Bimal K. Kad

Deformation mechanism transitions in nanoscale fcc metals

We consider possible mechanisms that lead to transitions in the mechanisms of deformation in fcc metals and alloys. In particular, we propose that, when grain sizes are below a critical size (i.e. below 100 nm), deformation can occur via the emission of stacking faults from grain boundaries into the intragranular space. A model is developed that accounts for observed experimental data and which, in turn, shows how stacking-fault energy together with shear modulus determines achievable strength. A mechanism is proposed based on this model for transitions at both high and quasistatic strain rates, including grain-boundary sliding.

Laser-induced shock compression of monocrystalline copper: characterization and analysis

Controlled laser experiments were used to generate ultra-short shock pulses of approximately 5 ns duration in monocrystalline copper specimens with [001] orientation. Transmission electron microscopy revealed features consistent with previous observations of shock-compressed copper, albeit at pulse durations in the µs regime. At pressures of 12 and 20 GPa, the structure consists primarily of dislocation cells; at 40 GPa, twinning and stacking-fault bundles are the principal defect structures; and at a pressure of 55–60 GPa, the structure shows micro-twinning and the effects of thermal recovery (elongated sub-grains). The results suggest that the defect structure is generated at the shock front; the substructures observed are similar to the ones at much larger durations. The dislocation generation is discussed, providing a constitutive description of plastic deformation. It is proposed that thermally activated loop nucleation at the front is the mechanism for dislocation generation. A calculational method for dislocation densities is proposed, based on nucleation of loops at the shock front and their extension due to the residual shear stresses behind the front. Calculated dislocation densities compare favorably with experimentally observed results. It is proposed that simultaneous diffraction by Laue and Bragg of different lattice planes at the shock front can give the strain state and the associated stress level at the front. This enables the calculation of the plastic flow resistance at the imposed strain rate. An estimated strength of 435 MPa is obtained, for a strain rate of 1.3 × 10 7 s 1 . The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle–Grady relationship. The calculated threshold pressure for the [001] orientation is 16.3 GPa.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Shear boundaries in lamellar TiAl

Abstract Small-angle tilt boundaries and small-angle twist boundaries are familiar features of crystalline microstructures but small-angle shear boundaries are much less well known. Shear boundaries may be composed of cross-grids of either edge or screw dislocations. This paper reports some observations of shear boundaries on {111} planes in tetragonal TiAl which consist only of screw dislocations. In the electron microscope, shear boundaries look very similar to twist boundaries but they differ in that their two sets of dislocations have the same sign whereas twist boundaries have two sets with opposite signs. Asymmetric shear boundaries, composed of two sets of dislocations with different spacings, can be described as a superposition of a symmetric shear boundary and a twist boundary. An extreme case is a simple shear boundary which has just one set of screw dislocations.

On the contribution of climb to high-temperature deformation in single phase γ-TiAl

Abstract This study has assessed the contribution of climb to the plastic deformation of TiAl when deformed at elevated temperatures where significant ductility is realized. It is shown that the motion of dislocations with Burgers vectors, b, given by b = 1/2〈110] is achieved by either or both glide and climb, the degree to which these various mechanisms contribute depending on temperature and strain rate. Thus, at 900°C and strain rates less than approximately 10−2s−1, climb is found to contribute rather significantly to plastic deformation. A model is proposed which involves both glide and climb; initially, the deformation substructure is developed by glide until the appropriate link length for significant contributions from climb has been established. Calculations of the strain rate afforded by climb indicate that this form of deformation can contribute significantly to the deformation of TiAl at high temperatures.

Numerical simulations of plastic deformation and fracture effects in two phase γ-TiAl+α2-Ti3Al lamellar microstructures

Abstract Deformation characteristics of fully lamellar (FL) and nearly lamellar (NL) morphologies in two phase γ-TiAl(L10) + α2-Ti3Al(D019) polycrystalline aggregates are simulated by finite element methods. Polycrystalline stress-strain response is accurately predicted using, as input parameters, the range of soft (τhard crss) and hard (τsoft crss) mode critical resolved shear stresses obtained from single poly-synthetically twinned lamellar crystals, for shear parallel and perpendicular to the lamella. The deformation is severely inhomogeneous, due in part to the large difference in (τsoft crss) and (τhard crss), with the largest strain accumulations being encountered at grain boundaries, particularly at triple points. Such deformation incompatibilities between adjacent crystals create large hydrostatic stress concentrations at grain boundaries, which are likely nucleation sites for fracture, as experimentally verified for both tension and compression loading. Incorporating small volume fractions of γ-T...

Texture effects on shear response in Ti–6Al–4V plates

Abstract Due to the asymmetric nature of crystallographic slip and twinning in hexagonal-close-packed (hcp) materials, the bulk mechanical properties are strongly affected by the orientation distribution of single crystals in the aggregate. The following work is a description of in-plane shear response in Ti–6Al–4V polycrystalline aggregates nominally textured via routine deformation and thermal processing schedules. For rolled plates, a 2D constitutive model for slip and twinning (treated here as pseudo-slip) is derived for the hcp single crystal. The polycrystal is constructed by incorporating the material theory into a finite element model that explicitly represents a spatial distribution of single crystals. The polycrystal mechanical response is examined with respect to macroscopic shear loading as may take place during dynamic punch through processes.

Yield and fracture of lamellar γα2 TiAl alloys

Abstract Lamellar TiAl is formed from α 2 grains by Shockley dislocations sweeping alternate basal planes, thereby transforming the structure into the tetragonal γ phase which exhibits close-packed planes and directions aligned with the hexagonal substrate. Mismatches at the lamellar interfaces, of the order of 1%, give rise to internal stresses and interfacial dislocations. Both the flat plate geometry of the grains and the structure of the interfaces contribute to an extreme plastic anisotropy. The main trends evident in the plastic tensile properties are as follows: both the yield and fracture stresses are low when deformation occurs in the plane of the lamellae (soft mode) and are high when deformation occurs across the lamellae (hard mode); the ductility is high when the tensile axis lies close to the lamellar plane and is low when the tensile axis is nearly normal to the lamellar plane. The trends in the yield and fracture stresses and in the ductility may be understood by considering dislocations piled up at either lamellar (in hard mode) or domain (in soft mode) boundaries. Depending on the strength of the interface, which in turn depends on the type of interface and on the orientation of the adjacent grain, the stress field of the pile up may either cause dislocations to cross into the next grain (ductile, Hall-Petch behavior) or to nucleate cracks (semibrittle, Stroh behavior). The anisotropies of the yield and fracture stresses are explained principally by the effects of Schmid factors and anisotropies in the Hall-Petch (or Stroh) stresses resulting from the lamellar shapes.

Design perspectives for creep-resistant magnesium die-casting alloys

The microstructure of die-cast magnesium alloys is highly non-uniform, which leads to a non-uniform distribution of the solidus/homologous temperature in the α(Mg) phase and a non-uniform distribution of deformation stresses and strains in the specimen during creep testing. Experimental observations suggest that significant creep deformation occurs in the α(Mg) phase in and adjacent to the eutectic regions while deformation in the primary α(Mg) dendrites is less pronounced. This article addresses the effect of the non-uniform as-cast microstructure on the creep resistance of die-cast magnesium alloys. Computational thermodynamic simulations were carried out to determine solute segregation, solidus temperature, and the corresponding homologous temperature distribution in the α(Mg) phase. Transmission electron microscopy studies provided evidence of non-uniform creep deformation in the creep-tested specimens. The results suggest that the creep resistance of magnesium alloys is determined by the weakest aggregate and/or phase in the alloy, viz., the α(Mg) phase in and adjacent to the eutectic regions. Microstructural design efforts that increase the homologous temperature or reinforce the eutectic α(Mg) phase hold significant promise for increasing the creep resistance of magnesium alloys.

Deformation and fracture under compressive loading in lamellar TiAl microstructures

Abstract A physically based micromechanical model is applied to study finite compressive deformation behaviour and the development of failure modes in polycrystalline fully lamellar and nearly lamellar microstructures. Orientation-dependent yielding of lamellar TiAl single crystals is specifically modelled. Finite-element computations show that deformation is inherently non-uniform in the lamellar microstructure, in accordance with results presented earlier by Kad, Dao and Asaro. Intergranular fracture initiation is found to be expected at small aggregate strains (i.e. strains less than 5%), while fracture initiated by internal buckling is found to be increasingly likely to occur at larger aggregate strains (i.e. strains larger than 5–10%). Internal buckling is found in lamellar TiAl crystals whose lamellae are initially nearly parallel to the compressive loading. A weak basal texture, normal to the compression axis, is developed at only 20% aggregate strain. Subtle variations in microstructural constitue...

Through thickness dynamic impact response in textured Ti–6Al–4V plates

Abstract The following work is a computationally derived description of through thickness dynamic impact responses in Ti–6Al–4V polycrystalline aggregates nominally textured via routine rolling deformation and thermal processing schedules in the α + β or β phase fields. Prior work [M.S. Burkins, W.W. Love, J.R. Wood, U.S. Army Research Lab, ARL-MR-359, Aberdeen Proving Ground, MD, 1997] indicates that ballistic response, and the incidence of material plugging, is strongly affected by specific thermal/mechanical processing paths. We attempt to reconcile these response variations within the context of material textures. Thus, realistic processing textures viz. the basal, transverse as well as an idealized random texture are simulated via a 2D constitutive model for slip and twinning (treated here as pseudo-slip) prescribed for the hcp single crystal. The polycrystal is constructed by incorporating the material theory into a finite element model that explicitly represents a spatial distribution of single crystals. The polycrystal mechanical response is examined with respect to macroscopic shear loading as may take place during dynamic punch through processes. A ranking of the material textures is prescribed via numerically derived measures of external work performed.