Static annealing process of 423 K for 2.5 h is adequate to segregate alloying elements at {101¯2} twin boundaries in various Mg binary alloys. These segregated twin boundaries play a role in ...obstruction of dislocation slips; thus, they contribute to increase in hardness. Internal friction tests reveal that, irrespective of the solute elements, induced twin boundaries are effective in enhancing damping capacity, owing to their reversible motion, i.e., growth and shrinkage. In contrast, by comparison of the loss factor of specimens with/without twin boundary segregation, segregation leads to a decrease in damping capacity. The energy barrier required for twin boundary sliding to occur is closely related to the loss factor. When solute element having a characteristic of high (or low) energy barrier exists at twin boundaries, such an alloying element prevents (or enhances) the occurrence of twin boundary motion; as a result, shows a low (or high) loss factor.
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•The energy of prismatic core is 22.8 meV/b higher than that of the pyramidal core.•The energy difference between the prismatic and basal cores is much higher (127 meV/b).•That ...indicates the basal slip does not activate in pure Ti.•However, the Peierls barrier for motion in the basal plane is not as high (16 meV/b).•Both Al and V solutes facilitate dislocation motion in the basal plane.
The deformation mode of some titanium (Ti) alloys differs from that of pure Ti due to alloying elements in the α-phase. Herein, we investigated all possible slip modes in pure Ti and the effects of Al and V solutes as typical additive elements on the dislocation motion in α-Ti alloys using density functional theory (DFT) calculations. The stacking fault (SF) energy calculations indicated that both Al and V solutes reduce the SF energy in the basal plane. In contrast, Al solute increases the SF energy in the prismatic plane, making the slip motion in different planes more comparable. DFT calculations were subsequently carried out to simulate dislocation core structures. The energy landscape of the transition between all possible dislocation core structures and the barriers for dislocation glide in various slip planes clarified the nature of dislocation motion in pure Ti; i) the energy of prismatic core is higher than the most stable pyramidal core, and thereby dislocations need to overcome the energy barrier of the cross-slip (22.8 meV/b) when they move in the prismatic plane, ii) the energy difference between the prismatic and basal cores is higher (127 meV/b), that indicates the basal slip does not activate, iii) however, the Peierls barrier for motion in the basal plane is not as high (16 meV/b) once the dislocation exists stably in the basal plane. Direct calculations for the dislocation core around solutes revealed that both Al and V solutes facilitate dislocation motion in the basal plane by reducing the energy difference between the prismatic and basal cores. Thus, the effect of solutes characterizes the difference in the deformation mode of pure Ti and α-Ti alloys.
This review reports the research activity on the hydrogen embrittlement in high-strength aluminum alloys, especially focusing on hydrogen trapping at various trap sites and its influence on hydrogen ...embrittlement. We have investigated the three representative hydrogen embrittlement mechanisms in high-zinc-concentration Al-Zn-Mg alloys. One of the three mechanisms is the damage evolution originated from hydrogen precipitated as pores. We have paid marked attention to the existence of age-hardening precipitates as the major hydrogen trap site.Firstly, we have clarified the nanoscopic structures of a few MgZn2 precipitates and their interface by means of the high-resolution TEM technique. Such information has been utilized to perform a first principles simulation to know trap binding energy values for almost all the possible trap sites. At the same time, detailed fracture micromechanisms and microstructure-property relationships have been investigated by employing both the high resolution X-ray micro-tomography technique and the first principles simulation. The ultra-high-resolution X-ray microscope, which has been realized quite recently, has also been applied. Characteristic localized deformation and subsequent crack initiation and growth through deformed aluminum have been observed. It has also been revealed that hydrogen embrittlement has been suppressed when relatively coarse particles are dispersed. In-situ hydrogen repartitioning during deformation and fracture has been estimated by considering thermal equilibrium among the various trap sites together with the increase in trap site density during deformation. The relationship between the in-situ repartitioning of hydrogen and hydrogen embrittlement with the three different micromechanisms are discussed to explain realistic conditions for hydrogen embrittlement to occur.
Metastable β titanium alloys possess excellent strain-hardening capability, but suffer from a low yield strength. As a result, numerous attempts have been made to strengthen this important structural ...material in the last decade. Here, we explore the contributions of grain refinement and interstitial additions in raising the yield strength of a Ti-12Mo (wt.%) metastable β titanium alloy. Surprisingly, rather than strengthening the material, grain refinement actually lowers the ultimate tensile strength in this alloy. This unexpected and anomalous behavior is attributed to a significant enhancement in strain-induced α’’ martensite phase transformation, where in-situ synchrotron X-ray diffraction analysis reveals that this phase is much softer than the parent β phase. Instead, a combination of both oxygen addition and grain refinement is found to realize an unprecedented strength-ductility synergy in a Ti-12Mo-0.3O (wt.%) alloy. The advantageous effect of oxygen solutes in this ternary alloy is twofold. Firstly, solute oxygen largely suppresses strain-induced transformation to the α’’ martensite phase, even in a fine-grained microstructure, thus avoiding the softening effect of excessive amounts of α’’ martensite. Secondly, oxygen solutes readily segregate to twin boundaries, as revealed by atom probe tomography. This restricts the growth of {332} deformation twins, thereby promoting more extensive twin nucleation, leading to enhanced microstructural refinement. The insights from our work provide a cost-effective rationale for the design of strong yet tough metastable β titanium alloys, with significant implications for more widespread use of this high strength-to-weight structural material.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Display omitted
It is well known that body-centered cubic (bcc) transition metals such as α-Fe become brittle below the ductile to brittle transition temperature (DBTT). Although the fracture occurs on a macroscopic ...scale, it consists of a series of interatomic bond breaking events; therefore, accurate atomistic modeling is critical to understanding this phenomenon. In this work, atomistic simulations of curved crack fronts of α-Fe were performed using an interatomic potential generated by a machine learning technique. For this purpose, large simulation boxes consisting of ∼26 million atoms were used. We obtained evidence that the cleavage plane is {100}, and the cleavages are accompanied by crack tip plasticity at elevated temperature.
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