【作者】 张毅；

【导师】 董尚利；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 硕士

【摘要】 本文利用MTS材料试验机和MYBP疲劳试验机研究了锻造态Mg-Zn-Y-Zr合金在室温与低温(223K、173K、123K、77K)下的拉伸性能和室温大气、室温真空、低温真空环境下的疲劳性能,并采用光学显微镜、扫描电镜(SEM)和透射电镜(TEM)分析技术考察了该合金的低温拉伸与低温真空疲劳变形断裂行为。研究表明,锻造态Mg-Zn-Y-Zr合金主要由基体α-Mg、Mg3YZn6相和Mg3Y2Zn3相组成,Mg3YZn6相和Mg3Y2Zn3相可形成混合物,以较大尺寸的第二相颗粒分布于α-Mg晶界处,或以细小颗粒弥散分布在α-Mg晶粒内部。测试温度对Mg-Zn-Y-Zr合金的拉伸性能具有显著影响,随拉伸温度的降低,合金的抗拉强度和屈服强度增加,塑性降低,弹性模量增高。Mg-Zn-Y-Zr合金在室温下具有良好的塑性,延伸率可达15.5%,这与合金中稀土元素Y的存在有关,77K拉伸时合金的延伸率则降低到4.2%。随拉伸温度的降低,Mg-Zn-Y-Zr合金拉伸断口形貌逐渐表现出解理断裂的特征,拉伸温度越低,解理特征越明显。Mg-Zn-Y-Zr合金在低温下的变形主要以孪生为主,尽管仍可发现位错滑移的迹象。各种温度下,拉伸试样标距内的变形均较均匀。Mg-Zn-Y-Zr合金在低温真空环境下的疲劳性能较室温大气有很大提高,疲劳寿命显著增加,室温真空环境下的疲劳性能则介于二者之间。三种环境下,疲劳源总是产生于存在第二相颗粒或者有缺陷的试样表面或次表面。其中,室温大气和室温真空环境下,微裂纹萌生于试样表面或者接近表面,而低温真空环境下则萌生于试样次表面。合金在室温大气环境下的疲劳断裂以塑性断裂为主,并且发现沿晶界处第二相颗粒聚集区域断裂的现象;低温真空疲劳断口解理断裂特征明显,并可发现裂纹沿特定晶体学平面扩展的痕迹。三种疲劳试样断口均存在疲劳辉纹,低温环境下的疲劳辉纹最清晰。断口附近显微组织中则可发现大量的疲劳变形孪晶和数量有限的滑移带,表明三种环境下Mg-Zn-Y-Zr合金的疲劳仍以孪生变形为主。更多还原

【Abstract】 The tensile properties at 293K, 223K, 173K, 123K and 77K and fatigue properties in laboratory air, in vacuum at room temperature and in vacuum at cryogenic temperature of a forged Mg-Zn-Y-Zr alloy were investigated by use of universal material testing system (MTS) and fatigue testing machine. And the related tensile and fatigue deformation and fracture behaviors of this alloy were also investigated by means of optical microscope, scanning election microscope (SEM) and transmission electron microscope (TEM).The results show that as-forged Mg-Zn-Y-Zr alloy mainly consisted ofα-Mg, Mg3YZn6 and Mg3Y2Zn3 phases. The Mg3YZn6 and Mg3Y2Zn3 phases have been found to be present at boundaries ofα-Mg grain or disperse withinα-Mg matrix. Testing temperature has a remarkable effect on tensile properties of the foraged alloy. Decreasing with the testing temperature, the tensile strength, yield strength and Young’s modulus of Mg-Zn-Y-Zr alloy increases, while the fracture strain decreases. The alloy exhibited a largest fracture strain of 15.5% as tested at 293K, resulting from the addition of Y elements to the alloy, and a lowest value of 4.2% was obtained at 77K. The fracture pattern of the tested Mg-Zn-Y-Zr samples was found to gradually represent cleavage fracture character as the test temperature decreased, the lower of testing temperature, and the more evident of cleavage fracture features. The deformation of the Mg-Zn-Y-Zr alloy at cryogenic temperature mainly induced by twining, despite that sign of dislocation slip could also be observed. Tensile deformation was found to occur uniformly within gauge of all the tested samples.The fatigue property in cryogenic temperature-vacuum of Mg-Zn-Y-Zr alloy is the best under the three tested environments. The fatigue cracks were found to always initiate at surface or subsurface of the tested samples where second-phase particles or structure defects existed. The fatigue initiation sites were observed to be at surface of the samples tested in laboratory air or in vacuum at room temperature, while at the subsurface of the sample tested in vacuum at cryogenic temperature. Ductile fracture was mainly found in samples fatigued in laboratory air, and fracture of second phases at grain boundaries was also observed. Cleave fracture feature was evident in samples fatigued in vacuum at cryogenic temperature, and propagation of small cracks along the appropriate crystallographic plane was seen. Fatigue striations have been observed on fracture surface of all the samples tested in three environments, which are more apparent in vacuum at cryogenic temperature. Lots of twinning and limited slip bands were found in the area next to fracture surface of the fatigued samples, indicating that twinning seems to be the dominant deformation mechanism of Mg-Zn-Y-Zr alloy fatigued in these environments.更多还原