The role of twinning in the cavitation erosion of cobalt single crystals

The deformation characteristics contributing to the superior cavitation erosion properties of HCP cobalt single crystals have been determined. Results indicate that its erosion response is highly orientation sensitive. A homogeneous distribution of and glide occurs in {0110} crystals, whereas slip in the (0001) crystals is much more heterogeneous and consists mainly of dislocations. Continued exposure to cavitation nucleates a large number of twins, predominantly on the and planes in the and (0001) crystals respectively. The former twins are finer and more needle-like than the latter. The crystals are also significantly more erosion resistant than the (0001) crystals. The twin density increases continuously with cavitation exposure until a dense network of twins spans the entire exposed area. This fine-scale twinning is considered responsible for the superior erosion resistance of the metal.     

Phase Transition and Texture Evolution in the Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys Studied by a Neutron Diffraction Technique

The phase transition and influence of the applied stress on the texture evolution in the as-cast Ni-Mn-Ga ferromagnetic shape-memory alloys were studied by the time-of-flight (TOF) neutron diffraction technique. The neutron diffraction experiments were performed on the General Purpose Powder Diffractometer (Argonne National Laboratory). Inverse pole figures were determined from the neutron data for characterizing the orientation distributions and variant selections of polycrystalline Ni-Mn-Ga alloys subjected to different uniaxial compression deformations. Texture analyses reveal that the initial texture for the parent phase in the as-cast specimen was composed of , , , and , which was weakened after the compression deformation. Moreover, a strong preferred selection of martensitic-twin variants (and ) was observed in the transformed martensite after a compression stress applied on the parent phase along the cyclindrical axis of the specimens. The preferred selection of variants can be well explained by considering the grain/variant-orientation-dependent Bain-distortion energy. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Crystallography of directionally solidified magnesium alloy AZ91

The crystallographic direction of growth in directionally solidified magnesium alloy AZ91 has been studied by TEM and EBSP techniques in SEM. The main direction of growth is found to be . The dendrites have sixfold symmetry around the main direction, with secondary arms lying along the traces of the (0001), , and -planes, respectively. The secondary arms lying in the basal plane are crystallographically of the same type as the main direction: and .     

Microscopic Properties of Long-Period Ordering in Al-Rich TiAl Alloys

The ordering mechanism of long-period superstructures (LPSs) in Al-rich TiAl alloys has been investigated by high-resolution transmission electron microscopy (HRTEM). The LPSs are classified in terms of arrangements of base clusters with different shapes and compositions formed in Ti-rich (002) layers of L1₀-TiAl matrix: square Ti₄Al, fat rhombus Ti₃Al, and lean rhombus Ti₂Al type clusters. The HRTEM observations revealed that antiphase boundaries of long-range-ordered LPS domains and short-range-ordered microdomains are constructed by various space-filling arrangements of the base clusters. Such a microscopic property characterized by the base clusters and their arrangements is markedly analogous to that of the * special-point ordering alloys such as Ni-Mo. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.

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Interaction of deformation twin and 120 deg-rotational order fault domain boundary in the lamellar structure of two-phase TiAl-based alloys

Interactions between deformation twin and 120 deg-rotational domain boundary were studied by transmission electron microscopy in a two-phase TiAl-based alloy with fully lamellar structure deformed at room temperature. Three types of the interaction were observed, depending on the interaction geometry and crystallography faced by the incident twinning Shockleys. It was found that the incident twinning shear could be accommodated into the barrier domain by a reaction involving emission of 1/2 {111} _B slip in all the three types of interactions presumably since the slip required a small critical resolved shear stress (CRSS) and was always favored by the pile-up stress. Several reaction schemes involving 1/2 {111} _B slip for each type of the interactions were proposed by considering whether the reaction resulted in a reduced elastic energy and if the dissociated dislocations were able to glide away to minimize the total elastic energy associated with a long-range stress field of a pileup of the incident twinning partials. It is suggested that whether a reaction scheme is feasible would depend on behavior of other product dislocation except 1/2 {111} _B .