Interfacial structures and energetics of the strengthening precipitate phase in creep-resistant Mg-Nd-based alloys

Abstract

The extraordinary creep-resistance of Mg-Nd-based alloys can be correlated to the formation of nanoscale-platelets of β 1 -Mg 3 Nd precipitates, that grow along 〈1120〉 Mg in bulk hcp-Mg and on dislocation lines. The growth kinetics of β 1 is sluggish even at high temperatures, and presumably occurs via vacancy migration. However, the rationale for the high-temperature stability of precipitatematrix interfaces and observed growth direction is unknown, and may likely be related to the interfacial structure and excess energy. Therefore, we study two interfaces-{112} β1 /{1100} Mg and {111} β1 /{1120} Mgthat are commensurate with β 1 /hcp-Mg orientation relationship via first principles calculations. We find that β 1 acquires plate-like morphology to reduce small lattice strain via the formation of energetically favorable {112} β1 /{1100} Mg interfaces, and predict that β 1 grows along 〈1120〉 Mg on dislocation lines due to the migration of metastable {111} β1 /{1120} Mg . Furthermore, electronic charge distribution of the two interfaces studied here indicated that interfacial-energy of coherent precipitates is sensitive to the population of distorted lattice sites, and their spatial extent in the vicinity of interfaces. Our results have implications for alloy design as they suggest that formation of β 1 -like precipitates in the hcp-Mg matrix will require well-bonded coherent interface along precipitate broad-faces, while simultaneously destabilizing other interfaces.Magnesium (Mg) alloys have tremendous potential as structural materials for automotive applications in engine block, transmission parts due to their lightweight. The concomitant fuel savings is however, thwarted by their unfavorable creep strength 1-4 . Mg-rare earth (RE) alloys are exceptions with demonstrated excellent creep resistance 1-3 . Their superior creep properties are correlated to the formation of high volume fractions of strengthening precipitates based on RE intermetallic compounds 4,5 . However, the scarcity and high cost of RE elements limits the development of creep-resistant Mg-RE alloys. This work is motivated by gaining an atomistic understanding of commonly found strengthening precipitates in creep-resistant Mg-RE alloys, and inform Mg-alloy design approaches by providing energetics and bonding characteristics associated with their presence within the hcp-Mg matrix. Therefore, we have investigated the structure of interface between intermetallic precipitate phase β 1 (an ordered cubic structure) and hcp-Mg. This precipitate typically forms in Mg-Nd-based alloys like Mg-Nd, Mg-Nd-Y and Mg-Nd-Y-Zr, e.g. commercial WE43 and WE54 5-7 . The coherent β 1 phase has a plate-like morphology, and shares an (0001) Mg [5][6][7][8][9][10][11][12] . TEM results also indicate that plate-like β 1 precipitates always forms with a high aspect ratio, and its broad-face {211} β1 parallel to {1100} Mg 4-11 . Furthermore, β 1 nucleating within the Mg-matrix is associated with the orthorhombic β ′ precipitate phase 5,7 , or as self-accommodating β 1 tr...

Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

“…This observation is significant because the γ″ precipitates are better bonded to its surrounding parent Mg lattice than β′ in Mg–Nd, and the enhanced covalent bonding of the γ”/Mg interfacial regions can likely inhibit diffusing vacancies during creep. Additionally, our recent study of precipitate–matrix interfaces in a Mg–RE alloy indicated that lower interfacial energy correlates with higher interfacial valence charge density (also see Supplementary Note 3 ) 42 . Taken together, the stiffer bonds near the solute species and γ″/Mg interfacial regions will form a distribution of covalently bonded pockets within the matrix of Mg–Nd–Zn.…”

Particle curvature effects on microstructural evolution during solid-state sintering: phenomenological insights from phase-field simulations

“…Particularly in the configurations of three grains, i.e., cases III, IVb and V, where they exhibit very different t disappear norm for a given c S c GB , especially when c S is ''significantly'' greater than c GB . Furthermore, it should be noted that we have examined sintering of multiparticle systems using a phenomenological approach using c S [ c GB (Table 1); a relationship exhibited in many materials [54,84,85]. Therefore, the demarcation of sintering kinetics into different regimes will hold for many materials systems (Sect.…”