| Mg-Gd based rare-earth magnesium alloys have excellent mechanical properties,no matter at room temperature or at elevated temperature,because of their remarkable ageing hardening response,and have attracted a great deal of attention in aerospace,automotive industries where weightsaving is of great importance.So far,most of previous studies focused on the microstructures and mechanical properties of Mg-Gd based alloys.However,there are limited studies on the solidification behaviors of Mg-Gd based alloys,and thus the quantitative relationship among the solidification conditions,solidification characteristics and mechanical properties has not been effectively established.In this thesis,Mg-Gd based alloys were chosen as the targeted alloy.Firstly,3D structure of the liquid Mg-10Gd(-0.4Zr)alloys were studied using synchrotron X-ray diffraction technique and Empirical Potential Structure Refinement(EPSR)modelling;secondly,the solidification process of Mg-10 Gd binary alloy and growth behavior of α(Mg)grains were studied in-situ using synchrotron X-ray imaging technique;thirdly,the effects of composition and solidification condition on the microstructures and mechanical properties of directionally solidified Mg-Gd binay alloys were investigated to establish the quantitative link among composition,solidification condition,microstructure and mechanical property;fourthly,the hot tearing susceptibity of Mg-Gd binary alloys were systematically studied,and the factors influencing the hot tearing susceptibility and the underlying mechanism were clarified;finally,based on the above studies,the substitution effect of Gd with Nd on the microstructure,mechanical properties and strengthening mechanism of Mg-(10-x)Gd-x Nd-Zr alloys were studied,attempting to provide guidelines for developing commercial Mg-Gd based alloys with excellent mechanical properties and low Gd content using multiple-component alloying.The structure of liquid Mg-10Gd(-0.4Zr)alloys were studied by using synchrotron X-ray diffraction technique.The results showed that the liquid Mg-10 Gd and Mg-10Gd-0.4Zr alloys exhibited typical amorphous structure with short-range ordered structure and long-range disordered structure.EPSR modelling results indicated that,when adding 0.4Zr into liquid Mg-10 Gd alloy,the distance of Mg-Mg atomic pair in Mg-10 Gd alloy increased,while the surrounding Mg and Gd atoms moved towards Mg-Mg atomic pair to coordinate the increase of the distance of Mg-Mg atomic pair,thus caused the increase in the nearest neighbours of Mg atom and the triangle of Mg-Mg-Mg linkage.In situ synchrotron X-ray imaging studies of Mg-10 Gd alloy during solidification showed that the grains exhibited typical six-folded symmetry structure,but the growth rate of six dendrite branches and their lengths varied,referred to as ‘‘self-poisoning’’effect.The ejected solute was carried downwards by gravity-driven fluid flow,leading to the suppression growth of dendrite arms situated aside and below during solidification.During directional solidification,the growing direction of some dendrites diverged from the temperature gradient direction,and a liquid pocket with higher Gd content formed between the diverged dendrite and the vertical dendrite,which facilitated the remelting of the secondary dendrite arms.The directionally solidified microstructures and mechanical properties of three Mg-x Gd(x=0.60,5.21 and 9.58)alloys with low,medium and high level of Gd content were investigated systematically.The results showed that,in the range of solidification conditions(the temperature gradient of 30 K/mm and the withdrawal rate of 10-200 μm),only cellular grains were observed in Mg-0.60 Gd alloy with low Gd content,and only dendritic grains were observed in Mg-9.58 Gd alloy with high Gd content,while the cellular-to-dendrite transition was observed in Mg-5.21 Gd alloy with medium Gd content.The dimensionless temperature gradient and withdrawal rate analysis indicated that solute content has a significant effect on the morphology of solid/liquid interface.Only cellular morphology exhibited in Mg-0.60 Gd alloy was attributed to the low solute content and the high temperature gradient used in this experiment.For the other two alloys with medium and high level of Gd content,the morphology of solid/liquid can be predicted by cellular-to-dendrite transition model proposed by Kurz-Fisher.For the directionally solidified Mg-x Gd binary alloy,the microstructural parameters,including cellular spacing/primary dendrite arm spacing,secondary dendrite arm spacing,decreased with the increase of the withdrawal rate,and the exponents of the cellular spacing/primary dendrite arm spacing,secondary dendrite arm spacing obtained from the experiments were very close to the theoretical values of 0.25 and 1/3,respectively.Meanwhile,the cellular spacing/primary dendrite arm spacing and secondary dendrite arm spacing can be predicted by Trivedi model and Kattamis-Flemings model respectively.Furthermore,with the increase of the Gd content and the withdrawal rate,the ultimate tensile strength of the directionally solidified Mg-Gd alloys increased and the elongation decreased.It was found that there is a typical ‘‘Λ’’ relationship between the hot tearing susceptibility and Gd content in Mg-Gd binary alloys,i.e.the hot tearing susceptibility increase firstly,and then decreased with the increase of Gd content,which has a good agreement with the predicted results by Clyne-Davies model.Mg-1.5Gd alloy had the maximum hot tearing susceptibility,which was mainly attributed to the coarse columnar dendrites,the high susceptible freezing range,and the thin and continuous liquid film as well as the hard and brittle intermetallics precipitated along the grain boundaries.Results showed that adding grain refiner(Zr)into Mg-Gd alloy can significantly reduce the hot tearing susceptibility,since the finer equiaxed grains can not only efficiently accommodate the deformation caused by the thermal contraction,but also increase the refilling capacity to heal the tears that formed previously.The substitution effects of Gd with 1%,3% and 5% Nd on the microstructures and mechanical properties of Mg-10Gd-0.4Zr alloy were studied.It was found that the microstructure of Mg-10Gd-0.4Zr alloy was composed of the equiaxed α(Mg),α(Mg)+ Mg5 Gd eutectic structure and a little amount of REH2 phase.And there was an extra Mg12 Nd phase observed in the Nd-containg alloys.After solution heat treatment,when the Nd content was less than 3.07%,the secondary phases were dissolved intoα(Mg)matrix completely.While there were still some secondary phases left in the solution-treated Mg-5Gd-5Nd-0.4Zr alloy,which was mainly attributed to the high content in that alloy,beyond the maximum solubility of Nd in Mg.Substitution of Gd with Nd can not only remarkably shorten the incubation period of ageing hardening,but also enhance the peak-hardness.Peak-aged Mg-7Gd-3Nd-0.4Zr alloy showed the improved mechanical properties with the ultimate tensile strength,the yield strength and the elongation of 294 MPa,195 MPa and 3.9%,respectively,which was due to the dense β’’ and β’ precipitates formed during ageing.Furthermore,the contributions of individual strengthening mechanisms in the experiemental alloys after different heat-treatments were quantitatively calculated,and the results showed that the secondary phase strengthening was the dominant strengthening mechanism in the as-cast alloys,while the key strengthening mechanism was the precipitate strengthening in peak-aged alloys,especially in peak-aged Mg-7Gd-3Nd-0.4Zr alloy,accouting for 60.6%. |
PDF Full Text Request