Hydrogen as a clean fuel is increasingly being used to propel gas turbines and to power combustion engines. Metallic materials including Ni-based alloys are commonly used in conventional gas turbines and combustion engines. However, hydrogen may cause embrittlement in these materials, depending on their chemical composition. In this work, the hydrogen embrittlement behavior of Ni-based alloys containing up to 50 wt.% Fe has been investigated using slow strain rate tensile testing, under cathodic hydrogen charging at room temperature. It was found that the larger the Ni equivalent concentration in an alloy, the more severe the hydrogen embrittlement. It was also found that solid solution alloys have less severe hydrogen embrittlement than precipitation alloys of the same Ni equivalent concentration. In solid solution alloys, hydrogen embrittlement led to cleavage type fracture, which is in line with literature where hydrogen enhanced planar deformation. In precipitation alloys, hydrogen embrittlement resulted in a typical intergranular fracture mode.

In this study, creep rupture behaviors and rupture mechanisms of dissimilar welded joint between Inconel 617B and COST E martensitic steel were investigated. Creep tests were conducted at 600 ℃ in the stress range 140-240 MPa. Scanning electron microscopy (SEM) and micro-hardness were used to examine the creep rupture behaviors and microstructure characteristics of the joint. The results indicated that the rupture positions of crept joints shifted as stress changed. At higher stress level, the rupture position was located in the base metal (BM) of COST E martensitic steel with much plastic deformation and necking. At relatively lower stress level, the rupture positions were located in the fine-grained heat affected zone (FGHAZ) of COST E or at the interface between COST E and WM both identified to be brittle fracture. Rupture in the FGHAZ was caused by type Ⅳ crack due to matrix softening and lack of sufficient precipitates pinning at the grain boundaries (GBs). Rupture at the interface was related to oxide notch forming at the interface.

Small punch creep testing (SPCT) is a small-scale, accelerated creep test that allows for the determination of creep data using a limited amount of material. The question, however, remains how the data generated by this technique correlate to more established techniques such as uniaxial testing and ultimately to predictions regarding the remaining service life of a plant component. This empirical study investigated the microstructure-to-property relationship of welded 9-12%Cr steels as measured using SPCT. Virgin P91 (X10CrMoVNb9-1) steel was joined to service exposed X20 (X20CrMoV12-1) steel using two different filler materials (X20 and P91) via fusion welding. Site-specific samples were extracted from the parent plates, heat affected zones and weld metals using electro-discharge machining. Small punch creep testing were performed using a 276 N load at a temperature of 625°C. The untested sample microstructures were quantitatively characterized using a range of electron microscopy techniques to determine the precipitate (M 23 C 6 , MX) spacing, subgrain sizes and dislocation densities for each region of the weldments. Multiple linear regression analysis found that the subgrain size (λsg) played the largest contribution to the SPCT rupture life. The heat affected zones had the lowest SPCT rupture times (49-68 hours), which corresponded to the largest subgrain sizes (1.1-1.3 μm). The P91 parent plate material had the longest SPCT rupture time (349 hours), which corresponded to the lowest subgrain size (0.8 μm). The P91 weld metal sample showed lower initial deflection rates during the SPC testing, however the presence of non-metallic SiO 2 inclusions in this zone contributed to accelerated brittle failure.

Effects of microstructure constituents of α 2 -Ti 3 Al/γ-TiAl lamellae, β-Ti grains and γ grains, with various volume fractions on room-temperature ductility of γ-TiAl based alloys have been studied. The ductility of the alloys containing β phase of about 20% in volume increases to more than 1% as the volume fraction of γ phase increases to 80%. However, γ single phase alloys show very limited ductility of less than 0.2%. Microstructure analysis have revealed that intragranular fracture along γ/γ grain boundary occurred in γ single phase alloy whereas it does not along β/γ interphase in alloys containing β phase. In addition, local strain accumulations along β/γ interphase have been confirmed. The present results, thus, confirmed the significant contribution of β phase, especially the existence of β/γ interphase to enhancement of the room-temperature ductility in multicomponent TiAl alloys.

High temperature strength of a nickel-based superalloy, Alloy 740H, was investigated to evaluate its applicability to advanced ultrasupercritical (A-USC) power plants. A series of tensile, creep and fatigue tests were performed at 700°C, and the high temperature mechanical properties of Alloy 740H was compared with those of other candidate materials such as Alloy 617 and Alloy 263. Although the effect of the strain rate on the 0.2% proof stress was negligible, the ultimate tensile strength and the rupture elongation significantly decreased with decreasing strain rate, and the transgranular fracture at higher strain rate changed to intergranular fracture at lower strain rate. The time to creep rupture of Alloy 740H was longer than those of Alloy 617 and Alloy 263. The fatigue limit of Alloy 740H was about half of the ultimate tensile strength. Further, Alloy 740H showed greater fatigue strength than Alloy 617 and Alloy 263, especially at low strain range.

The extrapolation of short-term laboratory test results to predict long-term high-temperature component failure remains challenging, particularly for P91 steel due to its phase transformation during extended service and susceptibility to type IV cracking. While the NSW model successfully predicts creep crack growth bounds using short to medium-term test data (<10,000 hours), recent literature suggests materials may exhibit more brittle behavior and reduced failure strain in longer-term tests. This study examines whether the NSW model, using short-term uniaxial data, can effectively predict these long-term behavioral changes for more accurate service life assessment.

HR6W (23Cr-44Ni-7W) is a candidate material for application in the maximum temperature locations of A-USC boilers. In this study the creep rupture properties of plastic deformed, notched, and weldment materials were investigated in comparison with those of solution treated material, in order to clarify the capability of HR6W as a material for A-USC plant application. The deterioration of long term creep rupture strength has been reported with respect to metastable authentic stainless steel due to cold working. However the creep strength of the 20% pre-strained HR6W increased. HR6W creep strength showed notch strengthening behavior. The creep ruptured strength of the GTAW joints was nearly the same as that of the solution treated material, and all specimens fractured within the base metal. The creep ductility of the solution treated materials decreased under low stress conditions. The intergranular fracture is considered to be caused of ductility drop. This tendency is the same as for austenitic stainless steel. The potential of HR6W as a material for A-USC was revealed from the standpoint of creep rupture properties.

Microstructural analyses by FE-SEM and TEM were performed on a ferritic heat-resisting steel that contained 12mass% chromium and 2mass% tungsten to characterize its multi-scale structure, consisting of prior austenite grains, packets, blocks, subgrains and precipitates. The size distributions of the block, subgrains and precipitates were quantitatively evaluated before and after a creep-fatigue test to relate them to their creep-fatigue property. Our results showed that the occupancy of precipitates on prior austenite grain boundaries increased markedly and subgrains became coarse during the creep-fatigue test, while block size did not change. It is suggested that the growth of grain boundary precipitates and coarse subgrains plays an important role in the intergranular fracture mechanism caused by creep-fatigue.

Theoretical and experimental investigations, including fracture tests, acoustic emission (AE) studies, fractography, micro-sclerometric analyses, and spectral/chemical analyses of specimens, have established the possibility of revealing, recognizing in-service acquired, age-related, and prefabricated flaws based solely on AE data. Results show a linear dependence between AE and mechanical deformation power of steel specimens in original and creep stage 3a-3b conditions, decreasing fracture load and J1c value for aging steel, creep processes at stage 3a-3b having J-integral value below 0.05J1c, possibility of assessing and distinguishing different flaw development stages with ≥87% accuracy, revealing zones of tough and brittle fracture, and recognizing inclusions/pre-fabricated flaws and assessing individual/interacting flaws. Experiments confirmed the absence of the Kaiser effect under repeated loading of flawed specimens and demonstrated using AE for defect revelation. Analysis showed that creep-associated AE is mainly continuous, with repeated loading decreasing burst AE contribution during plastic deformation development.