Precipitation Processes in Mg–Th,Mg–Th–Mn,Mg–Mn,and Mg–Zr Alloys

Abstract

A study has been made of precipitation processes in selected magnesium-base alloys by means of electron microscopy. The principal part of the investigation was concerned with the effect of manganese precipitation in a Mg-Th alloy. It is shown that the sequence of precipitation in a Mg–2.0percnt; Th–1.0% Mn alloy (HM21) is not as complex as that occurring in a Mg–3.0% Th–0.5 Zr alloy (HK31), in that only one coherent transition precipitate precedes the formation of an incoherent precipitate. The coherent precipitate forms as discs on the {0001}_Mg and has an hexagonal crystal structure a = 5.56Å (√3 × a Mg) and c =31.6Å (6 × a Mg) and the orientation relationship is <disp-formula> <mml:math> <mml:mrow> <mml:msub> <mml:mrow> <mml:mo>{</mml:mo> <mml:mn>0001</mml:mn> <mml:mo>}</mml:mo> </mml:mrow> <mml:mtext>ppte</mml:mtext> </mml:msub> <mml:mo>‖</mml:mo> <mml:msub> <mml:mrow> <mml:mo>{</mml:mo> <mml:mn>0001</mml:mn> <mml:mo>}</mml:mo> </mml:mrow> <mml:mtext>Mg</mml:mtext> </mml:msub> </mml:mrow> </mml:math> </disp-formula> <disp-formula> <mml:math> <mml:mrow> <mml:msub> <mml:mrow> <mml:mo>{</mml:mo> <mml:mn>11</mml:mn> <mml:mover> <mml:mn>2</mml:mn> <mml:mi>¯</mml:mi> </mml:mover> <mml:mn>0</mml:mn> <mml:mo>}</mml:mo> </mml:mrow> <mml:mtext>ppte</mml:mtext> </mml:msub> <mml:mo>‖</mml:mo> <mml:msub> <mml:mrow> <mml:mo>{</mml:mo> <mml:mn>10</mml:mn> <mml:mover> <mml:mn>1</mml:mn> <mml:mi>¯</mml:mi> </mml:mover> <mml:mn>0</mml:mn> <mml:mo>}</mml:mo> </mml:mrow> <mml:mtext>Mg</mml:mtext> </mml:msub> </mml:mrow> </mml:math> </disp-formula>

This transforms on continued ageing to a cigarshaped equilibrium precipitate parallel to 〈1120〉_Mg. Additionally, the three binary alloys Mg–Th, Mg–Mn, and Mg–Zr were studied in order to obtain a better understanding of the metallography of the two ternary systems Mg–Th–Mn and Mg–Th-Zr.