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Breakdown of Schmid's law is a long-standing problem for exploring the orientation-dependent deformation mechanism in intermetallics. The lack of atomic-level understanding of the selection rules for the plastic deformation modes has seriously limited designing strong and ductile intermetallics for high-temperature applications. Here we put forward a transparent model solely based on first principles simulations for mapping the deformation modes in γ -TiAl polysynthetic twinned alloys. The model bridges intrinsic energy barriers and different deformation mechanisms and beautifully resolves the complexity of the observed orientation-dependent deformation mechanisms. Using the model, one can elegantly reveal the atomic-level mechanisms behind the unique channeled flow phenomenon in lamellar TiAl alloys. [ABSTRACT FROM AUTHOR]

The orientation of austenite (γ) reverted from lath martensite has been studied using electron backscatter diffraction (EBSD) in Fe-2%Mn-1.5%Si-0.3%C (mass%) alloy. Two morphologies of γ, acicular and globular, are formed during reversion. Nucleation sites for acicular γ are lath, block and sub-block boundaries, while those for globular γ are prior γ grain boundaries, as well as inside packets. Both acicular and globular γ hold a near Kurdjumov-Sachs (K-S) orientation relationship with at least one of its surrounding tempered martensite blocks. Most acicular γ have almost identical orientations with the prior γ, while only a part of acicular γ have a specific twin-related variant with the prior γ. Furthermore, grain boundary globular γ has almost identical orientations with the prior γ into which it does not grow. The formation of acicular and grain-boundary globular γ was attributed to strong variant selection during γ nucleation at lath, block and sub-block boundaries. On the other hand, orientation relationship analyses among intragranular globular γ, tempered martensite and cementite suggest that the nucleation of γ at cementite and tempered martensite interfaces causes weakening of the variant selection for the formation of intragranular globular γ. [ABSTRACT FROM AUTHOR]

Atomistic modeling is used to investigate the mechanical response to compressive and tensile straining of the Bagaryatskii orientation relationship between ferrite and cementite within pearlite. A range of interlamellar spacings and ferrite to cementite ratios are considered and values for important mechanical properties, including elastic modulus, yield stress, flow stress, and ductility, are determined. These values are fit to simple elasto-plastic models, thus allowing for the easy interpolation of states not simulated. Transverse loading can be described using simple 1-D composite theory, while longitudinal loading requires the consideration of the strain compatibility of the interface. The mechanical properties are shown to be largely dependent on the volume ratios of the cementite and ferrite, with the interlamellar spacing having an increasing role as it reaches smaller values. Additionally, the effect of the interface is discussed and characterized, including its role as a nucleation site for dislocations in both the ferrite and cementite. [ABSTRACT FROM AUTHOR]

Interactions between neighboring grains influence the macroscale behavior of polycrystalline materials, particularly their deformation behavior, damage initiation and propagation mechanisms. However, mapping all of the critical material properties normally requires that several independent measurements are performed. Here we report the first grain mapping of a polycrystalline foil using a pixelated energy-dispersive X-ray area detector, simultaneously measuring X-ray fluorescence and diffraction with the Maia detector in order to determine grain orientation and estimate lattice strain. These results demonstrate the potential of the next generation of X-ray area detectors for materials characterization. By scanning the incident X-ray energy we investigate these detectors as a complete solution for simultaneously mapping the crystallographic and chemical properties of the sample. The extension of these techniques to broadband X-ray sources is also discussed. [ABSTRACT FROM AUTHOR]

The microstructure in a lamellar γ-based Ti-45Al-8.5Nb alloy has been investigated by transmission electron microscopy (TEM). The results show that there exists a nano-scale modulated structure with tweed contrast within α 2 laths and the orthorhombic phase as a constituent of the modulated structure has the O1 structure with a random occupancy of Ti and Nb atoms on the Wyckoff sites 8 g and 4c2. The orthorhombic phase has a thin plate-like morphology with orientation relationships of {001} O //{0001}α 2 and <110> O //< 11 2 ¯ 0 >α 2 . The habit plane for orthorhombic phase variants lies at {350} O //{ 13 4 ¯ 0 }α 2 , and the O1/α 2 interface is not atomic flat. The results further exhibit an enrichment of Nb in the orthorhombic phase in comparison with the α 2 parent phase. Based on the TEM evidences, the formation of the orthorhombic phase arises from a phase separation reaction in the α 2 parent phase, which is dominated by a diffusion mechanism. [ABSTRACT FROM AUTHOR]

In the present study, sub β-transus static annealing was carried out on a (α+β)-warm-rolled (700 °C, 90% thickness reduction) Ti-6Al-4V alloy at 900 °C for various durations (15, 30, 45, 60, 75 and 90 min). The key observation was that the spheroidization response of the constituent α-colonies from the starting warm-rolled material differs significantly during subsequent (α+β)-annealing treatment. While some of the α-colonies underwent an early morphological conversion from lamellar to equiaxed, some others stayed stable for prolonged durations. This unique phenomenon has been examined, for the first time, from an orientation perspective by coupling slip activation, boundary formation, and interfacial energy anisotropy of individual α-colonies with their response to static spheroidization. Orientation of the α-colonies in reference to the loading directions dictate the nature of slip activation (single versus multiple slip; basal or prism slip plus pyramidal 〈c+a〉 slip) during prior (α+β)-rolling. During subsequent static (α+β)-annealing, this factor translates into the relative ease of boundary splitting and thermal grooving for them. Formation of longitudinal boundaries, then set the order of spheroidization for diffusion based coarsening processes during long term annealing. Anisotropy in interfacial energy, owing to the loss of coherency during prior deformation (warm-rolling), creates further orientation dependency in the spheroidization sequence. An immediate consequence of this orientation dependent spheroidization response is realized in the formation of the macro-zones for two-phase Titanium alloys after secondary thermomechanical processing. Some remedial processing strategies are highlighted therein. [ABSTRACT FROM AUTHOR]

By compression along the normal direction of rolled pure titanium sheet, primary { 11 2 ¯ 2 } type compression twins were observed, followed by secondary { 10 1 ¯ 2 } extension twins. The latter can be classified into three groups according to their misorientation with respect to the parent matrix grains: 41.34° around < 5 143 ¯ > (Group I), 48.44° around < 5 ¯ 50 3 ¯ > (Group II), and 87.85° around < 7 43 ¯ 0 > (Group III). The experimental observations revealed the following activity frequency of these groups: Group II is the most frequent followed by Group I, and only a few secondary twins can be seen for Group III. When the classical Schmid factor (SF) analysis is applied, the smallest activity is predicted for Group III, in agreement with the experimental observations. However, the SF based criterion fails to distinguish between Group I and Group II variants. Similarly, the twin-shear accommodation based variant selection model proposed earlier for titanium [Qin and Jonas, Acta Mater. 75 (2014) 198–211] is not effective because the easiest accommodation by prismatic slips favors Group III variants which are nearly absent in the experiment. A possible explanation can be based on the smallest inclination angle of the secondary twinning habit plane with respect to the primary one, proposed by Barnett et al. [Acta Mater. 56 (2008) 5–15] which clearly favors Group II. It cannot make distinction between variants of the same group but can predict the right variants if complemented with the SF criterion. In the present work a new approach is proposed in order to disclose the preference for Group II variant over Group I. It is based on the special twinning geometry that applies to Group II; the intersection line of the primary and secondary twin planes lies in an active prismatic plane in the primary twin. Consequently, dislocation reactions are possible for the selection of secondary twin variant in Group II. Namely, prismatic < a > type dislocations in the primary twin can produce partial dislocations that activate the corresponding secondary twin variant. Nevertheless, a further selection rule has to be applied to choose one out of the two possible Group II variants; this is based on the higher Schmid factor of the two secondary variants. By contrast, pyramidal dislocations would be required for the formation of Group I twins, which is less likely due to their relatively high critical resolved shear stress. [ABSTRACT FROM AUTHOR]

Experimental pole figures are an important input for microstructure homogenization models. In this paper, we derive an exact analytical formulation to quantify the uncertainties in homogenized properties due to uncertainty in the experimentally measured pole figures. The pole figures are acquired from a set of Ti-7Al alloy samples. These samples were obtained from the same process: by compressing a beta forged ingot at room temperature followed by annealing. The samples were taken from different regions of the original ingot, and this created variability in the resulting pole figures. The joint multivariate probability distributions of the computed orientation distribution function (ODF) is then found using the method of characteristic functions based on a Gaussian model of the pole figures. Engineering properties such as elastic modulus can be obtained by volume averaging over the ODF. We also show that uncertainty in elastic properties can be analytically obtained using direct transformation of random variables in the homogenization approach. [ABSTRACT FROM AUTHOR]

Atomistic modeling was used to investigate the energetics and structure of the Bagaryatskii orientation relationship between ferrite and cementite within pearlite. The atomic level results show that the interface structure consists of a rectangular array of dislocations that lie along the high symmetry directions of the interface. The interface can be constructed using three different atomic terminating planes in the cementite structure, which dictates the chemistry and registry of the interface and controls the interfacial energy. The FeC-Fe terminating plane is always the lowest energy because the interfacial dislocations are most easily able to spread on these planes, thus reducing the interfacial energy. These atomistic results compare favorably with results from a continuum model based on O-lattice theory and anisotropic continuum theory. The dislocation spacing and interfacial energies predicted from the continuum level agree well with the atomistic simulation results. [ABSTRACT FROM AUTHOR]

We present the orientation dependent indentation response of pure magnesium during single grain indentation. A conical indenter and maximum loads between 50 mN and 900 mN were employed. Indent topographies were acquired by confocal microscopy. The indents were also characterized by electron backscatter orientation microscopy for their microstructures. Pronounced activation of specific twinning systems was observed around the impressions. The resulting data were compiled into the inverse pole figure presentation of indent microstructures and topographies after Zambaldi and Raabe, Acta Mater. (2010). Three-dimensional crystal plasticity finite element simulation of the indentation deformation supports the interpretation of the orientation dependent slip and twinning patterns around the indents. The match between the activation of observed and simulated twinning variants is discussed with respect to the conditions for nucleation and growth of extension twins. Furthermore, the compatibility of the twinning strains with the imposed deformation is discussed based on the expanding cavity model of indentation. The orientation dependent response of magnesium during indentation is compared to the literature data for indentation of alpha-titanium and beryllium. Recommendations are given on how to exploit the characteristic nature of the observed indentation patterns to rapidly assess the relative activity of deformation mechanisms and their critical shear stresses during alloy development. [ABSTRACT FROM AUTHOR]

The anisotropic tensile response of fully processed cold-rolled grain-oriented (CRGO) steel was studied for two crystallographic orientations: (1 1 0) 〈0 0 1〉 and (1 1 0) 〈1 1 ¯ 1〉. They showed remarkably different stress–strain behavior and corresponding developments in deformed microstructures. The (1 1 0) 〈0 0 1〉 oriented CRGO steel specimens retained their orientation stability until ε (true strain) = 0.07. On the other hand, (1 1 0) 〈1 1 ¯ 1〉 oriented specimens underwent significant reorientation and displayed formation of strain localizations by ε = 0.03. Discrete dislocation dynamics (DDD) simulations were carried out – three-dimensional (3-D) for “limited” (∼10 −4 ) plastic strain and two-dimensional (2-D) for the experimentally imposed strain – to investigate the orientation effects on the tensile response of the CRGO steel specimens. 2-D DDD simulations were able to provide qualitative and quantitative estimates of the post-yield tensile behavior of (1 1 0) 〈0 0 1〉 and (1 1 0) 〈1 1 ¯ 1〉 samples. Direct comparison between experimental and simulation results confirmed the orientation effect on the overall macroscopic response of the specimens. It was observed that the response of (1 1 0) 〈1 1 ¯ 1〉 specimens showed features of reorientation and textural softening and these were captured by the Taylor type deformation simulations. [ABSTRACT FROM AUTHOR]

The early stages of recrystallization have been systematically characterized in single crystal metals of medium and low stacking fault energy. Goss {1 1 0}〈0 0 1〉 and brass {1 1 0}〈1 1 2〉 oriented samples of Ni, Cu and Cu–2 wt.% Al alloy were deformed in a channel die to a logarithmic strain of 0.51 to develop a homogeneous structure composed of two sets of symmetrical primary microbands and then lightly annealed. Scanning electron microscopy/electron backscattered diffraction analyses demonstrate a strong relation between as-deformed orientations and the limited number of recrystallized grain orientations. The disorientation angles across the recrystallization front are mostly grouped in the ranges of 25–35° and 45–55° around axes located near, but not at, the normals of all four {1 1 1} planes. The original nuclei possess disorientation angles of 25–35°. The orientation of the growing new grain quickly transforms through the formation of a first generation twin, leading to disorientations of 45–55°. The most frequent situation occurs when the normal of the twinning face plane is situated near the rotation axis, around which the crystal lattice of the “primary nuclei” rotates. Based on the anisotropy of grain growth, a possible mechanism of orientation generation and grain growth by thermally activated movement of dislocation families on {1 1 1} planes is proposed. [ABSTRACT FROM AUTHOR]

The orientation dependence of hardness and nanoindentation-induced deformation mechanisms of differently orientated tungsten carbide (WC) grains in WC–Co hardmetal were studied. Electron backscatter diffraction, atomic force microscopy and scanning electron microscopy investigations were performed to determine the grain orientation, and to study the surface morphology and the resulting deformation fields around the indents. The hardness of the differently orientated WC grains showed significant angle dependence from the basal towards the prismatic directions, but there was only a slight change in hardness between the two types of prismatic orientations ( ( 1 0 1 ¯ 0 ) and ( 2 1 ¯ 1 ¯ 0 ) ). Sink-in and pile-up effects, together with highly deformed regions and dislocation steps, were revealed around the imprints in the case of basal and prismatic orientations, respectively. A theoretical model is proposed in which the critical force for slip activation is determined as a function of orientation, based on the possible slip systems of WC. The predictions of the present model concerning the measured hardness values and the deformation field around the indents together with the sink-in effect are in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

When Widmanstätten ferrite is produced by mechanical activation, it forms in pairs of self-accommodating plates of near-identical orientation. An explanation is proposed for the lack of orientation difference between these pairs. This done by deriving all the possible rotation axes and angles associated with the Kurdjumov–Sachs, Nishiyama–Wassermann, Pitsch, Greninger–Troiano and inverse Greninger–Troiano relationships. It is shown that whereas the misorientations associated with these relationships are conventionally described in terms of minimum angles, in every case an alternative rotation axis is available around which the angle is in the range 90.0–90.12°. Given that the plates form as a result of opposite shears, the misorientations between two plates in a pair formed according to any of the usual orientation relationships all fall in the range 180–180.24°. [ABSTRACT FROM AUTHOR]

Abstract: During Pb-free soldering, βSn often requires a high nucleation undercooling and there is an ongoing effort to develop nucleation catalysts. It is shown here that NiSn4, PdSn4 and PtSn4 are heterogeneous nucleants for βSn, reducing the nucleation undercooling to ∼4K when these intermetallics are present either in the bulk solder or as the interfacial layer. Nucleation catalysis occurs by βSn nucleating on the (008) facet of XSn4 crystals with an orientation relationship (OR) (100)Sn||(008)XSn4 and [001]Sn||[100]XSn4 where there is a planar lattice match of ∼5%. This OR is also the origin of well-aligned lamellar βSn–XSn4 eutectic morphologies, even though the eutectics contain less than 2vol.% of faceted NiSn4, PdSn4 or PtSn4. [Copyright &y& Elsevier]

Abstract: Line profile analysis was carried out on neutron diffraction patterns collected by the energy-dispersive method for an in situ tensile-deformed AISI-316 stainless steel specimen. The experiments were carried out at the VULCAN engineering beam line of the spallation neutron source of the Oak Ridge National Laboratory. Both the dislocation densities and the local stresses in grains oriented with different h k l crystal directions along the tensile axis were determined. The work-hardening equation of Taylor was tested for the h k l-dependent phenomenological constant α. The grain-orientation-dependent α values were directly related to the heterogeneity of dislocation distribution in correlation with previous transmission electron microscopy data. [Copyright &y& Elsevier]

Abstract: A transmission-electron-microscope-based orientation mapping technique that makes use of beam precession to achieve near-kinematical conditions was used to map the phase and crystal orientations in nanolamellar Cu/Nb composites with average layer thicknesses of 86, 30 and 18nm. Maps of high quality and reliability were obtained by comparing the recorded diffraction patterns with pre-calculated templates. Particular care was taken in optimizing the dewarping parameters and in calibrating the frames of reference. Layers with thicknesses as low as 4nm were successfully mapped. Heterophase interface plane and character distributions (HIPD and HICD, respectively) of Cu and Nb phases from the samples were determined from the orientation maps. In addition, local orientation relation stereograms of the Cu/Nb interfaces were calculated, and these revealed the detailed layer-to-layer texture information. The results are in agreement with previously reported neutron-diffraction-based and precession-electron-diffraction-based measurements on an accumulated roll bonding (ARB)-fabricated Cu/Nb sample with an average layer thickness of 30nm as well as scanning-electron-microscope-based electron backscattered diffraction HIPD/HICD plots of ARB-fabricated Cu/Nb samples with layer thicknesses between 200 and 600nm. [Copyright &y& Elsevier]

Abstract: Epitaxially grown thin films with nominal composition Ni50Mn30Ga20 and thickness 1.5μm were prepared on MgO(100) substrate with a Cr buffer layer by DC magnetron sputtering. The surface layer microstructures of the as-deposited thin films consist of non-modulated (NM) martensite plates with tetragonal structure at ambient temperature, which can be classified into the low and high relative contrast zones of clustered plates (i.e. plate colonies) with parallel or near-parallel inter-plate interface traces in secondary electron images. Orientation analyses by electron backscatter diffraction revealed that individual NM plates are composed of alternately distributed thicker and thinner lamellar variants with (112)Tetr compound twin relationship and coherent interlamellar interfaces. In each plate colony, there are four distinct plates in terms of the crystallographic orientation of the thicker lamellar variants and therefore, in total, eight orientation variants. For the low relative contrast zones, both thicker and thinner lamellar variants in adjacent plates are distributed symmetrically across their inter-plate interfaces (along the substrate edges). At the atomic level, there are no unbalanced interfacial misfits and height misfits, resulting in long and straight inter-plate interfaces with homogeneous contrast. However, in the high relative contrast zones, the thicker and thinner lamellar variants in adjacent plates are oriented asymmetrically across their inter-plate interfaces (at ∼45° to the substrate edges). Significant atomic misfits appear in the vicinity of the inter-plate interfaces and in the film normal direction. The former result in bending of the inter-plate interfaces, and the latter give rise to the high relative contrast between adjacent plates. [Copyright &y& Elsevier]