【作者】 李润霞；

【导师】 李荣德； 胡壮麒；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2004， 博士

【摘要】 高强度Al-Si-Cu-Mg合金的铸造性能优良，通过热处理可以大幅度提高材料的力学性能。本文研究了该类合金的热处理强化特性，通过考察热处理工艺参数、主要合金元素和微量元素对热处理过程的影响，分析了合金的强化机理。 本文研究了铸造Al-Si-Cu-Mg合金的时效行为，发现在一定温度以上时效处理时，Al-Si-Cu-Mg合金的硬化曲线上出现了“双峰”现象，两个硬度峰值之间存在明显的“谷”。175℃时效，合金分别在6h和12h出现了第一个和第二个硬度峰。 DSC和TEM观察发现，铸造Al-Si-Cu-Mg合金的主要时效析出相为θ相和Q相。θ′相在合金基体上非均匀形核，在位错、晶界等能量高的位置优先形核和长大。而Q′相在合金基体中均匀分布，不受位错、晶界等缺陷的影响。在时效处理过程中，Q相的分布和形貌变化不大，稳定性很好，而θ序列析出相的结构变化较大，因此合金时效的“双峰”现象主要来源于θ相结构的变化。 时效硬化的单、双峰现象与合金的时效析出序列密切相关，GPⅡ区/θ″和亚稳相θ′都能有效强化Al-Si-Cu-Mg合金，但高密度的GP区（主要是GPⅡ区）的溶解将这一时效强化过程分为两个阶段。GP区向亚稳相转变的明显间隔是造成Al-Si-Cu-Mg合金时效双峰现象的主要原因。而在Al-Si-Mg合金的析出过程中，GPⅡ区和亚稳相的形成重叠交叉进行，造成了时效曲线上的硬度平台；在Al-Si-Cu合金的时效过程中，由于大量位错抑制了GP区的形成而促进了θ′相的非均匀形核，造成了时效曲线上的单硬度峰。 在固溶处理过程中，共晶硅相形貌发生了显著的变化，并对合金的性能产生了很大的影响；通过定量金相分析，共晶硅相形貌的演变及其对合金力学性能的影响可划分为三个阶段：固溶初期硅相的熔断和钝化使合金的塑性得到显著的提高；固溶中期以粒化为主，合金的力学性能达到了峰值；固溶后期硅相的粗化符合LSW粗化模型，硅相形貌呈现棱角小面特性，合金性能降低。 实验发现，淬火后共晶硅在合金基体中引入了大量的位错，从而抑制了GP的形成，促进了亚稳定θ′相的析出，降低了合金的时效硬化能力，且使得合金的时效过程难以控制。另一方面，固溶处理后过饱和的Si原子以纯硅质点方式析出，为强化相的均匀形核提供了良好的环境，从而加快了合金的时效强化过程。 在时效过程中，Ti与其它元素形成了弥散分布的Al₃Ti相和Al-Si-Cu-Ti四元化合物相，对合金基体起弥散强化作用。在含Mn合金的时效组织中，存在α-Al₁₅（MnFe）₃Si₂沈阳工业大学博士学位论文弥散质点，均匀、弥散分布在合金的基体上，强烈地钉扎位错，阻碍位错的运动，其本身对合金也有一定的弥散强化作用。 在铸造Al一Si一Cu.Mg合金中分别添加微量元素Cd、sn、Ag，以考察它们对固态相变过程的影响。实验发现:Cd和Sn抑制GP区的形成且促进亚稳相的析出，从而促进了合金的时效过程，提高了合金的峰时效硬度，加快了合金的硬化速度。由于Cd和Sn对GPn区的抑制和亚稳相的促进，也使得GPll区向亚稳相的转变间隔变小甚至消失。在合金的峰时效组织中，GPn区和亚稳相同时存在，且尺寸细小，分布弥散，从而提高了其硬化速度，引起了合金时效峰的显著增高，有效地促进了合金的时效析出过程，也使合金的时效“双峰”现象消失。 元素Ag和Ni也对合金时效过程中的e相析出序列产生显著的影响，Ag和Ni的加入促进了合金时效过程中GPll区/e’‘的形成，而且抑制了合金中亚稳相e’的形成与分解，并强烈阻碍了稳定相e的形成。因此，Ag和Ni的添加提高了合金的时效硬化速度，并阻碍了合金过时效的发生。更多还原

【Abstract】 Among the commercial cast aluminum alloys, Al-Si-Cu-Mg cast alloy is extensively used mainly because of its good castability and excellent mechanical properties in the heat-treated condition. The heat treatment behavior of the alloy during heat treatment was investigated in this paper through adjusting heat treatment processing parameters, and inspecting the influence of main alloying elements and trace elements on heat treatment process, and corresponding strengthening mechanism was also carefully analyzed.It is found that double aging peaks are present in the age-hardening curves of Al-Si-Cu-Mg alloy aged above certain temperature and the first peak is higher than the second one, whereas an aging hardening plateau and single aging peak is present in age-hardening curves of Al-Si-Mg and Al-Si-Cu alloys respectively. The results of DSC analysis and TEM observation reveal that the age hardening behavior is related to the precipitation sequence of alloys.It is shown that two precipitation sequences, namely asss@GP zones and asss- Q’- Q, are mainly responsible for the precipitation hardening of Al-Si-Cu-Mg alloy. The Q’ phase homogeneously precipitates in a matrix, regardless of dislocations and grain boundaries, and has excellent structural stability during aging treatment. On the contrary, ’ phase preferentially precipitates on the dislocations, and structure transforms dramatically during aging treatment, which should be responsible for the double aging peak phenomenon of the Al-Si-Cu-Mg alloy. In ageing microstructure of the Al-Si-Cu-Mg alloy, the first aging peak is corresponding to the high-density GP zones (especially GP II zones), while the second one is due to the formation of metastable phases. Obvious interval during the transition from the GP zones to metastable phase in DSC curve, caused by the dissolution of GP zones and the nucleation of metastable phases on dislocation, probably is the main reason of the appearance of double aging peaks. The aging plateau of the Al-Si-Mg alloy is corresponding to the continuous transition from the GP zones to metastable phases, while single aging peak of the Al-Si-Cu alloy is caused by the formation of metastable phases.Through the morphological observation on silicon particles of Al-Si-Cu-Mg cast alloy, it is found that during solution heat treatment the evolution of eutectic silicon morphology and their effect on mechanical property can be classified into three stages. In the initial stage, necking, stubbing and fragmentation of silicon particles results in an improvement in plasticity of the alloy. In the intermediate stage, the mechanical properties of the alloy attain peak values due to the spheroidization of silicon particles. In the final stage, the drop of hardness and strength is related to the deterioration of silicon morphology. The facets and lap are occurred in siliconparticles and the coarsening process of silicon follows LSW model. On the other hand, a lot of dislocations are introduced by eutectic silicon after quench, which stimulates the formation of but suppresses the GP zone. At the same time, the excess Si atoms affect the process of aging precipitation and lead to a fine and highly dense distribution of GP zones/ phase.During ageing treatment, the addition of element Ti forms compounds such as Al3Ti, Al-Si-Cu-Ti, which have dispersion hardening effect for the alloy with other elements. The particles of a-Al15(MnFe)3Si2 are uniformly dispersed in the matrix, and can pin-up dislocations effectively.The addition of Cd and Sn remarkably increases the peak aging hardness and reduces the time to reach aging peak in the Al-Si-Cu-Mg alloy through suppressing the formation of GP zone and stimulating the formation of ’ and phases. Due to the suppression of GP zone formation and the stimulation of formation, trace elements Cd and Sn cause the shortening of the interval between GP II and metastable phase. In the peak microstructure, phases coexist with phase and then cause effectively hardening of the alloy and eliminate the double aging pe更多还原