced during milling and the recovery behavior of the metal. X-ray diffraction and thermal analysis of the nanocrystalline powders reveal that the crystal size refinement is accompanied by an increase in atomic-level strain and in the mechanically stored enthalpy in comparison to the undeformed state. The excess stored enthalpies of 10–40% of the heat of fusion exceed by far the values known for conventional deformation processes. The contributions of the atomic-level strain and the excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth in the nanocrystalline fcc metals are investigated by thermal analysis. The activation energy for grain boundary migration is derived from a modified Kissinger analysis, and estimates of the grain boundary enthalpy are given.

Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel

Abstract: The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100 °C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained aus- tenite stability.

On the strain and strain-rate dependence of the fraction of plastic work converted to heat: An experimental study using high speed infrared detectors and the Kolsky bar

Abstract: The conversion of plastic work to heat at high strain rates gives rise to a significant temperature increase which contributes to thermal softening in the constitutive response of many materials. This investigation systematically examines the rate of conversion of plastic work to heat in metals using a Kolsky (split Hopkinson) pressure bar and a high-speed infrared detector array. Several experiments are performed, and the work rate to heat rate conversion fraction, the relative rate at which plastic work is converted to heat, is reported for 4340 steel, 2024 aluminum and Ti-6A1-4V titanium alloys undergoing high strain and high strain rate deformation. The functional dependence of this quantity upon strain and strain rate is also reported for these metals. This quantity represents the strength of the coupling term between temperature and mechanical fields in thermomechanical problems involving plastic flow. The experimental measurement of this constitutive function is important since it is an integral part of the formulation of coupled thermomechanical field equations, and it plays an important role in failure mode selection — such as the formation of adiabatic shear bands — in metals deforming at high strain rates.

Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl

Abstract: Tensile deformation behavior of polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of the angle o between the lamellar boundaries and tensile axis at room temperature. Tensile elongation to fracture strongly depends on the angle o but its dependence on the angle o is not symmetrical with respect to o = 45°. A tensile elongation as large as 20%, which is far larger than any other reported values on TiAl-based compounds, has been obtained for o = 31°. The yield stress also strongly depends on the angle o but any particularly significant tension-compression asymmetry in yield stress has not been observed. Fracture has been found to occur in a brittle manner without showing any local contraction even after deformation to more than 10%. When the tensile axis is perpendicular or inclined to the lamellar boundaries, fracture occurs in a cleavage-like mode with a habit plane parallel to the lamellar boundaries while fracture occurs in a zigzag across the lamellar boundaries when the tensile axis is parallel to the lamellar boundaries.

Tensile and tensile fatigue behaviour of concrete; experiments, modelling and analyses

Abstract: A model based on nonlinear fracture mechanics is presented for the fatigue behavior of plain concrete. The tensile behavior of concrete for a monotonic increasing deformation is described. The softening relation is described and the way the fracture mechanics parameters are influenced by several variables is shown. The behavior of a softening material subjected to a uniaxial tensile test is explained. Based on experimental results, a constitutive model for the crack cyclic behavior of concrete is proposed. The adequacy of the model is shown by numerical simulation of notched beams under 4-point bending. It is shown by experiments, that localization and non-uniform crack opening occurs in tensile fatigue tests just as in monotonic loaded tensile tests.

Morphological alterations during texture-producing plastic plane strain compression of high-density polyethylene

Abstract: Numerous specimens of a single linear polyethylene sample were deformed by plane strain compression in a channel die to very large plastic strains. The mechanisms of deformation were elucidated by density measurements, polarized light microscopy, TEM and wide-angle and small-angle X-ray diffraction.

Deformation and adhesion of elastic bodies in contact

Abstract: The elastic deformation and adhesion of two convex bodies that interact via surface forces of finite range are calculated self-consistently. Hertz theory is compared to the result of an exponential repulsion, and it is found to be valid in the limit of short-ranged forces and high loads. A Lennard-Jones law is used to examine the classical theories of adhesion, which relate the surface energy to the pull-off force, and their regime of validity is explored. Explicit expressions are given for the displacement prior to contact, and for the jump instabilities due to elastic deformation, which occur for compliant bodies with rapidly changing surface forces. The loading-unloading cycle is shown to be hysteretic for large adhesions, and this is correlated with the onset of jumps. In these cases the pull-off force is demonstrated to depend upon the history of the sample, and it increases with increasing maximum applied loads.

Superplasticity of Alloys, Intermetallides and Ceramics

Abstract: 1. Structural Superplasticity.- 1.1 Phenomenology and Influence of the Initial Microstructure.- 1.2 Structure Evolution and Fracture Modes.- 2. The Nature of Superplastic Flow.- 2.1 Experimental Investigations of Superplastic Deformation.- 2.1.1 Grain Boundary Sliding.- 2.1.2 Intragranular Deformation.- 2.1.3 Diffusion Creep.- 2.1.4 The Interrelationship and the Role of Different Deformation Mechanisms in Superplastic Flow.- 2.2 Micromechanisms and the Theory of Superplastic Deformation.- 2.2.1 Physical Models.- 2.2.2 Grain Boundaries in the Deformation Processes.- 2.2.3 A New Model of Structural Superplasticity.- 3. Microcrystalline Materials and Microstructure Refining.- 3.1 Structure and Properties of Microcrystalline Materials.- 3.2 Metallurgical Methods of Refinement.- 3.3 Heat Treatment.- 4. Superplastic Deformation of Magnesium Alloys.- 4.1 Effect of Chemical Compositon, Structure, and Temperature-Rate Conditions of Deformation on Ductility.- 4.2 Refining of the Magnesium Alloy Structure.- 4.3 Effect of Superplastic Deformation on the Properties of the Alloys.- 5. Superplasticity of Commercial Aluminium Alloys.- 5.1 Effect of Structure, Conditions of Deformation, and Chemical Composition on the Superplasticity of Aluminium Alloys.- 5.1.1 Effect of Structure and Conditions of Deformation. Strain Rate.- 5.1.2 Effect of Chemical Composition.- 5.2 Processing Methods for Obtaining Microcrystalline Structure in Commercial Aluminium Alloys.- 5.3 Effect of Superplastic Deformation on the Structure and Properties of Aluminium Alloys.- 5.4 Effect of Grain Size on the Structural Strength of Aluminium Alloys.- 6. Superplasticity of Titanium Alloys.- 6.1 Plasticity and Superplasticity of the Titanium Alloys in the Single-Phase Region.- 6.2 Superplasticity of Titanium Alloys in the Two-Phase Region.- 6.3 Methods of Obtaining Microcrystalline Structure in Titanium Alloys.- 6.4 The Effect of Superplastic Deformation on the Mechanical Properties of the Titanium Alloys.- 7. Structural Superplasticity of Steels.- 7.1 Superplasticity of Iron, Plain Carbon, and Alloyed Steels.- 7.2 Preparation of Fine-Grained Microstructure in Steels.- 7.3 Effect of Superplastic Deformation on Mechanical Properties.- 8. Superplasticity of Nickel-based Superalloys.- 8.1 Effect of Structure, Deformation Temperature and Strain Rate on the Ductility of Alloys.- 8.2 Methods for Converting Alloys into a Superplastic State.- 8.3 High-Temperature Properties of Alloys After Processing in the Superplastic Condition.- 9. Superplasticity of Intermetallic Compounds.- 9.1 Superplasticity of Alloys with Both Intermetallic and Metallic Phases.- 9.2 Superplasticity of Intermetallic Compounds.- 9.3 Refining of Microstructure in Intermetallics.- 9.4 Effect of Superplastic Treatment on the Physico-mechanical Properties of Intermetallics.- 9.4.1 Mechanical Properties Under Ductile-Brittle Transition.- 9.4.2 Mechanical Properties at Room Temperature.- 9.4.3 Magnetic Properties.- 10. Superplasticity of Ceramics.- 10.1 Influence of Structure and Strain Conditions on the Superplasticity of Ceramics.- 10.2 Mechanisms of Superplastic Deformation of Ceramics.- 10.3 Methods for Creating Microcrystalline Structures in Ceramics.- 10.4 Influence of Superplastic Deformation on the Properties of Ceramics.- References.- Author Index.

Deformation mechanisms and plastic resistance in single-crystal-textured high-density polyethylene

Abstract: The plastic resistances for the three most important mechanisms actig in HDPE, i.e. the (100)[001] and (010)[001] chain slips and (100)[010] transverse slip was investigated and the ranges of crystal odrientation in which a single one of these mechanisms dominares the observed deviation behavior were explored. An attempt was also made to detect the (110)[001] chain slip, which according to theoretical predictions should also be an active mechanism in the deformation of polyethylene crystals

Evolution of the crystalline texture of high-density polyethylene during uniaxial compression

Abstract: The formation of an axially aymmetrical texture in HDPE was studied through unixial compression at room temperature. The orientation of crystallographic axes was probed at various stages of the deformation process, up to an equivalent strain of 1.86, by means of WAXS pole figures. Additionally the lamellar orientation was studied using SAXS and TEM. The results of these structural and morphological studies demonstrated that the major deformation mechanism involved in plastic deformation of the crystalline phase were (100)[001] chain slip

Mechanical behavior and interface design of MoSi2-based alloys and composites

Abstract: The mechanical behavior of hot pressed MoSi2-based composites containing M05Si3, SiO2, CaO and TiC as reinforcing second phases was investigated in the temperature regime 1000-1300 °C. The effects of strain rate on the flow stress for M05Si~-, SiO2- and CaO-containing composites are presented. Effects of several processing routes and microstructural modifications on the mechanical behavior of MoSi2-M05Si ~ composites are given. Of these four composite additions, M05Si 3 and CaO produce strengthening of MoSi 2 in the temperature range investigated. SiO 2 greatly reduces the strength, consistent with the formation of a glassy phase at interface and interphase boundaries. TiC reduces the flow stress of MoSi 2 in a manner that suggests dislocation pumping into the MoSi 2 matrix. The strain rate effects indicate that dislocation creep (glide and climb) processes operate over the temperature range investigated, with some contribution from diffusional processes at the higher temperatures and lower strain rates. Erbium is found to be very effective in refining the microstructures and in increasing the hardness and fracture properties of MoSi2-MosSi 3 eutectics prepared by arc melting. Initial results on microstructural modeling of the deformation and fracture of MoSi2-based composites are also reported.

Periodic instabilities during compression or extension of the lithosphere 1. Deformation modes from an analytical perturbation method

Abstract: We argue that the plastic rheology of the lithosphere, rather than its “recoverable” elastic properties, is responsible for the systematic development of periodic instabilities during compression or extension. We use a linear perturbation model with analytical solutions to calculate the instability modes for various rheologies. The growth of such periodic instabilities is enhanced by the highly nonlinear stress-strain rheologies encountered in the brittle layers of the lithosphere. On the contrary, ductile layers, deforming according to high temperature creep flow laws, tend to inhibit these instabilities. For oceanic domains, we assume that the only brittle layer is the upper part of the lithosphere. In compression, the only valid instability is a buckling whose wavelength is around 4 times the thickness of the brittle layer. Calculated wavelengths and growth rates are consistent with observations available for the Indian Ocean. For continental domains, a reasonable assumption is the existence of two plastic layers, one in the upper crust, the other in the upper mantle, for Moho temperatures between 450°C and 600°C. In compression and extension, two instabilities develop a long wavelength instability involving the whole lithosphere (coupling mode) and a short wavelength instability involving the crust and controlled by the upper brittle layer (intrinsic crustal mode). In compression, the coupling mode is a whole-lithosphere buckling, with a wavelength about 4 times the thickness of the active lithosphere (the two plastic layers plus the intermediate ductile layer). In extension, the coupling mode is a boudinage of opposite phase in the two plastic layers and a folding of the intermediate ductile layer. The intrinsic crustal mode is a crustal buckling in compression, crustal boudinage in extension. Neither deflects the Moho. The intrinsic crustal mode is favored by an increase in thermal gradient and by a decrease in strength of the ductile lower crust.

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

Abstract: The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10−5/s to 10−3/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.

A Visco-Plastic Constitutive Model for 60/40 Tin-Lead Solder Used in IC Package Joints

Abstract: A new unified visco-plastic constitutive model for the 60 Sn-40 Pb alloy used in solder joints of surface-mount IC packages and semiconductor devices is proposed. The model accounts for the measured stress-dependence of the activation energy and for the strong Bauschinger effect exhibited by the solder. The latter is represented by a back stress state variable which, in turn, evolves according to a hardening-recovery equation. Based on the observed hardening behavior, it is assumed that the isotropic resistance to plastic flow does not evolve within the deformation range covered in this study (e< 3 percent). The deformation phenomena associated with the solder’s monotonic and steady-state cyclic responses are accurately predicted for −55°C≦T≦150°C and 8 x 10−2 s−1 ≦ e ≦ 8 x 10−5 s−1 . The model also predicts well the overall trend of steady-state creep behavior. The constitutive model is formulated within a continuum mechanics framework and is therefore well suited for implementation into finite element or other structural codes.

An investigation of the creep processes in tin and aluminum using a depth-sensing indentation technique

Abstract: Constant load creep experiments were conducted using a depth-sensing indentation instrument with indentation depths in the submicron range. Experiments were conducted on polycrystalline Sn and sputtered Al films on Si substrates. The results show that the plastic depth versus time curves and the strain rate versus stress plots from these experiments are analogous to those obtained from conventional creep experiments using bulk specimens. The value of the stress exponent for Sn is close to the reported values from uniaxial creep tests. Tests on Al films showed that the stress exponent is dependent on the indentation depth and is governed by the proximity to the film/substrate interface. Load change experiments were also performed and the data from these tests were analyzed. It is concluded that indentation creep experiments may be useful in elucidating the deformation properties of materials and in identifying deformation mechanisms.

On the measurement of local strain and temperature during the formation of adiabatic shear bands

Abstract: A series of experiments was performed to study the process of adiabatic shear band initiation and formation in steels. The steels include a low carbon cold-rolled steel and three martensitic steels (HY-100 and two tempers of AISI 4340 VAR steel of varying hardness). In each case the specimens are machined as thin-walled tubes that are deformed dynamically in a torsional Kolsky bar (torsional split Hopkinson bar). Shear band initiation and formation are observed by ultrahigh-speed photography of a fine grid pattern deposited on the specimen's surface. It is shown that the critical strain for shear band initiation depends on the magnitude of a preexisting defect, in accordance with the predictions of Molinari and Clifton, J. Appl. Mech., 54 (1991) 806–812. Ultrahigh-speed photographs of the grid pattern show that local strains of 100–1000% may be attained and that the local strain rates reach 10 5 s −1 . In addition, the local temperature in the shear band is measured by employing an array of small high-speed infrared detectors that provide a plot of temperature as a function of time and position. Within the shear band region, temperatures of 600 °C have been measured.

Development of Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed martensitic transformations: Part I. shape memory behavior

Abstract: As part of a study on the newly developed Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed (fcc-hcp) martensitic transformations, transfor-mation behavior is characterized utilizing a combination of electrical resistance, dilatometry, and magnetic susceptibility measurements. The characteristics of thermally induced and strain-induced e martensitic transformations under the influence of antiferromagnetism are discussed based on the experimental results. The variations of shape memory properties with prestraining temperature are interpreted in terms of the transformation characteristics. It is shown that the e martensite can be readily strain-induced under the stabilization effect of the antiferromagnetism which strongly suppresses the thermally induced transformation. The strain-induced transfor-mation of e martensite is more preferred as a predominant deformation mechanism at low tem-peratures under a combined influence of the antiferromagnetism and other physical factors, whereas the irreversible deformation mode is more likely with prestrain at relatively high tem-peratures. The transformation characteristics can be significantly changed by alloying and mechanical /thermal treatments. This offers a possibility of developing new practical Fe-based shape memory alloys with a wide range of mechanical and physical properties.

Deformation resistance in oriented nylon 6

Abstract: Initially spherulitic nylon 6 was textured into a quasi-single crystal form by plane-strain compression in deep channel-die, followed by cooling under pressure to room temperature. The resulting material, having orthotropic symmetry with quasi-single crystalline perfection, had a dual morphology in which the normals of the crystallographic (100) planes equipartition at ±20° with respect tot he constraint direction of the planne-strain deformation. This highly textured material, has relatively large dimensions in all directions, was used to study the elastic and plastic anisotropy of the actual constituent lamellar crystallites

Micromechanics of crack growth into a craze in a polymer glass

Abstract: The problem of craze failure near the tip of a crack embedded inside a craze is investigated. The micromechanics model is based on the presence of cross-tie fibrils in the craze microstructure. These cross-tie fibrils give the craze some small lateral load-being capacity so that they can transfer stress between the main fibrils. This load-transfer mechanism allows the normal stress on the fibrils directly ahead of the crack tip in the center of the craze to reach the breaking stress of the chains. An exact solution is obtained for the deformation field near the crack tip, and this solution is used to relate craze failure to the external loading and microstructural quantities such as the draw stress, the fibril spacing, and the chain breaking force

An analysis of the isothermal hot compression test

Abstract: The nominally isothermal, uniaxial hot compression test has been analyzed with special reference to the effects of temperature nonuniformities and friction on sample deformation and flow stress estimates. A simple one-dimensional analysis was used to establish the influence of initial temperature nonuniformities, strain rate, and the temperature dependence of the flow stress on flow localization tendencies. Noticeable strain concentrations were predicted to occur only at high strain rates (∼10 s−1) in materials such as titanium alloys, but not in steels, for typical values of the initial temperature nonuniformity. More extensive numerical (finite element method) simulations of the compression test with various values of the friction shear factor corroborated the conclusions of the flow localization analysis. In addition, it was established that initial temperature nonuniformities, as well as friction, have an almost negligible effect on flow stress data deduced from measurements of average pressurevs true height strain, at least for reductions of the order of 50 pct. The analysis results were supported by observations of the deformation behavior of a near-gamma titanium aluminide and a low-alloy steel.