Study On The Microstructure, Mechanical Properties And Strengthen Mechanism Of Mg-Nd-Zn-Zr Alloys

The extremely low density, high specific strength and stiffness of Mg alloy make itattractive for engineering applications. It has been demonstrated that rare earth metals(RE) are the most effective elements to improve the strength properties of magnesiumalloys especially at elevated temperatures. More recently, a lot of work has been focusedon magnesium alloys containing heavy rare earth elements, such as Mg-Gd based alloys.Though these alloys show high strength, high content of heavy rare earth elementsenhances alloys’ cost, which would probably limit their engineering application. Rationaluse of Neodynium (Nd) could make it possible to develop a high strength and low costmagnesium alloy. Nd is one of light rare earth elements, with maximum solubility in solidMg of3.6wt.%at eutectic temperature545℃. Mg-Nd binary alloys have already hadsignificant strengthening effect. The commercial Russia ML10and Chinese ZM6alloys aredeveloped from Mg-MM alloy, in which MM is Nd rich Misch Metal. Addition of smallamount Zn to Mg-Nd alloy can further increase its peak-aged hardness and creepstrength, but more addition can reduce the peak-aged hardness. The strengthenmechanisms of Zn addition in Mg-Nd alloy are unclear yet. Therefore, it is necessary to dosystematic research on Mg-Nd-Zn alloys in order to optimize the alloy’s strength andreveal the strengthen mechanisms.By inductively coupled plasma analyzer (ICP), optical microscopy (OM), X-raydiffractometer (XRD), scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM), hardness, tensile, compression and creep test, the microstructure andmechanical properties of gravity cast Mg-2.75Nd-xZn-Zr (x=0,0.2,0.5,1.0,2.0wt.%) andMg-yNd-0.2Zn-Zr (y=1.25,1.75,2.25,2.75,3.0,3.25wt.%) alloys in as-cast,solution-treated and200℃peak-aged conditions are investigated at room temperatureand the optimal chemical compositions are determined. The effect of trace amount of Znaddition on the plastic deformation mechanism of the alloy at room temperature, theprecipitates, mechanical behavior and strengthen mechanism in typical aged optimized alloy, and the effect of hot extrusion on the optimized alloy are investigated anddiscussed. The results show:When x=0.2, y=3.0wt.%, the cast Mg-yNd-xZn-Zr alloy has the best combination ofroom temperature strength and elongation in as-cast, solution-treated and200℃peak-aged conditions, and is considered as optimized cast alloy. The best mechanicalproperties of optimized cast alloy at room temperature are: yield strength (YS)140MPa,ultimate tensile strength (UTS)300MPa, elongation11%. The best mechanical propertiesof hot extruded alloy, whose chemical composition are the same as the optimized castalloy, at room temperature are: YS314MPa, UTS325MPa, elongation19.3%.Trace amount of Zn addition can significantly enhance Mg-Nd-Zn-Zr alloys’ ductility andincrease the UTS in as-cast, solution-treated and200℃peak-aged conditions at roomtemperature. By comparison the surface morphology variation of solution-treatedMg-2.75Nd-Zr (NK-T4) and Mg-2.75Nd-0.2Zn-Zr (NZK-T4) alloy tensile samples in in-situtensile test in SEM at room temperature, it is found that the enhancement of ductility ofNZK-T4alloy is probably due to the frequent activity of plastic deformation mechanismrepresented by the wave slip lines. The investigation of dislocation distribution in5%tensile deformed NK-T4and NZK-T4samples in TEM shows that, besides basal <a>dislocations, non-basal basal <a> dislocations are investigated in both NK-T4and NZK-T4alloys, however, the dislocation density is much higher in NZK-T4alloy than that in NK-T4alloy, specially near the grain boundaries. Also, in some grains in NZK-T4alloy,<a+c> or<c> dislocations are investigated and their density are high. Therefore, the wave slip linesinvestigated on the surface of tensile samples in in-situ tensile test are probably causedby the non-basal dislocations investigated in TEM. Nd solute atoms in magnesium alloycan enhance the activity of non-basal dislocations (mainly non-basal <a> dislocations).Trace amount of Zn addition can further promote the activity of non-basal dislocations,inclusion non-basal <a> dislocations and <a+c> or <c> dislocations, which improve thealloys’ both ductility and UTS at room temperature.Cast Mg-3.0Nd-0.2Zn-Zr (NZ30K)(wt.%) alloy has different precipitates under differentaging process: the precipitates are the β’ metastable phase when aged at250℃for0.5to500h; the precipitates are mainly the β” metastable phase when aged at200℃for10to14h (peak-aged). The200℃peak-aged alloy has the best mechanical properties atroom temperature.The contribution of precipitate strengthening in cast-T6alloy is about60%. After hot extrusion and200℃peak-aging, the absolute strengthening contribution of precipitatestays about90MPa as cast-T6alloy, but the contribution percentage reduces to30%. Thestrengthening contribution of grain refinement and secondary phase produced during hotextrusion is more than50%in hot extruded alloy.As-extruded NZ30K alloy composes of recrystallized grains, un-recrystallized area andsecond phases precipitated during hot extrusion. The recrystallized grains show thebimodal grain size distribution. The fine grains are less than1μm, while the coarse grainsare several micrometers in size. The (0001) planes of the un-recrystallized area areparallel to the extrusion direction, which is probably the main reason of the ring fibertexture. As-extruded NZ30K alloy shows typical ductile fracture characteristics during thetensile test at room temperature: the yield drop phenomenon, the yield point elongation,necking in the late period of tensile test and dimple fracture surface. The yield dropphenomenon is caused by the localized plastic deformation at the beginning of tensiletest. The localized plastic deformation is closely associated with the non-homogeneousdistribution of the un-recrystallized area and fine grains, the bimodal grain sizedistribution, the enhanced plastic deformation ability of the individual grains by Nd andZn solute atoms and the low density of mobile dislocation in as-extruded alloy. Thepropagation of localized plastic deformation on the gauge length of tensile samples leadsto the yield point elongation, which are mainly caused by the basal and non-basal <a>dislocations. The homogeneous plastic deformation follows the yield point elongation,which are also mainly caused by the basal and non-basal <a> dislocations. Localizeddeformation zone and dislocation walls are investigated in the grain interiors in this stage.After UTS, necking happens and lots of dislocations containing <c> component areinvestigated. In this stage, the alloy has five individual slip systems and ductile fracturehappens. During the whole tensile deformation, twinning is only investigated in somecoarse grains, hence, only takes small part of the whole plastic deformation.