Equal-channel angular extrusion of beryllium

The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions have been successfully completed on Ni-canned billets of Be at approximately 425 °C. No cracking was observed in the billets, and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed for a single-pass extrusion, including evidence of 〈c〉 and 〈c+a〉 dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation processing technique and the underlying physical processes which sustain the deformation. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Equal-channel angular extrusion of beryllium

The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions have been successfully completed on Ni-canned billets of Be at approximately 425°C. No cracking was observed in the billets, and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed for a singlepass extrusion, including evidence of <c> and <c+a> dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation processing technique and the underlying physical processes which sustain the deformation. S.R. AGNEW, formerly with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115 This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Determination of dislocation densities in HCP metals from X-ray diffraction line-broadening analysis

X-Ray diffraction (XRD) line-broadening analysis has been performed on highly textured Zr-2.5Nb specimens which had been deformed in tensile tests to produce well-controlled dislocation structures. An iterative deconvolution method has been applied to extract the broadening function for the material, using as standards, a Zr single crystal and a 0 pct deformed specimen. In both cases, for specific tensile tests, a significant contribution to the basal line broadening was observed, which was clearly not directly related to the dislocation structure generated by the deformation, i.e., so-called c-component dislocations having a component of their Burgers vectors perpendicular to the basal plane. Calculations showed that the extent of basal line broadening cannot be attributed to the secondary effect of strain from a-type dislocations, i.e., dislocations with Burgers vectors parallel with the basal plane. It is concluded that most of the line broadening observed was the result of intergranular strain distributions. These distributions are most prominent for grains oriented with their c-axes perpendicular to the tensile-deformation axis and resulted in basal-plane line broadening even when there were few, if any, c-component dislocations present. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Determination of dislocation densities in HCP metals from X-ray diffraction line-broadening analysis

X-Ray diffraction (XRD) line-broadening analysis has been performed on highly textured Zr-2.5Nb specimens which had been deformed in tensile tests to produce well-controlled dislocation structures. An iterative deconvolution method has been applied to extract the broadening function for the material, using as standards, a Zr single crystal and a 0 pct deformed specimen. In both cases, for specific tensile tests, a significant contribution to the basal line braodening was observed, which was clearly not directly related to the dislocation structure generated by the deformation, i.e., so-called c-component dislocations having a component of their Burgers vectors perpendicular to the basal plane. Calculations showed that the extent of basal line broadening cannot be attributed to the secondary effect of strain from a-type dislocations, i.e., dislocations with Burgers vectors parallel with the basal plane. It is concluded that most of the line broadening observed was the result of intergranular strain distributions. These distributions are most prominent for grains oriented with their c-axes perpendicular to the tensile-deformation axis and resulted in basal-plane line broadening even when there were few, if any, c-component dislocations present. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Nonbasal deformation modes of HCP metals and alloys: Role of dislocation source and mobility

A review is presented on the role of dislocation cores and planar faults in activating the nonbasal deformation modes, 〈c+a〉 pyramidal slip and deformation twinning, in hcp metals and alloys and in D0₁₉ intermetallic compounds. Material-specific mechanical behavior arises from a competition between altemate defect structures that determine the deformation modes. We emphasize the importance of accurate atomistic modeling of these defects, going beyond simple interatomic energy models. Recent results from both experiments and theory are summarized by discussing specific examples of Ti and Mg single crystals; Ti-, Zr-, and Mg-base alloys; and Ti₃Al ordered alloys. Remaining key issues and directions for future research are also discussed. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM and TMS committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Nonbasal deformation modes of HCP metals and alloys: Role of dislocation source and mobility

A review is presented on the role of dislocation cores and planar faults in activating the nonbasal deformation modes, <c + a> pyramidal slip and deformation twinning, in hcp metals and alloys and in D0₁₉ intermetallic compounds. Material-specific mechanical behavior arises from a competition between alternate defect structures that determine the deformation modes. We emphasize the importance of accurate atomistic modeling of these defects, going beyond simple interatomic energy models. Recent results from both experiments and theory are summarized by discussing specific examples of Ti and Mg single crystals; Ti-, Zr-, and Mg-base alloys; and Ti₃Al ordered alloys. Remaining key issues and directions for future research are also discussed. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans Louisiana, under the auspices of the following ASM and TMS committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

First-principles investigation of perfect and diffuse antiphase boundaries in HCP-based Ti-Al alloys

First-principles thermodynamic models based on the cluster expansion formalism, Monte Carlo simulations, and quantum-mechanical total energy calculations are employed to compute short-range-order (SRO) parameters and diffuse-antiphase-boundary energies in hcp-based α-Ti-Al alloys. Our calculations unambiguously reveal a substantial amount of SRO is present in α-Ti-6 Al and that, at typical processing temperatures and concentrations, the diffuse antiphase boundaries (DAPB) energies associated with a single dislocation slip can reach 25 mJ/m². We find very little anisotropy between the energies of DAPBs lying in the basal and prism planes. Perfect antiphase boundaries in DO₁₉-ordered Ti₃Al are also investigated and their interfacial energies, interfacial stresses, and local displacements are calculated from first principles through direct supercell calculations. Our results are discussed in light of mechanical property measurements and deformation microstructure studies in α-Ti-Al alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Grain-boundary diffusion by vacancy mechanism in α-Ti and α-Zr

Minimum-energy structures for the symmetric and twin grain boundaries (GBs), as well as for two nonsymmetric GBs that exhibit dislocations, are obtained for the hcp structure by computer modeling. Central force potentials constructed within the embedded-atom method are used to represent atomic interactions. Vacancy-formation energies and entropies for different sites are calculated, and the properties of various vacancy jumps are investigated. Unstable vacancy sites, located in the GB dislocation cores, are observed. The random-walk approach, combined with simulation results, is applied to study tracer diffusion by a vacancy mechanism in the twin GBs; higher diffusivity values than those for the lattice are obtained, in qualitative agreement with experiments. Correlation effects, taken into account by the matrix method, determine the main features of GB diffusion to be contributed by jumps in a narrow region. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001 in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

First-principles investigation of perfect and diffuse antiphase boundaries in HCP-based Ti-Al alloys

First-principles thermodynamic models based on the cluster expansion formalism, Monte Carlo simulations, and quantum-mechanical total energy calculations are employed to compute short-range-order (SRO) parameters and diffuse-antiphase-boundary energies in hcp-based α-Ti-Al alloys. Our calculations unambiguously reveal a substantial amount of SRO is present in α-Ti-6 Al and that, at typical processing temperatures and concentrations, the diffuse antiphase boundaries (DAPB) energies associated with a single dislocation slip can reach 25 mJ/m². We find very little anisotropy between the energies of DAPBs lying in the basal and prism planes. Perfect antiphase boundaries in DO₁₉-ordered Ti₃Al are also investigated and their interfacial energies, interfacial stresses, and local displacements are calculated from first principles through direct supercell calculations. Our results are discussed in light of mechanical property measurements and deformation microstructure studies in α-Ti-Al alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Bulk and interface boundary diffusion in group IV hexagonal close-packed metals and alloys

Bulk and grain boundary (GB) self-diffusion and substitutional solute diffusion in group IV hexagonal close-packed (hcp) metals (α-Ti, α-Zr, and α-Hf) are reviewed. The recent results obtained on high-purity materials are shown to approach closely the “intrinsic” diffusion characteristics. The enhancement effect of fast-diffusing impurities (such as Fe, Ni, or Co) is discussed for both self-and substitutional bulk solute diffusion in terms of the interstitial solubility of the impurity atoms. In GB self-diffusion, the impurity effect is found to be less dramatic. The results obtained on high-purity hop materials can be interpreted in terms of intrinsically ‘normal’ vacancy-mediated GB diffusion, with the ratio of GB to volume diffusion activation enthalpies of Q _gb /Q ≈ 0.6. The GB self-diffusion in group IV hcp metals reveals distinct systematics. Bulk self-diffusion and fast interstitial solute diffusion (Fe and Ni) in the hcp phase α ₂-Ti₃Al are reviewed. Interphase boundary diffusion of Ti in the unidirectional lamellar α ₂/γ structure of the two-phase Ti₄₈Al₅₂ alloy is analyzed with respect to the phase boundary structure and GB self-diffusion in α ₂-Ti₃Al. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Microstructure evolution in Zr under equal channel angular pressing

Pure polycrystalline Zr was deformed by equal channel angular pressing (ECAP), and the microstructural characteristics were analyzed. By repeated alternating ECAP, it was possible to refine the grain size from 200 to 0.2 μm. Subsequent annealing heat treatment at 550 °C resulted in a grain growth of up to 6 μm. Mechanical twinning was an important deformation mechanism, particularly during the early stage of deformation. The most active twinning system was identified as 85.2 deg {10 2}〈 011〉 tensile twinning, followed by 57.1 deg {10 1}〈 012〉 compressive twinning. Crystal texture as well as grain-boundary misorientation distribution of deformed Zr were analyzed by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). The ECAP-deformed Zr showed a considerable difference in the crystallographic attributes from those of cold-rolled Zr or Ti, in that texture and boundary misorientation-angle distribution tend toward more even distribution with a slightly preferential distribution of boundaries of a 20 to 30 deg misorientation angle. Furthermore, unlike the case of cold rolling, the crystal texture was not greatly altered by subsequent annealing heat treatment. Overall, the present work suggests ECAP as a viable method to obtain significant grain refining in hexagonal close-packed (hcp) metals. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Transmission electron microscopy investigation of 〈c+a〉 dislocations in Mg and α-solid solution Mg-Li alloys

The ductility of Mg alloys is limited due to a shortage of independent slip systems. In particular, c-axis compression cannot be accommodated by any of the easy slip or twinning modes. Basal-textured samples of pure Mg and Mg-15 at. pct Li were examined for the presence of 〈c+a〉 dislocations by post-mortem transmission electron microscopy (TEM) after a small deformation, which forced the majority of grains to compress nearly parallel to their c-axes. A higher density and more uniform distribution of 〈c+a〉 dislocations is found in the Li-containing alloy. Because the 1/3〈11 3〉 {11 } pyramidal slip mode offers five independent slip systems, it provides a satisfying explanation for the enhanced ductility of α-solid solution Mg-Li alloys as compared to pure Mg. The issue of 〈c+a〉 dislocation dissociation and decomposition remains open from an experimental point of view. Theoretically, the most feasible configuration is a collinear dissociation into two 1/2〈c+a〉 partial dislocations, with an intervening stacking fault on the glide plane. It is speculated that Li additions may lower the fault’s energy and, thereby, increase the stability of this glissile configuration. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Bulk and interface boundary diffusion in group IV hexagonal close-packed metals and alloys

Bulk and grain boundary (GB) self-diffusion and substitutional solute diffusion in group IV hexagonal close-packed (hcp) metals (α-Ti, α-Zr, and α-Hf) are reviewed. The recent results obtained on high-purity materials are shown to approach closely the “intrinsic” diffusion characteristics. The enhancement effect of fast-diffusing impurities (such as Fe, Ni, or Co) is discussed for both self- and substitutional bulk solute diffusion in terms of the interstitial solubility of the impurity atoms. In GB self-diffusion, the impurity effect is found to be less dramatic. The results obtained on high-purity hcp materials can be interpreted in terms of intrinsically ‘normal’ vacancy-mediated GB diffusion, with the ratio of GB to volume diffusion activation enthalpies of Q _gb /Q ≈ 0.6. The GB self-diffusion in group IV hcp metals reveals distinct systematics. Bulk self-diffusion and fast interstitial solute diffusion (Fe and Ni) in the hcp phase α ₂-Ti₃Al are reviewed. Interphase boundary diffusion of Ti in the unidirectional lamellar α ₂/γ structure of the two-phase Ti₄₈Al₅₂ alloy is analyzed with respect to the phase boundary structure and GB self-diffusion in α ₂-Ti₃Al. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S—B) twinning mechanism is conservative. The possible wider significance of the S—B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Lousiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S-B) twinning mechanism is conservative. The possible wider significance of the S-B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Observations of room-temperature creep recovery in titanium alloys

Conventional α(hcp) and α(hcp)/β(bcc) titanium alloys exhibit significant primary creep strains at room temperature and at stresses well below their macroscopic yield strength. It has been previously reported in various materials systems that repeated unloading during primary creep testing may either accelerate or retard the accumulation of creep strains. These effects have been demonstrated to depend on both microstructure and the applied stress. This article demonstrates that significant room-temperature recovery occurs in technologically relevant titanium alloys. These recovery mechanisms are manifested as a dramatic increase in creep rates (by several orders of magnitude) upon the introduction of individual unloading events, ranging from 1 minute to 365 days, during primary creep tests. Significant increases in both creep rate and the total accumulated creep strain were observed in polycrystalline single α-phase Ti-6Al, polycrystalline α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and individual α/β colonies of Ti-6242. Based on transmission electron microscopy (TEM) studies of the active deformation mechanisms, it is proposed that the presence of significant stress concentrations within the α phase of these materials, in the form of dislocation pileups, is a prerequisite for significant room-temperature recovery. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Diffusion along grain and interphase boundaries in alpha Zr and Zr-2.5 wt pct Nb alloy

Diffusion parameters of Cr diffusion along the α/β interphase boundaries of a Zr-2.5 wt pct Nb alloy are presented. The conventional radiotracer technique combined with serial sectioning of the samples was applied. In the Arrhenius plot, it is possible to consider only one straight line (with Q=133 kJ/mol for 615<T<953 K) or two zones (with Q=230 kJ/mol for 773<T<953 K and Q=77 kJ/mol for 615<T<773 K). An analysis is made of these results together with previous data concerning diffusion along short circuits paths in α-Zr (grain boundaries) and Zr-2.5 wt pct Nb (interphase boundaries): Zr and Nb as the alloy component elements and Ni, Fe, and Co as other relevant impurities. Different mechanisms are proposed: a vacancy mechanism for Zr and Nb and an interstitial-like mechanism for the impurities, for both kind of boundaries. The influence on diffusion and the estimated values of the impurities segregation in the α phase are discussed in the work. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Deformation behavior of HCP Ti-Al alloy single crystals

Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for 〈c+a〉 slip observed in compression loading. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Computational modeling of through-thickness dynamic impact response in cross-rolled Ti-6Al-4V plates

The through-thickness ballistic impact response for extra-low-interstitial (ELI)-grade Ti-6Al-4V crossrolled and heat-treated plates is numerically simulated, taking into account specific material textures. Ballistic tests suggest that the incidence of material plugging is affected by specific thermal/mechanical processing paths above and below the β-transus temperature. Ti-6Al-4V polycrystalline aggregates are nominally textured via routine rolling-deformation and thermal-processing schedules in the α+β or β-phase fields. Thus, realistic processing textures, viz., the basal, transverse, as well as an idealized random texture, are simulated via a two-dimensional (2D) constitutive model for slip and twinning (treated here as pseudoslip) 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 polycrystalline mechanical response is examined with respect to macroscopic shear loading, such as that which may take place during dynamic punch-through processes. A ranking of the material textures is prescribed via numerically derived measures of external work performed. Results indicate that transverse textures generated by thermal/mechanical processing in the β-phase field are particularly susceptible to impact failures. Such microstructural and orientation-sensitive rankings are a keen marker of material performance and offer a refinement over the quasistatically generated Mil-A-4077 acceptance criterion for ballistic impact applications. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following AMS committeers: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.