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This study reports on the anisotropic indentation response of α-titanium. Coarse-grained titanium was characterized by electron backscatter diffraction. Sphero-conical nanoindentation was performed for a number of different crystallographic orientations. The grain size was much larger than the size of the indents to ensure quasi-single-crystal indentation. The hexagonal c-axis was determined to be the hardest direction. Surface topographies of several indents were measured by atomic force microscopy. Analysis of the indent surfaces, following Zambaldi and Raabe (Acta Mater. 58(9), 3516–3530), revealed the orientation-dependent pileup behavior of α-titanium during axisymmetric indentation. Corresponding crystal plasticity finite element (CPFE) simulations predicted the pileup patterns with good accuracy. The constitutive parameters of the CPFE model were identified by a nonlinear optimization procedure, and reproducibly converged toward easy activation of prismatic glide systems. The calculated critical resolved shear stresses were 150 ± 4, 349 ± 10, and 1107 ± 39 MPa for prismatic and basal 〈a〉-glide and pyramidal〈c + a〉-glide, respectively.
We report evidence for graphene layer rearrangements in heavy ion interactions with carbon onions at 140 MeV and 70 MeV per nucleon kinetic energies. Graphene layer rearrangements have been recently predicted in spherical and cylindrical multi-layer graphene systems. The implications of graphene layer rearrangement on the tribological performance of multi-layer nano-carbons in extreme environments are discussed.
Subgrain boundaries (subboundaries) in a creep deformed investment cast Ti-48Al-2Nb-2Cr specimen were characterized. A multi-stress drop test was performed at temperature 765 °C and stresses from 276 MPa to 103 MPa. The stress exponent n = 7. Subboundaries were observed both in equiaxed γ grains and within γ laths in lamellar grains. Dislocations within the subboundaries are characterized to be ordinary 1/2<110] dislocations. Subboundaries are found to occur on many crystallographic planes and no preferred crystallographic planes are found for subboundary formation in the crept TiAl specimen. Misorientations across the subboundaries are less than 1°. Analysis of subboundary formation mode indicates that the creep deformation in TiAl is pure metal type.
IN90211 has exhibited superplastic elongations above 500% at high homologous temperatures (0.76–0.82 Tm). A high strain rate and flow stress for optimum elongation was measured (1–5/sec, 20–60 MPa, 425–485 °C). The apparent strain rate sensitivity of m≈0.25 differs from the usual m≈0.5 observations of superplastic deformation. An analysis of the data at several strains indicates a highly temperature dependent threshold stress is present, with either a n=2 or n=3 assumption for the stress exponent. The magnitude of the threshold stresses in IN90211 are smaller than usually observed in a dispersion strengthened matrix (1–20% instead of ≈50% of the Orowan stress). Experimental evidence from creep experiments supports the n=3 deformation mechanism as the rate limiting step of deformation.
To gain a better understanding of the ductility limitations in TiAl alloys, the mechanisms involved in deformation strain transfer and/or microcrack initiation at grain boundaries have been examined in an equiaxed near-γ alloy. These studies have been carried out on both in-situ and ex-situ deformed bulk samples using scanning electron microscopy (SEM) techniques for both orientation analysis and deformation defect imaging. Selected area electron channeling patterns (SACPs) have allowed determination of grain orientations, eliminating ambiguity between the a and c axes. Deformation twins and dislocations have been imaged in the bulk samples using electron channeling contrast imaging (ECCI). A combination of ECCI contrast analysis and trace analysis based on orientations determined from SACP has allowed identification of the active deformation systems. Microcracks have been found to initiate at γ-γ boundaries as a result of an inability to adequately transfer twin strain from grain to grain. Once initiated, cracks propagate through cleavage and re-nucleation of grain boundary microcracks in front of the advancing crack. A geometric based predictive factor has been developed that accounts for microcrack initiation at γ-γ boundaries based in deformation twinning and strain accommodation by ordinary dislocations.
Growth orientation and type of internal structures are both observed to change abruptly as a function of growth temperature in catalyst free growth of gallium nitride nanowires. In the present work, corresponding temperature-dependent changes in the growth matrix substrate that can affect the availability of nucleation sites and influence the reactivity of constituent adatom materials in catalyst-free nanowire growth are investigated. The influence of Ga vapor pressure and an abrupt change in the availability of single versus molecular adatom constituents is identified as a possible controlling parameter.
As a part of an ongoing study of creep deformation mechanisms, mechanical twinning behavior was investigated in the lamellar region of a creep specimen. A multi-stress jump creep test was performed and the microstructures before and after deformation were investigated using optical and transmission electron microscopy. The identification of mechanical twins in a lamellar microstructure is discussed. Extensive mechanical twinning was observed in lamellar regions, in addition to slip and subgrain formation. The occurrence of mechanical twinning depended on the lamellar orientation with respect to the tensile axis. The mechanical twins are analyzed and discussed in terms of possible crystallographic twinning systems. In this case, a maximum resolved shear stress criterion for mechanical twinning is proposed to account for the observed orientations.
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