發(fā)布時間：2018-08-27 08:01

【摘要】：鈹銅合金的使用性能與其顯微組織密切相關,尤其是合金中析出物的狀態(tài)。通過優(yōu)化析出物的的狀態(tài),可以獲得性能更佳優(yōu)異的合金。本文對Cu-Be-Co-Ni合金合金高溫壓縮變形行為、不同單級和雙級時效熱處理工藝下的顯微組織、相變規(guī)律、斷裂韌性、疲勞行為以及時效硬化機理進行了系統(tǒng)研究,得到的主要結(jié)論如下：Cu-Be-Co-Ni合金高溫壓縮變形是一個熱激活的過程,主要分為加工硬化、動態(tài)回復和動態(tài)再結(jié)晶三個階段。合金的應力峰值隨應變速率的增加而上升,隨著變形溫度的增加而降低。結(jié)合XRD測試與導電率法,對固溶軟態(tài)及硬態(tài)Cu-Be-Co-Ni合金的時效析出動力學規(guī)律進行了研究,獲得了相應的時效動力學方程。固溶軟態(tài)合金320℃時效過程中,析出物均勻形核,具有很長的相變孕育期：固溶硬態(tài)合金320℃時效過程中,析出物在界面非均勻形核,時效初期就可達到很高的相轉(zhuǎn)變率。通過320℃預時效30 min,隨后在280℃時效360 min的斷續(xù)時效處理,可以顯著改善Cu-Be-Co-Ni合金的韌性。與普通峰值時效合金相比,斷續(xù)時效處理后合金的抗拉強度僅降低了3.3%,而均勻伸長率以及平面應力斷裂韌度分別提高了17.1%和23%,斷裂過程中裂紋萌生所需能量提高了84%,裂紋擴展所需能量提高了將近2倍。合金中析出物以密集分布的小尺寸顆粒狀態(tài)存在并且與位錯間相互作用機制為位錯剪切析出粒子時,合金將具有更高的塑性和斷裂韌性。Cu-Be-Co-Ni合金的疲勞裂紋擴展速率與顯微組織密切相關,根據(jù)裂紋尖端反向塑性區(qū)(RPZ)尺寸與合金顯微組織參數(shù)之間的關系可以很好地解釋疲勞裂紋擴展機理。應力場強度因子水平較低時,RPZ尺寸小于晶粒尺寸,疲勞裂紋在晶粒內(nèi)部及晶界附近局部區(qū)域獨立擴展,主要受到晶內(nèi)析出物的影響,合金的疲勞裂紋擴展速率隨合金中可變形析出物含量的增加而降低。應力場強度因子水平較高時,RPZ尺寸約為晶粒尺寸的1～2倍,疲勞裂紋沿晶界擴展,晶界的特征成為影響疲勞裂紋擴展速率的主要因素。胞狀不連續(xù)脫溶產(chǎn)物和母相間連續(xù)的相界面將有效地阻礙疲勞裂紋的擴展。通過對不同時效狀態(tài)下Cu-Be-Co-Ni合金中析出物的特征進行定量研究,建立了合金的屈服強度模型,揭示了合金的析出強化機理。對于僅含有不可變形析出物的普通峰值時效和過時效合金來說,析出強化的貢獻來自于Orowan機制。而雙級時效和欠時效的合金中同時含有可變形及不可變形析出物,兩者的臨界轉(zhuǎn)換尺寸為1.5 nm,析出強化效果由位錯剪切析出粒子以及Orowan機制共同提供,其中Orowan機制起主要作用。根據(jù)Cu-Be-Co-Ni合金單向拉伸及拉伸-壓縮變形(Bauschinger)行為,分別研究了合金各向同性強化和隨動強化對合金應變硬化的貢獻,建立了合金析出物微觀特征與宏觀應力-應變行為之間的模型。該模型表明合金的抗拉強度和均勻伸長率取決于兩個相互矛盾的因素：位錯動態(tài)回復速率和存儲在析出粒子周圍的位錯密度增長速率,優(yōu)化合金性能的關鍵在于平衡這兩個相互矛盾的因素。研究結(jié)果表明,含有可變形及不可變形混合析出物的合金可達到這兩個因素間的良好平衡,從而獲得最佳的性能。
[Abstract]:The service properties of beryllium-copper alloys are closely related to their microstructure, especially the state of precipitates in the alloys. By optimizing the state of precipitates, better alloys with better properties can be obtained. Fracture toughness, fatigue behavior and aging hardening mechanism of Cu-Be-Co-Ni alloy are systematically studied. The main conclusions are as follows: high temperature compression deformation of Cu-Be-Co-Ni alloy is a thermal activation process, which is divided into three stages: work hardening, dynamic recovery and dynamic recrystallization. The ageing kinetics of solid solution soft and hard Cu-Be-Co-Ni alloys was studied by XRD and conductivity method, and the corresponding ageing kinetics equations were obtained. The toughness of Cu-Be-Co-Ni alloy can be significantly improved by pre-aging at 320 C for 30 min and then aging at 280 C for 360 min. The tensile strength of Cu-Be-Co-Ni alloy after intermittent aging treatment is stronger than that of normal peak aging alloy. The crack initiation energy is increased by 84% and the crack propagation energy is increased by nearly two times. The precipitates in the alloy exist in densely distributed small size particles and interact with dislocations as dislocations. The fatigue crack growth rate of Cu-Be-Co-Ni alloy is closely related to the microstructure. The mechanism of fatigue crack growth can be well explained by the relationship between the size of RPZ and the microstructure parameters of the alloy. The fatigue crack propagates independently in the grain interior and the local area near the grain boundary. The fatigue crack propagation rate decreases with the increase of the content of deformable precipitates in the alloy. The RPZ size is about 1 of the grain size when the stress field intensity factor is high. Fatigue cracks propagate along grain boundaries, and grain boundaries are the main factors affecting the growth rate of fatigue cracks. The discontinuous cell-like desolvation products and the continuous phase interface between parent phase will effectively inhibit the growth of fatigue cracks. The yield strength model reveals the precipitation strengthening mechanism of the alloy. The contribution of precipitation strengthening to the normal peak aging and over aging alloys with only undeformed precipitates comes from the Orowan mechanism. At 1.5 nm, the precipitation strengthening effect is provided by dislocation shear precipitation particles and Orowan mechanism, in which Orowan mechanism plays a major role. Based on the uniaxial tensile and tensile-compressive deformation (Bauschinger) behavior of Cu-Be-Co-Ni alloy, the contributions of isotropic strengthening and follow-up strengthening to strain hardening of the alloy are studied, and the alloy is established. The model shows that the tensile strength and uniform elongation of the alloy depend on two contradictory factors: the dynamic recovery rate of dislocation and the growth rate of dislocation density stored around the precipitated particles. The results show that the alloy containing deformable and non-deformable mixed precipitates can achieve a good balance between the two factors and obtain the best properties.
【學位授予單位】：北京科技大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TG146.11

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