【摘要】：鎂稀土合金因其比強度高、耐腐蝕性能和生物相容性優(yōu)良,引起國內(nèi)外越來越多的重視,但現(xiàn)有的高強鎂稀土合金大都采用價格昂貴的重稀土元素作為主要強化元素,大大提高了這類合金的成本。為拓展鎂稀土合金應用領域,急需開發(fā)一種低成本高性能的鎂稀土合金。本文提出采用價格較低的輕稀土元素Nd和Sm聯(lián)合強化鎂合金的新思路,同時配合少量的Zn和Zr元素進一步改善組織。目前,基于混合輕稀土的鎂合金研究尚處于起步階段,Mg-Nd-Sm-Zn-Zr合金的微觀組織及力學性能尚不清楚。因此,本文系統(tǒng)開展了Mg-Nd-Sm-Zn-Zr合金成分優(yōu)化,深入研究了合金在固溶和時效過程組織演變,考察了正向擠壓聯(lián)合等通道轉角擠壓及其隨后時效處理對合金組織和力學性能影響。為優(yōu)化Mg-Nd-Sm-Zn-Zr合金成分,依據(jù)本課題組前期大量的相關研究工作,在已確定的Sm、Zn、Zr元素的最佳含量的基礎上,系統(tǒng)研究了稀土元素Nd對Mg-x Nd-2.0Sm-0.4Zn-0.4Zr(0≤x≤2.5)合金鑄態(tài)、固溶態(tài)和時效態(tài)的組織和性能的影響,揭示了合金的斷裂機制。結果表明:鑄態(tài)合金主要由α-Mg晶粒和共晶β相組成,β相主要呈網(wǎng)狀分布在晶界附近。鑄態(tài)合金經(jīng)固溶處理后,β相溶解于α-Mg基體中,形成過飽和固溶體,經(jīng)隨后的時效處理,在α-Mg的基體中形成大量的GP區(qū)、β′相和β1相,片狀β′相和β1相的慣析面主要為棱柱面。Nd元素可顯著增強析出強化效果,Nd元素含量為2.0 wt.%時,時效態(tài)合金可獲得較高的強度,屈服強度和抗拉強度分別為154 MPa和261 MPa,延伸率為5.8%。鑄態(tài)合金經(jīng)固溶處理和時效處理后的拉伸斷裂行為由沿晶斷裂向穿晶斷裂轉變。在合金成分優(yōu)化的基礎上,通過調控固溶溫度和時間,考察了固溶溫度和時間對Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr合金組織和性能的影響。結果表明:隨著固溶溫度的增加和固溶時間的延長,合金的晶粒尺寸逐漸增大,β相逐漸減少,強度先升高后降低,當固溶溫度高于515℃且固溶時間超過8 h,β相基本分解。確定了合金的最佳固溶處理工藝為515℃×8 h�；趦�(yōu)化的固溶處理工藝,進一步研究了時效溫度和時間對Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr合金組織和性能的影響。結果表明:隨著時效溫度的逐漸增加,β′相和β1相的尺寸逐漸增大,當時效溫度過高,析出物的密度明顯降低。合金的強度隨時效溫度的升高,先升高后降低,延伸率與強度的變化相反。隨著時效時間的延長,合金依次經(jīng)歷初始欠時效、半峰時效、峰時效和過時效階段。欠時效階段隨時效時間的延長β′相和β1相的尺寸和密度逐漸增大,峰時效階段β′相和β1相的密度最高,過時效階段β1相顯著粗化且密度降低。首次發(fā)現(xiàn)時效態(tài)合金中存在基于納米粒子和粗大片狀β1相的棋盤形雙相共構析出物。合金經(jīng)190℃×18 h時效處理后可獲得較高的強度,屈服強度和抗拉強度分別為152 MPa和266 MPa,延伸率為6.1%。為縮短Mg-Nd-Sm-Zn-Zr合金的時效處理時間,在時效前對固溶態(tài)合金進行冷壓縮預變形處理。結果表明:合金的硬度達到峰值所需時效時間僅為10 h,時效時間縮短近一半,且硬度峰值更高。發(fā)現(xiàn)預變形合金存在新型基面析出物βB′相,基于基面析出的βB′相及棱柱面析出的β′相所共同形成的封閉結構,可顯著增強時效強化效應。利用HAADF-STEM等分析測試技術,確定Mg-Nd-Sm-Zn-Zr合金的時效序列為:SSSS→GP區(qū)→β′相→β1相→β相,析出物主要以與基體結構相似度逐步降低的順序析出,析出物與基體的共格關系和位向關系隨析出物的轉變逐漸減弱。采用正向擠壓聯(lián)合等通道轉角擠壓技術制備了高性能Mg-Nd-Sm-Zn-Zr合金。結果表明:隨著Nd元素含量的增加,擠壓態(tài)合金的晶粒尺寸逐漸減小,強度逐漸提高,延伸率逐漸降低。擠壓態(tài)合金經(jīng)190℃×18 h時效處理后,析出了大量的析出物,且隨著Nd元素含量的增加,析出物數(shù)量逐漸增加,強度逐漸升高,塑性逐漸降低。Nd元素含量達到2.0 wt.%時強度較高,屈服強度和抗拉強度分別為187 MPa和315 MPa,延伸率為8.5%。綜上,本文設計并制備了低成本高性能Mg-Nd-Sm-Zn-Zr合金,系統(tǒng)研究了合金的組織演變和力學性能。開發(fā)的鑄態(tài)和固溶態(tài)合金具有中等強度和塑性,可廣泛應用于汽車、3C等領域;經(jīng)時效處理的合金具有較高的強度,在航空等輕量化領域具有廣闊的應用前景;采用正向擠壓聯(lián)合等通道轉角擠壓技術制備的高強度、高塑性的合金,在醫(yī)療等領域的應用潛力巨大。因此,該研究為擴大鎂稀土合金的應用,提供學術理論依據(jù),具有重要的應用價值。
[Abstract]:Because of its high specific strength, corrosion resistance and good biocompatibility, magnesium rare earth alloys have attracted more and more attention at home and abroad. However, the existing high strength magnesium rare earth alloys are mostly used as the main intensities of heavy rare earth elements, which greatly improve the cost of this kind of alloy, so it is urgent to develop the application field of magnesium rare earth alloys. A low cost and high performance magnesium rare earth alloy. In this paper, a new idea of combining Nd and Sm with low prices of light rare-earth elements to strengthen magnesium alloys is proposed. At the same time, a small amount of Zn and Zr elements are used to further improve the microstructure. At present, the research on magnesium alloys based on mixed light rare earth is still in the initial stage, the microstructure and force of the Mg-Nd-Sm-Zn-Zr alloy Therefore, the composition optimization of Mg-Nd-Sm-Zn-Zr alloy was systematically carried out in this paper. The microstructure evolution of the alloy in the solid solution and aging process was deeply studied. The influence of the forward extrusion combined equal channel angular extrusion and the subsequent aging treatment on the microstructure and mechanical properties of the alloy was investigated. In order to optimize the composition of Mg-Nd-Sm-Zn-Zr alloy, this course was based on this lesson. On the basis of the optimal content of Sm, Zn and Zr elements, the effects of rare earth element Nd on the structure and properties of Mg-x Nd-2.0Sm-0.4Zn-0.4Zr (0 < < x < 2.5) alloy are systematically studied on the basis of the optimum content of the determined elements. The results show that the alloy's fracture mechanism is revealed. The results show that the cast alloy is mainly based on the alpha -Mg. The crystalline grain is composed of the eutectic beta phase and the beta phase is mainly in the vicinity of the grain boundary. After solid solution treatment, the cast alloy is dissolved in the matrix of alpha -Mg to form a supersaturated solid solution. After the aging treatment, a large number of GP regions, beta 'phase and beta 1 phase are formed in the matrix of alpha -Mg. The inertial surface of the beta' phase and beta 1 phase is mainly.Nd element of the prism surface. When the content of Nd element is 2 wt.%, the aging state alloy can obtain higher strength, the yield strength and tensile strength are 154 MPa and 261 MPa respectively. The tensile fracture behavior of the cast alloy after solid solution treatment and aging treatment is changed from intergranular fracture to transgranular fracture, and the composition of the alloy is superior to the alloy. The effect of solid solution temperature and time on the microstructure and properties of Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr alloy was investigated by adjusting the temperature and time of solid solution. The results showed that the grain size of the alloy increased gradually with the increase of the solid solution temperature and the time of solid solution. The phase of the alloy gradually decreased, the strength first increased and then decreased, when the solid solution temperature was the solution temperature. Higher than 515 C and the solid solution time exceeding 8 h and the basic decomposition of beta phase. The optimum solution treatment process of the alloy was 515 C 8 h. based on the optimized solution treatment process. The effect of aging temperature and time on the microstructure and properties of Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr alloy was further studied. The results showed that the increasing of aging temperature, beta ' The size of the phase and beta 1 phase increases gradually. When the aging temperature is too high, the density of the precipitates decreases obviously. The strength of the alloy increases first and then decreases with the aging temperature. The elongation is the opposite of the strength. With the aging time prolonging, the alloy goes through the initial under aging, half peak aging, peak aging and over aging stage. Under aging stage. The size and density of beta 1 phase and beta 1 phase increased gradually with the aging time. The density of beta and beta 1 phase was the highest at the peak aging stage. The phase of beta 1 was coarsely coarsened and the density decreased in the over aging phase. It was found for the first time that there was a disk shaped double phase co structure precipitate based on nanoparticles and large large beta 1 phase in the aging state alloy. The alloy was 190 C 18 h. High strength is obtained after aging, yield strength and tensile strength are 152 MPa and 266 MPa respectively. The elongation is 6.1%. to shorten the aging time of Mg-Nd-Sm-Zn-Zr alloy, and the cold compression predeformation treatment is done to the solid solution alloy before aging. The results show that the aging time required for the hardness of the alloy is only 10 h, and the aging time is only 10. The time shortening is nearly half and the peak hardness is higher. It is found that the pre deformed alloy has a new base surface precipitate of beta B '. The closed structure formed by the precipitated beta B phase of the base surface and the beta phase precipitated from the prism surface can significantly enhance the aging hardening effect. The aging method of HAADF-STEM is used to determine the aging of the Mg-Nd-Sm-Zn-Zr alloy. The sequence is: SSSS, GP region, beta 'phase, beta 1 phase to beta phase. The precipitates precipitate mainly in the order of gradually decreasing the similarity between the matrix structure and the matrix structure. The relationship between the precipitates and the matrix and the orientation relation gradually weaken with the transition of the precipitates. The high performance Mg-Nd-Sm-Zn-Zr alloy was prepared by the forward extrusion combined with equal channel angular extrusion technology. The results show that with the increase of the content of Nd elements, the grain size of the extruded alloy gradually decreases, the strength increases and the elongation gradually decreases. A large number of precipitates have been precipitated in the extruded alloy after 190 C 18 h aging treatment, and with the increase of the content of Nd elements, the number of precipitates gradually increases, the strength increases gradually, and the plasticity gradually reduces.Nd yuan. The strength of the element is 2 wt.%, the yield strength and tensile strength are 187 MPa and 315 MPa respectively, and the elongation is 8.5%.. The low cost and high performance Mg-Nd-Sm-Zn-Zr alloy is designed and prepared. The microstructure evolution and mechanical properties of the alloy are systematically studied. The developed and solid solution alloys are of medium strength and plasticity, and can be widely used. The pan should be used in the fields of automobile, 3C, and other fields. The aging alloy has a high strength and has a broad application prospect in the field of Aeronautical light weight. The high strength and high plasticity alloy prepared by forward extrusion combined with equal channel angular extrusion technology has great potential in the application of medical and other fields. Therefore, this study is to expand the combination of magnesium rare earth and rare earth. The application of gold provides academic theoretical basis and has important application value.
【學位授予單位】：哈爾濱理工大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TG146.22

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