發(fā)布時(shí)間：2018-03-31 12:29

  本文選題：高鈮TiAl合金　切入點(diǎn)：蠕變　出處：《北京科技大學(xué)》2016年博士論文

【摘要】：高鈮TiAl合金以其優(yōu)異的高溫力學(xué)性能和較低的密度在航空、航天以及汽車發(fā)動(dòng)機(jī)等領(lǐng)域顯示了巨大的發(fā)展?jié)摿?是Ni基高溫合金潛在的替代材料。目前,高鈮TiAl合金已被列為我國(guó)重點(diǎn)發(fā)展的航空發(fā)動(dòng)機(jī)材料之一,受到了國(guó)家“973”及軍工“863”項(xiàng)目的資助,其在成分設(shè)計(jì)、組織控制、制備成型以及加工焊接等方面取得了一系列進(jìn)展,但在性能表征以及可靠性評(píng)估方面的研究還不夠充分,尤其是在高溫服役條件下,由疲勞和蠕變交互作用引起的損傷失效則更缺乏深入系統(tǒng)的研究,嚴(yán)重影響了該合金的進(jìn)一步應(yīng)用和設(shè)計(jì)開(kāi)發(fā)�；�于上述研究背景,本文圍繞近片層組織高鈮TiAl合金的高溫疲勞—蠕變交互作用對(duì)其相關(guān)的高溫力學(xué)性能展開(kāi)了研究。具體內(nèi)容包括高溫拉伸及斷裂韌性的研究、高溫蠕變性能的研究、高溫疲勞性能的研究以及高溫疲勞—蠕變交互作用的研究。主要結(jié)論如下：高鈮TiAl合金的高溫拉伸性能和斷裂韌性受顯微組織和裂紋萌生及擴(kuò)展行為的影響。SEM原位觀察及斷口觀察表明,近片層組織的高鈮TiAl合金在拉伸過(guò)程中裂紋主要在片層團(tuán)界處萌生并且沿著片層團(tuán)界擴(kuò)展,相反,全片層組織在拉伸過(guò)程中裂紋主要在片層界面處萌生并且沿著片層界面擴(kuò)展。由于裂紋沿晶界萌生和擴(kuò)展降低了局部的應(yīng)力集中,因此近片層組織的拉伸性能優(yōu)于全片層組織。而由于裂紋沿晶界擴(kuò)展的阻力小于裂紋沿片層界面或穿片層界面擴(kuò)展的阻力,因此表現(xiàn)出近片層組織的斷裂韌性低于全片層組織的特點(diǎn)。高鈮TiAl合金的高溫蠕變性能研究結(jié)果表明,隨著溫度或蠕變應(yīng)力的增加,其最小蠕變速率(εmin)增加,蠕變壽命(Tr)降低。其蠕變的壽命預(yù)測(cè)公式為：logTr(h)+0.94×logεmin(%/h)=0.07SEM原位觀察表明,其蠕變變形的三階段與裂紋的萌生、擴(kuò)展及相互連接相互對(duì)應(yīng)。在穩(wěn)態(tài)蠕變階段主要表現(xiàn)為裂紋的萌生和擴(kuò)展,而在加速擴(kuò)展階段則主要表現(xiàn)為裂紋的相互連接。微觀機(jī)制分析表明,對(duì)應(yīng)于不同的應(yīng)力水平,其蠕變變形機(jī)制不同：低應(yīng)力區(qū)為晶格擴(kuò)散,中等應(yīng)力區(qū)為位錯(cuò)滑移、高應(yīng)力區(qū)為孿晶變形。高鈮TiAl合金在高溫疲勞變形時(shí),應(yīng)力比(R)對(duì)其疲勞壽命及變形機(jī)制有顯著的影響。當(dāng)0.1≤R≤0.4時(shí),疲勞壽命(NT)受疲勞—蠕變交互作用控制,表現(xiàn)為極小值特征,其壽命預(yù)測(cè)公式為其中,σa為循環(huán)應(yīng)力幅,σm為平均應(yīng)力。當(dāng)0.4≤R≤1時(shí),疲勞壽命(Nf)由蠕變變形控制,并且隨R增加N減小。其相應(yīng)的壽命預(yù)測(cè)公式為：Nf=1.17×1020σm-5.46SEM原位觀察表明,隨著R的增加,疲勞斷裂方式由R=0.1時(shí)的穿晶開(kāi)裂轉(zhuǎn)變?yōu)镽=0.2和0.3時(shí)的穿晶和沿晶混合開(kāi)裂,再到R≥0.4時(shí)的沿晶開(kāi)裂。相應(yīng)地,微觀機(jī)制分析表明,疲勞變形機(jī)制由位錯(cuò)滑移和位錯(cuò)攀移轉(zhuǎn)變?yōu)槲诲e(cuò)滑移和孿晶變形,再轉(zhuǎn)變?yōu)閷\晶變形。并且,加載頻率對(duì)其疲勞性能也有一定的影響作用。隨著加載頻率(D的降低,疲勞斷裂方式由f=10 Hz和1 Hz時(shí)的穿晶開(kāi)裂轉(zhuǎn)變?yōu)閒=0.05 Hz和0.025 Hz時(shí)的沿晶和穿晶開(kāi)裂；相應(yīng)地,疲勞變形機(jī)制由位錯(cuò)滑移和位錯(cuò)攀移轉(zhuǎn)變?yōu)閷\晶變形和位錯(cuò)滑移。不同加載頻率下的疲勞壽命公式為：Nf=118887.96(f)1.01高鈮TiAl合金疲勞—蠕變交互作用研究表明,隨著有效保載時(shí)間(△t/tp)的增加,其壽命(Nf)呈線性降低。其相應(yīng)的壽命預(yù)測(cè)公式為：Nf=N10-Ktp/ΔtSEM原位觀察表明,隨著有效保載時(shí)間的增加,裂紋在片層團(tuán)界面處的萌生幾率明顯增大,并且其裂紋擴(kuò)展方式與純疲勞和純?nèi)渥冏冃螘r(shí)的裂紋擴(kuò)展方式顯著不同,表現(xiàn)為混合的裂紋擴(kuò)展特征。這種混合的裂紋擴(kuò)展特征加速了裂紋的擴(kuò)展速率,導(dǎo)致其壽命急劇下降。微觀機(jī)制分析表明,位錯(cuò)滑移和孿晶變形共存是其疲勞—蠕變交互作用的典型特征。
[Abstract]:High Nb TiAl alloy with excellent mechanical properties at high temperature and low density in aviation, aerospace and automotive engine fields show a great potential for development, is the substitute of Ni based high temperature alloy potential. At present, the high Nb TiAl alloy has been listed as one of our focus on the development of aero engine materials by the national "973" and "863" military funded project, the organization control in the composition design, preparation, molding and welding processing has made a series of progress, but the study of characterization and reliability evaluation is not enough, especially the service conditions at high temperatures, caused by the action of fatigue and creep interaction the damage is more a lack of systematic research, has seriously affected the further application of the alloy design and development. Based on the above research background, this paper focuses on the high temperature nearly lamellar microstructure of high Nb TiAl alloy. High temperature fatigue creep interaction on the mechanical properties of the studied. The specific contents include research on high temperature tensile and fracture toughness, creep property research, study of high temperature fatigue properties and fatigue creep interaction. The main conclusions are as follows: the high temperature tensile properties and fracture toughness of high Nb TiAl alloy the microstructure and crack initiation and propagation behavior of the influence that.SEM in situ observation and fracture observation, the crack of high Nb TiAl alloy near lamellar structure during stretching mainly in the lamellar boundary of initiation and expand, lamellar along circles instead, lamellar microstructure under tensile deformation mainly in the lamellar interface the initiation and propagation along lamellar interface. The crack initiation and propagation along grain boundaries reduces the local stress concentration, so nearly lamellar microstructure tensile performance is better than that of FL group Fabric. Because the cracks propagate along the grain boundaries of the resistance is less than crack along lamellar interface or lamellar interface extended wear resistance, thus showing near the fracture toughness of microstructure than lamellar microstructure characteristics. The creep properties of high Nb TiAl alloy shows that with the increase of temperature and creep stress, the minimum creep rate (min) increased, the creep life (Tr) decreased. The creep life prediction formula: logTr (H) +0.94 * log e min (%/h) =0.07SEM in situ observation shows that the three stage and the crack initiation of creep deformation, expansion and mutual connection mutual corresponding in steady-state creep stage. Mainly for the initiation and propagation of cracks in the accelerated expansion stage is mainly connected to crack. The microscopic mechanism analysis showed that correspond to different levels of stress, the creep deformation mechanism is different: the low stress region for lattice diffusion, medium Stress is dislocation slip, high stress area is twin deformation. The high Nb TiAl alloy in high temperature fatigue deformation, stress ratio (R) has a significant effect on the deformation mechanism and its fatigue life. When R = 0.1 ~ 0.4, the fatigue life (NT) under the control of fatigue creep interaction. For the minimum feature, the life prediction formula for the sigma a cyclic stress amplitude, sigma m is the average stress. When R = 0.4 ~ 1, the fatigue life (Nf) controlled by creep deformation, and with the increase of R N decreases. The corresponding fatigue life prediction formula: Nf=1.17 * 1020 m-5.46SEM in situ observation showed that with the increase of R, the fatigue fracture mode by R=0.1 transgranular cracking into R=0.2 and 0.3 of transgranular and intergranular cracking mixture, and then to R more than 0.4 of the intergranular cracking. Accordingly, the micro mechanism analysis showed that the mechanism of fatigue deformation by dislocation slip and dislocation transfer to climb dislocation slip and twinning And then to the deformation, deformation twinning and loading frequency have certain effect on the fatigue performance. With the loading frequency (D decreased by f=10 Hz and fatigue fracture mode 1 Hz transgranular cracking into crystal cracking; f=0.05 Hz and 0.025 Hz in intergranular and accordingly. The mechanism of fatigue deformation by dislocation slip and dislocation climb into the fatigue life formula of deformation twinning and dislocation slip. Under different loading frequencies were as follows: Nf=118887.96 (f) on the interaction of fatigue and creep - 1.01 high Nb TiAl alloy shows that, with the effective holding time (t/tp) increased linearly with its life (Nf) reduced. The corresponding fatigue life prediction equation: Nf=N10-Ktp/ tSEM in situ observation showed that with the increase of effective holding time, crack initiation at the lamellar interface rate increases obviously, and the crack and deformation of pure fatigue and pure creep When the crack propagation of different performance for mixed crack propagation characteristics. This kind of mixed crack propagation characteristics of accelerated crack propagation rate, resulting in a sharp decline in life expectancy. The microscopic mechanism analysis showed that the dislocation slip and deformation twinning is a typical feature of the coexistence of fatigue creep interaction.

【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】：2016
【分類號(hào)】：TG146.2

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