Research Of The Long Period Stacking Ordered Phase (LPSO)and Mechanical Properties Of Mg-y-zn Based Magnesium Alloys

As one of the lightest structure materials,magnesium alloys have attracted great attention with regard to their application in field such as automotive,aerospace,electronic communications and defense,named as the 21 st century green engineering materials.Mg-Y-Zn based alloys have shown great potential in developing Mg products with high strength and high ductilites and have received widespread concern by global researchers.A study regarding the effect of Sn and Y element on long period stacking ordered(LPSO)phase and mechanical properties of the Mg-Y-Zn base alloys is performed,with the aid of optical microscopy(OM),X-ray fluorescence(XRF),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffractometer(XRD),X-ray energy dispersive spectroscopy(EDS)and mechanical testing machine.The main results are summarized as follows:The LPSO phase is a key strengthening precipitate phase in the Mg-Y-Zn based alloys.Fully controlled by the chemical ordering and stacking faults,the LPSO phase could be strongly modified by alloying elements.This work explored the effect of trace Sn addition on the Mg-6Y-2Zn alloy,with an emphasis on the LPSO phase and corresponding changes of mechanical properties in the as-cast,as-annealed and as-extruded states.Results show that the 18 R LPSO phase forms as needles and distributes at grain boundaries.Meanwhile,its length and density are stimulated dramatically by the Sn addition in the as-cast state.After annealing,the content of the 18 R increases accompanied by the change of morphology shapes from needle to block.Additionally,fine 14 H LPSO lamellas are precipitated both in grains and between 18 R LPSO phases.In contrast to the as-annealed Sn-free alloy,the area fraction of 18 R LPSO phase in the Sn-containing Mg alloy increases from 35% to 58%,the 14 H LPSO phase is also promoted to a certain extent.Tensile tests conducted on as-extruded state revealed remarkable mechanical properties improvement in the Sn-containing alloy over its ternary equivalents.The relevant mechanisms are also investigated in detail.Besides,this work successfully developed alloys containing only 14 H LPSO phase,18 R LPSO phase and 14H+18R LPSO phases.Results show that the Y content increasing induces the formation of bulk-like 18 R LPSO phase in the Mg-Y-Zn-Sn alloys,while discourages that of the lamellar 14 H LPSOphase.The alloy containing 14 H LPSO phase exhibits the smallest dynamic recrystallized grain size and the highest elongation to failure,> 18%.While the highest yield strength and ultimate tensile strength are possessed by the 18 R LPSO phase containing alloy.The 14 H LPSO phase-containing alloy is mainly strengthened and toughed by the combination of the coherent interface,the suppression of dislocation movement,kink deformation and the load transfer mechanism,while the 18 R LPSO phase cannot be regarded as short-fibers,the main strengthening mechanisms for the 18 R LPSO phase-containing alloy mainly originated from fine particle(coherent interface)strengthening,and the 18 R LPSO phase itself directly acts as a strong reinforcement.The 18 R LPSO phase is more effective to strengthen the alloy,whereas the 14 H lamellas is more effective to toughen the alloy.