Dual fatigue failure modes in Ti–6Al–2Sn–4Zr–6Mo and consequences on probabilistic life prediction

The variability in fatigue life of the Ti–6Al–2Sn–4Zr–6Mo (Ti-6-2-4-6) alloy was investigated. Cumulative life distribution plots were found to be composed of two failure mechanisms. The data could be closely represented by a cumulative distribution function (CDF) resulting from the superposition of the CDFs of the individual mechanisms. An approach for life prediction based on the data due to the worst-case mechanism is suggested.     

Creep modeling for life evaluation and strengthening mechanism of tungsten alloyed 9–12% Cr steels

Recently, high strength tungsten (W) alloyed steels have been developed for use in power plants with higher steam conditions for environmental reasons as well as the improvement of thermal efficiency resulting in lower fuel costs. In order to establish a creep modeling of high strength martensitic steel and to understand the basic role of W in tungsten alloyed 9–12Cr steels, conventional martensitic steels (X20CrMoV121, X20CrMoWV121, and Mod9Cr-1Mo) and tungsten alloyed steels (NF616 and HCM12A) were employed for creep tests and creep behavior analyses by the Ω method. The proposed creep model, which takes into account both primary and tertiary creep, satisfactorily described the creep curves and accurately predicted creep life, as martensitic steel undergoes a relatively large amount of primary creep, up to nearly 30%, over its normal life. The tungsten alloyed steels exhibited a smaller minimum creep rate and a larger stress exponent compared to the conventional steels. In addition, in tungsten alloyed steel, the Ω value features strong stress dependence such that creep life is prolonged at lower stresses due to high Ω values. The importance of the Ω value from the standpoint of creep strengthening in primary and tertiary creep is discussed.     

Repair of NiCrAlYSi overlay coating on Ni3Al base alloy IC6

The effect of repair of NiCrAlYSi coating after long time use on the coherence of coating to substrate and mechanical properties of Ni₃Al base alloy IC6 has been studied. The alloy with NiCrAlYSi coating was stressed under the condition of 900 °C/100 MPa for 200 h to simulate the service condition of IC6 turbine vanes. The results showed that the coherence of base alloy/original coating, original coating/first repair coating, first repair coating/second repair coating or base alloy/second repair coating was firm. The interfaces between them had no cracks and porosity, for either partly or entirely repaired coatings. However, there were more Mo rich particles in the affected zone compared with original coating. Compared with IC6 alloy aged at 900 °C for 200 h, the yield strength at ambient temperature of IC6 alloy with first repair coating was almost equivalent, while the elongation decreased slightly; its stress rupture life under 1100 °C/90 MPa condition was about 100 h. For secondary coating repair, the room temperature tensile properties of base alloy had no obvious change, while stress rupture lives decreased, but still were rather long, compared to the alloy with first repair coating. Therefore, NiCrAlYSi coating repair is feasible to prolong the service lives of IC6 turbine vanes.     

Effects of aging temperature on the precipitation behavior of Ω phase in an Al-Cu-Mg-Ag alloy

Microstructures and mechanical properties of an Al-Cu-Mg-Ag alloy aged for 1 h at temperatures in a range 25 °C to 450 °C were characterized in the present work by means of hardness tests, electrical conductivity measurements, and transmission electron microscopy (TEM). In-situ X-ray diffraction (XRD) was also employed to examine the precipitation behavior of Ω phase in a temperature range of 25 °C to 400 °C The in-situ Xray diffraction peak at 2θ = 26°–28° detected at elevated aging temperatures between 165 °C and 400 °C was attributed to the formation of Ω phase. TEM observations demonstrated the existence of Ω phase in the alloy when aged for 1 h at temperatures in a range 145 °C to 450 °C.     

Creep data analyses of a columnar-grained nickel-base superalloy by conventional and Β-envelope methods

Creep-rupture properties of a columnar-grained nickel-base superalloy have been evaluated over a wide temperature range (1033 to 1311 K) and stress levels (80 to 850 MPa). Creep data analyses based on the conventional approach as well as on a new graphical method—the Β-envelope method (Ref 1)—have been carried out for creep strain and life estimation purposes. The relation between minimum creep rate of the alloy with the applied stress obeys simple power law, whereas the rupture data of the alloy fits well to the Larson-Miller parameter. Also, the Monkman-Grant relation between the minimum creep rate and the rupture life produces a trend with some degree of scatter in the data. The latter relation in its generalized form by the Β-envelope method exhibited the best correlation with significantly reduced scatter in the data.     

Effect of high-temperature annealings and creep on structure and properties of single- and double-layer heat-resistant coatings

The evolution of the structure, phase, and chemical composition in the double-layer refractory heat-resistant plasma coatings and surface layers of ZhS6U alloy [Ni-(8–9)Cr-2,4Ti-5,3Al-(9–10)Co-(1,2-2,4)Mo-10,5W-1,0Nb–0,02B-(0,18–0,2)C] under a coating was investigated during high-temperature annealings and creep. The internal oxidation was observed in ZhS6U alloy under a single-layer coating of Ni-Cr-Al-Y alloy after high-temperature annealings and creep in air. The possibilities of inhibiting internal oxidation by the application of an additional barrier layer for oxygen diffusion were considered. It was shown that ion implantation does not lead to the suppression of internal oxidation in the alloy under a coating and does not significantly affect the long-term strength during creep at 1273 K. An additional ceramic layer delays the internal oxidation of ZhS6U alloy by 50–60 h, which leads to an increase in the time until destruction at a stress of 170 MPa from 100 h for an alloy with a single-layer coating to 120 h. The application of a sublayer of refractory alloys completely suppresses internal oxidation. A refractory alloy with a double-layer coating has the same long-term strength for 100 h both in air and under a vacuum.     

Formation of submicrocrystalline structure in titanium and titanium alloys and their mechanical properties

The effect of deformation temperature on the grain size in titanium VT1-0 and double-phase titanium alloy VT6 is studied. The temperature-and-rate modes of formation of submicrocrystalline structure are determined. The mechanical behavior and the evolution of the microstructure in the course of warm severe deformation is investigated by means of successive compression of specimens over three orthogonal directions at a temperature of 400°C for titanium and 550°C for alloy VT6. The mechanisms of grain refinement in severe deformation are determined. It is shown that they differ depending on the composition of the alloy. The method of “abc”-deformation is used for fabricating large-mass preforms from titanium VT1-0 and alloy VT6 with submicrocrystalline structure; large-size sheets are produced from alloy VT6. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 19–26, February, 2006. Grant of Russian foundation for fundamental research No. 05-08-65396-a.     

Superplastic Behavior of Copper-Modified 5083 Aluminum Alloy

An AA5083 aluminum alloy was modified with two different levels of Cu additions, cast by direct-chill method, and thermo-mechanically processed to sheet gauge. Copper additions reduced sheet grain size, decreased tensile flow stress and significantly increased tensile elongation under most elevated temperature test conditions. The high-Cu (0.8 wt.%) alloy had the finest grain size 5.3 μm, a peak strain-rate sensitivity of 0.6 at a strain-rate of 1 × 10⁻² s⁻¹, and tensile elongation values between 259 and 584% over the temperature range, 400-525 °C, and the strain rate range, 5 × 10⁻⁴ to 1 × 10⁻² s⁻¹, investigated. In biaxial pan forming tests, only the Cu-containing alloys successfully formed pans at the higher strain rate 10⁻² s⁻¹. The high-Cu alloy showed the least die-entry thinning. Comparison of ambient temperature mechanical properties in O-temper state showed the high-Cu alloy to have significantly higher yield strength, ultimate strength, and ductility compared to the base 5083 alloy. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

Study on Fatigue Properties of Turbine Disk Groove Modeling Specimens of GH4720 Alloy

基于航空发动机涡轮盘榫槽结构特点及其工作状态,采用榫槽模拟件对GH4720合金的疲劳失效机理和裂纹扩展寿命进行了实验研究和理论分析。研究结果表明:GH4720合金榫槽模拟件的疲劳失效表现为3个阶段:(i)模拟涡轮盘榫槽处由于较高的应力集中而产生滑移,进而萌生裂纹;(ii)随着应力集中和循环载荷的持续,相邻晶粒间位错开动、发生滑移,裂纹在晶粒间传递;(iii)随着应力强度因子范围增大,剪应力和主应力交互作用、滑移系开动及位错在不同滑移系间的运动,裂纹快速扩展。在实验基础上建立了GH4720合金的疲劳裂纹扩展寿命模型,基于有限元分析的榫槽处的应力和裂纹扩展寿命模型得到的裂纹扩展寿命与实验结果相符,表明该裂纹扩展寿命模型可用于工程中预测涡轮盘的剩余寿命。     

The Fretting Fatigue of Commercial Hard Anodized Aluminum Alloy

An investigation has been carried out in order to study the fretting fatigue behavior of a 2014-T6 aluminum alloy, which has been coated with a commercial hard anodizing of approximately 20-25 μm in thickness. The hardness (HV) was significantly improved up to about 380 after hard anodizing coating while the hardness value of original 2014-T6 was 175. Fretting reduced drastically the fatigue life of samples in both conditions, substrate and coated conditions. The application of such a coating to the substrate may increase the fretting fatigue life in comparison with the uncoated samples in low-stress region for rotating bending fatigue loading while at higher stresses the effect of anodizing is reversed. This may be result from early initiation of cracking of hard anodizing film due to high-stress concentration resulting from bulk stresses. On the other hand, the increase in fretting fatigue life in low-stress region may be probably attributed to low coefficient of friction that prevents metal-to-metal contact, which may result in higher fretting fatigue life because of retardation of crack initiation resulting from lower stress concentration compared to the substrate.     

In-situ neutron diffraction during shape-memory behavior in Fe-Mn-Si-Cr

The internal stress change in the austenite matrix during shape-memory behavior in a ferrous alloy was examined by means of an in-situ neutron diffraction technique. After tensile deformation accompanying the austenite to epsilon martensite transformation, the internal stress is yielded. The averaged internal elastic strain in 〈111〉 oriented austenite grains was measured. The internal stress remained after tensile straining and decreased after subsequent reverse transformation on heating, i.e., shape recovery process. When the tensile straining and subsequent annealing was performed repeatedly, which is a kind of “training,” the measured recoverystrain increased and stress-induced transformation started at a lower applied stress in subsequent cycles. These findings suggest that a key point for improving the shape-memory effect is to increase the ability of the alloy to accumulate oriented internal stress generated by the martensitic transformation. For more information, contact Y. Tomota, Faculty of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki, 316-8511 Japan; +81-294-38-5055; fax +81-294-38-5226; e-mail tomota@ipc.ibaraki.ac.jp.     

High-Temperature Deformation Behavior of Ti-6Al-4V Alloy without and with Hydrogenation Content of 0.27 wt.%

Isothermal compression of Ti-6Al-4V alloy without and with hydrogenation content of 0.27 wt.% was carried out on Gleeble-1500D thermal simulation machine at deformation temperature between 760 and 1000 °C and strain rate from 0.001 to 1 s⁻¹. The experimental results show that hydrogenation can decrease the deformation temperature or increase the strain rate of Ti-6Al-4V alloy. The apparent activation energy was determined to be 667 kJ mol⁻¹ for isothermal compression of the Ti-6Al-4V alloy without hydrogenation content of 0.27 wt.% in the α + β phase region (760-960 °C), and this value was about 655 and 199 kJ mol⁻¹ for the alloy with 0.27 wt.% of hydrogenation content in the α + β phase region (760-840 °C) and β phase region (840-960 °C), respectively. Constitutive equation was developed for the high-temperature deformation of Ti-6Al-4V alloy both without and with hydrogenation content of 0.27 wt.%.     

Hot deformation behaviors of Fe-microalloyed Ti-6Al-4V based on experiments and calculations

Our previous results have shown that comprehensive mechanical properties of titanium alloys can be effectively improved by addition of Fe[1]. We systematically investigate hot deformation behaviors of Ti-6Al-4V-0.35Fe in this study, which is significant to improve plastic deformation ability of titanium alloys. In experiment, we use a Gleeble 3800 thermo-mechanical simulator to obtain the relationship between thermomechanical parameters and flow stress in a range of temperatures (800-950 °C) and strain rates (0.001-10 s-1). The single-peak profiles of the flow curves indicate that dynamic recrystallization (DRX) mechanism dominates the deformation. TEM analysis indicate that the grain size in DRX changes under different deformation temperatures, and finer grains are formed at relatively lower temperature due to the dynamic globularization. The dislocation walls are formed in subgrain boundaries due to dislocation slipping-climbing. The Avrami-type DRX model and the strain compensated multivariable regression model have been applied to fit the experimental stress-strain data during hot deformation. A comparative study between these two types of constitutive models is conducted to represent the flow behavior. It is found that both models have good accuracy in predicting the flow stress of Ti-6Al-4V-0.35Fe alloy. A processing map based on dynamic material model (DMM) at the strain of 0.8 (steady-state flow stage) has been established to identify the flow instability regions and stability regions. The strain rate range of stability region is 0.001-0.6s-1 which has been expanded compared to the range of 0.0003-0.1s-1 of Ti-6Al-4V. Optimal hot working parameters are confirmed to be 920-950 °C and 0.001-0.005 s-1, and nearly complete DRX has taken place. Our results indicate that hot working property of Fe-microalloyed Ti-6Al-4V is better than that of Ti-6Al-4V alloy in 800-950 °C temperature scale, and processing cost has been decreased.     

The fracture toughness and fatigue behavior of DRA

‘In this study, the fracture toughness and elevated-temperature tensile and fatigue behavior of discontinuously reinforced aluminum (DRA) were examined. The effects of heat treatment and specimen thickness on fracture toughness were studied in a 7093/SiC_p composite. The toughness of the DRA was lowest in the peak-aged condition, but increased considerably in the overaged condition. The highest toughness was obtained at a critical value of specimen thickness; this critical value was used to fabricate a laminated composite consisting of alternating layers of DRA and unreinforced alloy. Elevated-temperature tensile and fatigue behaviors were investigated in a 2080-T6/SiC_p composite at different volume fractions and particle sizes. Increasing reinforcement volume fractions resulted in increases in tensile and fatigue strength. Exposure tests for 300 h at 150°C produced no significant reduction in ultimate tensile strength or yield strength, indicating good thermal stability of the 2080 matrix. For more information, contact A.B. Pandey, Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433; (937) 255-1320; fax (937) 255-3007; e-mail awadh.pandey@ml.afrl.af.mil.     

基于灰色系统理论的7075铝合金板材疲劳寿命研究

在航空用飞机蒙皮制造过程中,7075铝合金是十分理想的材料。为了更精确地研究并预测7075铝合金的疲劳寿命,对7075-T-651铝合金板材进行了疲劳试验,在应力比R=0.1条件下,测得120~320 MPa不同应力下7075-T-651铝合金板材的疲劳寿命。以试验数据为基础,将灰色系统理论运用到7075-T-651铝合金板材的疲劳寿命预测中,生成GM(1,1)模型,在与试验相同的条件下进行疲劳寿命预测,并将得到的疲劳寿命预测模型S-N曲线与试验数据S-N曲线进行对比。研究表明,所建立的灰色系统理论GM(1,1)模型S-N曲线与试验所得S-N曲线基本吻合,因此,可以通过GM(1,1)模型来预测7075铝合金板材的疲劳寿命。     

Deformation behavior of TC6 alloy in isothermal forging

Isothermal compression of the TC6 alloy was carried out in a Thermecmaster-Z (Wuhan Iron and Steel Corporation, P.R. China) simulator at deformation temperatures of 800∼1040 °C, strain rates of 0.001∼50.0 s⁻¹, and maximum height reduction of 50%. The deformation behavior of the TC6 alloy in isothermal forging was characterized based on stress-strain behavior and kinetic analysis. The activation energy of deformation obtained in the isothermal forging of the TC6 alloy was 267.49 kJ/mol in the β phase region and 472.76 kJ/mol in the α+β phase region. The processing map was constructed based on the dynamic materials model, and the optimal deformation parameters were obtained. Constitutive equations describing the flow stress as a function of strain rate, strain, and deformation temperature were proposed for the isothermal forging of the TC6 alloy, and a good agreement between the predicted and experimental stress-strain curves was achieved.     

基于CDM法AZ31B镁合金焊接接头疲劳评定

采用临界距离法(CDM)中的点法和线法对AZ31B镁合金对接接头和横向十字接头进行疲劳评定.对两种焊接接头进行了疲劳试验,采用ANSYS有限元软件建模分析,计算出两种评定方法的局部应力参量Δσloc,拟合出相应的S-N曲线,并与疲劳试验结果进行了对比分析.结果表明,采用CDM法可以对镁合金焊接接头进行疲劳评定,其评定结果与疲劳试验结果相符合.依据应力集中大小可以有效预测疲劳断裂位置,在2×106循环次数下,两种焊接接头点法的疲劳强度分别为79.55和49.10MPa,线法的疲劳强度分别为79.01和44.38MPa.     

应变速率条件下第二相对7A52铝合金搅拌摩擦焊焊缝应力腐蚀性能的影响

采用慢应变速率拉伸试验,结合扫描电镜、能谱分析仪研究了第二相对7A52铝合金搅拌摩擦焊(FSW)焊缝应力腐蚀性能的影响.结果表明,FSW焊缝在3.5%NaCl溶液中,应力腐蚀开裂(SCC)均发生在后退侧的热影响区,断裂位置与显微硬度曲线中硬度最低区域相对应.焊缝的SCC敏感性在1×10-6s-1条件下最高,在1×10-5s-1条件下最低,不同第二相对FSW焊缝应力腐蚀裂纹形核和扩展的影响是不同的:Al9Fe0.84Mn2.16Si容易引发裂纹的萌生和扩展,Mg2Si被腐蚀留下的点蚀坑引发的裂纹相对较小.     

含缺口TC21钛合金腐蚀疲劳性能分析

研究了TC21钛合金缺口试样在两种腐蚀环境（油箱积水、3.5%NaCl水溶液）与室温空气环境下的疲劳性能与断裂机理。并与光滑试样在室温空气环境下疲劳性能进行对比。结果表明,室温空气环境下,当两种试样疲劳寿命均达到5×105次循环时,缺口试样的循环应力值较光滑试样下降了52.7%;相同环境下随着应力水平降低,试样疲劳寿命增加;相同应力条件下,3.5% NaCl水溶液环境下试样疲劳寿命最低,油箱积水环境下次之,室温空气中TC21钛合金试样疲劳寿命最高;当应力较低时,差异更为显著。在腐蚀环境下,溶液中离子与金属原子发生电化学反应,加速了裂纹的萌生与扩展,3.5% NaCl水溶液中离子浓度较大,电化学反应更为剧烈     

High Temperature Creep Deformation Mechanisms of a Hot Corrosion-Resistant Nickel-based Superalloy

Creep properties of the experimental superalloy were investigated in the temperature range 1073–1223 K and stress range 110–550 MPa. The observations of dislocation structures during different creep conditions reveal that in the high stress region, particle-shearing mechanisms including stacking fault formation and antiphase boundary creation are operative and in the low stress region, the dislocation climb mechanism is dominant. From the plot of minimum creep rate versus applied stress, a very low stress region with exponent n < 2, which is related to diffusional creep, is found. Based on the experimental results, a stress–temperature creep deformation mechanism map for the alloy is constructed. On the basis of particle hardening theories and various dislocation-creep theories, the dislocation-creep transitions in terms of internal stress are discussed and calculated threshold stresses of various creep deformation mechanisms indicates that the particle shearing is easier to operate than Orowan looping at high stresses, and general climb is easy to happen at low stresses.