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• High cycle fatigue strains mapped by nanoscale resolution digital image correlation. • Fine bands of deformation in lamellar γ-TiAl alloy formed ∼50 nm from interfaces. • Unlike monotonic loading, high cycle fatigue strains remain highly localised. • Strain gradients develop along slip bands, increasing in magnitude with cycling. Microcompression testpieces of a lamellar γ-TiAl alloy were high cycle fatigued up to 105 cycles at stress ratios of 0.5–0.7, to maximum stresses above and below the yield stress. The localised deformation was studied by nano-scale digital image correlation strain mapping of a Pt patterned side-face of the pillars. With crystal orientation mapping, the fatigue deformation lines were found to locate at the γ/γ and γ/α 2 lamellar interfaces. The deformation lines were closely spaced slip lines in transmission electron imaging. Gradients of shear strain developed along the deformation lines, increasing in magnitude as cycling progressed. Extensive transverse twinning also occurred. [ABSTRACT FROM AUTHOR]

Lamellar γ-TiAl has a microstructure that spans multiple length scales: lamellar thickness (10 nm–1 μm), γ domain length (100 nm–10 μm) and colony size (100 μm–1 mm). Only by characterizing the deformation across the different scales can one understand how the microstructure influences plasticity and damage formation in this material. Here, we use digital image correlation and electron backscatter diffraction to map strain and lattice rotation with both sub-colony and sub-lamella spatial resolution in a polycrystalline lamellar γ-TiAl alloy with TiB 2 inclusions. Two lamellar thicknesses and temperatures are tested. It is shown that the hard mode-oriented colonies undergo minimal plastic strain. However, as the temperature is increased, macroscopic deformation bands develop across colonies of different orientations, independent of their local lamellar orientations. Increasing magnification, it is apparent that some segments of these bands occur by slip or twinning parallel to the lamellae, and others by transverse lattice rotation. In the thin lamellar condition, such macroscopic deformation bands also occur along colony boundaries; this causes a significant inhomogeneity in the strain distribution. Such strain inhomogeneities are also further characterised here at the microstructural scale of single lamellae, and locally at boride particles. • Nanoscale resolution digital image correlation (HR-DIC) strain mapping up to 700 °C. • Characterisation of plastic deformation and mechanical damage across multiple length scales in lamellar γ-TiAl. • Macroscopic bands of heightened deformation developed, focused at grain boundaries. • Methods to repress deformation bands by microstructural modifications are discussed. [ABSTRACT FROM AUTHOR]

Abstract The distribution of strain in hard mode oriented lamellar stacks of the two-phase γ-TiAl/α 2 -Ti 3 Al alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol% TiB 2 was measured at several temperatures up to 633 °C by in situ micropillar compression, complemented by electron backscatter diffraction orientation mapping and digital image correlation strain mapping of a thermally stable surface Pt speckle pattern. Post-mortem transmission electron microscopy further identified the finest scale deformation structures. It was found that slip and twinning transverse to the lamellae operates within discreet bands that zigzag across the lamellar structure. The shear strain within each band is approximately constant across the pillar width. This is inconsistent with current energetic models for transverse twin formation in γ-TiAl, which assume independent, non-interacting twins. This is explained using a mathematical formulation for the stress required to operate this transverse mechanical twinning as a function of strain. This study has elucidated how the multi-scale combination of several transverse twinning systems on different {111} planes in γ-TiAl lamellae can relieve the elastic stresses generated at a lamellar interface by the primary (highest Schmid factor) twinning system. It is thought that the facilitation of this mechanism will promote the ductilisation of lamellar γ-TiAl alloys. This is crucial for an increased damage tolerance and ease of component manufacture, leading to a more widespread use of γ-TiAl alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The occurrence of plasticity below the macroscopic yield stress during tensile monotonic loading of nearly lamellar Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB 2 at both 25 °C and 700 °C, and in two conditions of lamellar thickness, was measured by digital image correlation strain mapping of a remodelled Au surface speckle pattern. Such initial plasticity, not necessarily related to the presence of common stress concentrators such as hard particles or cracks, could occur at applied stresses as low as 64% of the general yield stress. For a same applied strain it was more prominent at room temperature, and located as slip and twinning parallel to, and near to or at (respect.) lamellar interfaces of all types in soft mode-oriented colonies. These stretched the full colony width and the shear strain was most intense in the centre of the colonies. Further, the most highly operative microbands of plasticity at specimen fracture were not those most active prior to yielding. The strain mapping results from polycrystalline tensile loading were further compared to those from microcompression testing of soft-mode stacks of lamellae milled from single colonies performed at the same temperatures. Combined with post-mortem transmission electron microscopy of the pillars, the initial plasticity by longitudinal dislocation glide was found to locate within 30–50 nm of the lamellar interfaces, and not at the interfaces themselves. The highly localised plasticity that precedes high cycle fatigue failure is therefore inherently related to the lamellar structure, which predetermines the locations of plastic strain accumulation, even in a single loading cycle. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The stress required to activate twinning of the longitudinal < 11 2 ¯ ] { 111 } system in the lamellar γ-TiAl phase of the alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol.% TiB 2 was measured at several temperatures up to 700 °C by in situ micropillar compression of soft mode oriented γ-TiAl/α 2 -Ti 3 Al lamellar stacks. The lamellae undergoing deformation twinning were identified by electron backscatter diffraction orientation mapping. In some cases, such lamellae were not constrained by domain or colony boundaries and longitudinal twinning was the only deformation mechanism observed based on digital image correlation strain maps. The resolved shear stress for such unconstrained twinning was found to increase monotonically with temperature from 25 °C to 700 °C. This is consistent with the stacking fault energy increasing with temperature as found in many metallic alloys, suggesting that the increased ease of deformation twinning at high temperature in bulk TiAl alloys is due to the increased ease with which the twinning shear can be accommodated by the neighbouring domains and lamellae with increasing temperature, rather than a thermal softening of the intrinsic twinning mechanism. [ABSTRACT FROM AUTHOR]

In this paper the occurrence of twinning parallel to the lamellae during compression at 700 °C of a polycrystalline nearly lamellar commercial γ-TiAl alloy, Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB 2 , has been studied and shown to lead to the formation of cracks at colony boundaries. However, the occurrence of this longitudinal twinning mode was less common by at least a factor of ten in tests at room temperature. Furthermore, the debonding of colony boundaries caused by the shear strain of longitudinal twinning is exacerbated when the same γ-TiAl variant favourably oriented for twinning occurs repeatedly in the lamellar structure. This effect was caused by the preferential nucleation of certain γ-TiAl variants in the presence of TiB 2 boride reinforcement. It is shown that the boride additions increase the probability of double, triple or even higher order multiply stacked γ-variants. This increases the resulting shear strain that must be accommodated and hence the probability of crack nucleation. [ABSTRACT FROM AUTHOR]

The occurrence of longitudinal twinning in the engineering alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol.% TiB 2 has been studied by measuring the changes in crystallographic orientation within individual lamellae during microcompression. Twinning in this alloy appeared to be a nucleation-limited process with the twins growing from lamellar boundaries at resolved shear stresses as low as 100 MPa, consistent with observations elsewhere. However, instead of forming twins ~ 10–200 nm in thickness, as in polysynthetically twinned crystals, the longitudinal twins in this alloy were initiated at a lamellar boundary and then spread through the whole lamella. [ABSTRACT FROM AUTHOR]

The development and validation of crystal plasticity models requires the ability to map deformation at the microstructural scale. Here, a new method of high-resolution deformation mapping is used to measure strain, material rotation and lattice rotation in austenitic stainless steel at sub-micron resolution. Electron back-scatter diffraction maps are used to link the deformation to the microstructure. Deformation occurs in domains, in which most of the plastic strain originates from the activation of a single slip system with high resolved shear stress. Within domains, slip is localized in lamellar regions that increase in number with strain. The deformation incompatibility between grains that develops as a consequence of this single crystal like behaviour is accommodated by either a gradient in slip intensity and the consequent development of lattice curvature at the grain boundary or the activation of an additional high stressed slip system and the consequent formation of a complementary deformation domain within the grain. In many cases, however, lattice curvature across grain boundaries is small because the deformation domains in neighbouring grains are compatible. The implications of these observations for continuum crystal plasticity modelling are discussed. [ABSTRACT FROM AUTHOR]

• Nanoscale digital image correlation (HR-DIC) strain mapping of HCF up to 670 °C. • Microplasticity mainly in softmode colonies: parallel to lamellae, near interfaces. • Poor strain transfer across colony boundaries at both temperatures. • At 25 °C, maximum stress influenced the slip band count more than did the cycle count. Microplasticity in a lamellar TiAl alloy upon high cycle tensile fatigue was measured by high resolution digital image correlation strain mapping at several stress and cycle increments, at 25 °C and 670 °C, for two lamellar thicknesses. Plastic deformation occurred primarily in soft-mode colonies at both temperatures, operating by slip parallel to the lamellae, and near lamellar interfaces. Plastic strains generally decreased to zero by the colony boundary; strain transfer across such boundaries was rare at both temperatures. At 25 °C, the maximum applied stress influenced the number of slip bands more so than did the number of loading cycles. [ABSTRACT FROM AUTHOR]

Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of γ / α 2 lamellar microstructure with the γ / α 2 interfaces being aligned 25 o and 55 o to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25 o micropillar while the anisotropy of the 55 o micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25 o micropillar less localised than that in the 55 o micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at γ / α 2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins. Deformation behaviour of (ϕ = 25 o )- and (ϕ = 55 o )-pillars: in-situ observation versus CP-FEM results. Image 1 • Crystal plasticity of lamellar TiAl is studied by in-situ testing and modelling. • CP-FEM well predicts the constitutive response and localisation of lamellar TiAl. • This study reveals the micro-mechanisms of anisotropy of lamellar TiAl. • Deformation twinning significantly relieves internal stresses at γ / α 2 interfaces. [ABSTRACT FROM AUTHOR]