ACTA

There is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. To better understand the material characteristics that control or exacerbate this behaviour, evaluation of the time dependent plasticity of the main prismatic and basal slip systems is critical. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem. In this work, characterisation of the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75 °C, 145 °C and 250 °C) was performed during stress relaxation using synchrotron X-ray diffraction. Key parameters that govern the dislocation motion were determined for the major prismatic and basal slip systems as a function of temperature and oxygen content by calibrating a crystal plasticity finite element model with the measured lattice strain relaxation responses. From the temperatures assessed, 75 °C was found to be the worst-case scenario, where the macroscopic plastic strain accumulation was significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems. As the temperature increases, the contribution of thermal energy becomes greater than mechanical energy for dislocation glide. Oxygen was found to have a stronger strengthening effect on prism slip over basal slip, through a significant change in their respective critical resolved shear stresses. This effect becomes more significant in high oxygen content commercially pure Ti.

ACTA

Cycling of a metallic glass between ambient and cryogenic temperatures can induce higher-energy states characteristic of glass formation on faster cooling. This rejuvenation, unexpected because it occurs at small macroscopic strains and well below the temperatures of thermally induced structural change, is important, for example, in improving plasticity. Molecular-dynamics simulations elucidate the mechanisms by which thermal cycling can induce relaxation (reaching lower energy) as well as rejuvenation. Thermal cycling, over tens of cycles, drives local atomic rearrangements progressively erasing the initial glass structure. This arises mainly from the heating stage in each thermal cycle, linked to the intrinsic structural heterogeneity in metallic glasses. Although, in particular, the timescales in MD simulations are shorter than in physical experiments, the present simulations reproduce many physically observed effects, suggesting that they may be useful in optimizing thermal cycling for tuning the properties of metallic glasses and glasses in general.

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Melts of hypomontectic Ti₇₅Y₂₅, near-monotectic Ti₇₀Y₃₀ and hypermonotectic Ti₆₀Y₄₀ composition were undercooled and solidified using an electrostatic levitation facility. Liquid-liquid phase separation, solidification sequence and dendritic growth velocities in undercooled melts were investigated in situ using high-energy X-ray diffraction (HEXRD) and optical imaging with a high-speed video camera. HEXRD observations confirm that undercooled melts of the three Ti−Y compositions are separated into a Ti-rich liquid and a Y-rich liquid prior to solidification. The solidification sequence of the separated liquids depends on nucleation of α-Y with hcp structure from the Y-rich liquid and on nucleation of β-Ti with bcc structure from the Ti-rich liquid. Optical imaging observations suggest that the dendrite growth velocities of α-Y and β-Ti during recalescence are often time dependent and that they do not show any consistent trends with increasing nucleation undercooling. These unusual growth kinetics can be attributed to dynamic interactions between growing dendrites and droplets of a separated liquid. Scanning electron microscopy studies show that the phase-separated samples solidify into a core-shell structure or an island-and-sea-like structure. These microstructures can be understood in terms of the liquid-liquid phase separation and the droplet-dendrite interactions during solidification.

ACTA

向上滑动阅览英文摘要

Al₃Ti and TiB₂ particles were made in-situ from adding halide salts in molten Al-4.5 wt % Cu alloys. The molten halide salts also dissolved the oxide in the molten metal to ensure that the newly formed Al₃Ti and TiB₂ particles had intimate interfaces with the molten metal. Excess amounts of titanium were added into the melt to ensure that the wetting angles between the particles and the molten alloy were small and the particles were able to significantly reduce the grain sizes of the aluminum alloy. Such particle/matrix systems should have a negative total interfacial energy change when a particle is engulfed into the solid by an advancing solid-liquid interface, resulting in particles being engulfed by an advancing solid-liquid interface according to widely accepted theories on particle pushing. Experimental results indicate that these particles are actually pushed by the growing dendrites into the interdendritic regions, regardless of the growth rates being positive, close to nil, or even negative. It seems that the particle pushing mechanisms based on interfacial energy interactions are not applicable for these particle/matrix systems under experimental conditions of this study. Analytical analysis suggests that these particles are pushed by the growing dendrites due to fluid flow before they could get into close contact with the solid-liquid interface within a several interatomic layers where the van der Waals forces become significant. The shrinkage driven flow in the liquid of the mushy zone is sufficient to dislodge a particle from a depression and rolls the particle on the solid-liquid interface.

Vol. 213，1 Jul. 2021, 116982

Vol. 213，1 Jul. 2021, 116949