Ordered magnesium-lithium alloys: First-principles predictions

Abstract

Magnesium-lithium ͑Mg-Li͒ alloys are among the lightest structural materials. Although considerable work has been done on the Mg-Li system, little is known regarding potential ordered phases. A first and rapid analysis of the system with the high-throughput method reveals an unexpected wealth of potentially stable low-temperature phases. Subsequent cluster expansions constructed for bcc and hcp superstructures extend the analysis and verify our high-throughput results. Of particular interest are those structures with greater than 13 at. % lithium, as they exhibit either partial or complete formation as a cubic structure. Order-disorder transition temperatures are predicted by Monte Carlo simulations to be in the range 200-500 K. I. MOTIVATION AND BACKGROUNDEmerging technologies increasingly depend on the production of ultralight-weight materials. Magnesium-lithium ͑Mg-Li͒ alloys are the lightest metallic alloys, having densities near that of plastics, 1-3 and are strong enough to be used in a variety of high-performance applications. In particular, Mg-Li alloys are good candidates for material applications in industries such as aerospace and automotive manufacturing. 4 As an alloying agent in magnesium, lithium is advantageous principally because of its low density and cubic crystal structure. The addition of lithium converts the hexagonalclose-packed ͑hcp͒ structure of natural magnesium to a more ductile and formable body-centered-cubic ͑bcc͒ alloy. Addition of 13 at. % Li partially converts the alloy to a cubic structure and content exceeding 33 at. % results in total conversion. 5 Although the Mg-Li system has been studied extensively, 5-11 evidence suggesting the formation of ordered phases is notably sparse. Diffusionless transformations of the martensitic type have been observed in pure Li, and similar transformations occur in low-temperature magnesium alloyed lithium at certain concentrations. 11,12 Binary ordered phases have not been conclusively identified, however, and any instances contained in the literature are indeterminate. 6,9,13,14 The results of high-throughput ͑HT͒ ab initio calculations performed with the AFLOW package ͑described later͒ show, however, that the heat of formation is negative for a large number of potential structure types, implying the existence of at least one thermodynamically stable ordered phase ͑Fig. 1͒. II. FIRST-PRINCIPLES METHODFollowing the HT results, we have made predictions of Mg-Li ordered phases using the cluster-expansion ͑CE͒ method-a first-principles-based approach in which input data is mapped to a truncated Ising-type Hamiltonian. The configurational Hamiltonian allows for a fast ground-state search ͑GSS͒ over many derivative superstructures. 16 The CE method was applied to the Mg and Li lattice configurations ͑bcc and hcp, respectively͒. Although minimum-energy configurations may exist outside those considered, superstructures derived from the parent lattices of the alloy constituents generally include the lowest-energy configurations. This assumption is ...

AFLOW: An automatic framework for high-throughput materials discovery

“…IV. Occasionally, fully relaxed calculations by Aflow (cell volume and shape and the basis atom coordinates inside the cell) turn up as ground state structures that do not yet have known experimental prototypes [7,[16][17][18][19][20][21][22][23][24][25]. When identified, they are also added to the structure database.…”

“…The HT concept has already become an effective and efficient tool for materials discovery [1][2][3][4][5][6][7][8] and development [9][10][11][12][13]. Examples of computational materials HT applications include combinatorial discovery of superconductors [1], Pareto-optimal search for alloys and catalysts [14,15], data-mining of quantum calculations applying principle-component analysis to uncover new compounds [5][6][7][16][17][18][19][20][21][22][23][24][25], Kohn-anomalies search in ternary lithiumborides [26][27][28], and multi-optimization techniques used for the study of high-temperature reactions in multicomponent hydrides [29][30][31].…”

Guiding the experimental discovery of magnesium alloys

“…This includes most of the transition metals and several other alloys including some of industrial importance (e.g., Al, Ca). Although also systems of interest, Ag-Mg, Hf-Mg, InMg, and Li-Mg were not included because one or more of the authors have already reported HT ab initio data on these systems 5,10,11,22 . In the systems Al-, Ge-, and Si-Mg, anomalously low energies (many meV below the next lowest energies) were obtained for the Be 2 Zn structure.…”

Stabilization of bcc Mg in Thin Films at Ambient Pressure: Experimental Evidence andab initioCalculations

“…A notable exception is the Mg-Li system, in which an fcc solid solution is stabilized with only 30 at.% Li. [1][2][3] Under non-bulk thermodynamic constraints (i.e. in thin film or nano-structures), phase stability can be modified through the introduction of coherency constraints [4] and it is likely that Mg-based solid solutions with a wider range of alloying elements can be stabilized.…”