晶界结构和晶界析出相对Al-Zn-Mg-Cu合金断裂及腐蚀的影响

Precipitates easily aggregate and discontinouously distribute along large-angle grain boundaries when high-density precipitation strengthening is attained in super-strength Al-Zn-Mg-Cu series alloys, which seriously decreases fracture toughness and corrosion resistance of the super-strength Al-Zn-Mg-Cu alloys. This project aims to enhance fracture toughness and corrosion resistance of super-strength Al-Zn-Mg-Cu series alloys. By regulating grain-boundary precipitation and structure, new heat treatment and multiple micro-alloying are proposed to adjust the micro-orientation of matrix grain and the composition and distribution of grain-boundary precipitates and decrease potential and structural difference between grain interior and grain boundary, and enhance corrosion resistance. Moreover, by regulating multi-size dispersed phases and aging precipitates, the microstructural characteristics of super-strength and high-toughness aluminium alloys is explored and discribed, and the uniformity of deformation resistance is enhanced. A new route is provided in order to solve the contradictory between strengthning and fracture toughness as well as corrosion resistance of super-strength Al-Zn-Mg-Cu alloys by organically combining the adjustment of grain-bounday precipitates and micro-orientation of matrix grains. The content of this project is mainly included as followed: laws and characteristics of grain-interior and grain-boundary precipitation after pre-precipitation and the second aging process, and their effect on the corrosion and fracture toughness of super-strength Al-Zn-Mg-Cu alloys, and grain-boundary precipitates toughening mechanism；new multi-compound dispersed phases and their crystalline structure, heat stability and law on inhibiting recrystallization；Characteristics of pre-precipitation and the second aging precipitation along small-angle sub boundary.

Al-Zn-Mg-Cu超强合金高密度析出强化的同时，析出相易在大角度再结晶晶界上富集、连续分布，导致晶界断裂特征显著，严重降低断裂韧性和抗腐蚀性能。本课题以提高Al-Zn-Mg-Cu超强合金断裂和腐蚀性能为目标，从调控晶界析出和晶界结构入手，提出新型热处理及新型多元复合微合金化调控基体晶粒微取向和晶界析出相成分及分布，降低晶内晶界电位差结构差，提高腐蚀抗力，同时调控多尺度弥散相与时效相组态，提高变形阻力均匀性，寻求并确立超强高韧耐蚀特征微结构；本课题将晶界析出相与基体晶粒微取向调控有机集成，为解决Al-Zn-Mg-Cu超强合金强化与断裂及腐蚀抗力的矛盾提供新途径。本研究主要包括：预析出及二次时效晶内晶界析出特征与规律，及其对合金腐蚀与断裂的调控作用与晶界析出韧化机理；新型弥散相的多元合金化及其晶体结构、热稳定性和抑制再结晶规律的研究；形变回复微取向细晶基体小角晶界预析出及二次时效析出特征。

AlZnMgCu超强合金广泛应用于航空航天领域和地面运载工具，在国民经济和国防军事领域具有不可或缺的作用。AlZnMgCu超强合金元素含量高，纳米尺度相在晶内高密度析出大幅提高强度的同时，晶界析出相易在大角度再结晶晶界富集连续析出，晶界断裂特征显著，严重降低材料的断裂韧性和抗腐蚀性能。本课题研究了新型热处理制度晶内晶界析出特征与规律，及其对合金腐蚀与断裂的调控作用与晶界析出韧化机理；新型弥散相的多元合金化及其晶体结构、热稳定性和抑制再结晶；形变回复微取向细晶基体新型热处理小角晶界析出特征及对合金断裂与腐蚀的影响与作用。研究结果表明：①合金经465℃高温近平衡析出处理后，在保持合金时效状态强塑性的同时，明显提高合金抗腐蚀性能。经高温近平衡析出后合金晶界析出相明显粗化、离散分布且Cu含量显著增加。合金经过新型时效TXX处理后，强度与T6状态接近，断裂韧性、腐蚀性能有较大提高。相较于T6时效，TXX时效合金晶内强化析出相细小密集，极少量长大，晶界析出相粗大离散，Cu含量增加，无明显PFZ，且在晶界析出细小析出相。②常规低温时效处理的合金PFZ和基体电位较正，成为微电池腐蚀的阴极；晶界析出相电位较负，成为阳极。晶界析出相Cu含量增加，析出相电极电位正移。新型热处理提高晶界析出相Cu含量，降低了晶界析出相与PFZ和基体间的电位差，是新型热处理制度提高合金应力腐蚀性能的重要原因。③在纯Al及合金中复合添加Zr-Cr-Yb/Ti/Pr，形成大量与铝基体共格的(Al,Cr)3(Zr, Yb/Ti/Pr) 细小弥散相，显著抑制基体亚晶长大和再结晶，使合金基体保持以小角晶界为主的形变回复组织。复合添加Zr-Cr-Yb/Ti/Pr在提高合金硬度、强度和断裂韧性的同时，显著改善合金的抗腐蚀性能。④多元复合微合金化AlZnMgCu合金经过新型热处理后，强硬度相较于T6状态略低，但相差不多；断裂韧性和抗腐蚀性改善较大，这与多元复合微合金化及其形成的形变回复细晶基体小角晶界和新型热处理方式改变晶界析出的协同调控有关。经过新型热处理后，合金仍保留回复亚晶组织，晶内分布大量 (Al,Cr)3(Zr,X) 细小弥散相和位错，强化相η'细小密集均匀分布，小角度亚晶界椭圆状析出相分离、不连续，PFZ不明显。本课题确立了超强高韧耐蚀特征微结构，为合金保持超高强度的同时提高断裂及腐蚀抗力提供了新方向和新途径。