定向凝固下Mg-xZn-Y合金力学性能及导热性能研究

发布时间：2018-06-08 11:06

本文选题：Mg-Zn-Y合金 + 定向凝固　；参考：《太原科技大学》2017年硕士论文

【摘要】：目前,采用普通凝固技术获得的镁合金室温、高温力学性能,蠕变抗力,导热、导电等性能往往难以满足性能需求,而定向凝固作为一种新型凝固技术,通过控制单向热流获得单向凝固组织,可以提高镁合金各项性能。本文设计并制备了高导热、高比强的Mg-xZn-Y(x=1、3、5wt.%)合金,通过定向凝固装置、光学显微镜、扫描电镜、XRD衍射仪、材料试验机以及硬度计等设备来研究铸态和定向凝固工艺下Mg-Zn-Y合金力学性能和导热性能的变化规律以及影响机理,设置定向凝固参数为:温度梯度70 K/cm、100 K/cm;生长速度5μm/s、10μm/s、25μm/s、50μm/s、100μm/s。通过研究得出以下结论:(1)Mg-xZn-Y合金组织主要由α-Mg基体相与第二相W相、I相组成,基体相呈花瓣状的等轴晶,第二相呈白色粒状弥散分布于基体或晶界上。定向凝固能够使铸态合金组织由等轴晶向柱状晶演化转变,与铸态组织相比,沉淀析出的第二相数量增多。(2)铸态条件下,Zn可以细化合金组织,当Zn含量增大时,晶粒细化,沉淀析出第二相数量增多,分布也更加均匀。定向凝固条件下,当生长速度恒定时,温度梯度增大,柱状晶更粗大、均匀、方向性也更好;当温度梯度恒定时,生长速度增大,柱状晶平均宽度逐渐变窄,晶粒分布的均匀性逐渐降低,晶粒生长的方向性、连续性也逐渐变差。(3)定向凝固条件下,当生长速度恒定时,温度梯度增大,抗拉强度、伸长率和硬度都呈现出增大的趋势,通过对比发现,高温度梯度下的抗拉强度较之低温度梯度下的抗拉强度提高了约9.3%,伸长率提高了约15.0%,合金硬度提高了约11.3%;当温度梯度一定,生长速度逐渐增大时,抗拉强度、伸长率和硬度呈现出先增后降的趋势,当温度梯度在100 K/cm,生长速度达到50μm/s时,抗拉强度、伸长率、硬度达到了最大值,分别为245.0 MPa、13.3%、71.7 HV,相比于铸态合金,分别提高了35.7%、49.4%,26.0%。(4)当Zn含量在(1~5 wt.%)范围内增大时,铸态合金热导率降低,并且,Zn含量的增大对热导率的影响十分明显,呈线性下降趋势。定向凝固条件下,当生长速度恒定时,温度梯度增大,合金的热导率呈现小幅增长的趋势;当温度梯度恒定时,生长速度增大,热导率呈现逐渐下降的趋势。但是,相比于铸态合金,定向凝固合金的热导率均有所提高。以Mg-3Zn-Y合金为例,当温度梯度为100 K/cm,生长速度为5μm/s时,合金的热导率最高达到了149.73 W/(m?K),比铸态合金提高了36.0%;当温度梯度为100 K/cm,生长速度为100μm/s时,定向凝固热导率最低为128.64 W/(m?K),比铸态合金提高了13.1%。
[Abstract]:At present, the mechanical properties, creep resistance, thermal conductivity and electrical conductivity of magnesium alloys obtained by ordinary solidification technology are often difficult to meet the requirements of performance, and directional solidification is a new solidification technology. The properties of magnesium alloy can be improved by controlling unidirectional heat flux to obtain unidirectional solidified microstructure. In this paper, a high thermal conductivity and high specific strength Mg-xZn-YTX (1) -5 wt.alloy was designed and prepared. The alloy was characterized by directional solidification, optical microscope, scanning electron microscopy and XRD diffraction. The mechanical properties and thermal conductivity of Mg-Zn-Y alloy under as-cast and directional solidification conditions were studied by material testing machine and hardness tester. The directional solidification parameters were set as follows: temperature gradient 70 K / cm ~ (100) K / cm; growth rate 5 渭 m / s ~ (10) 渭 m / s ~ (25) 渭 m / s ~ (50) 渭 m 路s ~ (-1) ~ 100 渭 m 路s ~ (-1). It is concluded that the microstructure of the alloy consists of 伪 -Mg matrix phase and the second phase W phase I phase, the matrix phase is petal-like equiaxed crystal, the second phase is white granular dispersion on the matrix or grain boundary. Directional solidification can change the structure of as-cast alloy from equiaxed to columnar. Compared with the as-cast structure, the amount of the second phase precipitated by precipitation can be increased under the condition of as-cast. Zn can refine the microstructure of the alloy, and when Zn content increases, the grain size will be refined. The number of precipitated second phases increased and the distribution was more uniform. Under the condition of directional solidification, when the growth rate is constant, the temperature gradient increases, the columnar crystal is thicker, uniform, and the directivity is better, and when the temperature gradient is constant, the growth rate increases and the average width of columnar crystal becomes narrower. Under the condition of directional solidification, when the growth rate is constant, the temperature gradient increases and the tensile strength, elongation and hardness increase. It is found by comparison that tensile strength at high temperature gradient is about 9.3% higher than that at low temperature gradient, elongation increases 15.0%, hardness of alloy increases about 11.3%, tensile strength increases gradually when temperature gradient is constant and growth rate increases gradually. The elongation and hardness increased first and then decreased. When the temperature gradient was 100K / cm and the growth rate was 50 渭 m / s, the tensile strength, elongation and hardness of the alloy reached the maximum value, which were 245.0 MPA / 13.3 and 71.7HVrespectively, compared with the as-cast alloy. The thermal conductivity of as-cast alloys decreased with the increase of Zn content in the range of 1wt.) and the effect of Zn content on the thermal conductivity was obvious, which showed a linear decreasing trend. Under the condition of directional solidification, when the growth rate is constant, the temperature gradient increases, the thermal conductivity of the alloy increases slightly, and when the temperature gradient is constant, the growth rate increases and the thermal conductivity decreases gradually. However, compared with the as-cast alloy, the thermal conductivity of the directionally solidified alloy is improved. Taking Mg-3Zn-Y alloy as an example, when the temperature gradient is 100K / cm and the growth rate is 5 渭 m / s, the thermal conductivity of the alloy reaches the highest level of 149.73 W / m ~ (-1), which is 36.0% higher than that of the as-cast alloy, and when the temperature gradient is 100 K / cm and the growth rate is 100 渭 m / s, the thermal conductivity of Mg-3Zn-Y alloy is 36.0% higher than that of the as-cast alloy. The lowest thermal conductivity of directional solidification is 128.64 W / m ~ (-1), which is 13.1% higher than that of as-cast alloy.
【学位授予单位】：太原科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.22;TG292

【参考文献】