DOI: 10.1002/adem.201200297 Grain Refinement of Magnesium Alloys by Mg–Zr Master Alloys: The Role of Alloy Chemistry and Zr Particle Number Density** By Ming Sun, Mark A. Easton, * David H. StJohn, Guohua Wu, * Trevor B. Abbott and Wenjiang Ding Grain refinement of cast Mg alloys by potent nucleant particles is the first effective step towards a fine, uniform and equiaxed grain structure. [1] This refinement imparts to castings structural uniformity and consistency in perfor- mance [2] improving strength, ductility, corrosion resistance, and in some Mg-alloys, creep resistance. [3–5] Al-free magnesium alloys such as Mg–RE(–Zn) alloys can be grain refined effectively by Mg–Zr master alloys. [5,6] Relatively high addition levels of master alloy are required due to rapid settling of undissolved Zr particles as the density of Zr (6.52 g cm 3 ) is much higher than liquid Mg (1.74 g cm 3 ). [8] Thus, these master alloys are expensive increasing the cost of manufactur- ing Zr-bearing Mg alloys. [2,6,7] The Zr alloying efficiency and the grain refining perfor- mance of Mg–Zr master alloys is related to the distribution of Zr particle size, [9] and it has been proposed from qualitative examination of as-cast microstructures that the Zr particles which act as nucleants for Mg grains, are in the range 1–5 mm. [10,11] Therefore, Mg–Zr master alloys with more particles in this size range should exhibit better refining efficiency. The contribution of total Zr (Zr T ) to grain refinement is dictated by both soluble Zr (Zr S ) and insoluble Zr (Zr I ). [3,12] The undissolved Zr particles act as heterogeneous nucleation sites during solidification, while the dissolved Zr strongly restricts the growth of Mg grains. [3,7,13–15] This combined effect COMMUNICATION [*] Dr. M. Sun, Prof. G. Wu, Prof. W. Ding National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China E-mail: ghwu@sjtu.edu.cn Dr. M. Sun, Dr. M. A. Easton CAST Cooperative Research Centre, Department of Materials Engineering, Monash University, Victoria 3800, Australia E-mail: mark.easton@monash.edu Prof. D. H. StJohn CAST Cooperative Research Centre, Centre for Advanced Materials Processing and Manufacturing (AMPAM), School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia Dr. T. B. Abbott Magontec Ltd, Sydney, New South Wales 2000, Australia [**] The work is sponsored by National Natural Science Foundation of China (No. 51275295), the Australian CAST Co-operative Research Centre, ARC Discovery Grants DP120101672 (DStJ) and DP2010000071 (ME), and Funded Projects of SAST-SJTU Aerospace Advanced Technology Joint Research Centre (No. USCAST2012-15). The authors acknowledge the Monash Centre for Electron Microscopy (MCEM) and thank Dr. Mark Gibson, Mr. Andrew Yob at CSIRO and Mr. Darren Gandel at Monash University for help with experiments. Although refinement of the as-cast grain size of magnesium alloys by Zr is well established commercially, little research has been undertaken to optimize this refinement technology and to quantify the relative performance of different Mg–Zr master alloys. The performance of Mg–Zr master alloys was found to be related to (1) the number density of Zr particles between 1 and 5 mm in size where the master alloy with the largest number density of these particles exhibits the best refinement, and (2) the alloy’s growth restriction factor (Q) where a linear relationship between grain size and 1/Q, was found to exist for each master alloy. An equation for predicting grain size based on the Interdependence model was developed. Further, the Interdependence Model was improved to be able to cater for an increasing Zr particle number density with increasing values of Q. ADVANCED ENGINEERING MATERIALS 2013, 15, No. 5 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 373