Thermodynamics-based design of creep resistant Mg solid solutions using the Miedema scheme

Abstract

Most current creep resistant alloy design methods for Mg alloys aim at incremental strengthening of existing alloys such as AZ91 thorough addition of ternary and quaternary elements, or alternatively through RE's additions. The most remarkable improvements has been through either Mg-Y or recently developed Mg-Gd-Y-Zr and Mg-Y-Nd-Zr alloys, although still below the strength of MgTh alloys, which are being phase out due to Th's radioactivity.Many micromechanisms proposed to account for the observed improvements in either hardness or creep strength of Mg alloys including solid solution and/or precipitation hardening, dynamic precipitation, solute dragging/solute clustering and more importantly atomic size controlled diffusivity more often than not are little more than ad-hoc, after-the-fact explanations, valid at best for the particular system under consideration. Hence, extrapolation to other alloy-systems is either not possible or contradictory. In practice, a feasible path for systematic, either precipitationhardened or creep-resistant Mg alloys, selection and design is still lacking.The aim of the present work was to use atomic level thermodynamical arguments that suggest that extending the athermal regime through short range order (SRO) is a most feasible path to increasing the creep strength of Mg alloys. Potential solutes were sorted out and ranked based on their tendency to developing SRO using the Miedema's phenomenological scheme. The predictions were corroborated by experiments involving dilute binary alloys as well as through a collection of data from the literature.Monotonic compression and stress relaxation tests were carried out on specimens of 6 cast binary alloys with (at.%) 2.5 Al, 0.6 Sn, 2.2 Zn, 0.8 Gd, 1.3 Y and 0.9 Nd, and of a similarly cast AZ91D alloy for reference. The solute concentration in the binary alloys was kept deliberately low to limit precipitation hardening effects during the testing. Compression testing was carried out at 298K (25°C), 373K (100°C) and 453K (180°C) The relative tendency or lack of it, of most solutes to develop SRO rationalizes a number of observations in current multicomponent Mg alloys while it disputes the viability of several other micromechanisms often considered.The feasibility of strengthening HPDC Mg alloys through a combination of percolating eutectics and residual solute in solution, or, more generally, age hardenable alloys through either homogeneously nucleated, coherent with the Mg matrix, precipitates, or via internally ordered precipitates mimicking Mg-Th alloy system, is also considered by making parallels with either the Al-Zn or the Al-Cu alloy systems. Examples of these approaches were discussed using data from the literature, such as experiments on alloys combining Mn and Sc to introduce order in the soft Mn precipitates, or the use of Na to provide finely dispersed, homogeneously nucleated clusters with the aim of refining the subsequent precipitation in alloys such as Mg-Sn. III Declaration by authorThis thesis is composed of my or...

A Review on Developments in Magnesium Alloys

“…Short range order was also reported in earlier studies on binary Mg alloys, namely, in Mg-tin (Sn) (Henes and Gerold, 1962;van der Planken and Deruyttere, 1969), in Mg-(6.4 at%) Er and local order in Mg-(2.9 at.%) Gd (Gencheva et al, 1981), and in Mg-Zn (Akhtar and Teghtsoonian, 1969). Since then, a number of relatively recent studies on lean binary Mg alloys also focused on SRO (Blake and Cáceres, 2008;Abaspour and Caceres, 2013;Abaspour andCáceres, 2015, 2016;Abaspour, 2014;Yang et al, 2014;Zhang et al, 2014). According to Abaspour and Caceres (2013), SRO was also claimed in previous studies (Henes and Gerold, 1962;Safranova et al, 1977) based on small-angle xray scattering.…”

“…Short range order can be viewed as a case where B-B bonds are not allowed in a binary A-B system. The phenomenon of temperature-independent flow stress in some Mg-X alloys, that is, "athermal component of solid solution strengthening, " has been attributed to SRO (Akhtar and Teghtsoonian, 1969;Abaspour andCáceres, 2015, 2016;. In essence, the phenomenon is an extra strengthening mechanism that cannot be explained based on simple solid solution strengthening (van der Planken and Deruyttere, 1969).…”

“…Consequently, presence of SRO can become especially useful for Mg to overcome its shortcomings such as low strength and elasticity modulus and poor creep resistance. The contribution of SRO to strength with high resistance to changes in temperature was demonstrated via compression and stress relaxation tests conducted at various temperatures (Abaspour and Caceres, 2013;Abaspour, 2014; and via creep experiments (Abaspour and Cáceres, 2015).…”

“…The following experimental observations were reported by Abaspour et al for Mg-X [X: Y, Gd, Ca, Zn, Sn, antimony (Sb), and Al] binary systems: an initial linear strengthening at constant temperature tests; for some alloys, an increasing strength at higher temperatures and little or no stress relaxations. Studies by Abaspour (2014) and Abaspour andCáceres (2014, 2016) showed that temperature dependence of the strength of some Mg-X [X: Al, Zn, Sn, Y, Gd, neodymium (Nd)] alloys displayed an athermal regime at low strain levels up to some different temperature range and to varying levels. The same workers further claimed that there was even a strength increase within a temperature range in the case of Mg-X (X: Y, Gd, Nd) (see Figures 2, 3 in .…”