The Effect Of Long Period Stacking Ordered Phases On Mechanical Properties In Magnesium Alloys Containing Rare Earth Elements

To date, the applications of Magnesium alloys are still limited due to their lowstrength, low ductility as well as low thermal stability. Magnesium alloys containingrare earth (RE) in conjunction with other alloying elements, such as Zn, have drawnincreasing interest due to their high strength, high ductility and high creep-resistancewith the existence of long period stacking ordered (LPSO) phases. Although theLPSO phases have been studied for over a decade, the question remains how an LPSOphase impacts the mechanical properties of Mg-RE-X alloys. Using high-resolutiontransmission electron microscopy (HRTEM), we systematically studied the sizedistribution and formation mechanism of LPSO phases in different as-cast andheat-treated states. On the basis of the modified Zener’s model, the present work aimsto clarify the intrinsic effect of LPSO phases on mechanical properties in Mg alloyscontaining RE elements. The main results are as follows:1. As-cast Mg alloys with nominal compositions of Mg-10Y-5Gd-1.5Zn-0.35Zr(WGZ1051K, wt.%) and Mg-12.5Y-5Gd-1.5Zn-0.35Zr (WGZ1251K, wt.%) weresolution-treated at535°C for16hours, then were aged at225°C for different times,ranging from10minutes to1200hours. These two alloys exhibit a similar ageingprocess, with an sequence of α-Mg (SSSS)â†'β’’(D019)â†'β’(cbco)â†'β1(fcc). Themajor hardening precipitate is found to be β’ phase, which forms on{1010}αprismatic plane and has a c-axis base-centered orthogonal (cbco) structure with latticeparameters of aβ’=2aα-Mg=0.642nm, bβ’=43aα-Mg=2.224nm and cβ’=cα-Mg=0.521nm.The orientation relationship between the β’ phase and the α-Mg matrix is(010)β’//{1010}α,[001]β’//[0001]αrather than that of(100)β’//{1210}α，[001]β’//[0001]αreported in literature. Although the LPSO phases have littlecontribution to age-hardening, they can produce the refined grain strengthening byinhibiting grain boundary movement and keep the excellent ductility of alloys byaccommodating a considerable amount of dislocations moving from the matrix. An ideal microstructure model, which utilizes the LPSO phase to inhibit the growth ofgrains and β’ particles, is established and used to design a new generation of advancedMg-RE-X alloys that combine high strength, high ductility and high creep-resistanceat room and elevated temperatures.2. As-cast Mg alloys with nominal compositions of Mg-10Y-2Zr (W10K,wt.%), Mg-10Y-1Zn-2Zr (WZ101K, wt.%) and Mg-10Y-1Al (WA101, wt.%) weresolution-treated at520°C for72hours and at550°C for48hours. Both Type I LPSOphase that forms during solidification in the WZ101K alloy and Type II LPSO phasethat forms during solution treatment in the WA101alloy can effectively inhibit graingrowth at elevated temperatures. Although the Zener’s model was successfully used topredict the limiting grain sizes of Mg alloys with spherical pinning particles (such asZr and Al2Y), it cannot be used to predict the limiting grain sizes of Mg alloyscontaining LPSO phases. Based on the plate-shaped features of the LPSO phases, amodified Zener’s model is established, which well explains the strong pinning effectof LPSO phases on grain growth at elevated temperatures.3. An18R-LPSO phase with pseudo-twinning satellite reflections was firstlyfound in the Mg-Y-Al alloy. It has a monoclinic structure with lattice parameters ofa=23aα Mg=1.112nm, b=6aα Mg=1.926nm, c≈18c α Mg=4.69/sinβ=4.72nmand β=83.2°. Using high-angle annular dark field scanning transmission electronmicroscopy (HAADF-STEM) viewed along[1210]α Mg, the stacking sequence of18R structure is identified to be ABCACACABCBCBCABAB, which consists of threeABCA-type building blocks separated by two consecutive (0002)αplanes. However,these building blocks present four different types of in-plane ordering when viewedalong[1010]α Mg, designated as α, β, γ and δ, where the last three are firstly found inan ordered18R structure. This indicates that the presently discovered18R structure is far more complicated than the18R structures determined previously.4. On the basis of studying the LPSO phases in the as-cast WZ101K (Type I18R),520ST WZ101K (Type II14H) and550ST WA101(Type I18R) alloys, thesimilarity and disparity of formation mechanism of Type I and Type II LPSO phasesare suggested as follows:(1) The formation of LPSO phases is based on thethickening of ABCA-type building blocks;(2) The transformation of a perfect18Rstructure to a perfect14H structure requires the short-range diffusion (shuffle) of theheavy RE and X atoms and long-range diffusion of Mg atoms, which belongs to thecategory of diffusional-displacive transformation;(3) Due to a higher diffusivecoefficient of RE and X atoms in solidification, the ordering of Type I18R structure(formed during solidification) is higher than that of Type II18R structure (formedduring heat treatment). Therefore, the well-ordered Type I18R structure can be easilytransformed into the Type II14H structure, while the less-ordered Type II18Rstructure shows four types of in-plane ordering (α, β, γ and δ) during heat treatment.The less-ordered Type II18R structure will transform into14H structure only whenthe RE and X atoms have sufficient time in diffusion.