【摘要】：在原子尺度揭示硬化析出相結(jié)構(gòu)和成分的變化是理解和調(diào)控復(fù)雜多相合金微觀結(jié)構(gòu)的關(guān)鍵,對開發(fā)高性能合金材料非常重要。本論文以各種先進的電子顯微學方法和性能測試為手段,結(jié)合計算分析,以重要的汽車用時效強化鋁合金為研究對象,系統(tǒng)研究了鋁合金中硬化納米析出相的結(jié)構(gòu)演變及其與性能之間的關(guān)系,主要結(jié)論如下:1.利用電子顯微學方法研究Al-Si合金時效過程Si顆粒界面處的析出情況,以及Al-Si-Mg合金Si顆粒球化處理對時效過程硬化相析出的影響。退火過程中共晶硅顆粒上可以析出連續(xù)多重{111}[112]型納米孿晶,其有時具有相當復(fù)雜的結(jié)構(gòu),比如三重和四重孿晶,以及多晶材料不多見的封閉五重孿晶。延長A356合金的球化處理時間對后續(xù)人工時效硬度響應(yīng)有不利影響,其本質(zhì)原因是Mg元素偏聚降低了基體中用于形成強化相的Mg含量。2.結(jié)合定量高分辨率電子顯微學、環(huán)形暗場掃描透射電子顯微學和計算分析建立了Al-Mg-Si-(Cu)合金中不同析出相在原子尺度的結(jié)構(gòu)演變物理冶金圖像。本文的發(fā)現(xiàn)對揭示Mg/Si含量比和Cu含量對Al-Mg-Si-Cu合金顯微結(jié)構(gòu)和力學性能的影響非常重要,通過適當?shù)臅r效工藝控制析出方式可以優(yōu)化合金性能。另外,本文提供的演變規(guī)律可以作為理解添加多余元素(如Ag和過渡金屬)的Al-Mg-Si合金析出機制的基礎(chǔ)。3.添加Cu會顯著提升Al-Mg-Si合金的早期時效硬化潛力和速率,即使Mg/Si比是2的Al-Mg-Si-Cu合金的時效動力學也很快。通過優(yōu)化合金成分配比,本文提出了一種新的用于制備汽車車身用Al-Mg-Si-Cu合金板材的成分范圍(Mg/Si含量比1~2),既提高了鋁合金板材的加工成形性和烤漆后的強度,又保證了其使用過程中性能的穩(wěn)定。4.結(jié)合形變和時效,提出一種調(diào)控鋁合金時效過程中納米析出相的新策略。通過控制軋制前合金元素的聚集和分布狀態(tài)以及后續(xù)時效工藝,可以使鋁合金強度顯著上升但塑性保持不變。通過控制冷軋前的化學狀態(tài),比如合金元素的聚集和分布,可以調(diào)控后續(xù)時效過程的析出,相對于T6峰值時效的Al-Mg-Si-Cu合金,改進的工藝可以使塑性保持不變而強度提升30%。經(jīng)后續(xù)時效后,自然時效預(yù)處理的樣品中存在兩類析出相:離散分布的板條狀Q"類型析出相以及尺寸更大且連續(xù)彎曲的析出相,后者成分接近Q'類型析出相。冷軋前短時間人工時效誘導(dǎo)的納米尺度共格顆粒(單斜β"相的GP區(qū))能夠促使均勻細密Q"類型析出相在后續(xù)時效過程的形成。本文提出的方案可以應(yīng)用到很多析出硬化的合金系統(tǒng)中,形變和時效結(jié)合的工藝極大地改變了制備合金的腐蝕特性,合金不但具有優(yōu)良的力學性能,其抗腐蝕性同時也得到了明顯改善。通過合理設(shè)計后續(xù)時效工藝,可以使硬度提高的同時導(dǎo)電率顯著提高。5.利用顯微硬度測試、拉伸測試、熱分析和透射電鏡觀察研究了后續(xù)時效工藝對冷軋Al-Mg-Si-Cu合金微觀結(jié)構(gòu)演變和力學性能的影響,揭示了變形合金時效時溶質(zhì)原子擴散和缺陷退化交互作用的物理圖像。析出相和晶粒結(jié)構(gòu)的轉(zhuǎn)變具有溫度依賴性,三類溶質(zhì)原子偏聚過程會發(fā)生,包括位錯亞胞界面偏聚、基體析出和晶界偏聚。隨時效溫度提高,時效動力學明顯加快。在70℃和120℃后續(xù)時效可以使合金硬度持續(xù)上升直到一個平臺,時效溫度高于150℃后,合金到達峰值后會發(fā)生明顯軟化。透射電鏡觀察和DSC分析揭示,冷軋合金時效析出特性與傳統(tǒng)T6時效過程顯著不同。70℃時效時溶質(zhì)原子參與了位錯釋放和重組的過程并形成了位錯胞晶面偏聚物,可以使強度和塑性同時升高,120℃以上時效時基體中析出大量板條相。調(diào)控同時發(fā)生的基體析出和位錯胞界面偏聚可以優(yōu)化冷軋Al-Mg-Si-Cu合金強度和塑性的結(jié)合。當時效溫度高于150℃時界析出會發(fā)生,強度和塑性明顯降低。在180℃時效后期晶界析出相(Q相和S相)幾乎耗盡了所有溶質(zhì)原子,晶粒內(nèi)部沒有位錯胞偏聚物和板條狀析出相,這可能是溶質(zhì)原子擴散受再結(jié)晶中晶界運動影響的結(jié)果。6.析出硬化的Al-Mg-Si-Cu合金在六個不同狀態(tài)下軋制:四個不同時間的自然預(yù)時效(合金內(nèi)部形成不同的溶質(zhì)團簇),欠人工時效處理和峰值人工時效處理,然后進行后續(xù)時效處理。峰值人工時效預(yù)處理降低了合金的軋制性能,引起了肉眼可見的邊緣開裂。后續(xù)時效過程的溶質(zhì)再析出隨變形前溶質(zhì)狀態(tài)變化很大,溶質(zhì)原子再析出可以補償強度損失并減輕晶體缺陷(位錯和晶界)對塑性的負面影響。相對于T6處理,自然時效和欠人工時效預(yù)處理的樣品經(jīng)后續(xù)時效后強度可以提升20-40%,峰值人工時效預(yù)處理的樣品強度可以提升40-50%,但塑性很小,欠人工時效預(yù)處理的樣品獲得了最優(yōu)的強度和塑性結(jié)合。軋制前的預(yù)時效對后續(xù)時效過程的析出反應(yīng)有很大影響,自然時效預(yù)處理的樣品后續(xù)時效時析出反應(yīng)類型相似,其差別是每個析出反應(yīng)過程形成析出相的總量,峰值人工時效預(yù)處理的樣品后續(xù)時效不會發(fā)生明顯的析出反應(yīng)。后續(xù)時效后自然時效預(yù)處理的樣品中形成兩類析出相:離散的板條狀Q"相和位于位錯胞界面長且彎曲的析出相。人工時效預(yù)處理樣品中的析出相經(jīng)軋制后位于局部區(qū)域并在后續(xù)時效時重新變得有序,軋制前欠人工時效預(yù)處理樣品中的早期β"相經(jīng)后續(xù)時效轉(zhuǎn)變成細小且均勻分布的板條狀Q"相。峰值時效預(yù)處理的樣品中軋制前含有單斜的β"相,軋制后析出相仍存在,但有序性顯著損失,經(jīng)后續(xù)時效,鋁基體處于高應(yīng)變狀態(tài)且Q'類型的析出相仍被大量位錯包圍。
[Abstract]:Revealing the changes of the structure and composition of hardened precipitates at the atomic scale is the key to understand and control the microstructure of complex multiphase alloys and is very important to develop high performance alloy materials. The main conclusions are as follows: 1. The precipitation at the interface of Si particles during aging of Al-Si alloy and the effect of spheroidizing treatment of Si particles on the precipitation of hardened phase during aging of Al-Si-Mg alloy were studied by means of electron microscopy. Continuous multiple {111} [112] type nanotwins can be precipitated on eutectic silicon particles during annealing, sometimes with rather complex structures, such as triple and quadruple twins, and closed quintuple twins, which are rare in polycrystalline materials. It is the segregation of Mg that reduces the content of Mg used to form the strengthening phase in the matrix. 2. Combined with quantitative high resolution electron microscopy, ring dark field scanning transmission electron microscopy and computational analysis, physical metallurgical images of the structural evolution of different precipitated phases in Al-Mg-Si-(Cu) alloys at atomic scale have been established. The effect of Cu content on the microstructure and mechanical properties of Al-Mg-Si-Cu alloy is very important. Proper aging process can optimize the properties of Al-Mg-Si-Cu alloy by controlling precipitation. In addition, the evolution law provided in this paper can be used as a basis for understanding the precipitation mechanism of Al-Mg-Si alloy with superfluous elements (such as Ag and transition metals). In order to improve the early aging hardening potential and rate of Al-Mg-Si alloy, even if the Mg/Si ratio is 2, the aging kinetics of Al-Mg-Si-Cu alloy is very fast. By optimizing the alloy composition ratio, a new composition range (Mg/Si ratio 1~2) for preparing Al-Mg-Si-Cu alloy sheets for automotive body is proposed. Combining deformation and aging, a new strategy is proposed to control the nano-precipitates in the aging process of aluminum alloy. By controlling the aggregation and distribution of alloy elements before rolling and the subsequent aging process, the strength of aluminum alloy can be significantly increased, but the strength of aluminum alloy can be improved. By controlling the chemical state before cold rolling, such as the aggregation and distribution of alloy elements, the precipitation of subsequent aging process can be controlled. Compared with T6 peak aging Al-Mg-Si-Cu alloy, the improved process can keep the plasticity unchanged and increase the strength by 30%. Precipitate-like phase: Discretely distributed strip-like Q "type precipitates and larger and continuously curved Q" type precipitates whose composition is close to that of Q'-type precipitates. Nano-scale coherent particles (GP region of monoclinic beta phase) induced by artificial aging before cold rolling can promote the formation of uniform and fine Q "type precipitates" in the subsequent aging process. The proposed scheme can be applied to many precipitation hardening alloys. The combination of deformation and aging process has greatly changed the corrosion characteristics of the alloys. The alloys not only have excellent mechanical properties, but also their corrosion resistance has been significantly improved. The effect of subsequent aging on microstructure evolution and mechanical properties of cold rolled Al-Mg-Si-Cu alloy was investigated by means of microhardness test, tensile test, thermal analysis and transmission electron microscopy. The physical image of interaction between solute atom diffusion and defect degradation during aging was revealed. The structure transformation is temperature dependent, and the segregation of three solute atoms occurs, including dislocation subcellular interface segregation, matrix precipitation and grain boundary segregation. The aging kinetics is obviously accelerated with the increase of aging temperature. The results of transmission electron microscopy and DSC analysis show that the precipitation characteristics of cold rolled alloys are significantly different from those of traditional T6 aging process. The solute atoms participate in the process of dislocation release and recombination and form dislocation cell surface segregates, which can increase the strength and plasticity at the same time and aging above 120 C. The strength and plasticity of cold rolled Al-Mg-Si-Cu alloy can be optimized by controlling matrix precipitation and interfacial segregation of dislocation cells. The strength and plasticity of cold rolled Al-Mg-Si-Cu alloy decrease significantly when the aging temperature is higher than 150 C. The precipitates at grain boundaries (Q phase and S phase) are almost exhausted at the later stage of aging at 180 C. Dislocation cell segregates and strip-like precipitates are absent in solute atoms, which may be the result of solute atom diffusion affected by grain boundary movement in recrystallization. 6. Precipitated and hardened Al-Mg-Si-Cu alloys are rolled in six different states: four natural pre-aging at different time (different solute clusters are formed in the alloy), under artificial time. The rolling properties of the alloy were reduced and the visible edge cracking was caused by the peak aging pretreatment. The solute re-precipitation during the subsequent aging process changed greatly with the solute state before deformation. The solute atom re-precipitation could compensate the strength loss and reduce the crystal defect. Compared with T6 treatment, the strength of samples treated by natural aging and under-artificial aging can be increased by 20-40% after subsequent aging, and the strength of samples treated by peak artificial aging can be increased by 40-50%, but the plasticity is very small. The samples pretreated by less artificial aging have the best strength and plasticity. Pre-aging before rolling has a great influence on the precipitation reaction of the subsequent aging process. The precipitation reaction types of the samples after natural aging pretreatment are similar. The difference is the total amount of precipitation phases formed in each precipitation reaction process. The precipitation reaction of the samples after peak artificial aging pretreatment is not obvious. Two types of precipitates are formed in the samples pretreated by natural aging after aging: the discrete strip Q "phase and the long and curved precipitates at the interface of dislocation cells. After subsequent aging, the precipitates in the samples pretreated by peak aging contain monoclinic beta phase before rolling, but there is a significant loss of order. After subsequent aging, the aluminum matrix is in a high strain state and the Q'type precipitates are still surrounded by a large number of dislocations.
【學位授予單位】：湖南大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG146.21

【參考文獻】

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1 季凱;祖國胤;姚廣春;;一種新型可焊耐蝕6×××系鋁合金材料[J];中國有色金屬學報;2010年10期

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本文編號：2219725