Study On The Microstructural Evolution, Properties And Fracture Behavior Of Mg-Gd-Y-Zr(-Ca) Alloys

High-performance magnesium-heavy rare earth alloys, such as those based on Mg-Gd, Mg-Dy, Mg-Tb systems and so on, are very attractive for aerospace, armament and racing automotive industries because of their high specific strength and good thermal stability. Among them, Mg-Gd system is one of the most promising candidates due to the remarkable age-hardening response and very good thermal stability of the main strengthening phase at up to 250oC. Recently, Mg-Gd alloys with the addition of Y, Nd, Sm, Sc, and Zn, were investigated to further improve the mechanical properties or reduce the high content of expensive Gd, which exhibited the development trend of multi-elements alloying. However, there is lack of general and systematic investigation for Mg-Gd and Mg-Gd-Y alloys and the relationship among the compositions, microstructure and properties, especially the precipitation process and its relationship with mechanical properties, which will obstruct the development of the complex Mg-Gd series alloys.Several Mg-(6-12)Gd-(1-3)Y-Zr, Mg-15Gd and Mg-8Y (wt%) alloys were prepared. Effects of variant content of Gd, heat treatment and thermal-mechanical process on the microstructure, mechanical properties, creep resistance, corrosion resistance and fracture behavior of Mg-xGd-3Y-Zr (6≤x≤12) alloys were mainly investigated, by computer data collection system, optical microscopy (OM), image analysis apparatus with a image analysis software, X–ray diffractometer (XRD), inductively coupled plasma analyzer (ICP), Differential Thermal Analysis (DTA), Differential Scanning Calorimeter (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy dispersive X-ray analyses (EDAX) and microdiffraction etc.. The strengthening mechanism of the alloys was analyzed and discussed, and microstructural evolution during aging, including the morphology, structure, size and distribution of the precipitates, was studied in detail. Furthermore, the effect of a small Ca addition on microstructure and properties, especially creep resistance and corrosion resistance, was investigated for the first time. The purpose of the present work is to provide theoretical and practical results for the development of high-performance magnesium-heavy rare earth alloys.The microstructure evolution of cast Mg-Gd-Y-Zr(-Ca) alloys from as-cast to T4 to T6 conditions involves solid solution + eutectic compound→supersaturated solid solution + cuboidal phase→solid solution +β′precipitates + cuboidal phase. In addition, Zirconium cores exist in all these conditions. The cuboidal phase is Gd and Y rich solid solution with a f.c.c. structure (Fm3m cubic, a = 5.25?), which are observed mostly in grain interior but partially at grain boundary. The eutectic compound has a b.c.c. structure with a = 11.2? and a composition of Mg24(Gd, Y)5.The peak tensile properties of cast-T6 Mg-xGd-3Y-Zr (6≤x≤12) alloys is such that the ultimate tensile strength (UTS), tensile yield strength (TYS) and elongation is 370MPa, 241MPa and 4.0% respectively, which is attained by the optimized heat treatment. UTS and TYS of Mg-xGd-3Y-Zr(-Ca) alloys containing a high Gd content with x≥9% are remarkablely superior to those of WE54, especially in the temperature range from room temperature to 200oC. A very high strength of extruded-T5 GW123K alloy with UTS=491MPa, TYS=436MPa and elongation=3.6%, is achieved by appropriate hot extrusion process, the following 6% cold working hardening and age strengthening at room temperature. The strengths of Mg-xGd-3Y-Zr(-Ca) both in cast-T6 and extruded-T5 conditions decline very slowly from room temperature to 200oC. However, at the temperature of 250oC or more, the strengths steeply decrease, and the difference in strength between cast-T6 and extueded-T5 becomes small at 300oC. The instant tensile strengths of extruded-T5 GW123K and GW102K alloys are higher than those of forged-T6 2618 aluminum alloy and forged-T5 WE54 magnesium alloys at the temperatures from 20oC to 250oC.The decomposition ofα-Mg supersaturated solid solution (S.S.S.S., cph) in Mg-Gd-(Y)-Zr alloys with increasing aging time is as follows:β″(D019)→β′(cbco)→β1(fcc)→β(fcc), which is similar to that of Mg-Gd-Nd, Mg-Dy-Nd and Mg-Y-Nd alloys, but different from previously reported three stage sequence: S.S.S.S.→β″(D019)→β′(cbco)→β(fcc). It is found that the metastableβ″andβ′phases coexist in the matrix at the very early stage of ageing. Peak age-hardening is attributed to the precipitation of prismaticβ′plates in a triangular arrangement. At the over-aged stage,β1 phase appears to take place via an in situ transformation from a decomposedβ′phase but grows in a direction different from the previous one ofβ′phase. Continued ageing makes theβ1 phase transform in situ to the equilibriumβphase and the orientation relationship between the precipitate and matrix phases is retained through the in situ transformation of theβ1 phase.The atomic models based on both the microdiffraction and crystallography analysis indicate that the stacking type of atoms inβ″andβ′phases are the same as that of the hcp matrix and the precipitation makes progress simply by long-period ordering of RE atoms such as Gd and Y. Both of them keep a perfect coherency with the matrix. The interface between theβ1 phase and matrix is near to perfectly coherent in the ( 1 100)αhabit plane of theβ1 plates, but only semi-coherent in the (1120 )αwhich is vertical to the habit plane. Probably so does the interface between theβphase and matrix. In 200oC/200MPa test condition within 300h, the creep resistance of cast-T6 alloys increases in the following order: GW63K < GW83K < WE54 < GW103K < GW113K, where the creep resistance of GW103K or GW113K alloy is higher than that of WE54 alloy, and the minimum creep rate of GW113K is lower than that of WE54 alloy by over 40%. Under a constant stress of 200MPa, Mg-Gd-Y-Zr(-Ca) alloys can endure the applying temperature lower than 175oC.Comparative investigation on the creep resistance of cast and extruded Mg-Gd-Y alloys has been carried out. The minimum creep rates of extruded-T5 alloys with fine grains can be two orders of magnitude more than those of cast-T6 alloys. Under the test condition of 200oC/160MPa, the extruded-T5 alloys have exhibited the characteristic of diffusion creep, and formed a very wide precipitate free zone (PFZ), which results in the main large creep strain.Within the temperature and applied stress range of 175-200oC/160-200MPa, the activation energy for creep of the cast-T6 and extruded-T5 alloys similarly varies from 190-256kJ/mol, which is higher than the activation energy for self diffusion of magnesium. The stress exponent n of the cast-T6 alloys (n=6-10) is higher than that of the extruded-T5 alloys (n=4-6). It can be showed from the result of TEM observation that the non-basal slip planes of dislocations, including the first order pyramidal plane {1011} and the second order plane {1012 }, and the cross-slip between them and basal plane, can be activated under the creep condition of 200oC/160MPa.The weight loss corrosion rate of cast-T6 Mg-xGd-3Y-Zr alloys in the salt spray test increases first and decreases later, and the corrosion resistance of the alloys descends in the following order: GW63K < GW83K < GW123K < GW103K. The corrosion resistance of cast-T6 GW63K or cast-T6 GW83K alloy is superior to that of as cast AZ91D, and the corrosion resistance of cast-T6 GW103K or cast-T6 GW123K alloy is comparable with that of as cast AZ91D. Compared with cast-T6 alloys, the self-corrosion potential and weight loss corrosion rate of cast-T5 alloys increase dramatically due to the obvious grain refinement.The rupture of cast-T4, cast-T6 and ectruded-T5 alloys mainly belongs to quasi-cleavage fracture, and the inter-granular fracture cleavage fracture only occurs in the as-cast alloys with high Gd content. With the elevated temperature, or the grain refinement by hot extrusion, the fracture style of the alloys transforms gradually from brittleness to toughness. From room temperature to 200oC, the rupture of the cast-T6 and extruded-T5 alloys is mainly quasi-cleavage fracture; over 200oC, the proportion of the fracture due to microcavities coalescing increases with the elevated temperature; the rupture of the alloys consists of mixed microcavities coalescing and quasi-cleavage fracture at 250oC, but microcavities coalescing becomes the main fracture mechanism at 300oC.The dislocations piling-up model can be successfully applied to explain the initiation mechanism of cleavage microcracks, and the modified Griffith equation to explain well the resistance to crack propagation which vary with the composition and status of the alloys.Two contributions to the TYS above that of pure Mg, can be identified for the cast-T4 alloys, namely solid solution strengthening and grain refinement strengthening, where the solid solution strengthening plays the main strengthening role in the cast-T4 alloys. The relationship between TYS (σs) and the atom concentration (c) is such that ? s ? 905c2/3. In spite of sacrificing most of solid solution strengthening, the TYS of cast-T6 alloys is greatly elevated from cast-T4 alloys due to the remarkable precipitation strengthening, which is main strengthening contribution over 50% of total TYS to all the cast-T6 alloys. The contribution from grain refinement strengthening increases and the one from texture strengthening occurs in the extruded-T5 alloys, and both of these two contributors lead to the increase of TYS in the extruded-T5 alloys, compared to cast-T6 alloys. Although the proportion of strengthening contribution due to precipitation strengthening decreases, it keeps as the largest contributor.At the temperature lower than 200oC, the outstanding precipitation strengthening in aged Mg-Gd-Y alloys is mainly attributed to the favorable shape and orientation ofβ′precipitates which make them obstruct effectively the basal dislocation slip, the bigger volume fraction of the precipitates, perfectly coherent interface between the precipitates and the matrix, and good thermal stability of the precipitates. Coherent strengthening and Orowan mechanism both contribute to the peak hardness of aged Mg-Gd-Y alloys. However, Orowan mechanism becomes the main contributor to TYS at the over-aged stage.A small addition of 0.4-0.6wt% Ca improves the creep and corrosion resistances of Mg-Gd-Y-Zr alloys but severely deteriorates the elongation, which is related to the presence of the Ca segregation at grain boundaries.