Molecular dynamics investigation of deformation twinning in ?-TiAl sheared along the pseudo-twinning direction

In spite of being sheared along the so-called pseudo-twinning direction, ?-TiAl undergoes true twinning under zero pressure or hydrostatic tension by means of a specific combination of , and shears in two consecutive (1 1 1) matrix planes allowing the adjacent twin to thicken over one (1 1 1) atomic layer. The corresponding total shear strain of is four times as large as that generated by conventional deformation twinning or during the L10 to L11 transformation by or shears, respectively. This shear is substantially more effective in accommodating stress concentration and high strain rate than conventional deformation twinning. The conditions under which twinning by dislocations operates are interpreted based on a modified gamma-surface and discussed in terms of zonal partial dislocations.

Dongsheng Wanga?Rui Yanga?Patrick Veyssièreb
[a]Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China;[b]LEM, CNRS-ONERA, BP 72, 92322 Chatillon, France

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Deformation in metals after low-temperature irradiation: Part I – Mapping macroscopic deformation modes on true stress–dose plane

Macroscopic deformation modes, elastic, uniform plastic, and unstable plastic deformation modes, are mapped in tensile true stress–dose space for more than two dozen metallic materials consisting of 13 body-centered cubic (bcc), 11 face-centered cubic (fcc), and two hexagonal closed packed (hcp) metals. The boundaries between different deformation zones are set by the true stress versus dose curves: the yield stress (YS), plastic instability stress (PIS), and true fracture stress (FS) plotted as functions of dose. Values for these true stresses are obtained from uniaxial tensile tests or calculated from engineering tensile data using a linear strain-hardening model for necking deformation. The relatively low-strength annealed fcc metals display large uniform plasticity regions, while unstable deformation regions are dominant in the harder bcc and hcp metals. PIS values for all materials are independent of dose except for the precipitation-hardened IN718 alloy, where a decrease of PIS occurs due to an irradiation-induced change in second phases. In the bcc materials for high-temperature application, such as 9Cr ferritic/martensitic steels, sintered molybdenum, vanadium, and tantalum, the radiation-induced embrittlement is characterized in terms of FS decreasing with dose at relatively high doses. FS is nearly dose-independent below the critical dose for embrittlement. It is concluded that the tensile stress-based deformation mode maps effectively integrate mechanical property information and characterize differences in radiation effects between crystalline structures or material groups.

Thak Sang Farrella?Meimei Lia
[a]Oak Ridge National Laboratory, P.O. Box 2008, MS-6151, Oak Ridge, TN 37831, USA

Orientation stability in equal channel angular extrusion. Part II: Hexagonal close-packed materials

The orientation stability in equal channel angular extrusion (ECAE) of hexagonal close-packed (hcp) crystals using a 90° die is investigated based on the three-dimensional lattice rotation fields from rate-dependent crystal plasticity simulations. The results show that for the major slip and twinning modes considered, the relatively stable orientations in ECAE are distributed along the h1? to h6? fibers in the Euler space and feature characteristic alignments of the a- or c-axis with respect to the macroscopic deformation axes. The application of such simulations is demonstrated by comparing the predictions with experimental ECAE textures in high-purity titanium and other hcp polycrystalline materials, including commercial-purity zirconium, beryllium and magnesium alloys. The simulation results for ECAE are also applied to derive the relatively stable orientations in conventional simple shear deformation.

[a]School of Mechanical Engineering, South China University of Technology, Guangzhou 510640, China

Orientation stability in equal channel angular extrusion. Part I: Face-centered cubic and body-centered cubic materials

Stability of crystallographic orientations is a key aspect in the characterization and understanding of texture evolution during plastic deformation. In this study, a rate-dependent crystal plasticity model was applied to investigate orientation stability during equal channel angular extrusion (ECAE) of face-centered cubic (fcc) and body-centered cubic (bcc) crystals. The stability of experimentally observed ideal orientations was examined according to lattice rotation fields computed at and around the orientations. It is shown that these ideal orientations are meta-stable under rate-sensitive conditions, and their stability generally increases with the decrease of strain rate sensitivity. The results also reveal a well-preserved duality in the lattice rotation and orientation stability between the two types of crystal structure. The stability results simulated at low strain rate sensitivities agree well with the experimental observations in one-pass ECAE of Al and Cu single crystals. In Part II of the paper, this analysis is extended to hexagonal materials.

[a]School of Mechanical Engineering, South China University of Technology, Guangzhou 510640, China

Asymmetric surface intermixing during thin-film growth in the Co–Al system: Role of local acceleration of the deposited atoms

Surface intermixing behavior during thin-film deposition in the Co–Al system was investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. Asymmetry of the surface intermixing was observed: Al deposition on a Co substrate resulted in an Al thin-film with an atomically sharp interface, while a Co thin-film deposited on an Al substrate had an interfacial intermixing layer of B2 structure. This phenomenon is discussed in terms of the kinetics of atomic intermixing on the surface. A kinetic criterion for the atomic intermixing is whether the increased kinetic energy of the deposited atom near the surface is larger than the energy barrier to atomic intermixing on the surface. Local acceleration of the deposited atoms near the surface provides an explanation of the puzzling phenomenon of the significant intermixing under low-energy deposition conditions such as thermal evaporation or molecular beam epitaxy.

Sang-Pil Kima?Seung-Cheol Leea?Kwang-Ryeol Chungb
[a]Computational Science Center, Future Fusion Research Laboratory, Korea Institute of Science and Technology, Seoul 136–791, Republic of Korea;[b]Division of Materials Science Engineering, Hanyang University, Seoul 133–791, Republic of Korea

A computational analysis of the deformation mechanisms of a nanocrystal–metallic glass composite

Simulations of a monatomic model amorphous matrix embedded with approximately 37% of a body-centered cubic phase demonstrate mechanisms by which nanocrystallites can alter the mechanical response of metallic glass. Three effects affect the resulting ductility: (i) the presence of weak amorphous–crystalline interfaces, (ii) the fraction of nanocrystallites oriented to prevent twinning relative to the loading stress, and (iii) the shear-induced growth and dissolution of the nanocrystallites when they are impinged by shear bands. While the first effect dominates in these simulations due to system size limitations, the third effect appears to be crucial for understanding the ductility of experimental samples. These simulations indicate that shear-induced growth of existing nanocrystallites, rather than nucleation of new crystalline regions, may account for the observed enhancement in ductility.

Yunfeng Shia?Michael L.
[a]Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA

The fatigue behavior of I-phase containing as-cast Mg–Zn–Y–Zr alloy

The fatigue behavior of as-cast Mg–12%Zn–1.2%Y–0.4%Zr alloy has been investigated. The S–N curve showed that the fatigue strength at 107 cycles was 45 MPa. Scanning electron microscopy observations on the surfaces of the failed and unfailed specimens (after up to 1 × 107 cycles) suggested that the slip bands could act as preferential sites for non-propagating fatigue crack initiation, and the I-phase could effectively retard fatigue crack propagation (FCP). The macro fracture morphology clearly indicated that the overall fracture surface was composed of three regions, i.e. a fatigue crack initiation region (Region 1), a steady crack propagation region (Region 2) and a tearing region (Region 3). High-magnification fractographs showed that only porosities can act as the crack initiation sites for all specimens. Moreover, for specimens with fatigue lifetimes lower than 2 × 105 cycles, the cracks mostly initiated at the subsurface or surface of the specimen. However, when the fatigue lifetime was equal to or higher than 2 × 105 cycles, the fatigue crack initiation sites transferred to the interior of the specimen. The maximum stress intensity factors corresponding to the transition sites between Regions 1, 2 and 3 were 2 and 4.2 MPa m1/2, respectively. When the maximum stress intensity factor Kmax was lower than 4.2 MPa m1/2, in the steady crack propagation region, due to the retarding effect of I-phase/?-Mg matrix interfaces, the fatigue cracks tended to pass the I-phase/?-Mg matrix eutectic pockets directly and propagated through the grain cells, resulting in the formation of many flat facets on the fracture surface. However, when the maximum stress intensity factor was higher than 4.2 MPa m1/2, in the sudden failure region, the rigid bonding of I-phase/?-Mg matrix interfaces was destroyed and the cracks preferentially propagated along the interfaces, which resulted in the fracture surface being almost completely composed of cracked I-phase/?-Mg matrix eutectic pockets. Based on microstructural observation and the fracture characteristics of the two regions, it is suggested that with an increase in crack tip driving force, the FCP mode changes from transgranular propagation to intergranular propagation.

D.K. Xua?L. Liub?Y.B. Xub?E.H.
[a]Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China;[b]Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Phase stabilities and thermal decomposition in the Zr1?xAlxN system studied by ab initio calculation and thermodynamic modeling

The ab initio density functional theory is used to calculate the lattice constants, total energy and bulk modulus of binary hexagonal close packed (hcp) and face-centered cubic (fcc) ZrN, AlN and ternary Zr1?xAlxN phases. The calculated results are supported by previously reported experimental and theoretical data. The lattice stabilities of binary phases and demixing energies of ternary phases calculated by the ab initio method are then used in thermodynamic modeling to construct the Gibbs free energy diagram of the immiscible quasi-binary ZrN–AlN system at different temperatures. The results show that, for the composition x 0.472, Zr1?xAlxN solid solutions are more stable in fcc than in hcp structure, which is in a good agreement with the experimentally reported value of x < 0.43. The constructed chemically binodal and spinodal decomposition curves show that fcc Zr1?xAlxN solution coatings should undergo phase decomposition into fcc ZrN and fcc AlN. However, considering the relatively large lattice mismatch between the fcc ZrN and the fcc AlN, the coherent spinodal decomposition may probably be hindered due to rapid phase transformation from fcc AlN to the stable hcp AlN. This is supported experimentally in that no intermediate fcc AlN has so far been observed in the Zr–Al–N system. S.H. Shenga?R.F. Zhanga?S. [a]Department of Chemistry, Technical University of Munich, Lichtenbergstrasse 4, D-85747 Munich, Germany