Zr基块体非晶合金的胀形和弯曲行为

发布时间：2018-10-09 17:37

【摘要】：本文选择Zr50.7Cu28Ni9Al12.3块体非晶合金作为研究对象,结合非晶合金在实际成形和应用方面面临的实际应力条件问题,分别进行气体自由胀形和单边缺口梁法三点弯曲试验,系统研究了Zr基非晶合金在高温和室温下处于复杂应力和应变状态时的变形过程。通过控制变形温度、气体压力和加压时间三个参数研究了不同因素对非晶合金气体自由胀形过程的影响。借助金相显微镜(OM)对非晶合金胀形高度和厚向应变进行研究。结果表明,随着变形温度和气体压力的增加,非晶合金的变形能力增强,胀形高度和厚向应变增加,但是由于材料的流变导致其厚度不均匀程度加剧。在压力为2 MPa时非晶合金胀形件容易发生破裂失效。借助差示扫描量热仪(DSC)和透射电子显微镜(TEM)研究了非晶合金胀形过程中的结构演变规律。研究发现,随着变形温度的升高和加压时间延长,非晶合金的晶化程度加剧,同时在非晶合金胀形件的不同部位也出现明显的晶化程度的差异,这是由于应变诱发晶化导致的,非晶合金胀形件越接近其胀形最高点,所受到的应变越大,晶化程度亦相应增加。借助TEM观察了非晶合金胀形件的微观结构,结果表明,在变形温度较低时,晶化相以块状存在,随着变形温度的升高晶化相尺寸逐渐增加。当温度继续升高到一定程度时从非晶基体中析出针状晶化相,这将导致非晶合金的强度和塑性等力学性能的显著下降。借助数字图像相关方法(DIC)研究了单边开不同宽度U型缺口的非晶合金弯曲试样在三点弯曲过程中缺口附近的应变场的变化规律。弯曲试验的载荷-位移曲线表明,在三点弯曲过程中,Zr基非晶合金并没有出现明显的屈服现象,随着缺口尺寸减小,非晶合金三点弯曲的最大载荷和最大弯曲位移逐渐减小,这是由于应变集中程度增加导致韧性下降。利用DIC得到的缺口附近的应变场分布结果表明,缺口附近区域的线应变和切应变均出现应变集中现象,且应变值随着时间的增加呈现逐渐增加的趋势。随着缺口尺寸减小,应变集中现象更加明显,但缺口宽度的减小会造成缺口附近区域最大应变值的减小。利用扫描电子显微镜(SEM)观察了缺口附近区域和断口两侧的剪切带分布特征。剪切带增殖和扩展是局部切应变超出弹性应变极限造成的,研究发现,随着缺口尺寸减小,缺口附近区域剪切带尺寸和数量减小。借助SEM对非晶合金弯曲试样断口进行观察,分析发现,弯曲试样断口存在裂纹扩展区和快速断裂区两个截然不同的区域。随着缺口尺寸减小,应变集中程度增加,非晶合金弯曲试样的韧性下降,因此裂纹扩展区的宽度和快速断裂区的韧窝状微观结构的尺寸均呈逐渐下降的趋势。
[Abstract]:In this paper, Zr50.7Cu28Ni9Al12.3 bulk amorphous alloy is chosen as the object of study. Combined with the actual stress conditions of the amorphous alloy in actual forming and application, the gas free bulging test and one-sided notched beam three-point bending test are carried out, respectively. The deformation process of Zr based amorphous alloy in complex stress and strain state at high temperature and room temperature was systematically studied. The effect of different factors on the gas free bulging process of amorphous alloy was studied by controlling deformation temperature, gas pressure and pressure time. The bulging height and thick strain of amorphous alloy were studied by metallographic microscope (OM). The results show that with the increase of deformation temperature and gas pressure, the deformation ability of amorphous alloy increases, the bulging height and thick strain increase, but the thickness inhomogeneity is aggravated due to the rheology of the material. When the pressure is 2 MPa, the bulging part of amorphous alloy is prone to fracture failure. By means of differential scanning calorimeter (DSC) and transmission electron microscope (TEM), the structure evolution of amorphous alloy during bulging was studied. It is found that the crystallization degree of amorphous alloy increases with the increase of deformation temperature and pressure time, and there are obvious differences of crystallization degree in different parts of bulging part of amorphous alloy, which is caused by strain induced crystallization. The closer the bulging part of amorphous alloy is to its bulging peak, the greater the strain is and the degree of crystallization increases accordingly. The microstructure of bulging part of amorphous alloy was observed by TEM. The results show that the crystalline phase exists as a block when the deformation temperature is low, and the size of crystallization phase increases gradually with the increase of deformation temperature. When the temperature continues to rise to a certain extent, a needle-like crystallization phase is precipitated from the amorphous matrix, which will lead to a significant decrease in mechanical properties such as strength and plasticity of the amorphous alloy. By means of digital image correlation method (DIC), the variation of strain field near the notch in three point bending process of amorphous alloy bending specimens with different widths of U notches is studied. The load-displacement curve of bending test shows that there is no obvious yield phenomenon in Zr-based amorphous alloy during three-point bending. With the decrease of notch size, the maximum load and maximum bending displacement of three-point bending of amorphous alloy decrease gradually. This is due to an increase in strain concentration resulting in a decrease in toughness. The results of the strain field distribution near the notch obtained by DIC show that both the linear strain and the shear strain in the region near the notch show the phenomenon of strain concentration and the strain value increases gradually with the increase of time. With the decrease of the notch size, the strain concentration becomes more obvious, but the decrease of the notch width will result in the decrease of the maximum strain value in the region near the notch. The distribution characteristics of shear bands near the notch and on both sides of the fracture were observed by scanning electron microscopy (SEM). The propagation and propagation of shear band are caused by the local shear strain exceeding the elastic strain limit. It is found that the size and number of shear band near the notch decrease with the decrease of notch size. The fracture surface of amorphous alloy bending specimen was observed by SEM. It was found that there were two distinct zones of crack growth zone and fast fracture zone on the fracture surface of amorphous alloy. With the decrease of notch size and the increase of strain concentration, the toughness of amorphous alloy bending specimens decreases, so the width of crack growth zone and the size of dimple microstructure in fast fracture zone tend to decrease gradually.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG139.8

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