【作者】 邹东利；

【导师】 甄良；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 本文研究了固溶态AM60B镁合金高速撞击条件下的变形及损伤行为。采用火药炮、一级轻气炮和二级轻气炮进行高速撞击试验,研究了不同撞击速度和不同碰撞副下镁合金靶板的成坑过程;通过光学显微镜、扫描电子显微镜、透射电子显微镜等分析手段对高速撞击条件下弹坑附近不同深度、不同区域的变形组织进行了表征;同时利用显微压痕、霍普金森压杆和热模拟试验机对撞击后弹坑附近材料的力学性能进行了测试,并利用原位拉伸试验研究了高速撞击诱发的缺陷对主裂纹扩展过程的影响规律。研究表明钢弹/镁靶碰撞副的成坑过程不同于铝弹/镁靶碰撞副。随着撞击速度的增加,钢弹/镁靶碰撞副形成的弹坑形貌经历了球冠形→半球形→圆柱形+半球形→半球形过渡,而铝弹/镁靶碰撞副在撞击成坑过程中弹坑形貌由球冠形逐渐过渡到半球形。在弹道撞击速度范围内,弹坑深度是钢弹/镁靶碰撞副的主要侵彻形式,而弹坑体积是铝弹/镁靶碰撞副的主要侵彻形式。当撞击速度达到超高速撞击时,弹坑体积是镁合金靶板的主要侵彻方式,与碰撞副的类型无关。高速撞击的成坑过程明显不同于准静态压缩成坑,撞击成坑过程所消耗的弹丸动能始终大于准静态压缩成坑所做到的功,且随着弹坑深度的增加,两者的差距增大。撞击成坑过程计算表明撞击瞬间弹靶接触界面形成的冲击波压力和温升随着撞击速度的增加显著增大,当撞击速度达到5000 m/s,形成的冲击波压力超过材料强度2个数量级,冲击波温升超过材料的熔点,甚至沸点。撞击成坑过程中撞击方向上的材料承受了最严重的动态变形,45°撞击方向的材料次之,垂直撞击方向上的材料变形程度最轻。弹坑周围变形组织研究表明撞击方向上变形组织分布区域最宽,45°撞击方向上分布次之,垂直撞击方向上变形组织分布最窄,形成了椭球状组织分布。随着撞击速度的增加,弹坑周围变形组织的分布区域均有展宽的现象。相近撞击速度下,钢弹/镁靶碰撞副弹坑周围变形组织的分布区域宽于铝弹/镁靶碰撞副。弹道撞击条件下,弹坑周围的变形组织可划分为三个区域:高密度孪晶区、中等密度孪晶区和低密度孪晶区,而超高速撞击条件下,弹坑周围出现了细晶区,其变形组织可划分为四个区域:细晶区、细晶+高密度孪晶区、高密度孪晶区和低密度孪晶区,其中低密度孪晶区贯穿整个30 mm厚的靶板。由于高速撞击可在弹坑底部提供梯度性的应变、应变速率载荷变化,通过弹坑周围不同区域变形组织的表征,揭示了弹坑附近细晶的形成过程,建立了弹坑附近细晶形成的物理模型。绝热剪切带研究表明高速撞击条件下绝热剪切带的形成需要一定的临界撞击速度,随着撞击速度的增加,弹坑附近材料经历了均匀塑性变形→应变局部化(形变带)→白色侵蚀带(转变带)→裂纹的演化过程,即形变带和转变带是不同变形阶段、变形程度的产物。通过转变带在两种侵蚀剂下呈现的光学形貌发现,转变带的光学形貌与侵蚀剂的类型密切相关,因而基于光学显微镜下白色侵蚀的特征来辨别转变带的类型存在一定偏差。高速撞击条件下弹坑附近材料由于应变局部化的差异,一定撞击速度下弹坑附近可同时形成形变带和转变带。形变带内部以严重变形、碎化的晶粒为主,而转变带内部以细小等轴的再结晶晶粒为主。孪生和位错滑移在转变带内部细晶的形成过程中起着重要的作用。高速撞击条件下镁合金转变带内细晶的形成应归结于孪晶诱发的旋转动态再结晶机制。弹坑附近典型的微观结构研究表明高速撞击条件下镁合金中主要形成了{1012}拉伸孪晶和{1 0 11}压缩孪晶,孪生方向分别为< 1011>和< 1012>。高的应力应变水平、低的临界剪切应力、小的切变量是两类变形孪晶形成的主要原因。高速撞击条件下弹坑附近超细晶Mg晶粒内部存在高密度的位错结构,位错滑移是超细晶Mg晶粒进一步塑性变形的主要方式。高速撞击条件下弹靶界面材料在冲击高温、高压的共同作用下形成了多种与熔化相关的微观组织,如铝晶粒、镁铝化合物以及非晶组织。非晶组织的形成是熔化、快速凝固的结果。撞击后弹坑附近材料的力学性能研究表明随着撞击速度的增加,撞击后弹坑附近材料的动态屈服强度逐渐增大,而材料的动态抗压强度在一定的撞击速度下存在极大值。钢弹/镁靶碰撞副撞击后弹坑附近材料达到最大动态抗压强度的临界撞击速度为590 m/s,铝弹/镁靶碰撞副为2500 m/s。超过临界撞击速度,撞击后材料的动态抗压强度随着撞击速度的继续增加而降低。随着与弹坑边沿距离的增加,撞击后材料的动态屈服强度逐渐降低,而材料的动态抗压强度则存在临界变形程度,超过临界值时,材料的动态抗压强度在弹坑底部一定距离上存在极大值。原位拉伸试验研究表明撞击诱发的微裂纹、微孔洞、绝热剪切带及孪晶界是主裂纹形核和扩展的主要路径,大量缺陷的形成降低了材料抵抗继续变形的能力。更多还原

【Abstract】 Deformation and damage behaviors of solution treated AM60B Mg alloy under high velocity impact have been investigated in this paper. High velocity impact experiments were carried out on power gun, one-stage and two-stage gas guns, and the cratering processes in Mg alloy target impacted by different projectiles under different impact velocities were studied. The deformed microstructure at different zones and depth levels adjacent to the crater under high velocity impact were characterized by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The mechanical properties of the materials near the crater after impact were measured by indentation test, Hopkinson bar and thermal simulation test, and the in-situ tensile tests were used to investigate the influence of defects induced by high velocity impact on the primary crack propagation.The results show that the cratering process in the Mg alloy target impacted by steel projectile was different from that impacted by Al projectile. With the impact velocity increasing, several crater morphologies including spherical cap, hemispherical, columned + hemispherical and hemispherical crater were experienced sequentially in Mg alloy target impacted by steel projectile, while the crater morphologies transformed from the spherical cap to hemispherical crater were experienced sequentially in Mg alloy target impacted by Al projectile.Under ballistic impact, the crater depth was the main penetration behaviors in the Mg alloy target impacted by steel projectile, while the crater volume was the main penetration behaviors in the Mg alloy target impacted by Al projectile. When the impact velocity was speeded up to the hypervelocity impact, the crater volume was the main penetration behaviors in Mg alloy target impacted by various projectiles. The impact cratering process was different from quasistatic compressive cratering, and kinetic energy of projectile dissipated by the impact cratering was always larger than the work dissipated by quasistatic compressive cratering. With the crater depth increasing, the dissipated energy gap between the impact and quasistatic cratering increased. The calculated impact cratering process shows that the formed shock pressure and temperature rising increased obviously at the interface between the projectile and target at impact moment as impact velocity increasing. When the impact velocity was reached 5000 m/s, the calculated shock pressure was two times magnitude higher than the materials strength, and shock temperature rising was exceeded the materials melting point, even boiling point. During the impact cratering process, the materials paralleled to the impact direction suffered the most severe plastic deformation, and the deformed levels of the materials 45°and vertical to impact direction decreased sequentially.The investigated deformed microstructures near the crater show that the microstructural distribution paralleled to the impact direction was widest, and the distribution scope 45°and vertical to impact direction decreased sequentially, and the ellipsoid-like microstructural distribution was formed. With the impact velocity increasing, the microstructural distribution zones near the crater became much wider. At almost same impact velocities, the microstructural distribution near the crater in Mg alloy target impacted by steel projectile was larger than that formed by Al projectile. Under ballistic impact, the deformed microstructure near the crater could be classified into three zones: high density twin zone, medium density twin zone and low density twin zone. Under hypervelocity impact, the ultrafine grain zone was appeared near the crater, and the deformed microstructure near the crater could be classified into four zones: ultrafine grain zone, ultrafine grain + high density twin zone, high density twin zone and low density twin zone. The low density twin zone was spread in the whole target with thickness of 30 mm. High velocity impact could provide a gradient variation of the strains and strain rates from the crater rim to the deep matrix, thus the ultrafine grain evolution adjacent to the crater could be obtained through the characterization of the deformed microstructure at different depth levels, and the physical model of the ultrafine grains formed near the crater was constructed.The investigated adiabatic shear bands show that the critical impact velocity was required for the formation of the adiabatic shear bands. With impact velocity increasing, the materials near the crater suffered uniform plastic deformation, strain localization (deformed bands), white-etching bands (transformed bands) and cracks sequentially, thus the deformed and transformed bands were the different stage products. The metallographic observation of the transformed bands etched by two etchants shows that the optical morphology of the transformed bands was closely related with etchants, thus the white-etching characteristic in metallographic observation to distinguish the transformed bands was inaccuracy. The different stain localization the materials suffered near the crater could lead to the formation of the deformed and transformed bands simultaneously in Mg alloy targets impacted at a critical velocity. The deformed bands composed of deformed, fragmental grains were confirmed, while the transformed bands composed of the ultrafine and equiaxed recrystallized grains were observed. The twinning and dislocation slipping played an important role for the formation of the ultrafine grains in the transformed bands, and its formation should be attributed to the twining-induced rotational dynamic recrystallization mechanism.The investigated typical microstructures near the crater show that the {1 0 12} extension twins with twinning direction along < 1011> and {1 0 11} contraction twins with twinning direction along < 1012> were the main twin modes in Mg alloy under high velocity impact. High stress and strain levels, low critical shear stress and small shear displacement should be responsible for the formation of two twin modes. High density dislocation structure have been confirmed in the ultrafine Mg grains formed near the crater under high velocity impact, which indicated that dislocation slipping was the main mechanism of the further plastic deformation of the ultrafine Mg grains. Under high velocity impact, high temperature and pressure the materials suffered at the interface between the projectile and target led to the formation of various molten-related microstructures, such as Al grains, MgAl compound and amorphous microstructure. The formation of the amorphous microstructure was a product of the melting and rapid solidification.The mechanical properties of the materials after impact show that dynamic yield strength of the materials near the crater after impact increased as the impact velocity increasing, and the maximum dynamic compressive strength of the materials was obtained at critical impact velocity. The critical values were obtained in Mg alloy target impacted by steel and Al projectiles at the velocities of 590 m/s and 2500 m/s, respectively. Beyond the critical values, the dynamic compressive strength of the materials after impact decreased as the impact velocity increasing. As the distance approaching to the crater rim, the dynamic yield strength of the materials after impact increased obviously, and the critical plastic deformation was existed for the dynamic compressive strength of the materials. Beyond the critical value, the maximum dynamic compressive strength of the materials was obtained at a certain distance from the crater rim. The situ tensile tests show that the microcracks, microvoids, ASBs and twin boundary induced by high velocity impact were the preferential sites for the nucleation and propagation of the primary cracks, and the formation of various defects led to the further deformability of the materials decreasing.更多还原