电解加钛Al-Mg-Si合金板材组织和性能的研究

【作者】 王文焱；

【导师】 谢敬佩；

【作者基本信息】 郑州大学 ， 凝聚态物理， 2007， 博士

【摘要】 本文以电解加钛Al-Mg-Si合金为研究对象，通过SEM、TEM、XRD、HRTEM、DSC差热分析及力学性能测试等，对合金的微观组织和性能进行了全面分析。重点研究了电解加钛Al-Mg-Si合金的半连续铸造组织，铸态及均匀化的结晶相：研究了热处理工艺对Al-Mg-Si合金组织和性能的影响，对时效组织进行了的高分辨分析；分析了固溶态合金的DSC曲线对应的放热峰组织并计算了T4态合金β”和β’相的激活能；研究了不同过剩Si含量，Mg₂Si含量及Ti／B对电解加钛Al-Mg-Si合金组织和性能的影响；对热处理态电解加钛Al-Mg-Si合金的SEM原位拉伸过程中裂纹萌生与扩展进行了观察，并阐明了萌生和扩展机理。研究成果对电解加钛Al-Mg-Si合金的开发与工业应用具有重要的理论意义和实际意义。应用电解加Ti的方法配制了Al-Mg-Si合金，分别研究了不同Mg含量对铸态、均匀化处理和T4态合金结晶相的影响，描述了结晶相在铸态、均匀化及T4态的类型、形态及数量变化规律。铸态合金组织面扫描和线扫描结果表明，Mg、Si元素在晶界处偏聚，形成非平衡组织。电解加钛Al-Mg-Si合金铸态结晶相主要为Al_1.9CuMg_4.1Si_3.3、Al_0.75MnSi_1.25、Mg₂Si相，结晶相中Mn、Fe具有互相代替作用。Al_1.9CuMg_4.1Si_3.3相呈颗粒状，均匀化处理可使Al_1.9CuMg_4.1Si_3.3相溶入基体。AlMnSi类结晶相主要包括Al_0.75MnSi_1.25和Al₅Mn₁₂Si，其形态均为不规则条状。随着Mg含量增加，仅AlMnSi类结晶相数量减少。铸态合金中Mg₂Si相为不规则的黑色块状，提高Mg含量时，合金中Mg₂Si相增多；均匀化处理时，Mg₂Si相变化不大。首次研究了预时效、预时效＋模拟烘烤、预时效＋预拉伸＋模拟烘烤等工艺对Al-Mg-Si合金组织和性能的影响。电解加钛Al-Mg-Si合金水淬之后立即进行短时预时效，可使过饱和固溶体中溶质的分布不均匀，抑制随后自然时效硬化效果，减轻淬火后停留时间对合金力学性能的影响；预时效过程形成的大量的β”核心，在随后的人工时效（即烤漆过程）时，将促进β”相的均匀析出，从而产生明显的烤漆硬化效应。优化出4号合金最佳热处理工艺为550℃×30min固溶＋170℃×5min预时效＋5天自然时效＋180℃×30min模拟烘烤，强度达337MPa，延伸率达19.9％，与熔配加钛6009变形铝合金在T6态下强度345MPa接近，塑性超过该热处理条件下的数值（12％）。预拉伸使组织中产生大量的位错，位错缠结使合金强度得到提高。8号合金最佳热处理工艺为550℃×30min固溶＋170℃×5min预时效＋3％预拉伸＋5天自然时效＋180℃×30min模拟烘烤，强度达到373MPa，延伸率达到22.2％，高于熔配加钛6009变形铝合金在T6态下强度345MPa和塑性（12％）。时效组织的高分辨分析表明，180℃时随着时效时间的增加，调幅分解逐渐形成，在180℃×8h的人工时效条件下，调幅分解呈现花格尼状。经过计算，调幅组织的晶格相晶面间距比基体晶面间距大35％。这是由于Al的原子半径为0.1432nm，Mg的原子半径0.1602 nm，Mg、Si原子在α（Al）中（200）晶面偏聚，使晶格产生畸变造成的。通过DSC差热分析，发现电解加钛Al-Mg-Si合金的时效初期（150℃左右）其组织为G．P．区，形貌为针状或粒状。随着加热温度的升高，到250℃左右，开始形成与基体完全共格的针状β”相。继续升高温度，在320℃左右形成了与基体部分共格的β’相，最后形成稳定的β（Mg₂Si），时效析处序列为GP→β″→β’→β（Mg₂Si）。推导出时效过程中析出产物激活能的热力学方程，，通过该方徘出了T4态β”和β’激活能分别为63KJ／mol、121KJ／mol，说明试验合金容易形成β”相，β”的强化作用优于β’相。电解加钛Al-Mg-Si合金，随过剩Si含量的增加，模拟烘烤前板材强度提高明显，延伸率也有一定程度增加。模拟烘烤后板材强度增加趋势变小，延伸率没有明显变化，说明适量的过剩Si有利于板材的成形性和强度提高。不同Mg₂Si含量的电解加钛Al-Mg-Si合金板材，模拟烘烤前后强度均有所提高。模拟烘烤前强度的提高主要是通过过剩Si形成附加的团簇沉淀起强化作用，模拟烘烤后主要通过Mg₂Si的沉淀强化起主要作用。Ti和B的加入有效的细化电解加钛铝合金，随着Ti／B比的减小，铸态组织得到细化。电解加钛铝合金中，Ti分布弥散，加入B元素有效的提高细化能力。首次运用扫描电镜原位拉伸台对电解加钛变形Al-Mg-Si合金进行了原位拉伸观察，电解加钛变形铝合金的断裂过程与一般塑性材料基本相同。局部应力集中和应变造成粗大的分散相和基体分离及缺陷是裂纹优先萌生的部位。裂纹的扩展主要是平行于滑移条纹扩展和沿晶扩展，当裂纹尖端有较大Mg₂Si颗粒存在时，微裂纹会在Mg₂Si／基体界面再形核而扩展，最终导致合金断裂。更多还原

【Abstract】 Microstructure and properties of an electrolytic titanium aluminum alloy （Al-Mg-Si） were investigated by Transmission Electron Microscope（TEM）, Scanning Electron Microscope（SEM） incorporating energy dispersive X-ray analysis（EDX）, High Resolution Transmission Electron Microscope（HRTEM）, Differential Thermal Analyzer （DSC） and Electronic Tensile Tester. The microstructure of the half-continuously cast electrolytic titanium aluminum alloy （Al-Mg-Si） and the phase structure of the cast and uniform heat-treated electrolytic titanium aluminum alloy （Al-Mg-Si） were investigated emphatically. The influence of heat treatment parameters on microstructure and properties of the Al-Mg-Si alloys was studied and the HRTEM analysis of aging-treated Al alloy was carried out. The activation energies ofβ" andβ’ phase in a T4 heat-treated state were calculated by measuring the exothermic peak of the corresponding DSC curves. The effect of excess Si, Mg₂Si and Ti/B contents on the microstructure and properties of the Al-Mg-Si alloy was analyzed. An in-situ SEM observation of the fracture process during tensile test was carried out to understand the initiating and propagating mechanism of cracks. The results obtained present theoretical and practical significance to the research and development of the electrolytic titanium aluminum alloy （Al-Mg-Si）.The influence of the Mg content, uniform heat treatment and T4 status on the microstructure and phase structure of the electrolytic titanium aluminum alloy （Al-Mg-Si） was investigated by SEM, EDX and XRD. The area and line scanning analysis by SEM revealed that the Mg and Si clustering at the grain boundaries generated non-equilibrium microstructure. The crystalline phases of cast electrolytic titanium aluminum alloy （Al-Mg-Si） is consisted of Al_1.9CuMg_4.1Si_3.3、Al_0.75MnSi_1.25、Mg₂Si, and element Mn and Fe are of mutual replacement. Al_1.9CuMg_4.1Si_3.3 is sphere-like and dissolvable into the matrix. A1MnSi type phases include Al_0.75MnSi_1.25 and Al₅Mn₁₂Si with irregular morphology, and the content of these phases reduces with an increase in Mg content. The cast alloy is consisted of Mg₂Si phase with black block appearance, the corresponding content increase with an increase in Mg content and uniform heat-treatment has no obvious influence on Mg₂Si phase.The effect of pre-aging treatment, pre-aging treatment plus simulatively baking and pre-aging treatment + pre-tensile + simulatively baking on the microstructure and properties of the Al-Mg-Si alloy were investigated for the first time. A short time pre-aging treatment of the alloy after water quenching resulted in inhomogeneous distribution of elements in the Al-matrix solid solution, restrained the hardening effect during natural aging process, and relieve the effect of residence time on the properties of the alloy. The formation of a large amount ofβ" nucleus induced by pre-aging treatment promoted the precipitation ofβ" phase and increased the hardening effect significantly during the sequent artificial aging process. A good combination of both high strength （337MPa） and good plasticity （19.9%） （marked alloy 4） was achieved by the optimal treating parameters, i.e., solid solution treatment at 550℃for 30min+ pre-aging treatment at 170℃for 5min+natural aging treatment for 5 days+silulatively-baking treatment at 180℃for 30min, which nearly approachs to the strength of wrought aluminum alloy 6009 heat-treated at T6 status （345MPa）. The optimal heat treatment parameters for pre-tensile treatment of alloy 8 is solid solution treatment at 550℃for 30min+pre-aging treatment at 170℃for 5min+natural aging treatment for 5 days+silulatively-baking treatment at 180℃for 30min, by which the strength and extensibility are 373 MPa and 22.2% respectively, which is a little higher than the strength （345MPa） and the extensibility （12%） of wrought aluminum alloy 6009 heat-treated at T6 status.HRTEM analysis revealed that the interplanar distance of the spinodal-decomposed phase is 35% larger than that of the matrix since the clustering of Mg and Si on the plane of （200） ofα（Al） （the atomic radius of Ai and Mg is 0.1432nm and 0.1602 nm respectively.） resulted in distortion of crystal lattice.DSC analysis revealed that the primary aging stage of the electrolytic titanium aluminum alloy （Al-Mg-Si） （at around 150℃） was characterized by GP zone with needle- or sphere-like appearance. The needle-likeβ" phase having a total coherence relationship with the matrix appeared at about 250℃, a partial coherenceβ’ phase formed at 320℃, and a stableβ（Mg₂Si） phase formed finally, which means that the aging sequence is GP→β"→β’→β（Mg₂Si）. The thermodynamic equation of activation energy of precipitating phase by aging treatment is, by which the activation energy ofβ" andβ’ in T4 status is 63KJ/mol, 121 KJ/mol respectively, it indicates that theβ" is easier to form thanβ’ phase as well as the strengthening effect ofβ" is superior toβ’.The strength of the alloy is significantly improved and extensibility of the alloy is increased to a certain extent by an increase in excess Si content. The enhance extent of the strength of the electrolytic titanium aluminum alloy （Al-Mg-Si） before simulatively baking treatment was reduced but the corresponding extendibility of sheet material kept constant with an increase in excess Si content. The mechanism of improving strength of the alloy can be understood to be of the clustering precipitation of excess Si and the precipitation of Mg₂Si. The addition of Ti and B into the alloy effectively refined the grain size; the grain size of the as-cast alloy was refined with a decrease in the ratio of Ti to B.The in-situ tensile test was firstly carried out under SEM observation for the electrolytic titanium aluminum alloy （Al-Mg-Si）. The fracture process of the alloy is similar to normal plastic materials. Cracks initiated at locations of stress concentration and the interface between the coarse precipitating phase and the matrix. Cracks propagated parallel to slipping stripes and along the grain boundaries. Cracks meeting coarse Mg₂Si phase propagated along the interface between Mg₂Si and the matrix. The conect of cracks resulted in final fracture.更多还原