高速剪切作为一种复杂极端条件，普遍存在于航空航天、高速制造、国防工业等领域，如冲压成型、高速切削、弹体侵彻、爆炸破片等。局部化剪切带是金属材料在高速剪切加载下的一种常见的失效破坏行为。由于加载作用时间极短，且涉及高温、大变形等复杂条件，剪切局部化是一个涉及大变形、高应变率的热-力耦合现象，也是一个具有高度非线性特征的复杂力学问题。近年来，一大批先进金属材料不断涌现，对其在高速剪切条件下热-力耦合行为的研究仍然很匮乏，极大地限制了它们的广泛应用。本论文围绕两种典型的先进金属材料（单主元钛合金和多主元高熵合金）的高速剪切行为开展了一系列研究，取得了如下主要创新性成果。

（1）揭示了Ti6Al4V钛合金在高速切削变形中切削温度随切削速度演化规律，验证了金属高速切削中Salomon假设的存在性。基于课题组发展的高速瞬态测温技术并结合基于轻气炮的超高速切削实验装置原位测量了钛合金在超高速切削过程中的瞬态温升。实验结果显示，随着切削速度从7.5 m/s增大到212.6 m/s，切刀刀尖温度先升高至最大值之后不断降低，验证了Salomon假设在Ti6Al4V钛合金中的存在性。进一步，结合SEM 观测和理论模型计算，发现随着切削速度增大主剪切区塑性变形产热减少，而次剪切区摩擦产热增多；刀具-切屑接触时间缩短，热量传递给刀具的时间减少；物质对流效应增强，切屑带走了更多的热量。结合切削热量产生变化、刀具-切屑接触时间缩短、材料对流效应等因素，揭示了刀具刀尖温度演化背后的物理机制，从而验证了Salomon假设。

（2）获得了多主元CoCrFeMnNi高熵合金在冲击变形过程中塑性功转化为热的比例系数β，揭示了高熵合金在高速冲击变形中功热转化的微观机制。利用高速瞬态测温系统和霍普金森压杆实现了高熵合金动态压缩过程中的原位测温，发现CoCrFeMnNi高熵合金在冲击变形过程中具有较高的应变硬化率而温升仅为15 ℃，并且功热转化系数β值随应变增大而先增大后减小，最大值约为0.55。通过限位变形冻结实验和XRD、EBSD微结构表征发现CoCrFeMnNi高熵合金在高应变率条件下形成大量高密度位错，既增强了高熵合金的应变硬化能力，又储存了更多的冷功储存能。

（3）实验获得了CoCrFeMnNi高熵合金高速切削中的剪切带演化行为。通过基于霍普金森压杆的高速切削实验装置，首次对CoCrFeMnNi高熵合金开展了切削速度15 m/s的高速切削实验。利用EBSD和TEM对锯齿状切屑中的剪切带微观组织形貌进行了精细的观测与分析，并讨论了剪切带内微观组织结构的演化机制。发现CoCrFeMnNi高熵合金在高速切削过程中易形成锯齿状切屑。切屑中非均匀的变形梯度分布导致了复杂的微结构特征：在剪切带基体区域形成拉长的晶粒，而在剪切带中心区域形成等轴的超细晶。CoCrFeMnNi高熵合金剪切带内的微观组织结构演化过程主要有位错胞形成、拉长的亚晶形成、亚晶破碎旋转和等轴细小动态再结晶产生等阶段。

（4）揭示了CoCrFeMnNi高熵合金帽形试样动态冲击剪切过程中的剪切带演化行为。采用帽形剪切试样和霍普金森压杆系统对CoCrFeMnNi高熵合金开展了15 m/s的动态剪切实验，发现CoCrFeMnNi高熵合金在帽形试样剪切过程中具有较高的应变硬化率，形成剪切带的临界应变为6.5，展现出非常强的抗剪切局部化能力。设计限位冻结实验得到了不同变形状态的剪切区域的微观组织结构特征。在剪切带外部区域晶粒沿剪切方向拉长，而在剪切带中心区域晶粒破碎细化为等轴的细小再结晶晶粒。CoCrFeMnNi高熵合金剪切带内的微观组织结构演化过程主要经历位错胞、拉长的亚晶、亚晶破碎细化和亚晶旋转形成等轴超细晶等阶段，这与高速切削中的微结构演化过程基本一致。

High speed shearing, as a complex extreme condition, widely exists in aerospace, high-speed manufacturing, defense industry, and other fields, such as stamping, high-speed cutting, projectile penetration, explosive fragments, and so on. Localized shear band is a common failure behavior of metal materials under high-speed shear loading. Due to the extremely short loading time and complex conditions such as high temperature and large deformation, shear localization is a thermal-mechanical coupling phenomenon involving large deformation, high strain rate, and is also a complex mechanical problem with highly nonlinear characteristics. In recent years, a large number of advanced metallic materials have been emerging, but the research on their thermal-mechanical coupling behavior under high-speed shear conditions is still not clear, which greatly limiting their wide application. In this dissertation, In this thesis, a series of studies have been conducted on the high-speed shear behavior of two typical advanced metal materials (single principal component titanium alloy and multi principal component high entropy alloy), and the main innovative achievements are as follows..

(1) The evolution of cutting temperature with cutting speed during high-speed cutting deformation of Ti6Al4V titanium alloy was revealed, and the existence of Salomon hypothesis in high-speed metal cutting was verified. Based on the high-speed transient temperature measurement technology developed by the research group and combined with a light gas gun-based ultra-high-speed cutting setup, the transient temperature rise of titanium alloy during ultra-high speed cutting was in-situ measured. The experimental results show that the cutting temperature first increases to the maximum value and then continuously decreases as the cutting speed increases from 7.5 m/s to 212.6 m/s, verifying the existence of the Salomon hypothesis in Ti6Al4V titanium alloy. Further, combined with SEM observation and theoretical model calculation, it is found that with the increase of cutting speed, heat generation due to the plastic deformation in the primary shear zone decreases, while heat generation due to the friction in the secondary shear zone increases. The tool-chip contact time decreased as the cutting speed increased. There is not enough time for the heat generated by all heat sources to transfer into the tool. With an increase in cutting speed, the material convection effect is enhanced. More heat was effectively carried away by the high-speed chip flow through material convection. Combined with the heat generation, the tool-chip contact time decreases, and the material convection effect, the physical mechanism behind the cutting temperature evolution was revealed, thereby the Salomon hypothesis is verified.

(2) The fraction of plastic work converted to heat β during impact deformation of a multi-principal component CoCrFeMnNi high entropy alloy was obtained. The microscopic mechanism of the conversion of plastic work to heat for high entropy alloys during high-speed impact deformation is revealed. Using a high-speed transient temperature measuring system and a Hopkinson pressure bar, in-situ temperature measurement of high entropy alloys during dynamic compression was achieved. It was found that CoCrFeMnNi high entropy alloys had a high strain hardening rate during impact deformation, while the temperature rise was only 15 ℃. And the conversion coefficient of plastic work to heat (β) first increases and then decreases as the strain increases, with a maximum value of about 0.55, which is significantly different from traditional FCC alloys. The microstructures of specimens obtained by“freezing” interrupted experiments were characterized using XRD and EBSD, and the results showed that high-density dislocations occur during high strain rates deformation. The high-density dislocations are responsible to high strain hardening ability more stored energy of cold work in the high entropy alloy.

(3) The shear band evolution behavior of CoCrFeMnNi high entropy alloy in high speed cutting was obtained through experiments. The high-speed cutting experiment with a cutting speed of 15 m/s was first conducted for CoCrFeMnNi high entropy alloy by high-speed cutting device based on a Hopkinson pressure bar. The microstructure evolution of shear bands in serrated chips were carefully analysed using EBSD and TEM. The saw-teeth-like chips are prone to occur during high-speed cutting for CoCrFeMnNi high entropy alloy. The deformation gradient distribution was found to be heterogeneous in a chip sample, leading to complicated microstructure characteristics. The elongated grains grains were observed in the areas adjacent to the shear band. The microstructure inside the shear band was found to be composed of equiaxed ultrafine grains. The microstructure evolution process in the shear band of CoCrFeMnNi high entropy alloy mainly includes dislocation cell formation, elongated (sub)grains formation, (sub)grains subdivision and rotation, and equiaxed fine dynamic recrystallization generation.

(4) The evolution behavior of shear bands of CoCrFeMnNi high entropy alloy during dynamic impact shearing using hat-shaped specimens is revealed. A dynamic shear experiment was conducted on CoCrFeMnNi high entropy alloy using a hat-shaped shear specimen and a Hopkinson pressure bar system. It is found that the CoCrFeMnNi high entropy alloy exhibits high strain hardening ability during dynamic shear process. The alloy resists shear-band formation up to a critical shear strain of ~ 6.5, indicating high resistance to shear localization. The “freezing” interrupted experiments were designed to obtain the microstructure characteristics of the shear deformation zone at different shear deformation level. The grains were elongated along the shear direction outside shear band, and the microstructure with the shear band consisted of equiaxed recrystallized grains. The microstructure evolution inside the shear band mainly undergoes stages such as dislocation cells, elongated (sub)grains, (sub)grains breakup, and equiaxed ultrafine grains formation, which is consistent with the microstructure evolution process in high-speed cutting.