Nanocrystalline Mg-7.4%Al powder was prepared by mechanical alloying using a high-energy mill. The evolution of the various phases and their microstructure, including size and morphology of the powder particles in the course of milling and during subsequent annealing, were investigated in detail. Room temperature milling leads to a rather heterogeneous microstructure consisting of two distinct regions: Al-free Mg cores and Mg-Al intermixed areas. As a result, the material is mechanically heterogeneous with the Mg cores displaying low hardness (40–50 HV) and the Mg-Al intermixed regions showing high hardness of about 170 HV. The Mg cores disappear and the microstructure becomes (also mechanically) homogeneous after subsequent cryo-milling. Rietveld structure refinement reveals that the crystallite size of the milled powders decreases with increasing the milling time reaching a minimum value of about 30 nm. This is corroborated by transmission electron microscopy confirming an average grain size of ~25 nm.
References
[1]
Kainer, K.U. Magnesium-Alloy and Technology; Wiley-VCH: Weinheim, Germany, 2009.
[2]
Mordike, B.L.; Ebert, T. Magnesium: Properties—Applications—Potential. Mater. Sci. Eng. A 2001, 302, 37–45, doi:10.1016/S0921-5093(00)01351-4.
[3]
Youssef, K.M.; Wang, Y.B.; Liao, X.Z.; Mathaudhu, S.N.; Kecskes, L.J.; Zhu, Y.T.; Koch, C.C. High hardness in a nanocrystalline Mg97Y2Zn1 alloy. Mater. Sci. Eng. A 2011, 528, 7494–7499, doi:10.1016/j.msea.2011.06.017.
[4]
Wei, Y.; Anand, L. A constitutive model for powder-processed nanocrystalline metals. Acta Mater. 2007, 55, 921–931, doi:10.1016/j.actamat.2006.09.014.
[5]
Chua, B.W.; Lu, L.; Lai, M.O. Deformation behavior of ultrafine and nanosize-grained Mg alloy synthesized via mechanical alloying. Phil. Mag. 2006, 86, 2919–2939, doi:10.1080/14786430600660831.
[6]
Lu, L.; Lai, M.O.; Yan, C.; Ye, L. Nanostructured high strength Mg-5%Al-x%Nd alloys prepared by mechanical alloying. Rev. Adv. Mater. Sci. 2004, 6, 28–32.
[7]
Gleiter, H. Nanocrystalline materials. Prog. Mater. Sci. 1989, 33, 223–315.
El-Sherik, A.M. Thermal stability of nanocrystalline Ni. Mater. Sci. Eng. A 1995, 203, 177–186, doi:10.1016/0921-5093(95)09864-X.
[10]
Koch, C.C. Nanostructured Materials: Processing, Properties and Potential Applications; William Andrew Publishing: Norwich, NY, USA, 2002.
[11]
Koch, C.C. Mechanical Milling and Alloying. In Materials Science and Technology; Cahn R.W. Haasen, P., Kramer, E.J., Eds.; VCH Verlagsgesellschaft: Weinheim, Germany, 1991; Volume 15, pp. 193–246.
[12]
Lu, L.; Zhang, Y.F. Influence of process control agent on interdiffusion between Al and Mg during mechanical alloying. J. Alloys Compd. 1999, 290, 279–293, doi:10.1016/S0925-8388(99)00221-2.
[13]
Scudino, S.; Sakaliyska, M.; Surreddi, K.B.; Eckert, J. Mechanical alloying and milling of Al-Mg alloys. J. Alloys Compd. 2009, 483, 2–7, doi:10.1016/j.jallcom.2008.07.161.
[14]
Eckert, J.; Holzer, J.C.; Johnson, W.L. Thermal stability and grain growth behavior of mechanically alloyed nanocrystalline Fe-Cu alloys. J. Appl. Phys. 1993, 73, 131–141, doi:10.1063/1.353890.
[15]
Suryanarayama, C. Mechanical Alloying and Milling; Marcel Dekker: New York, NY, USA, 2004.
[16]
Scudino, S.; Eckert, J.; Yang, X.Y.; Sordelet, D.J.; Schultz, L. Conditions for quasicrystal formation from mechanically alloyed Zr-based glassy powders. Intermetallics 2007, 15, 571–582, doi:10.1016/j.intermet.2006.09.005.
