本文選題：包晶TiAl基合金 + 深過(guò)冷凝固�。� 參考：《西北工業(yè)大學(xué)》2016年博士論文

【摘要】：TiAl基合金是重要的包晶合金之一,其凝固組織對(duì)Al含量具有高度的敏感性,特別是較窄Al含量的包晶TiAl基合金。對(duì)于以鑄造為首選工藝的TiAl基合金而言,Al含量的變化嚴(yán)重影響了鑄錠和鑄件的凝固組織特征。通常鑄造屬于近平衡凝固的范疇,薄壁鑄件或鑄錠的邊緣由于液相到固相的相變過(guò)程進(jìn)行的非�？�,偏離了近平衡凝固,更接近于非平衡凝固。目前,對(duì)于包晶TiAl基合金非平衡凝固的研究工作主要集中于二元和三元以及少數(shù)多元TiAl基合金,而對(duì)高Al含量的多元包晶TiAl基合金非平衡凝固過(guò)程中的相選擇和組織演化規(guī)律的研究相對(duì)缺乏和有限�；诖�,本文系統(tǒng)地研究了多元Ti-xAl-2Cr-2Nb(x=46,47,48,49,50,51,52)(at.%)合金在近平衡凝固條件下初生β/α相凝固的Al含量臨界值、相組成以及凝固組織演化規(guī)律;并選定工業(yè)化應(yīng)用的Ti-48Al-2Cr-2Nb(at.%)合金和Al含量臨界值(初生β/α相凝固)附近的Ti-50Al-2Cr-2Nb(at.%)合金為研究對(duì)象,研究了過(guò)冷度和冷卻速率對(duì)這兩種合金非平衡凝固過(guò)程中的相選擇和組織演化規(guī)律的影響,探究了深過(guò)冷凝固過(guò)程中亞結(jié)構(gòu)和魏氏組織、羽毛組織以及塊狀組織等亞穩(wěn)組織形成和影響因素,探討了過(guò)冷和急冷耦合條件下,亞穩(wěn)相的析出和長(zhǎng)大,獲得了以下主要研究結(jié)果。對(duì)Ti-xAl-2Cr-2Nb(x=46,47,48,49,50,51,52)(at.%)合金近平衡凝固和非平衡凝固組織研究的結(jié)果表明:在近平衡凝固條件下,當(dāng)Al含量(CAl)≤50at.%時(shí),合金以β相為初生相進(jìn)行包晶凝固,相組成主要為α_2相和γ相以及少量的B2相;當(dāng)CAl50at.%時(shí),合金以α相為初生相進(jìn)行凝固,相組成主要為α_2和γ兩相。初生β/α相凝固的Al含量臨界值為50at.%;隨著CAl的增加,γ相的正方畸變度(c/a)隨之增加,α_2相的c/a值呈下降趨勢(shì);同時(shí)隨著CAl的增加,初生相的結(jié)晶溫度范圍先減小后增大,一次枝晶間距隨之變化。當(dāng)CAl=48at.%時(shí),前期為初生β相形核且未充分長(zhǎng)大,后期為包晶α相以β相為核心形核長(zhǎng)大的過(guò)包晶逐層凝固。棒狀試樣的凝固組織明顯細(xì)化,二次枝晶臂生長(zhǎng)受到抑制,一定程度也抑制了α→(α_2+γ)固態(tài)相變,并未改變初生β/α相凝固的Al含量臨界值。采用電磁懸浮熔煉的深過(guò)冷凝固技術(shù)實(shí)現(xiàn)了Ti-48Al-2Cr-2Nb(at.%)合金和Ti-50Al-2Cr-2Nb(at.%)合金的深過(guò)冷,獲得的最大過(guò)冷度分別為370K和290K。對(duì)于前者,在所獲得的過(guò)冷度范圍內(nèi),β相均能作為初生相優(yōu)先形核,這表明該合金凝固組織具有較高的穩(wěn)定性;而對(duì)于后者,在較小過(guò)冷度下(ΔT120K),β相均能作為初生相優(yōu)先形核。一旦超過(guò)臨界過(guò)冷度ΔT*=120K,亞穩(wěn)α相取代初生β相成為領(lǐng)先相。這表明Al含量臨界值(初生β/α相凝固)附近的合金更容易發(fā)生凝固模式的改變。此外,運(yùn)用經(jīng)典形核理論分析了Ti-48Al-2Cr-2Nb(at.%)合金中初生相和亞穩(wěn)相的形核與過(guò)冷度的關(guān)系,合理地解釋了各相的競(jìng)爭(zhēng)形核。運(yùn)用BCT模型分析了各相過(guò)冷度的分布;結(jié)合實(shí)驗(yàn)結(jié)果,較好地闡述了過(guò)冷凝固組織的演化過(guò)程。在較小過(guò)冷度下,由于已凝固枝晶重熔,導(dǎo)致晶粒細(xì)化,而在大過(guò)冷度下出現(xiàn)二次細(xì)化現(xiàn)象是由枝晶碎斷造成的,與過(guò)冷凝固過(guò)程中的內(nèi)應(yīng)力密切相關(guān)。在非平衡凝固組織中觀察到了豐富的亞結(jié)構(gòu),主要由變形孿晶、高密度位錯(cuò)以及少量層錯(cuò)組成,這表明過(guò)冷凝固過(guò)程中存在巨大的內(nèi)應(yīng)力。隨著過(guò)冷度增大,微觀應(yīng)變呈增大趨勢(shì)。TiAl合金的深過(guò)冷凝固過(guò)程相當(dāng)于塑性變形過(guò)程,其內(nèi)應(yīng)力釋放機(jī)制為孿生剪切變形,層片結(jié)構(gòu)中的γ相發(fā)生了塑性變形,形成大量的γ/γ孿晶(P型孿晶)和γ相層片內(nèi)部的孿晶(Q型孿晶),甚至出現(xiàn)了孿晶交割。在中等過(guò)冷度條件下,觀察到有魏氏組織(γ_w)的形成;在大過(guò)冷度條件下,能夠觀察到由羽毛組織(γ_f)、塊狀組織(γ_m)與片層耦合的組織形貌。γ_w和γ_f以及γ_m不是從過(guò)冷液相中直接凝固形成而是通過(guò)一系列固態(tài)相變形成。同時(shí),高密度位錯(cuò)與層錯(cuò)以及固態(tài)相變的過(guò)冷都為其在低冷卻速率下的形核提供了足夠的驅(qū)動(dòng)力,它們的形成與片層團(tuán)尺寸、過(guò)冷度、合金成分以及冷卻速率密切相關(guān)。采用電磁懸浮和銅模急冷鑄造相結(jié)合的方法制備了Ti-48Al-2Cr-2Nb(at.%)合金和Ti-50Al-2Cr-2Nb(at.%)合金急冷快速凝固錐形試樣,通過(guò)ANSYS軟件模擬估算出兩種合金的冷卻速率分別在2.9×103 K/s至2.6×104 K/s和3.6×103 K/s至2.6×104 K/s之間。通過(guò)線(xiàn)性擬合獲得一次枝晶間距λ_1和冷卻速率T的函數(shù)關(guān)系分別為:λ_1=323.40(T|-)~(-0.32)和λ_1=2.70×10~3(T|-)~(-0.53)。對(duì)于前者,隨著冷卻速率的增加至2.1×104 K/s時(shí),包晶轉(zhuǎn)變和共析轉(zhuǎn)變受到抑制;隨著冷卻速率的進(jìn)一步增加至2.6×10~4 K/s時(shí),出現(xiàn)了塊狀γ相,強(qiáng)急冷誘發(fā)大的起始過(guò)冷度和高密度位錯(cuò)有利于塊狀γ相的形成;對(duì)于后者,如果冷卻速率一旦超過(guò)臨界值(4.0×10~3 K/s),能夠獲得單一的包晶α相;在低的冷卻速率下(4.0×10~3 K/s),僅能在糊狀區(qū)觀察到初生β相的形核;在高的冷卻速率下(4.0×10~3 K/s),β相形核和包晶反應(yīng)受到抑制而亞穩(wěn)α相從非平衡液相中直接形核并長(zhǎng)大,最終獲得細(xì)化均勻的胞狀α相凝固組織。實(shí)驗(yàn)結(jié)果和動(dòng)力學(xué)分析表明:冷卻速率和Al含量的變化能夠引起初生β相和包晶α相的相選擇,可以獲得預(yù)期的相組成和顯微組織,包晶α相的直接形核和長(zhǎng)大受高冷卻速率下溶質(zhì)富集和形核過(guò)冷的控制;深過(guò)冷凝固和急冷凝固的耦合作用,有利于亞穩(wěn)α相的形核與長(zhǎng)大。
[Abstract]:TiAl based alloy is one of the most important peritectic alloys. Its solidification structure is highly sensitive to the content of Al, especially the peritectic TiAl based alloy with narrow Al content. For the TiAl based alloy with the preferred casting process, the change of Al content seriously affects the solidification structure of the ingot and the casting. In the category, the edge of the thin-walled castings or ingot is very fast due to the phase to solid phase transformation process, which deviates from the near equilibrium solidification and is closer to the non equilibrium solidification. At present, the research work on the non equilibrium solidification of the peritectic TiAl based alloys is mainly concentrated on two yuan and three yuan and a few multivariate TiAl based alloys, and the multiple packages of high Al content The study of phase selection and microstructure evolution of amorphous TiAl based alloy is relatively deficient and limited. Based on this, the critical value, phase composition and solidification structure of Al content of primary beta / alpha phase solidification of multiple Ti-xAl-2Cr-2Nb (x=46,47,48,49,50,51,52) (at.%) alloys under near equilibrium solidification conditions are systematically studied. The Ti-48Al-2Cr-2Nb (at.%) alloy and the Ti-50Al-2Cr-2Nb (at.%) alloy near the Al content (primary beta / alpha phase solidification) were selected as the research object. The effects of the supercooling and cooling rate on the phase selection and the microstructure evolution of the two alloys during non equilibrium solidification were studied. The formation and influence factors of the metastable tissues such as the Cheng Zhongya structure and the wechis structure, the feather tissue and the massive tissue are discussed. The precipitation and growth of the metastable phase are discussed under the conditions of supercooling and quench cooling. The following main research results are obtained. Study on the near equilibrium solidification and non equilibrium solidification structure of Ti-xAl-2Cr-2Nb (x=46,47,48,49,50,51,52) (at.%) alloy The results show that, under the condition of near equilibrium solidification, when the content of Al (CAl) is less than 50at.%, the alloy uses beta phase as the primary phase to solidify the peritectic phase. The phase composition is mainly alpha _2 phase and gamma phase and a small amount of B2 phase. When CAl50at.%, the alloy solidified with alpha phase as primary phase, and the phase composition is mainly alpha _2 and gamma phase. The critical Al content of primary beta / alpha phase solidification is critical. The value of the value is 50at.%, with the increase of CAl, the square aberration (c/a) of the gamma phase increases and the c/a value of the alpha _2 phase decreases. At the same time, with the increase of CAl, the crystallization temperature range of the primary phase decreases first and then increases, and then the primary dendrite spacing changes. When CAl=48at.%, the initial phase of the primary beta phase nucleus is not fully grown, and the later phase of the peritectic alpha phase is beta. The peritectic crystals of the core nucleation are solidified by layer by layer. The solidification structure of the bar like specimen is obviously refined, the growth of the two dendrite arm is restrained, the solid phase transformation of alpha to (alpha _2+) is suppressed to a certain extent, and the critical value of the Al content of the primary beta / alpha phase solidification is not changed. Ti-48Al-2Cr-2Nb is realized by the deep supercooling solidification technology of electromagnetic suspension melting. (at.%) the maximum supercooling degree of alloy and Ti-50Al-2Cr-2Nb (at.%) alloy is 370K and 290K. for the former. In the overcooling range, the beta phase can be used as the primary phase of the primary phase, which indicates that the solidification structure of the alloy has high stability, and for the latter, the beta phase is equal to the lower supercooling degree (delta T120K). As a primary nucleation, once the critical subcooling degree of T*=120K is exceeded, the metastable alpha phase is replaced by the primary beta phase as the leading phase. This indicates that the alloy near the critical value of the Al content (primary beta / alpha phase solidification) is more likely to change the solidification mode. In addition, the classical nucleation theory is used to analyze the primary phase and metastable phase in the Ti-48Al-2Cr-2Nb (at.%) alloy. The relationship between the nucleation of the phase and the supercooling degree is explained reasonably. The distribution of the supercooling degree of each phase is analyzed by the BCT model, and the evolution process of the supercooling solidification structure is explained well by the experimental results. The grain refinement is caused by the remelting of the solidified dendrite under the small supercooling, and two times under the large supercooling degree. The phenomenon is caused by the breakage of dendrite, which is closely related to the internal stress in the process of supercooling solidification. A rich substructure is observed in the non-equilibrium solidification structure, mainly composed of deformation twins, high density dislocation and a small amount of stacking faults. This indicates that there is a huge internal stress in the process of supercooling solidification. With the increase of supercooling, the microstrain shows the microstrain. The deep supercooling solidification process of the increasing trend of.TiAl alloy is equivalent to the plastic deformation process. The internal stress release mechanism is twin shear deformation. The plastic deformation occurs in the gamma phase in the layer structure, forming a large number of gamma / gamma twins (P twins) and the twins in the gamma phase layer (Q type twins), even the twin intertwinning. Below, the formation of Wechsler tissue (gamma _w) is observed; under the condition of excessive cooling, the microstructure of the coupling between the feather tissue (gamma _f) and the massive tissue (gamma _m) can be observed. Gamma and gamma _f and gamma _f and gamma _m are not directly formed from the supercooled liquid phase but are formed by a series of solid phase transitions. At the same time, high density dislocation and stacking fault and solid state are formed. The supercooling of the state phase transition provides sufficient driving force for its nucleation at low cooling rate. Their formation is closely related to lamellar size, supercooling, alloy composition and cooling rate. The Ti-48Al-2Cr-2Nb (at.%) alloy and Ti-50Al-2Cr-2Nb (at.%) alloy are prepared by the combination of electromagnetic suspension and copper die cold casting. By ANSYS software simulation, the cooling rates of two kinds of alloys are estimated from 2.9 x 103 K/s to 2.6 x 104 K/s and 3.6 x 103 K/s to 2.6 x 104 K/s, respectively. 53). For the former, with the increase of cooling rate to 2.1 x 104 K/s, the peritectic transformation and eutectoid transition are suppressed. As the cooling rate increases to 2.6 * 10~4 K/s, massive gamma phase appears. Strong chilling induced large initial undercooling and high density dislocation are beneficial to the formation of massive gamma phase; for the latter, if the cooling rate is one. A single peritectic alpha phase can be obtained at the above critical value (4 x 10~3 K/s); at a low cooling rate (4 x 10~3 K/s), only the nucleation of primary beta phase can be observed in the paste region; at a high cooling rate (4 x 10~3 K/s), the beta phase nucleus and the peritectic reaction are suppressed and the metastable alpha phase is nucleated directly from the nonequilibrium liquid phase and eventually obtained. The experimental results and kinetic analysis show that the change of cooling rate and Al content can lead to the phase selection of primary beta phase and peritectic alpha phase, and the expected phase composition and microstructure can be obtained. The direct nucleation of the peritectic alpha phase and the control of solute enrichment and nucleation supercooling under high cooling rate can be obtained. The coupling effect of deep undercooling and rapid solidification is beneficial to nucleation and growth of metastable alpha phase.

【學(xué)位授予單位】：西北工業(yè)大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類(lèi)號(hào)】：TG244.3

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2 叢韜;長(zhǎng)期大氣熱暴露環(huán)境中含鎢鈮細(xì)晶TiAl合金的組織和性能變化[D];西南交通大學(xué);2007年

3 李新安;含Mo高鈮TiAl合金高溫變形行為及組織性能研究[D];哈爾濱工業(yè)大學(xué);2011年

4 陳佳;定向凝固TiAl合金板狀坯微合金化及組織細(xì)化[D];哈爾濱工業(yè)大學(xué);2008年

5 李亮;TiAl合金液相擴(kuò)散連接工藝及機(jī)理研究[D];哈爾濱工業(yè)大學(xué);2011年

6 曹守臻;面層材料對(duì)熔模鑄造TiAl合金界面反應(yīng)和抗氧化性能的影響[D];哈爾濱工業(yè)大學(xué);2013年

7 黃財(cái)林;TiAl合金板材的制備與組織性能研究[D];哈爾濱工業(yè)大學(xué);2014年

8 司曉慶;高鈮TiAl合金釬焊工藝及接頭性能研究[D];哈爾濱工業(yè)大學(xué);2015年

9 林興濤;TiAl合金與Ti_3AlC_2陶瓷擴(kuò)散連接工藝及機(jī)理研究[D];哈爾濱工業(yè)大學(xué);2013年

10 李晶;長(zhǎng)期大氣熱暴露對(duì)不同含鎢量TiAl合金穩(wěn)定性的影響的研究[D];西南交通大學(xué);2007年

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