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Register a new userCold Creep of Titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations
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There is a long standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a significant fatigue life reduction. It is thought that localised time dependent plasticity in soft grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring hard grain poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. This work uses a novel approach of in situ synchrotron X-ray diffraction during stress relaxation tests, to quantify the time dependent plasticity. Measured lattice strains from multiple lattice families (21 diffraction rings) were compared with simulated lattice strains from crystal plasticity finite element (CPFE) simulations. The prism slip parameters were found to show stronger strain rate sensitivity compared to basal slip, and this has a significant effect on stress redistribution to hard grain orientations during cold creep.
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Crystal-level strain rate sensitivity and temperature sensitivity are investigated in Zircaloy-4 using combined of bending creep test, digital image correlation, electron backscatter detection and thermo-mechanical tensile tests with crystal plasticity modelling. Crystal rate-sensitive properties are extracted from room temperature microscale creep, and temperature sensitivity from thermal polycrystalline responses. Crystal plasticity results show that large microscale creep strain is observed near notch tip increased up to 50% due to cross-slip activation. Grain-level microscale SRS is highly heterogeneous, and its crystallographic sensitivity is dependent on plastic deformation rate and underlying grain-based dislocation slip activation. Pyramidal <c+a> slip and total dislocation pileups contribute to temperature-sensitive texture effect on yielding and strength hardening. A faithful reconstruction of polycrystal and accurate rate-sensitive single-crystal properties are the key to capture multi-scale SRSs.
We use three-dimensional discrete dislocation dynamics simulations (DDD) to study the evolution of interfacial dislocation network (IDN) in particle-strengthened alloy systems subjected to constant stress at high temperatures. We have modified the dislocation mobility laws to incorporate the recovery of the dislocation network by the climb. The microstructure consists of uniformly distributed cuboidal inclusions embedded in the simulation box. Based on the systematic simulations of IDN formation as a function of applied stress for prescribed inter-particle spacing and glide-to-climb mobility ratio, we derive a relation between effective stress and normalized dislocation density. We use link-length analysis to show self-similarity of immobile dislocation links irrespective of the level of applied stress. Moreover, we derive the dependence of effective stress on the ratio between mobile to immobile dislocation density based on the Taylor relation for strain hardening materials. We justify the relation with the help of a theoretical model which takes into account the balance of multiplication and annihilation rates of dislocation density.
Wehrenberg et. al. [Nature 550 496 (2017)] used ultrafast in situ x-ray diffraction at the LCLS x-ray free-electron laser facility to measure large lattice rotations resulting from slip and deformation twinning in shock-compressed laser-driven [110] fibre textured tantalum polycrystal. We employ a crystal plasticity finite element method model, with slip kinetics based closely on the isotropic dislocation-based Livermore Multiscale Model [Barton et. al., J. Appl. Phys. 109 (2011)], to analyse this experiment. We elucidate the link between the degree of lattice rotation and the kinetics of plasticity, demonstrating that a transition occurs at shock pressures of $sim$27 GPa, between a regime of relatively slow kinetics, resulting in a balanced pattern of slip system activation and therefore relatively small net lattice rotation, and a regime of fast kinetics, due to the onset of nucleation, resulting in a lop-sided pattern of deformation-system activation and therefore large net lattice rotations. We demonstrate a good fit between this model and experimental x-ray diffraction data of lattice rotation, and show that this data is constraining of deformation kinetics.
In their Letter, Haziot et al. [Phys. Rev. Lett. 110 (2013) 035301] report a novel phenomenon of giant plasticity for hcp Helium-4 quantum crystals. They assert that Helium-4 exhibits mechanical properties not found in classical plasticity theory. Specifically, they examine high-quality crystals as a function of temperature and applied strain, where the shear modulus reaches a plateau and dissipation becomes close to zero; both quantities are reported to be independent of stress and strain, implying a reversible dissipation process and quantum tunneling. In this Comment, we show that these signatures can be explained with a classical model of thermally activated dislocation glide without the need to invoke quantum tunneling or dissipationless motion. Recently, we proposed a dislocation glide model in solid Helium-4 containing the dissipation contribution in the presence of other dislocations with qualitatively similar behavior [Zhou et al., Philos. Mag. Lett. 92 (2012) 608].
New interatomic potentials describing defects, plasticity and high temperature phase transitions for Ti are presented. Fitting the martensitic hcp-bcc phase transformation temperature requires an efficient and accurate method to determine it. We apply a molecular dynamics (MD) method based on determination of the melting temperature of competing solid phases, and Gibbs-Helmholtz integration, and a lattice-switch Monte Carlo method (LSMC): these agree on the hcp-bcc transformation temperatures to within 2 K. We were able to develop embedded atom potentials which give a good fit to either low or high temperature data, but not both. The first developed potential (Ti1) reproduces the hcp-bcc transformation and melting temperatures and is suitable for the simulation of phase transitions and bcc Ti. Two other potentials (Ti2 and Ti3) correctly describe defect properties, and can be used to simulate plasticity or radiation damage in hcp Ti. The fact that a single EAM potential cannot describe both low and high temperature phases may be attributed to neglect of electronic degrees of freedom, notably bcc has a much higher electronic entropy. A temperature-dependent potential obtained from the combination of potentials Ti1 and Ti2 may be used to simulate Ti properties at any temperature.
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