不同温度下激光冲击TA2工业纯钛拉伸性能及微观强化机理研究

发布时间：2018-11-05 13:20

【摘要】：钛及钛合金因其密度低、比强度高、机械性能好和耐腐蚀性强等优点在航空航天、海洋开发和汽车工业等领域上的应用越来越广泛。通常钛及其合金结构件的中高温服役环境在500°C以下,而传统的表面处理方法无法满足钛及其合金结构件的中高温强化要求。激光冲击强化技术是一种新型的表面处理技术,广泛应用于金属材料的表面改性方面研究。目前尚未发现有关激光冲击强化工业纯钛在不同温度下的拉伸力学性能方面的研究,也未见关于激光冲击强化工业纯钛在不同温度下的微观组织演变和塑性变形行为方面的报道。针对以上问题,本文以TA2工业纯钛为研究对象,开展了激光冲击工业纯钛在不同温度下的拉伸性能和断口形貌特征,组织演化和显微硬度分布特点,以及激光冲击工业纯钛中高温拉伸塑性变形行为微观演变机制等方面的研究,具体研究内容如下:(1)研究了激光冲击TA2工业纯钛在不同温度下的拉伸性能和断口形貌特征,获得了中高温拉伸变形行为与断口形貌演变的规律:对TA2工业纯钛拉伸试样标距部分进行大面积激光冲击强化处理,对未冲击试样和冲击试样在20°C、150°C、250°C以及350°C下进行拉伸试验,结果表明工业纯钛抗拉强度随温度升高显示出降低的趋势。同一种温度下激光冲击试样断口颈缩现象比未冲击试样断口要明显,并且断口形貌显示激光冲击试样的塑性更好。温度变化对TA2工业纯钛断裂类型的影响非常明显,常温下TA2工业纯钛拉伸断裂类型属于脆性断裂,当拉伸温度逐渐提高时,工业纯钛表现出更加优异的塑性性能,断裂形式也逐渐转变为混合断裂和韧性断裂。(2)研究了激光冲击TA2工业纯钛微观组织和显微硬度分布,以及微观组织结构变化和显微硬度变化的内在联系:对不同拉伸温度的TA2工业纯钛断口区微观组织和显微硬度进行了深入的研究,结果表明激光冲击后的TA2工业纯钛晶粒细化明显,有大量变形孪晶、位错和“透镜状”相变马氏体??产生。激光诱导产生的马氏体相比于钢铁中的要细小很多,且性质不稳定。当温度逐渐升高至350°C后,晶内相变马氏体发生?→??逆相变,并且随着温度的升高逐渐消失。发现?晶粒都有不同程度的变大,但是幅度不是很均匀,亚晶粒长大较为明显。激光冲击强化处理显著提高了TA2工业纯钛的硬度,随着拉伸温度的提高硬度有所降低,但是幅度不是很大。又因为拉伸断裂产生严重塑性变形后,塑性变形过程中流动应力不断增加,并且有大量位错和形变孪晶产生,位错相互作用,在流动应力的作用下弥散速度加大,钉扎效应增强,又会有新的位错源产生导致显微硬度提高。因此,温度升高晶粒变大引起的软化和拉伸机械变形引起的硬化两者共同作用,使得硬度变化不是很大。(3)研究了激光冲击工业纯钛拉伸试样在不同温度下塑性变形微观演变机制以及同一试样不同区域位置的塑性变形演变机制:对激光冲击TA2工业纯钛拉伸试样在不同温度下拉伸区的TEM图像微观组织进行了全面的研究,研究发现不同温度下拉伸试样的变形行为模型可用位错分步激活和孪晶解体模型来解释。激光冲击使得TA2工业纯钛生成孪晶,由于工业纯钛的层错能低,中高温和变形外力对位错有激活作用,孪晶中积塞的位错被激活,不断穿过孪晶界,当温度和外力到达某个临界值,孪晶界基本解体,孪晶消失,积塞的位错弥散均匀分布。在这个过程中,孪晶界会吸纳其反应物——不全位错,从而提高材料塑性性能。当温度升高到350°C以上时,塑性变形行为的位错分步激活和孪晶解体模型基本结束,这段变形行为中位错的形核及运动在塑性变形过程中成为主要机制。不同断口区域的微观组织的不同,主要是孪晶界造成的结果,严重塑性变形区域的TA2工业纯钛内部生成大量孪晶簇,孪晶界的存在阻碍了受外应力激活位错的运动。
[Abstract]:Titanium and titanium alloy have the advantages of low density, high specific strength, good mechanical property, strong corrosion resistance and the like in the fields of aerospace, marine development and automobile industry. Generally, the high-temperature service environment of titanium and its alloy structural parts is below 500 掳 C, and the traditional surface treatment method can not meet the requirement of high-temperature strengthening of titanium and its alloy structural parts. Laser shock peening is a new kind of surface treatment technology, which is widely used in surface modification of metallic materials. At present, it has not been found that the research on the tensile mechanical properties of pure titanium under different temperatures has not been found in the laser shock peening industry, but it has not been reported on the microstructure evolution and plastic deformation behavior of pure titanium under different temperatures. Aiming at the above problems, this paper takes TA2 industrial pure titanium as the research object, and develops the characteristics of tensile properties and fracture morphology, microstructure evolution and microhardness distribution of pure titanium at different temperatures. The results are as follows: (1) The tensile properties and fracture morphology of pure titanium in laser shock TA2 industrial pure titanium at different temperatures are studied. The rule of the evolution of high temperature tensile deformation behavior and fracture morphology is obtained: a large area laser impact strengthening treatment is carried out on a TA2 industrial pure titanium tensile sample standard distance part, and the unimpact sample and the impact sample are subjected to a tensile test at 20 DEG C, 150 DEG C, 250 DEG C and 350 DEG C, The results show that the tensile strength of pure titanium decreases with the increase of temperature. At the same temperature, the fracture neck shrinkage of the laser impact specimen is obviously lower than that of the non-impact specimen, and the fracture morphology shows that the plastic of the laser impact specimen is better. The effect of temperature change on the fracture type of pure titanium in TA2 industry is very obvious. The tensile fracture type of pure titanium in TA2 industry belongs to brittle fracture at normal temperature. When the tensile temperature is gradually increased, the industrial pure titanium shows more excellent plastic property. The fracture form is also gradually transformed into mixed fracture and ductile fracture. (2) The inner relationship between microstructure and microhardness distribution of pure titanium in laser shock TA2 industrial pure titanium was studied. The microstructure and microhardness of TA2 industrial pure titanium fracture zone were studied deeply. The results show that the crystal grain refinement of TA2 industrial pure titanium after laser shock is obvious, and there are a large number of deformed columnar crystals and dislocations. "lenticular" Phase change martensite? Generates. Laser-induced martensite is much smaller and unstable than in steel. When the temperature gradually rises to 350 掳 C, the phase-change martensite in the crystal occurs? What's the matter? the inverse phase change and gradually disappears as the temperature increases. Discovery? The crystal grains have different degrees of change, but the amplitude is not very uniform, and the subgrain growth is obvious. Laser shock peening significantly improved the hardness of pure titanium in TA2 industry, but the hardness decreased with the increase of tensile temperature, but the amplitude was not very large. In addition, after severe plastic deformation is generated due to tensile fracture, the flow stress in the plastic deformation process is continuously increased, and a large number of dislocations and deformations are generated, dislocation interaction is generated, the dispersion speed is increased under the action of flow stress, and the pinning effect is enhanced, a new dislocation source may also result in an increase in microhardness. As a result, both the softening and stretching mechanical deformation caused by the large temperature increase grain function together so that the hardness variation is not large. (3) The mechanism of plastic deformation of pure titanium tensile specimen under different temperatures and the mechanism of plastic deformation evolution in different regions of the same specimen were studied. In this paper, a comprehensive study was conducted on the microstructure of TEM images of laser shock TA2 industrial pure titanium tensile test specimens at different temperatures. The results show that the deformation behavior model of tensile specimens under different temperatures can be explained by the dislocation step activation and the split-crystal breakdown model. The laser shock causes the TA2 industrial pure titanium to generate the polycrystalline silicon crystal, because the layer of the industrial pure titanium is low, the middle and high temperature and the deformation external force have an active effect on the dislocation, the dislocation of the product plug in the polycrystalline silicon crystal is activated, the crystal grain boundary is continuously passed, and when the temperature and the external force reach a certain critical value, the grain boundary of the polycrystalline silicon is basically disintegrated, the crystals disappear and the dislocation of the product plug is uniformly distributed. In this process, the grain boundary will absorb its reactant _ non-complete dislocation, thus improving the plastic property of the material. When the temperature rises above 350 掳 C, the dislocation step-by-step activation of plastic deformation behavior and the collapse model of crystal structure are basically finished, and the nucleation and movement of dislocations in this deformation behavior become the main mechanism during the plastic deformation process. The microstructure of different fracture zones is different, mainly because of the result of grain boundary, and a large number of crystal clusters are generated inside TA2 industrial pure titanium in severe plastic deformation area.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.23;TG665

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