本文選題：梯度結(jié)構(gòu) + 表面機(jī)械研磨處理��； 參考：《昆明理工大學(xué)》2017年碩士論文

【摘要】：梯度結(jié)構(gòu)材料在一定程度上較好地解決了傳統(tǒng)材料的強(qiáng)度-塑性“倒置”(trade-off)關(guān)系,不僅呈現(xiàn)出優(yōu)異的高強(qiáng)度高塑性匹配,還具備良好的表面耐磨性、抗疲勞等性能,因此也越來越受到更多關(guān)注。然而,目前有關(guān)梯度結(jié)構(gòu)的可控制備、梯度結(jié)構(gòu)材料的組織性能關(guān)系及變形力學(xué)行為等研究還不夠系統(tǒng),仍然存在諸多問題。本文采用表面機(jī)械研磨處理(Surface mechanical attrition treatment,SMAT)方法在純銅及不同層錯能的銅鋁合金中獲得梯度結(jié)構(gòu),梯度結(jié)構(gòu)的存在有效提高了材料強(qiáng)度,同時其塑性得到較好保持。對純銅分別在不同時間和溫度下進(jìn)行SMAT處理,SMAT處理時間越長、處理溫度越低得到的梯度結(jié)構(gòu)層更厚。溫度降低動態(tài)回復(fù)被抑制,晶粒細(xì)化更明顯,變形方式逐漸由位錯轉(zhuǎn)變?yōu)閷\生,低溫處理的樣品中有大量孿晶出現(xiàn),屈服強(qiáng)度高。經(jīng)SMAT處理得到的梯度結(jié)構(gòu)純銅,當(dāng)其梯度層所占體積分?jǐn)?shù)達(dá)到一定程度后,加工硬化率在低應(yīng)變范圍內(nèi)存在一個緩慢上升的階段,出現(xiàn)加工硬化率反轉(zhuǎn)(up-turn)現(xiàn)象,形成明顯的額外加工硬化。此現(xiàn)象與拉伸變形過程中的可動位錯運動有關(guān),相對可動位錯密度在拉伸過程中呈現(xiàn)先下降后上升的趨勢。將不同層錯能的銅鋁合金(Cu-2.2wt.%Al、Cu-4.5wt.%Al、Cu-6.9wt.%Al)在低溫下進(jìn)行SMAT處理獲得梯度組織,發(fā)現(xiàn)具有中等層錯能的Cu-4.5wt.%Al合金更適合于采用SMAT工藝來實現(xiàn)最佳的強(qiáng)度塑性配合。其屈服強(qiáng)度的提升主要由梯度(Gradient structure,GS)層貢獻(xiàn),均勻延伸率則受變形過程中的動態(tài)回復(fù)控制,根據(jù)Kocks-Mecking模型擬合得出,中等層錯能的Cu-4.5wt.%Al樣品中代表動態(tài)回復(fù)/位錯湮滅的K2值最小。應(yīng)力松弛實驗顯示,Cu-4.5wt.%Al樣品的可動位錯湮滅速率最慢,即位錯回復(fù)受到阻礙,可以推遲頸縮的產(chǎn)生,使樣品保持良好塑性。XRD結(jié)果表明,樣品在變形過程中產(chǎn)生的高密度孿晶可有效地降低可動位錯損耗,同時能夠在一定程度上使樣品保持較高的加工硬化能力。在梯度結(jié)構(gòu)材料中,根據(jù)Hall-Patch公式由簡單混合法則計算出的樣品屈服強(qiáng)度遠(yuǎn)小于實際樣品的屈服強(qiáng)度。梯度結(jié)構(gòu)的高強(qiáng)度并非完全由晶粒細(xì)化貢獻(xiàn),還由在梯度變化的晶粒中大量堆積的位錯產(chǎn)生的背應(yīng)力來提供。梯度結(jié)構(gòu)樣品的包辛格效應(yīng)比均勻樣品更明顯,應(yīng)變梯度越大,則產(chǎn)生的幾何必須位錯(Geometrically necessary dislocations,GNDs)越多,阻礙新的位錯產(chǎn)生和增殖,因此背應(yīng)力也更高,對屈服強(qiáng)度的貢獻(xiàn)更強(qiáng),強(qiáng)度得以明顯提升。梯度結(jié)構(gòu)材料主要依靠晶粒細(xì)化及梯度應(yīng)變產(chǎn)生的GNDs來實現(xiàn)協(xié)同強(qiáng)化。
[Abstract]:To some extent, gradient structural materials have solved the "inversion" relationship between strength and plasticity of traditional materials, showing not only excellent high strength and high plasticity matching, but also good surface wear resistance, fatigue resistance and so on.As a result, more and more attention has been paid to it.However, the studies on the controllable preparation of gradient structure, the relationship between microstructure and properties and deformation mechanical behavior of gradient structure materials are still not systematic, and there are still many problems.In this paper, the surface mechanical attrition treatment-Smatt method is used to obtain gradient structure in pure copper and copper aluminum alloy with different stacking fault energy. The existence of gradient structure improves the strength of the material and keeps the plasticity of the alloy.The longer the SMAT treatment time of pure copper is, the thicker the gradient structure layer is when the treatment temperature is lower.The decrease of temperature dynamic recovery was restrained, the grain refinement was more obvious, the deformation mode gradually changed from dislocation to twinning, a large number of twins appeared in the samples treated at low temperature, and the yield strength was high.When the volume fraction of gradient layer reached a certain degree, the work hardening rate of pure copper with gradient structure obtained by SMAT treatment increased slowly in the range of low strain, and the work hardening rate reversed up-turn phenomenon.The formation of obvious additional work hardening.This phenomenon is related to the movable dislocation movement during tensile deformation, and the relative movable dislocation density decreases at first and then increases during tensile deformation.Cu-2.2wt.Alncu-4.5wt.Alanum Cu-6.9wt.Al) with different stacking fault energy was treated with SMAT at low temperature to obtain gradient microstructure. It was found that the Cu-4.5wt.%Al alloy with medium stacking fault energy was more suitable to achieve the best strength and plastic fit by using the SMAT process.The increase of yield strength is mainly contributed by gradient structure GSlayer, and the uniform elongation is controlled by dynamic recovery during deformation. According to Kocks-Mecking model fitting, the K _ 2 value representing dynamic recovery / dislocation annihilation is the smallest in Cu-4.5wt.%Al samples with medium fault energy.The stress relaxation experiment shows that the dislocation annihilation rate of Cu-4.5wt.Al sample is the slowest, and the recovery of the spot fault is blocked, which can delay the occurrence of necking shrinkage and keep the sample in good plasticity. XRD results show that the dislocation annihilation rate of Cu-4.5wt. Al sample is the slowest.The high density twin produced in the process of deformation can effectively reduce the loss of movable dislocations, and at the same time, it can make the samples maintain high working-hardening ability to a certain extent.In gradient structural materials, the yield strength of the sample calculated by the simple mixing rule based on the Hall-Patch formula is much smaller than that of the actual sample.The high strength of gradient structure is not only contributed by grain refinement, but also by the back stress caused by dislocation accumulated in gradients.The Bauschinger effect of gradient structure samples is more obvious than that of homogeneous samples. The bigger the strain gradient is, the more geometric geometry must be generated, which hinders the generation and proliferation of new dislocations, so the back stress is higher and the contribution to yield strength is stronger.The strength was significantly increased.Gradient structure materials mainly rely on grain refinement and GNDs generated by gradient strain to achieve synergistic strengthening.
【學(xué)位授予單位】：昆明理工大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG146.11

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本文編號：1739977