Ordering in liquid and its heredity impact on phase transformation of Mg-Al-Ca alloys

It is a long-sought goal to achieve desired mechanical properties through tailoring phase formation in alloys,especially for complicated multi-phase alloys.In fact,unveiling nucleation of competitive crystalline phases during solidification hinges on the nature of liquid.Here we employ ab initio molecular dynamics simulations(AIMD)to reveal liquid configuration of the Mg-Al-Ca alloys and explore its effect on the transformation of Ca-containing Laves phase from Al2Ca to Mg_(2)Ca with increasing Ca/Al ratio(rCa/Al).There is structural similarity between liquid and crystalline phase in terms of the local arrangement environment,and the connection schemes of polyhedras.The forming signature of Mg_(2)Ca,as hinted by the topological and chemical short-range order originating from liquid,ascends monotonically with increasing rCa/Al.However,Al_(2)Ca crystal-like order increase at first and then decrease at the crossover of rCa/Al=0.74,corresponding to experimental composition of phase transition from Al_(2)Ca to Mg_(2)Ca.The origin of phase transformation across different compositions lies in the dense packing of atomic configurations and preferential bonding of chemical species in both liquid and solid.The present finding provides a feasible scenario for manipulating phase formation to achieve high performance alloys by tailoring the crystal-like order in liquid.

A corrosion-resistant and age-hardenable Mg-Al-Mn-Ca-Ce dilute alloy with fine-grained structure processed by controlled rolling

Low strength and poor corrosion resistance are two long-standing bottlenecks with dilute Mg alloys.Here,we report that the corrosion resistance of Mg-0.6Al-0.5Mn-0.2Ca(wt.%)alloy sheet can be significantly improved by micro-alloying with 0.3 wt.%Ce,and the strength can be considerably augmented after aging treatment.Simultaneous optimization of strength,ductility,and corrosion resistance is difficult due to the inherent trade-off in Mg alloys.Surprisingly,our aged Mg-0.6Al-0.5Mn-0.2Ca-0.3Ce alloy features with yield strength(YS)of∼194 MPa,ultimate tensile strength(UTS)of∼265 MPa,elongation to failure(EF)of∼17.2%,and corrosion rate(CR)of 2.2 mm y^(−1),the combination of which exhibits competitive advantage over other comparative dilute Mg alloys.It is found that Ce addition decreases the activity of cathodic phases,inhibits detrimental effects of Fe impurities,and forms a protective Ce-containing surface film.The high strength stems from the precipitation of ordered Guinier-Preston(G.P.)zones dispersed in uniform fine grains with an average grain size of∼8.2μm.The atomic-scale G.P.zones result in a noticeable age hardening response but do not act as crack initiation and propagation site,so our alloy also performs satisfactory ductility.This work sheds light on the design and fabrication of strong,ductile,and corrosion-resistant dilute magnesium alloys to be used in electronic products and automotive bodies.