Mg-Zn-Gd系镁合金的凝固特性及其力学性能研究

发布时间：2018-08-29 13:08

【摘要】：Mg-Zn-Gd系稀土镁合金具有良好的室温力学性能和优良的高温抗蠕变性能,受到航空航天、军工及汽车等工业领域的高度关注,是目前最具发展前景的耐热镁合金之一。尽管人们在提高Mg-Zn-Gd系镁合金性能方面进行了卓有成效的工作,但依然对Mg-Zn-Gd系的凝固特性、凝固组织控制,以及凝固组织特性与力学性能之间的关系等缺乏足够的认识和系统的研究。本文以Mg-Zn-Gd系镁合金为对象,首先采用定向凝固技术,系统地研究了稀土元素Gd含量、温度梯度、生长速度等对Mg-Gd二元合金和Mg-Zn-Gd三元合金凝固组织及室温力学性能的影响规律;随后,采用自由凝固方法,研究了不同冷却速度和合金成分的Mg-Zn-Gd三元合金自由凝固路径,建立了三元合金自由凝固路径预测模型;最后,设计和制备了Mg-5.5Zn-2.0Gd-0.6Zr(wt%)砂型铸造镁合金,探讨了热处理过程中合金的微观组织及力学性能的演变规律,并对合金的强韧化机理进行了分析。本文的主要结论如下:(1)提出了修正的Yokoyama自扩散系数模型,计算确定了Mg、Zn、Gd、Al、Si等纯金属元素的自扩散系数,发现其更接近实验结果。建立了多元合金的溶质液相扩散系数计算模型,通过对Al-Si-Mg、Al-Cu-Mg和Al-Fe-Ni合金的溶质液相扩散系数计算,发现计算结果与实验值吻合良好,证明本文模型合理可信,并由此确定了Mg-5.5Zn-2.0Gd(wt%)合金的溶质液相扩散系数值。(2)研究了Mg-xGd(x=1.38,2.35,4.38 wt%)合金在不同凝条件下的凝固组织演化特征。结果表明,在宽抽拉速度(10-200μm/s)范围内,Mg-1.38Gd和Mg-2.35Gd合金的凝固组织均为胞晶形貌,合金没有在Kurz-Fisher模型计算的胞-枝转变临界生长速度(vc-d)附近出现枝晶组织。分析发现,这与合金浓度较低和温度梯度较高有关。对于Gd含量较高的Mg-4.38Gd合金,当v=100~200μm/s时,合金中出现了枝晶组织。此外,通过非线性拟合,确定了Mg-xGd合金在G=20~30K/mm和v=10~200μm/s条件下胞晶间距(λ)和生长参数(G、v)之间的定量关系式。对比发现,λ的测量值与Trivedi模型的预测值一致。(3)确定了Mg-xGd(x=1.38,2.35,4.38 wt%)合金的凝固组织构成及微观偏析行为,对比发现,这与使用Scheil模型预测Mg-Gd二元合金的凝固路径和微观偏析一致。此外,力学性能实验表明,在相同冷却速率条件下,Mg-2.35Gd合金定向凝固试样的室温抗拉性能明显高于自由凝固试样。定向凝固参数对Mg-Gd合金的室温力学性能有显著影响,随着生长速率和温度梯度的增大,合金抗拉性能逐渐提高,塑性逐渐降低。(4)首次开展了Mg-5.5Zn-xGd(x=0,0.8,2.0,4.0wt%)合金在不同凝固条件下的凝固组织变化演化研究。实验发现,随着溶质Gd含量的增加,合金的胞-枝转变临界生长速度(vc-d)有降低趋势。此外,使用非线性拟合方法获得了合金枝晶间距(λ1,λ2)和生长速度(v)之间的关系式。λ1和λ2均随v呈指数关系降低,所得指数分别与理论模型预测值1/4和1/3接近。(5)研究了不同凝固参数下Mg-5.5Zn-xGd(x=0,0.8,2.0,4.0 wt%)合金的室温力学性能变化规律。当合金成分一定时,其抗拉强度随着生长速度的增大而增大;当生长速度相同时,其抗拉强度随Gd含量的增大呈先增大后降低的规律。对比发现,Mg-5.5Zn-2Gd合金的室温抗拉强度最优。(6)研究了Mg-5.5Zn-2.0Gd(wt%)合金的自由凝固路径。实验表明,当冷却速率小于等于0.75K/s时,合金首先生成的共晶为α(Mg)+W(Mg_3Zn_3Gd_2);当冷却速率大于等于7.71K/s时,合金首先生成的共晶为α(Mg)+I(Mg_3Zn_6Gd)。建立了综合考虑合金液相扩散和冷却速率因素的多元合金自由凝固路径计算模型,计算结果与实验结果吻合良好。计算及实验表明,Mg-Zn-Gd合金中Zn含量越高,冷却速率越大,越趋于形成I(Mg_3Zn_6Gd)相。反之,Gd含量越高,冷却速率越低,将更易于形成W(Mg3Gd2Zn3)相。力学性能表明,Mg-5.5Zn-2.0Gd合金的抗拉强度、屈服强度及延伸率均随冷却速度的提高而增强,其拉伸断口由冷却速度较低条件下的沿晶断裂特征转变为冷却速度较高时的穿晶断裂特征。分析发现,合金强度的提高除了晶粒细化的作用外,更多准晶I(Mg_3Zn_6Gd)相的形成对合金强度也起到了进一步增强的作用。(7)设计和制备了Mg-5.5Zn-2.0Gd-0.6Zr(wt%)砂型铸造镁合金,系统研究了合金在热处理过程中的组织及力学性能演化规律。合金铸态组织由α(Mg)、晶界α(Mg)+W(Mg_3Zn_3Gd_2)共晶和晶内的I(Mg_3Zn_6Gd)、Mg3Gd及α(Zr)相组成。固溶处理后,原铸态组织中α(Mg)+W(Mg_3Zn_3Gd_2)共晶大部分消失,残留W(Mg_3Zn_3Gd_2)相呈断续状分布。同时,合金基体内有棒状Zn2Zr3相形成。经T6热处理后,合金基体内有棒状β1′相和盘状β2′相析出。对合金室温强化机制分析表明,由于固溶强化和时效强化的综合作用,合金的室温抗拉强度和屈服强度由铸态的189 MPa和98 MPa提高至T6态的280 MPa和175 MPa。较ZK51A工业合金,实验合金T6态的室温抗拉强度提高了14.29%,屈服强度提高了26.81%。同时,实验合金的高温力学性能明显优于ZK51A工业镁合金。
[Abstract]:Mg-Zn-Gd rare earth magnesium alloys have good mechanical properties at room temperature and excellent creep resistance at high temperature. They have attracted great attention in aerospace, military and automotive industries. They are one of the most promising heat-resistant magnesium alloys at present. The solidification characteristics, microstructure control and the relationship between microstructure and mechanical properties of Mg-Zn-Gd alloys are still poorly understood and systematically studied in this paper. Secondly, the free solidification paths of Mg-Zn-Gd ternary alloys with different cooling rates and alloy compositions were studied by free solidification method, and the prediction model of the free solidification paths of Mg-5.5Zn-2.0Gd-0.6Zr (w) ternary alloys was established. Finally, the free solidification paths of Mg-5.5Zn-2.0Gd-0.6Zr (w) were designed and prepared. The main conclusions are as follows: (1) A modified Yokoyama self-diffusion coefficient model is proposed, and the self-diffusion coefficients of pure metal elements such as Mg, Zn, Gd, Al, Si are calculated and determined. The calculation model of solute liquid diffusion coefficients of multicomponent alloys is established. By calculating the solute liquid diffusion coefficients of Al-Si-Mg, Al-Cu-Mg and Al-Fe-Ni alloys, it is found that the calculated results are in good agreement with the experimental values, which proves that the model is reasonable and credible, and the solute liquid phase of Mg-5.5Zn-2.0Gd (wt%) alloy is determined. (2) The microstructure evolution of Mg-xGd (x=1.38, 2.35, 4.38 wt%) alloy under different solidification conditions was studied. The results show that the solidified microstructure of Mg-1.38 Gd and Mg-2.35 Gd alloys is cellular morphology in the wide pull-out rate range (10-200 micron/s), and the critical growth rate of CELL-DENDRITE transition calculated by Kurz-Fisher model is not found. Dendritic structure appears near vc-d. It is found that this is related to the lower alloy concentration and higher temperature gradient. For Mg-4.38G D alloy with higher G D content, dendritic structure appears in the alloy when v=100~200 micron/s. In addition, the intercellular spacing (lambda) and the growth of Mg-xGd alloy at the conditions of G=20~30K/mm and v=10~200 micron/s are determined by nonlinear fitting. The quantitative relationship between the long parameters (G, v) is found to be consistent with the predicted values of Trivedi model. (3) The solidification microstructure and micro-segregation behavior of Mg-xGd (x = 1.38, 2.35, 4.38 wt%) alloy are determined. The results show that the predicted solidification path and micro-segregation behavior of Mg-Gd binary alloy are consistent with those of Scheil model. Experimental results show that the tensile properties of directionally solidified Mg-2.35Gd alloy at room temperature are significantly higher than those of freely solidified Mg-2.35Gd alloy at the same cooling rate. (4) The evolution of solidification microstructure of Mg-5.5Zn-xGd (x=0,0.8,2.0,4.0wt%) alloy under different solidification conditions was studied for the first time. It was found that the critical growth rate of CELL-DENDRITE transition (vc-d) decreased with the increase of solute Gd content. (5) The mechanical properties of Mg-5.5Zn-xGd (x=0,0.8,2.0,4.0 wt%) alloy at room temperature were studied under different solidification parameters. The tensile strength of Mg-5.5Zn-xGd alloy increased with the growth rate when the alloy composition was constant. The tensile strength of Mg-5.5Zn-2Gd alloy increases at first and then decreases with the increase of Gd content at the same growth rate. It is found that the tensile strength of Mg-5.5Zn-2Gd alloy is the best at room temperature. (6) The free solidification path of Mg-5.5Zn-2.0Gd (wt%) alloy is studied. When the cooling rate is greater than or equal to 7.71K/s, the eutectic phase of the alloy is alpha(Mg)+I(Mg_3Zn_6Gd). A computational model for the free solidification path of multi-component alloy is established, which takes into account the factors of liquid diffusion and cooling rate. The calculated results are in good agreement with the experimental results. On the contrary, the higher the content of Gd and the lower the cooling rate, the easier W (Mg3Gd2Zn3) phase will be formed. The mechanical properties show that the tensile strength, yield strength and elongation of Mg-5.5Zn-2.0Gd alloy increase with the increase of cooling rate, and the tensile fracture surface of Mg-5.5Zn-2.0Gd alloy increases with the increase of cooling rate. The intergranular fracture characteristics under the alloy changed to transgranular fracture characteristics at higher cooling rate. It was found that besides grain refinement, the formation of more quasicrystalline I (Mg_3Zn_6Gd) phase enhanced the strength of the alloy. (7) Mg-5.5Zn-2.0Gd-0.6Zr (wt%) sand mold casting magnesium alloy was designed and prepared. The microstructure and mechanical properties of the alloy during heat treatment have been studied systematically. The as-cast microstructure of the alloy consists of a (Mg), a (Mg) + W (Mg_3Zn_3Gd_2) eutectic at grain boundary and I (Mg_3Zn_6Gd), Mg3Gd and a (Zr) phase in the crystal. After solution treatment, the eutectic of a (Mg) + W (Mg_3Zn_3Gd_2) in the as-cast microstructure disappears mostly and the residual W (Mg_3Zn_3Gd_2) phase disappears. At the same time, the rod-like Zn_2Zr_3 phase is formed in the alloy matrix. After T6 heat treatment, the rod-like beta_1'phase and the disc-like beta_2' phase precipitate in the alloy matrix. The analysis of the strengthening mechanism at room temperature shows that the tensile strength and yield strength of the alloy at room temperature are 189 MPa and 98 M due to the combination of solution strengthening and aging strengthening. Compared with ZK51A industrial alloy, the tensile strength at room temperature and yield strength of T6 alloy are increased by 14.29% and 26.81% respectively. Meanwhile, the high temperature mechanical properties of the alloy are obviously better than those of ZK51A industrial alloy.
【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22

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