In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4%(±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at-2% strain gradually increased from 26.3% to 43.5%;the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number;and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of-component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy.

Phonon broadening in high entropy alloys

Refractory high entropy alloys feature outstanding properties making them a promising materials class for next-generation hightemperature applications.At high temperatures,materials properties are strongly affected by lattice vibrations(phonons).Phonons critically influence thermal stability,thermodynamic and elastic properties,as well as thermal conductivity.In contrast to perfect crystals and ordered alloys,the inherently present mass and force constant fluctuations in multi-component random alloys(high entropy alloys)can induce significant phonon scattering and broadening.Despite their importance,phonon scattering and broadening have so far only scarcely been investigated for high entropy alloys.We tackle this challenge from a theoretical perspective and employ ab initio calculations to systematically study the impact of force constant and mass fluctuations on the phonon spectral functions of 12 body-centered cubic random alloys,from binaries up to 5-component high entropy alloys,addressing the key question of how chemical complexity impacts phonons.We find that it is crucial to include both mass and force constant fluctuations.If one or the other is neglected,qualitatively wrong results can be obtained such as artificial phonon band gaps.We analyze how the results obtained for the phonons translate into thermodynamically integrated quantities,specifically the vibrational entropy.Changes in the vibrational entropy with increasing the number of elements can be as large as changes in the configurational entropy and are thus important for phase stability considerations.The set of studied alloys includes MoTa,MoTaNb,MoTaNbW,MoTaNbWV,VW,VWNb,VWTa,VWNbTa,VTaNbTi,VWNbTaTi,HfZrNb,HfMoTaTiZr.

Prediction of pressure-promoted thermal rejuvenation in metallic glasses

Rejuvenation is the structural excitation of glassy materials,and is a promising approach for improving the macroscopic deformability of metallic glasses.This atomistic study proposes the application of compressive hydrostatic pressure during the glass-forming quenching process and demonstrates highly rejuvenated glass states that have not been attainable without the application of pressure.Surprisingly,the pressure-promoted rejuvenation process increases the characteristic short-and mediumrange order,even though it leads to a higher-energy glassy state.This‘local order’–‘energy’relation is completely opposite to conventional thinking regarding the relation,suggesting the presence of a well-ordered high-pressure glass/high-energy glass phase.We also demonstrate that the rejuvenated glass made by the pressure-promoted rejuvenation exhibits greater plastic performance than as-quenched glass,and greater strength and stiffness than glass made without the application of pressure.It is thus possible to tune the mechanical properties of glass using the pressure-promoted rejuvenation technique.

Temperature-dependent phonon spectra of magnetic random solid solutions

A first-principles-based computational tool for simulating phonons of magnetic random solid solutions including thermal magnetic fluctuations is developed.The method takes fluctuations of force constants due to magnetic excitations as well as due to chemical disorder into account.The developed approach correctly predicts the experimentally observed unusual phonon hardening of a transverse acoustic mode in Fe–Pd an Fe–Pt Invar alloys with increasing temperature.This peculiar behavior,which cannot be explained within a conventional harmonic picture,turns out to be a consequence of thermal magnetic fluctuations.The proposed methodology can be straightforwardly applied to a wide range of materials to reveal new insights into physical behaviors and to design materials through computation,which were not accessible so far.

Mechanical properties of Fe-rich Si alloy from Hamiltonian

The physical origins of the mechanical properties of Fe-rich Si alloys are investigated by combining electronic structure calculations with statistical mechanics means such as the cluster variation method,molecular dynamics simulation,etc,applied to homogeneous and heterogeneous systems.Firstly,we examined the elastic properties based on electronic structure calculations in a homogeneous system and attributed the physical origin of the loss of ductility with increasing Si content to the combined effects of magneto-volume and D03 ordering.As a typical example of a heterogeneity forming a microstructure,we focus on grain boundaries,and segregation behavior of Si atoms is studied through high-precision electronic structure calculations.Two kinds of segregation sites are identified:looser and tighter sites.Depending on the site,different segregation mechanisms are revealed.Finally,the dislocation behavior in the Fe-Si alloy is investigated mainly by molecular dynamics simulations combined with electronic structure calculations.The solid-solution hardening and softening are interpreted in terms of two kinds of energy barriers for kink nucleation and migration on a screw dislocation line.Furthermore,the clue to the peculiar work hardening behavior is discussed based on kinetic Monte Carlo simulations by focusing on the preferential selection of slip planes triggered by kink nucleation.