Analysis of high temperature deformation behavior of a high Nb containing TiAl based alloy

The effects of deformation temperature and strain rate on the hot deformation behaviors of as-cast Ti-45Al-8.5Nb-(W,B,Y) alloy were investigated. The results indicated that when deformation temperature is below 1250 °C, the flow stress decreases with the increase of deformation temperature and decrease of strain rate, once deformation temperature reaches 1250 °C, the flow stress is not sensitive to strain rate any more. A neural network model was established to predict the flow stress of this high Nb containing TiAl based alloy during hot deformation. The predicted flow stress curves are in good agreement with experimental results.     

Effect of stress ratio on fatigue crack growth in TiAl intermetallics at room and elevated temperatures

The fatigue crack growth behavior of γ-based titanium aluminides (TiAl) with a fine duplex structure and lamellar structure has been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. For the duplex structured material the fatigue crack growth rates are dominated by the maximum stress intensity, particularly at 750°C. The threshold stress intensity range for fatigue crack growth at 750°C is lower than that at room temperature for any corresponding stress ratio. The fatigue crack growth rate at 750°C is affected by creep deformation in front of the crack tip. The severe crack blunting occurs when the stress ratio is 0.5. For the lamellar structured material the scatter of fatigue crack growth data is very large. Small cracks propagate at the stress intensity range below the threshold for long fatigue crack growth. The effects of microstructure on fatigue crack growth are discussed.     

Microstructure degradation in high temperature fatigue of TiAl alloy

Low cycle fatigue properties of lamellar TiAl with 8 at.% Nb were studied at four temperatures: room temperature, 700, 750 and 800 °C. Up to 750 °C, stable cyclic behaviour is observed while cyclic softening is characteristic for 800 °C. The strength of the alloy is still high even at 800 °C. The TEM observation did not reveal any substantial changes in the microstructure due to the cycling at RT. At 750 °C, the lamellar structure was in some places destroyed by cyclic plastic straining and pure γ-phase islands with high density of dislocation debris were formed. At 800 °C, the domains without lamellar structure cover about 10% of volume and are almost dislocation free. The destruction of lamellar microstructure and possible annealing of dislocation debris is the reason for marked cyclic softening at 800 °C.     

高Nb-TiAl合金的凝固组织与力学性能研究

研究了Al含量变化对高Nb-TiAl合金的凝固组织与力学性能的影响.结果表明:随着Al含量的增加,TiAl合金晶粒尺寸呈增加趋势;当Al含量为45.7%时,凝固过程中局部区域发生包晶转变,使晶粒尺寸显著增大;室温及700℃高温拉伸强度随着Al含量的增加而呈增加的趋势,但发生包晶转变致使室温及700℃高温拉伸强度下降约200MPa;Al含量对延伸率不敏感,持久性能随Al含量的增加呈增加趋势.为控制铸锭凝固后的组织与力学性能,尽量避开包晶转变区,合金中Al含量应低于45.7%.     

Cyclic deformation and lattice strain distribution of high Nb containing TiAl alloy

Low cycle fatigue of lamellar TiAl with 8.5 at.-%Nb was studied with a total strain amplitude of 0.28% at three temperatures: room temperature, 750°C and 900°C. At room temperature, the material exhibited cyclic hardening and the fracture mode was mainly interlamellar. At 750°C and 900°C, the material showed cyclic softening and the fracture mode was translamellar. The lattice strain in γ phase was almost tensile and larger tensile lattice strain in γ phase seems detrimental. Besides, the opposite direction of {201}_γ and {100}_α2 lead to crack propagation along α₂/γ interfaces. B2/β_o phase always suffered compressive lattice strain in the tests. The destruction of lamellar microstructure was the reason for colony refinement at 750°C and 900°C.     

Effect of shot peening on fretting fatigue of Ti811 alloy at elevated temperature

The effects of compressive residual stress, surface roughness, microstructure hardening induced by shot peening (SP) on the fretting fatigue (FF) resistance of Ti811 titanium alloy at elevated temperature were evaluated. The results show that SP improves the FF resistance of Ti811 alloy at 350 °C, but decreases the resistance at 500 °C. Compressive residual stress is the predominant factor in improving the FF resistance of the alloy at 350 °C. Compressive residual stress induced by SP arrests crack growth. Microstructure hardening due to SP has a minor effect on FF resistance. Surface roughening induced by SP is detrimental to the FF resistance of the alloy at both 350 and 500 °C, as microcracks initiate easily in the roughened surface under these conditions.     

工程化高Nb钛铝合金锻造性研究

通过热模拟实验对高Nb-TiAl的锻造工艺参数进行探索,并根据该参数对工程化规模的铸锭进行了成功的锻造.同时通过金相显微镜和扫描电镜对材料锻造前、后的组织变化进行了分析.结果表明高Nb-TiAl合金有较好的锻造性能,变形抗力随变形温度的升高和应变速率的降低而降低.在α γ两相区对高径比为1.10～2.13的铸锭进行包套锻造,可以得到形状规则、无裂纹的饼材.锻造后饼材的大部分组织为几乎完全消除了原来片层团的细小的双态组织,而近表面区为残余铸造片层和再结晶晶粒的混合组织.     

Dissolution of TiAl alloy during high temperature brazing

An analytical model has been established to evaluate the thickness of the dissolution layer of base metal in a liquid brazing filler metal during a high temperature brazing process. The model was validated by a study of brazing of a TiAl alloy to a 42CrMo steel. Peak brazing temperatures were within a range of 1103–1203 K. The margin of deviation between predicted and experimentally determined dissolution thicknesses is within the range of 1–14 %, thus supporting the validity of the model well.     

加工方式对AZ31变形镁合金热拉伸流变应力的影响

针对不同方法制备的AZ31镁合金薄板,利用热拉伸试验机和金相显微镜对其在不同温度和变形速率下的流变应力进行了实验研究.结果表明:挤压、交叉、热轧和冷轧等方法制备的AZ31镁合金薄板的应力-应变曲线基本特征是相同的.峰值流变应力随变形温度的升高和应变速率的降低而降低,在低温时具有明显的厚度效应;当温度大于350℃时峰值流变应力几乎不随板材厚度变化而变化;应变速率小于1.0×10-2s-1,变形温度大于150℃下所有AZ31薄板的延伸率均δ≥45%;单向轧制薄板的各向异性随温度提高减小.     

TiAl基合金的高温压缩性能

利用Instron1185测量TiAl基合金在高温下的力学性能指标,得到其屈服应力随温度变化的关系曲线,并讨论其在高温下的力学特性。     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

Near-threshold fatigue crack growth at room temperature and an elevated temperature in Al---Li alloy 8090

The effects of slip distribution and crack tip shielding mechanisms on the near-threshold fatigue crack growth of the Al---Li alloy 8090 have been studied at both room temperature and an elevated temperature. The slip distribution has been varied by changing the distribution of the S phase, through prior stretching or by means of a duplex heat treatment. Fatigue crack growth (FCG) tests were conducted at a high stress ratio to reduce possible effects due to crack closure.

At room temperature the changes in FCG rates are interpreted as arising from the changes in the degree of planarity of slip in the materials.

At 150°C, the microstructural changes due to the long exposure to elevated temperature appear to dominate the effects observed. At lower ΔK, where the time at temperature is greatest, lower ΔK thresholds than those found at room temperature are obtained. These have been attributed to increased slip homogenization due to the increased precipitation and coarsening of the incoherent S phase together with loss of toughness due to the growth of coarse grain boundary phases and the formation of the associated δ′ precipitate free zone.

At higher ΔK, where the time at temperature is low and microstructural changes are minimal, slower FCG rates than those found at room temperature are obtaine. These are explained in terms of increased crack tip shielding which arises because of the increase in tortuosity of the crack path, the increased slip homogenization and the climb and cross-slip within the crack tip plastic zone.     

The effect of hold-time on fatigue crack growth behaviors of WASPALOY alloy at elevated temperature

The effect of hold-time on fatigue crack growth behaviors of WASPALOY alloy was investigated. It was found that the role of hold-time depends on the competition between the harmful environmental effect and the beneficial effect of creep. If temperature is not higher than 705 °C, fatigue crack growth rate of WASPALOY alloy increases with hold-time. On the contrary, hold-time plays a beneficial role on steady state fatigue crack growth of WASPALOY alloy at 760 °C and lower stress intensity factor. The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time. However, accumulated creep damages cause to cavity nucleation and growth at the grain boundaries, and then accelerate fatigue crack growth. Hold-time plays a harmful role during the final stage of fatigue crack growth.     

Ti-45Al-10Nb合金的高温氧化行为

研究了Ti—45A1—10Nb(原子分数，％)合金在800-960℃在氧气和空气中的氧化行为．结果表明，该合金具有较好的高温抗氧化性能，其氧化增重速率与铁基耐热不锈钢相似或略优．与Ti—50A1合金相反，Ti-45A1-10Nb合金在空气中的氧化增重速率明显低于在纯氧中的氧化增重速率．X射线衍射与能谱分析表明，Ti-45Al一10Nb合金在氧气中的氧化产物主要有TiO2和Al2O3，但在空气中的氧化产物中有TiN相．这是降低氧化速率的主要原因，同时，合金元素Nb稳定了氧化层中的TiN相，因而提高了合金在空气中的抗氧化性能．     

Acoustic emission and lifetime prediction during static fatigue tests on ceramic-matrix-composite at high temperature under air

The present work deals with two types of ceramic matrix composites CMC: SiC_f/[Si–B–C] and C_f/[Si–B–C], loaded in static fatigue at high temperature. An acoustic emission-based technique is proposed to predict the residual fatigue life. Indeed, two approaches based on the analysis of released energy are applied. A coefficient denoted R_AE is evaluated. Moreover, a cumulative Benioff law commonly used for pre-seismic activations is applied. Under constant stress, micro-cracks are created, which generate elastic waves in a manner similar to earthquakes. The law predicted satisfactorily the time-to-failure of SiC_f/[Si–B–C] composite under a constant load.     

Effects of frequency on fatigue crack growth at elevated temperature

The effects of frequency on fatigue crack growth behaviour have been studied in a prealloyed powder material, Udimet 720Li, at 650 °C. Fracture mode and fatigue crack growth behaviour were studied at frequencies ranging from 0.001 to 5 Hz using a balanced triangular waveform. Tests were carried out under constant Δ K control, with load ratio and temperature being held constant. A mechanism map was constructed where predominantly time, mixed and cycle-dependent crack growth behaviour were identified. The results were verified by SEM analyses. Cycle-dependent crack growth data were obtained at room temperature, while fully time-dependent crack growth data were generated under sustained loads at 650 °C.
It was found that mixed time/cycle-dependent behaviour is of most significance for this material at the temperature and frequencies studied. Data for other nickel-based superalloys from various sources in the literature were compiled and compared with those of U720Li alloy at a given stress intensity and temperature in the mixed regime. An analysis was developed to rationalize the observed effect of frequency on fatigue crack growth rate.     

Effect of surface finish on high temperature fatigue of a nickel based super alloy

The influence of residual stresses induced by machining, scratches and different processes of surface improvement, on fatigue resistance has been studied on Waspaloy at 426 °C under load control. Tests were performed on single edge notched samples. Scratches on notched samples exhibited a much larger effect, reducing fatigue life by an order of magnitude. Surface improvement processes, like blending of scratches and shot peening, substantially improved fatigue resistance as compared to notched samples with scratches.SEM analysis shows multiple crack initiation. The information on striation spacing in the given set of surface conditions was used to estimate initiation/stage I period, growth up to 400 μm deep crack and total fatigue life.     

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10 V-2 Fe-3 Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S-N relations of Ti-10 V-2 Fe-3 Al alloy in very high cycle fatigue(VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/βdecohesion, occur when the stress ratio(R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10~7 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from-0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.