高Nb-TiAl合金中有序ω相相变规律的研究

发布时间：2018-04-30 01:06

本文选题：高Nb-TiAl合金 + 有序ω相　；参考：《北京科技大学》2015年博士论文

【摘要】：高Nb-TiAl合金由于其优异的高温强度,高温抗氧化性和蠕变性能,已经成为一种先进的轻质高温结构材料。然而,在高Nb-TiAl合金凝固过程中产生的高温β相经有序化转变为室温脆性相B2相。虽然B2相降低了高Nb-TiAl合金的室温性能,然而近年来的研究发现无序的高温β相可以有效改善合金的热加工性能。另外,B2相在最近发展的Beta-GammaTiAl合金中的体积分数非常高,因此关于B2相的研究显得尤为重要；同时B2相中有序ω相的存在温度区间与高Nb-TiAl合金可能的服役温度区间重合,因此有序ω相的演变规律正逐步成为研究热点。本文主要研究了高Nb-TiAl合金中B2相区域内部有序ω相的相变规律,主要结论和创新点如下： 1、研究了高Nb-TiAl合金铸态组织中有序ω相的演变规律。结果表明,高Nb-TiAl合金铸态组织中的Nb偏析区一般会有有序ω相形成。衍射分析证明有序ω相存在B82-ωo相和ω”相的结构差异。阐明了Nb偏析区在铸锭冷却过程中的成分与相结构的演变机制,尺寸不同的有序ω相颗粒的出现是Nb和W的扩散及ω。相的析出温度高于ω”相造成的。 2、揭示了冷却速度对有序ω相形核长大的影响规律。不同的冷速导致不同结构的有序ω相形成。水冷样品中ω”相形核；空冷样品中ω。相和ω”相共存；而炉冷样品中ω。相长大,其间为ω”和B2相的混合组织。高Nb-TiAl合金中有序ω相的形成难以避免。 3、研究了热处理对有序ω相相变的影响规律。结果表明,中温(850℃)保温时ω。相大量析出,Nb偏析区实际上由ωo+γ相构成,ω。相变体间的界面过渡平滑。阐明了α2相向ω。相的相变机制,并用“边-边”法计算表明了两者只存在两种位向关系,其中[1120]α2//[0001]ωo；(0002)a2//(1120)ωo更稳定,与TEM观察一致。高温(950℃)短时保温即可使有序ωo相快速消失。 4、研究了合金元素对有序ω相的影响规律。结果表明,ω。相在Ti-45Al-(7-10)Nb合金中是中温平衡相。而添加少量合金元素即可有效控制有序ω相的数量和形貌：Mn可以强烈抑制有序ω相的生成；W、Cr的抑制作用稍弱；Ni则强烈促进有序ω相的生成。合金元素通过改变有序ω相的析出温度来控制其形核长大。 5、研究了有序ω相对高Nb-TiAl合金力学性能的影响。结果表明,少量有序ω相对合金力学性能的影响较小。在高温变形中,微米级ω。相难以强化合金；而以纳米级尺寸细小弥散分布的有序ω相可提高合金强度,这可以通过中温短时保温来实现。 6、炼制了Ti-34Al-13Nb合金以研究有序ω相相变规律。此合金在中温处于ωo+α2两相区,a2相由ωo。相中直接析出。α2相变体之间的界面可以自发调整至低能量状态,表现为共格界面或者类似孪晶的界面。 7、研究了B19相的相变规律。B19相在α2→ωo转变中于二者界面处出现,晶格常数为a=0.464nm、b=0.290nm、c=0.510nm。 HRTEM直接观察到B19相可作为α2相到Y相转变的过渡相,与同步辐射报道结果一致。B19相的析出温度在650℃-750℃之间,而高温短时保温即可消失。高Nb-TiAl合金中快冷获得α2相处于非平衡态。
[Abstract]:High Nb-TiAl alloy has become an advanced lightweight high temperature structural material because of its excellent high temperature strength, high temperature oxidation resistance and creep properties. However, the high temperature beta phase produced in the solidification process of high Nb-TiAl alloy is transformed into a brittle phase B2 phase at room temperature. Although B2 phase reduces the room temperature properties of high Nb-TiAl alloy, however, it is close to it. Recent studies have found that the disordered high temperature beta phase can effectively improve the thermal processing properties of the alloy. In addition, the volume fraction of the B2 phase in the recently developed Beta-GammaTiAl alloys is very high. Therefore, the study of the B2 phase is particularly important. At the same time, the existence of the temperature range of the ordered Omega phase in the B2 phase and the possible service temperature zone of the high Nb-TiAl alloy The evolution of ordered Omega phase is becoming a hot spot of research. This paper mainly studies the phase transition of ordered Omega phase in B2 phase in high Nb-TiAl alloy. The main conclusions and innovation points are as follows:
1, the evolution of ordered Omega phase in the cast microstructure of high Nb-TiAl alloy is studied. The results show that the ordered Omega phase in the Nb segregation region in the cast structure of high Nb-TiAl alloy is generally formed. The diffraction analysis shows that the structure difference between the B82- Omega o phase and the Omega phase exists in the ordered Omega phase. The composition and phase structure of the Nb segregation area in the cooling process of the cast ingot are clarified. The evolution mechanism is that the appearance of ordered Omega phase particles with different sizes is caused by the diffusion of Nb and W and the precipitation temperature of Omega phase.
2, the influence of cooling rate on the ordered Omega nucleation growth is revealed. Different cooling rates lead to the formation of ordered Omega phase of different structures. Omega phase nuclei in water cooled samples, Omega phase and Omega phase coexist in air cooled samples, and omega and B2 phase are mixed in cold samples. The ordered Omega phase in high Nb-TiAl alloy It's hard to avoid.
3, the effect of heat treatment on the ordered Omega phase transformation is studied. The results show that the phase of the Nb segregation is substantially precipitated at the medium temperature (850 degrees C). The phase transition between the phase transition and the phase transition is smooth. The phase transition mechanism of the phase transition between the phase transition and the phase transition is clarified, and there are only two potential relations between the two phases, which are calculated by the "side edge" method. Among them, [1120] alpha 2//[0001] Omega o; (0002) a2// (1120) Omega o is more stable, consistent with TEM observation. High temperature (950 degrees) can make the ordered Omega o phase disappear rapidly at short time.
4, the effect of alloying elements on the ordered Omega phase is studied. The results show that the phase in Ti-45Al- (7-10) Nb alloy is a medium temperature equilibrium phase, and the addition of a small amount of alloying elements can effectively control the number and morphology of the ordered Omega phase. Mn can strongly inhibit the formation of ordered Omega phase; W, Cr has a weak inhibition effect; Ni strongly promotes ordered Omega phase. Alloy elements control their nucleation and growth by changing the precipitation temperature of ordered Omega phase.
5, the influence of the mechanical properties of the ordered Omega relative high Nb-TiAl alloy was studied. The results showed that a small amount of ordered Omega has little influence on the mechanical properties of the alloy. In the high temperature deformation, the micrometer Omega is difficult to strengthen the alloy, and the ordered Omega phase with the nanometer size and dispersion can improve the strength of the alloy. This can be achieved through the medium temperature short time heat preservation. Now.
6, Ti-34Al-13Nb alloy is made to study the order of ordered Omega phase phase transition. The alloy is at Omega o+ alpha 2 phase and A2 phase precipitates directly from Omega O. phase. The interface between alpha 2 phase variants can be spontaneously adjusted to low energy state, showing a common interface or a similar twin interface.
7, the phase transition of B19 phase.B19 phase appears at the two interface in the transition of alpha 2 to omega o, the lattice constant is a=0.464nm, b=0.290nm, c=0.510nm. HRTEM can be observed directly as the transition phase of the alpha 2 phase to the Y phase transition, and the crystallization temperature of the.B19 phase is at 650 C -750, while the high temperature short time heat preservation is at the same time as the synchrotron radiation report. It can vanish. The fast cooling of high Nb-TiAl alloy obtains the alpha 2 and gets along with the nonequilibrium state.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.416

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