發(fā)布時(shí)間：2018-04-01 03:16

  本文選題：高鈮TiAl合金　切入點(diǎn)：疲勞　出處：《北京科技大學(xué)》2015年博士論文

【摘要】：具有高鈮含量的TiAl合金,以其優(yōu)異的高溫力學(xué)性能和較低的密度被視為航空航天及汽車發(fā)動(dòng)機(jī)等領(lǐng)域具有極大應(yīng)用前景的高溫結(jié)構(gòu)材料。經(jīng)過(guò)多年的研究,高鈮鈦鋁合金的制備和加工技術(shù),以及顯微組織和常規(guī)力學(xué)性能的控制技術(shù)已經(jīng)基本被掌握,但對(duì)于其高溫疲勞性能以及安全壽命設(shè)計(jì)方面的研究還比較欠缺。本文利用SEM原位觀察方法研究了高鈮TiAl合金的高溫疲勞性能中的一些關(guān)鍵問題,主要包括高鈮TiAl合金的循環(huán)硬化/軟化特征及其微觀機(jī)制；疲勞裂紋萌生及擴(kuò)展規(guī)律：顯微組織和微觀缺陷對(duì)其疲勞裂紋擴(kuò)展速率及疲勞壽命的影響；疲勞壽命預(yù)測(cè)等。主要的結(jié)論與新的成果如下： 近片層組織高鈮TiAl合金在750℃時(shí)的拉仲過(guò)程中表現(xiàn)出一定塑性和應(yīng)變硬化特征,拉伸過(guò)程中裂紋的擴(kuò)展、片層的扭轉(zhuǎn)以及位錯(cuò)的滑移是合金高溫塑性的主要來(lái)源,而拉伸過(guò)程中發(fā)生的位錯(cuò)釘扎、纏結(jié)與塞積是合金高溫拉伸過(guò)程中表現(xiàn)出一定的硬化特征的主要原因。透射分析表明合金在750℃時(shí)拉伸變形的主要機(jī)制是位錯(cuò)的滑移。 近片層組織高鈮TiAl合金在750℃循環(huán)變形時(shí)顯示出明顯的循環(huán)硬化特征；在循環(huán)變形過(guò)程中高鈮TiAl合金呈現(xiàn)出先硬化后穩(wěn)定的特征。結(jié)合透射觀察結(jié)果分析得出,高鈮TiAl合金在高溫循環(huán)變形中先以產(chǎn)生孿晶的方式進(jìn)行變形,而后以位錯(cuò)攀移的形式進(jìn)行變形。由于循環(huán)初期出現(xiàn)的變形孿晶與位錯(cuò)發(fā)生交互作用,從而導(dǎo)致合金在循環(huán)初期發(fā)生循環(huán)硬化。 750℃下近片層組織高鈮TiAl合金的疲勞裂紋萌生表現(xiàn)出多裂紋萌生特征。依據(jù)裂紋尖端微觀組織的不同,疲勞裂紋的擴(kuò)展過(guò)程呈現(xiàn)出兩或三個(gè)不同擴(kuò)展階段：當(dāng)疲勞裂紋平行于片層條帶,擴(kuò)展過(guò)程依次為快速擴(kuò)展、穩(wěn)定擴(kuò)展和加速擴(kuò)展三個(gè)階段；而當(dāng)疲勞裂紋垂直于片層條帶,擴(kuò)展過(guò)程則依次呈現(xiàn)為穩(wěn)定擴(kuò)展和加速擴(kuò)展兩個(gè)階段。 高鈮TiAl合金在高溫下具有良好的疲勞裂紋擴(kuò)展抗力。對(duì)十高鈮TiAl合金,高溫使合金的片層團(tuán)界面及片層間界而的強(qiáng)度降低,使得合金在高溫下的疲勞過(guò)程相比室溫條件下有更多的疲勞裂紋在片層團(tuán)界面與片層間界而萌生,于是高溫下也就相應(yīng)地有更多的裂紋分支、偏轉(zhuǎn)及橋接現(xiàn)象發(fā)生；同時(shí),也正是由于高溫下有更多的裂紋分支、偏轉(zhuǎn)及橋接發(fā)生,使得高鈮TiAl合金在高溫下的疲勞裂紋擴(kuò)展抗力遠(yuǎn)遠(yuǎn)高于室溫條件下的。 高溫下全片層組織高鈮TiAl合金比近片層組織高鈮TiAl合金具有更高的疲勞裂紋萌生抗力和斷裂韌性。高溫下,全片層組織高鈮TiAl合金疲勞斷裂以沿片層及穿片層為主；近片層組織高鈮TiAl合金疲勞斷裂主要集中在γ晶粒處,較多的裂紋沿Y晶粒邊界萌生和擴(kuò)展,隨著裂紋尺寸的增大,裂紋以沿片層、穿片層、沿γ晶及穿γ晶的混合方式進(jìn)行擴(kuò)展。 借助SEM原位觀察方法對(duì)高鈮TiAl合金的疲勞裂紋擴(kuò)展門檻值進(jìn)行了測(cè)定。經(jīng)測(cè)定,近片層組織Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金在應(yīng)力比P＝0.1的條件下,室溫與750℃下的疲勞裂紋擴(kuò)展門檻值分別為12.89MPa-m1/2與8.69MPa-m1/2。 高鈮TiAl合金在高溫下的疲勞裂紋萌生壽命Ni與疲勞總壽命Nf的比值(N/Nf)是恒定的。對(duì)于帶缺口試樣,其疲勞裂紋萌生壽命在整個(gè)疲勞斷裂壽命中所占的比例高達(dá)95%；對(duì)于無(wú)缺口試樣,疲勞裂紋萌生壽命占整個(gè)疲勞斷裂壽命的比例為43%。 根據(jù)Lemaitre的損傷力學(xué)模型提出高鈮TiAl合金疲勞裂紋萌生壽命和斷裂壽命預(yù)測(cè)公式,近片層組織高鈮TiAl合金在750℃下的疲勞裂紋萌生壽命和斷裂壽命預(yù)測(cè)公式分別為和。
[Abstract]:TiAl alloy with high Nb content, high temperature structural material has great application prospect because of its excellent mechanical properties at high temperature and low density is regarded as the aerospace and automobile engine parts. After years of research, high Nb Aluminum Alloy preparation and processing technology, and control technology and mechanical properties microstructure has been basically mastered, but for the study of high temperature fatigue performance and safe life design is still relatively lacking. Some key problems of high temperature fatigue properties by SEM in situ observation method of high Nb TiAl alloy was studied. The cyclic hardening / softening including high Nb TiAl alloy and its microscopic mechanism and propagation characteristics; fatigue crack initiation: effect of microstructure and micro defect propagation rate and the fatigue life of fatigue crack; fatigue life prediction. The main conclusions and new The results are as follows:
Show some plasticity and strain hardening characteristics of nearly lamellar microstructure of high Nb TiAl alloy at 750 DEG C when the tensile process of crack propagation during tensile test, lamellar torsion and dislocation slip is the main source of high temperature plasticity, and the dislocation occurred during stretching pinning, entanglement and plug the main reason is to show some characteristics of the hardening tensile process. TEM analysis showed that the main mechanism of tensile deformation in the alloy at 750 DEG is dislocation.
Nearly lamellar microstructure of high Nb TiAl alloy exhibits cyclic hardening characteristic at 750 DEG C during cyclic deformation; cyclic deformation in high Nb TiAl alloy in the process of showing the first hardening characteristics of stability. After combining the transmission observation results obtained, the high Nb TiAl alloy in high temperature cyclic deformation to twinning mode deformation then, the dislocation climb in the form of deformation. As the deformation twinning and dislocation loops emerged in the initial interaction occurs, resulting in alloy cyclic hardening occurred in the initial cycles.
Initiation showed fatigue crack initiation characteristics of more than 750 DEG C nearly lamellar microstructure of high Nb TiAl alloy. On the basis of crack tip microstructure, fatigue crack propagation process shows two or three different growth stages: when the fatigue crack parallel to the sheet strip, followed by the rapid expansion of the expansion process, stable growth and accelerate the expansion of three stages; and when the fatigue crack perpendicular to the lamellar strip, the expansion process are presented for stable growth and accelerate the expansion of the two stages.
High Nb TiAl alloy has good fatigue crack growth resistance at high temperature. The ten high Nb TiAl alloy, high temperature alloy to reduce lamellar interface and lamellar boundaries and the strength of the alloy at high temperature compared with the fatigue process under the condition of room temperature fatigue crack more in lamellar interface and tablets layer boundaries and the initiation, so high temperature corresponding to the crack branch more, deflection and bridging phenomenon; at the same time, it is because there are more high temperature crack branching, deflection and bridging occurs, the high Nb TiAl alloy fatigue crack growth resistance at high temperature is much higher than that at room temperature..
High temperature lamellar microstructure of high Nb TiAl alloy has the fracture toughness and fatigue crack initiation resistance higher than the nearly lamellar microstructure of high Nb TiAl alloy at high temperature, lamellar microstructure of high Nb TiAl alloy in fatigue fracture along the layers and wear layers; nearly lamellar microstructure of high Nb TiAl alloy fatigue fracture mainly in the austenite grain, more cracks along the grain boundary of Y initiation and propagation, with the increase of crack size, crack along the sheet layer, wear layer, extending along the Y and Y in mixed mode.
With the help of the fatigue crack of high Nb TiAl alloy SEM method in situ observation of growth threshold was determined. After the test, nearly lamellar microstructure of Ti-45Al-8Nb-0.2W-0.2B-0.1Y alloy in the stress ratio of P = 0.1 under the condition of fatigue crack growth at room temperature and 750 DEG C and the threshold value of 12.89MPa-m1/2 and 8.69MPa-m1/2. respectively.
The ratio of the initiation life of Ni and Nf of the total fatigue life of fatigue crack under high temperature high Nb TiAl alloy (N/Nf) is constant. The band notched specimens, the initiation life in the whole fatigue life of up to 95% the proportion of the fatigue crack; for unnotched specimens, fatigue crack initiation life total fatigue fracture the life of the ratio of 43%.
According to the damage mechanics model of Lemaitre, the prediction formula for fatigue crack initiation life and fracture life of high Nb TiAl alloy is put forward. The prediction formula of fatigue crack initiation life and fracture life of near niobium TiAl alloy at 750 temperature is respectively.

【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2015
【分類號(hào)】：TG146.23

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