Development of High Strength Magnesium Alloys with Long Period Ordered Structure

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(1) Long period stacking ordered (LPSO) structure was formed in Mg-TM-RE TM=Co, Ni, Cu, Zn, RE= Y, Dy, Ho, Er, Dy, Tb, Tm) alloys. The LPSO phase was formed during solidification for RE of Y, Dy, Ho and Er, and precipitated by annealing at 773 K for RE of Gd, Tb and Tm.(2) In Mg-TM-RE alloys, the criteria of TM and RE elements for forming of LPSO structure can be expressed by atomic radius, mixthing enthalpy, crystal structure and solid solubility in Mg.(3) The LPSO structure recognized in Mg-TM-RE alloys was 10H, 18R, 14H and 24R, in which two atomic layers enriched with TM and RE existed every 5, 6, 7 and 8 periods, respectively, on closed-packed plane.(4) Mg-TM-RE alloy with a yield strength of 440 MPa and an elongation of 6 % at room temperature and a yield strength of 330 MPa at 473 K was developed by extrusion of cast ingot. Moreover, Mg-TM-RE alloy with a yield strength of 515 MPa and an elongation of 3 % at room temperature and a yield strength of 367 MPa at 473 K was developed by extrusion-consolidation of chips.(5) The strengthening mechanism of LPSO Mg-TM-RE alloys through plastic working seems to be due to the formation of kink bands in LPSO phase with relatively high strength, and the grain refinement of matrix phase (α-M phase) by continuous dynamic recrystallization. This mechanism is different from the ordinary work hardening and new concept for strengthening mechanism of materials.