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首页> 外文会议>World conference on titanium >Isothermal Low-cycle and Thermo-mechanical Fatigue of a High-strength Multiphase Titanium Aluminide Alloy
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Isothermal Low-cycle and Thermo-mechanical Fatigue of a High-strength Multiphase Titanium Aluminide Alloy

机译：高强度多相铝化钛合金的等温低循环和热机械疲劳

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Most of the anticipated engineering applications of TiAl alloys involve components that are subjected to fluctuating or cyclic loading. This requires that the damage tolerance of the material with respect to intrinsic and service-generated defects be demonstrated. In this study isothermal straincontrolled low cycle fatigue (LCF) tests and thermomechanical fatigue (TMF) experiments were conducted at strain amplitudes of 0. 5. 0. 6 und 0. 7% in a temperature rang from 25 to 850t on a highstrength lamellar TiAl alloy. A very short fatigue life was found under out-of-phase TMF as compared to in-phase TMF and isothermal LCF. The life reduction can be attributed to the generation of positive mean stresses due to cyclic hardening at low temperatures and cyclic softening at high temperatures. Furthermore, out-of-phase tests in vacuum indicated a strong environmental effect of laboratory air leading to oxide scale formation at high temperature (compression) and a premature failure as a consequence of early crack formation in tension (low temperature part of TMF cycle). The micromechanisms controlling LCF and TMF of this alloy were characterized by conventional and highresolution transmission electron microscopy. Samples isothermally fatigued at 25 and 550°C exhibited dense structures of ordinary dislocations and debris that were accumulated in tangles. The dipole defects serve as additional glide obstacles and probably cause the cyclic hardening observed under these conditions. The thermal stability of the dipole defects was assessed by isothermal annealing and in-situ heating experiments performed inside the TEM. Another important fatigue mechanism,preferentially occurring at elevated temperatures,is the stress-induced transformation of an orthorhombic constituent into gamma phase combined with dynamic recrystallization. The transformation advances by a ledge mechanism and apparently provides some toughening. Samples isothermally fatigued at 850°C exhibit a degradation of the lamellar morphology due to this phase transformation combined with dynamic recrystallization.
机译：TiAl合金的大多数预期工程应用都涉及承受波动或循环载荷的组件。这就要求证明材料相对于固有缺陷和服务缺陷所产生的损伤容限。在这项研究中，在高强度层状TiAl上，在25至850t的温度范围内，以0. 5. 0. 6 und 0. 7％的应变幅度进行了等温应变控制的低周疲劳（LCF）试验和热机械疲劳（TMF）实验。合金。与同相TMF和等温LCF相比，异相TMF下的疲劳寿命非常短。寿命的减少可归因于由于低温下的循环硬化和高温下的循环软化而产生的正平均应力。此外，在真空中进行的异相测试表明实验室空气对环境的强烈影响会导致高温（压缩）时形成氧化皮，并且由于张力中的早期裂纹形成而导致过早失效（TMF循环的低温部分） 。通过常规和高分辨率透射电子显微镜表征了控制该合金的LCF和TMF的微机制。在25和550°C下等温疲劳的样品表现出通常错位的致密结构和碎屑，这些碎屑堆积在缠结中。偶极子缺陷会成为额外的滑行障碍，并可能导致在这些条件下观察到的循环硬化。偶极子缺陷的热稳定性通过等温退火和在TEM内部进行的原位加热实验进行评估。另一个重要的疲劳机制（最好在高温下发生）是应力诱导的正交晶成分转变为γ相并伴随动态重结晶。转变通过壁架机制推进，并且显然提供了一定的韧性。由于这种相变和动态重结晶，在850°C下等温疲劳的样品表现出层状形态的退化。

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