Computational high-throughput design of ductile magnesium alloys

Magnesium alloys have an outstanding position in the search for  the  next-generation  light-weight  materials.  However,  their  application  is  still  hampered  by  their  inherent  brittleness.  Based on an interdepartmental, computational high-throughput strategy,  scientists  at  the  MPIE have  found  a  solution,  which  makes the material both ductile and affordable.

Fig. 1: Computed values of the yttrium-similarity index, YsI (Eq. 1) for the 2850 solute pairs computed in this study and visualized in the form of a symmetric matrix with yellow indicating a high similarity and blue a low one.

Thinking  about  light-weight  structural  materials,  e.g.,  for  automotive applications, Al alloys are often considered  first. Although  magnesium is 1.7 times lighter than Al and has  a  comparable  price,  its  brittleness makes applications very rare. A few alloying strategies to improve the  situation  have  been  suggested  in  recent  years,  but  are  mostly  ex-pensive  due  to  the  involvement  of  rather expensive and environment-unfriendly rare-earth elements such as  yttrium.  therefore,  we  have  optimized  the  ductility  of  Mg  alloys, by  screening  several  thousand  alloying  concepts  and  identifying  the  most promising and economical solution [1].

The   materials   science   approach   required in the first place a reliable yet  easily  accessible  descriptor  for  optimum    mechanical    properties.    the experimental investigation performed  at  the  department  “Micro-structural Physics and Alloy Design“ identified the intrinsic stacking-fault energy as such a candidate. In con-trast  to  the  dislocations  processes  required  for  the  complete  description  of  the  underlying  plasticity  [2],  this  energy  can  be  directly  calculated  by  ab  initio  calculation  em-ployed  in  the  department  “Computational  Materials  Design”.  A  direct  screening of 18 transition metals as possible  individual  solutes  did  not  identify  better  candidates  than  the  previously identified rare-earth element yttrium [3]. Instead, it allowed us  to  identify,  which  physical  properties  distinguish  the  mechanical  impact of yttrium from other solutes. Based on these insights, the yttrium similarity   index   (YsI,   approach-ing  1  for  yttrium-similar  elements)  was  identified  as  a  new  figure  of merit,  which  subsequently  allowed  to  screen  2850  solute  pairs  within  Mg with reasonable numerical effort (Fig.  1).  Nevertheless,  the  evaluation of such widerange studies requires a reliable data management system.   Our   recently   developed   software   framework   pyiron   does   not only provide such a tool for automated  data  generation,  storage,  analysis  and  visualization,  but  at the same time an integrated developing  environment  for  new  materi-als concepts. As a result of this design strategy a Mg-Al-Ca system, namely, Mg-1Al-0.1Ca  (wt.%)  was  found  to  show  promising mechanical performance (YsI  >  0.95),  while  still  being  in-expensive  and  non-toxic.  Only  afterwards,  the  tensile  stress-strain behaviour of this alloy was compared with pure Mg and binary solid solution Mg-re and Mg-Y alloys (Fig. 2). the result was outstanding: a tensile elongation of about 20% (4x more ductile than  pure  Mg),  a  well-balanced  constant work hardening, and an ultimate tensile  strength  of  about  220  MPa  (40% higher than pure Mg) [1]. Such a performance makes Mg alloys highly attractive for future materials science as well as light-weight applications.

Fig. 2: Engineering stress-strain curves of the new Mg-Al-Ca alloy in comparison with not engineered (other than simple homogenization treatment) solid solution Mg-Y, Mg-RE, pure Mg and Mg-Al-0.3Ca.

References:

1.
S. Sandlöbes, M. Friák, S. Korte-Kerzel,  Z.  Pei,  J.  Neugebauer,  D. Raabe
A rare-earth free magnesium alloy with improved intrinsic ductility
Sci rep 7, 10458 (2017)
2.
Z. Wu,  W.A. Curtin
The  origins  of high hardening and low ductility in magnesium
Nature 526, 62 (2014)

Authors: T. Hickel, J. Neugebauer

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