First-principles study of energy and atomic solubility of twinning-associated boundaries in hexagonal metals

Abstract

Twinning-associated boundaries (TB), f1 0 1 ng coherent twin boundaries (CTB) and the coherent basal-prismatic (CBP) boundary in six hexagonal metals (Cd, Zn, Mg, Zr, Ti and Be) are studied using first-principles density function theory, with the focus on the structural character of TB and the solute's solubility at TB. Regarding the structure and energy of TB, the formation of TB is associated with the creation of an excess volume. All six metals show positive excess volume associated with ð1 0 1 1Þ and ð1 0 1 3Þ CTB, but the excess volume associated with ð1 0 12Þ CTB and CBP can be positive or negative, depending on the metal. The ð1 0 1 2Þ CTB has higher excess energy than ð1 0 1 1Þ and ð1 0 1 3Þ CTB for metals with c=a < ffiffiffiffiffiffiffi ffi 8=3 p , but lower for metals with c=a > ffiffiffiffiffiffiffi ffi 8=3 p . More interestingly, CBP has lower excess energy than ð1 0 1 2Þ CTB for all metals. This is consistent with the recent finding concerning the pure-shuffle nucleation mechanism of ð1 0 1 2Þ twins. To understand solubility at TB, the solubility of solute atoms in Mg, Ti and Zr is calculated for solute positions in bulk, ð1 0 1 2Þ CTB and CBP boundaries. In general, solute atoms have better solubility at CTB and CBP than in bulk. Interestingly, the solubility of solute atoms changes linearly with normal strain at CBP, increasing with normal strain for solute atoms with a greater metallic radius than the matrix, and decreasing with normal strain for solute atoms with a smaller metallic radius than the matrix. This suggests that the distribution of solute atoms in bulk, CTB and CPB boundaries varies with stress state and, in turn, affects the mobility of TB.

Solid solution strengthening of hexagonal titanium alloys: Restoring forces and stacking faults calculated from first principles

“…The most pronounced transition from covalent to metallic bonding takes place in basal SF, which results in large g SF (pure Ti) values of this system. Recent investigations revealed that a-Ti solutes have better solubility in twin boundaries (TB) than in bulk, due to the excess volume of the twin-bulk structure interface [19]. Since both TB and SF exhibit many common features, a-Ti SFs interplanar distance distortion r (difference of interplanar distance between distorted and unstrained crystal) is plotted in Fig.…”

“…[17,18] with the conclusion that O stabilizes/destabilizes basal/prismatic faults, respectively. The next important feature of the planar defects e twin boundaries (TB) reported lately [19], show better solubility of many Ti alloying atoms in TB than in bulk. This unique phenomenon was correlated with the excess TB volume delocalized within 4e5 atomic planes, implying relatively large TB e solute interaction distance.…”

Tensorial elastic properties and stability of interface states associated with Σ5(210) grain boundaries in Ni3(Al,Si)

“…It differs from vertical atomic-layer-resolved shifts that are analyzed, for example, in the recent study by Kumar et al [72]. The individual inter-layer distances significantly differ from the bulk equilibrium value (0.799 Å) with the extreme values of these deviations being (i) +0.33 Å for the distance between the GB plane and the first atomic layer off the GB plane and (ii) –0.26 Å between the first and second plane off the GB plane, respectively, in the Σ5(210) Al,Ni .…”

Twinning effects on strength and plasticity of metallic materials

“…Specifi cally, DFT calculations show that solute solubility has little dependence on the radius of solute atoms along CTB, but varies considerably with stresses on PBs. 69 …”