Search

The microstructure, mechanical properties, and high/very‐high cycle fatigue behaviors of 9Cr3W3Co turbine rotor steel and its welded joint were investigated. The welded nugget zone (WNZ) exhibited the highest microhardness value, followed by the heat‐affected zone (HAZ), while the base material (BM) showed the lowest hardness value. The WNZ exhibited a higher yield strength compared to the BM with reduced elongation, while maintaining similar ultimate tensile strength. The fatigue strength of the BM at 5 × 107 cycles is roughly 53.7% of the yield strength, whereas that of the WNZ increases to approximately 55.0%. In comparison to the fatigue strength of the BM at 5 × 107 cycles, the WNZ exhibits a notable increase of 10.5%. Smaller grain size and higher dislocation density were observed in the WNZ, which contributed to improved microhardness, yield strength, and fatigue resistance through grain boundary strengthening and dislocation hardening. Highlights: Microstructure and mechanical properties of 9Cr3W3Co steel welded joints were studied.Microhardness values of various zones are non‐uniform and the maximum value in WNZ.Dislocation and grain boundary strengthening contributed to strengthening in WNZ. [ABSTRACT FROM AUTHOR]

The microstructural evolution and mechanical properties of welded joints for Q450NQR1 and Q450AWR5 alloys were investigated. The inhomogeneous microstructures of Q450NQR1 and Q450AWR5 welded joints can result in heterogeneous mechanical responses. Microhardness distributions of the two welded joints are not uniform with the highest hardness value in the FZ, followed by the HAZ, and the lowest hardness value in the BM. The tensile strength of Q450AWR5 welded joints is slightly higher than the Q450NQR1 welded joints. The tensile fracture occurred in the BM of both welded joints, probably due to the strengthening effect of bainite with high dislocation density and lattice distortion. The fatigue life is dominated by the welded defects inside the welded joint. The location of fatigue failure occurs mainly near the fusion zone and is limited by the welded defects, independent of the microstructure. Highlights: Microstructure and mechanical properties of weathering steel welded joints were studied.Microhardness values of two welded joints are nonuniform and the maximum value in FZ.High dislocation density bainite strengthening contributed to failure occurring in BM.Welded defects are insensitive to tensile behavior but extremely sensitive to fatigue behavior. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 109 cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

• Complete change of crack initiation site. • Quantitative evaluation of grain-boundary retarding effect. • Crack type affects the formation of FGA. Ultrasonic fatigue tests are performed on a magnesium alloy with and without ultrasonic peening treatment (UPT). Surface enhancement layer leads to the complete change of crack initiation sites. However, crack initiation mechanism keeps the same and results in a single-faceted morphology at crack initiation site. Microcracks initiate as Mode II crack within the original grain, but deflect to Mode I crack outside of the original cracked grain. A threshold SIF value is proposed to evaluate the retarding effect of grain boundary on microcrack propagation. Outside of the original cracked grain, Mode I crack propagation below the threshold ΔK σ-th is responsible for the formation of fine granular area (FGA, a nano-grain layer). Based on the Numerous Cyclic Pressing (NCP) model, it is proposed that crack type should be another necessary condition for the formation of FGA. [ABSTRACT FROM AUTHOR]

• Very high cycle fatigue behavior of titanium alloy welded joints under asymmetric loading was investigated. • Mean stress causes the fatigue failure to transfer from the FZ to the BM. • The size of the plastic zone at the crack tip for FGA is related to the lamella α structures in FZ. • The crack non-propagation region and critical safe size of welded pores are obtained. Very high cycle fatigue behavior of electron beam welded TC17 titanium alloy joints under different stress ratios was investigated. The results revealed that the welded joints exhibited different failure modes depending on the stress ratios applied. As the stress ratio increased, the failure probability attributed to welded pores in the fusion zone (FZ) gradually decreased, shifting the failure mode towards fatigue failure induced by the primary α phase in the base metal (BM). Moreover, the stress intensity factor at the edge of fine granular area (FGA) was determined to be the threshold of long crack growth. It was observed that the size of the plastic zone at the crack tip of FGA was related to the acicular α phase structures precipitated through recrystallization in the FZ. The influence of effective size and location of welded pores on fatigue life was further discussed. To gain further insights, improvements were made to the K-T diagram. Consequently, the non-crack propagation zone and the critical safety size of welded pores responsible for initiating cracks in the welded joint were identified. Additionally, the Weibull probability function was employed to determine the average size of welded pores that led to fatigue failure in the welded joints. [ABSTRACT FROM AUTHOR]

• 1. Very high cycle fatigue behavior of LPBF TC4 was investigated at 400℃. • 2. Crack initiation competitions among defects at different locations was analyzed. • 3. Multiple slip systems activated at HT, even those with low Schmid factors. • 4. The formation of FGA is related to grain refinement caused by strain localization. The crack initiation mechanism of laser powder bed fusion (LPBF) Ti6Al4V was investigated at elevated temperature up to very high cycle fatigue (VHCF) regime. The competition concerning defect location is elaborated using the stress intensity factor range and Z-parameter model. Additionally, localized high stress near the defects is responsible for plastic strain localization in the non-prior α' region, following the grain refinement and fragmentation in the larger grains nearby, which is strongly associated with the formation of microcracks and fine granular area. Meanwhile, the process is facilitated by reducing dislocation resistance and activating multiple slip systems due to high temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • Very high cycle fatigue behavior of TC17 with stress ratios was investigated at 400℃. • Crack initiation depends on facets formation and is related to maximum shear stress. • Formation of facets is induced by basal or prismatic slips with high Schmid factor. • GNDs density is accumulated along with basal or prismatic slips, forming sub-GBs. Fatigue behavior of titanium alloy with various stress ratios was investigated at 400℃ in VHCF regime. Cracks initiation depends on the formation of facets and is related to the maximum shear stress. The formation of facets is induced by basal or prismatic slips with the high Schmid factor inside α p grains. The morphology of α p grains will affect the crack initiation. The high-density GNDs are found to be accumulated along with the basal or prismatic slips, then form subgrain boundaries (sub-GBs) inside α p grains. With the cyclic loading, the created sub-GBs will further form facets, thus initiating the microcracks. [ABSTRACT FROM AUTHOR]

Very high cycle fatigue failure of bimodal titanium alloy at 400 °C and high mean stress has been investigated. Many circular creep-fatigue voids (CFVs) were found underneath the crack initiation zone, which were distributed in the β transformed matrix and near the grain boundaries (GBs) of primary α grains. By the energy dispersive spectroscopy (EDS) element mapping, the oxygen element was found to be enriched around the CFVs. In addition, many sub-GBs of small-angle GBs could be observed inside the primary α grains. Based on these, a new mechanism of CFVs and sub-GBs assisted crack initiation and early propagation in bimodal titanium alloys was proposed in very high cycle fatigue regime. [ABSTRACT FROM AUTHOR]

Tensile and fatigue behaviors of a compressor blade titanium alloy TC17 at room and high temperatures (RT and HT) have been investigated up to very high cycle fatigue (VHCF) regime. Compared to RT, the HT tensile strength of TC17 titanium alloy decreases, but the elongation remains basically unchanged at HT. The fatigue S–N curves are changed from single linear mode at RT to bilinear mode at HT owing to the temperature effect, and the failure mode is transformed from the surface and subsurface crack initiations at RT to the surface crack initiation at HT. The brittle oxygen-enriched subsurface (BOES) layer on the specimen surface is responsible for the crack initiation at HT. The failure models of oxide shedding and oxide intrusion are proposed. Dislocation distribution indicates that the fatigue failure at HT is insensitive to the intrinsic material microstructure and is only related to the BOES layer on the specimen surface in the VHCF regime. • Very high cycle fatigue behavior of titanium alloy at room temperature and high temperature was investigated. • The oxide layer on the specimen surface is responsible for the fatigue cracks initiation at high temperature. • The dislocation distribution revealed that the material is insensitive to microstructure at high temperature. [ABSTRACT FROM AUTHOR]

The mechanical behavior of electron beam welded joints for titanium alloy has been investigated in static and cyclic loading up to very high cycle fatigue. It can be concluded that a higher welding voltage parameter and optimal heat treatment condition in combination will produce higher static and cyclic properties of titanium alloy welded joints. The metastable martensite α′ structures in the β grains of the fusion and heat-affected zones are wholly decomposed into small lamellar α phases, forming α + β phases through the post-weld heat treatment (PWHT). Hence, the welded joints by PWHT could transform the "concave" shape microhardness distribution into "convex" shape distribution. The fatigue cracks are initiated from the internal pores of the material in very high cycle fatigue regime. The higher voltage parameter applied to the welding process could reduce the density of pores in the material, which is in favour of the fatigue life of the welded joints. • A higher voltage and optimal heat treatment in combination produce higher mechanical properties of welded joints. • Microstructure and microhardness of welded joints were analyzed before and after heat treatment. • The higher voltage reduced the pores density in the welded joints, which improved fatigue life. [ABSTRACT FROM AUTHOR]

• The very high cycle fatigue strength of titanium alloy fabricated by additive manufacturing is much lower than that of the traditional process. • Combining with the weak zone, Δ K FGA is analyzed, and it is found that the size of the FGA plastic zone was similar to the width of the martensitic laths. • Failure critical dimension is obtained and a new fatigue life relationship is established based on defect characteristics. • Based on probability statistics, the influence of maximum defect size on fatigue behavior is discussed. Tensile and fatigue behaviors of selective laser melted titanium alloy were investigated in the as-built condition. The alloy has a high defect density, and its tensile properties are comparable to those of wrought alloys, but the fatigue strength is significantly reduced. The FGA plastic zone is similar to the martensite laths width. The critical size of fatigue failure is analyzed by the K-T diagram. A new value is obtained based on the defect characteristics. Using the probability statistical, the maximum effective size of the defect considering the shape was calculated, and the competitive relationship of the defects was analyzed by volume density. [ABSTRACT FROM AUTHOR]

• The formation of FGA is attributed to continuous dynamic recrystallization. • The formation of fine grains promotes the extension of microcracks. • A mechanism for VHCF crack initiation in Q450NQR1 weathering steel is proposed. The fatigue behavior of Q450NQR1 high-strength weathering steel under a loading ratio of R = 0.1 was experimentally investigated in the very high cycle fatigue (VHCF) regime, elucidating the formation mechanism of the fine granular area (FGA). The results indicated that Q450NQR1 high-strength weathering steel mainly fails due to non-inclusion-induced crack initiation (NIICI) in VHCF regime. Slip in {1 1 0} and {1 1 2} plane families dominates in ferrite. Intergranular cracking results in fatigue microcracking, which in turn leads to the formation of NIICI in Q450NQR1 high-strength weathering steel. The formation of FGA was attributed to the localized severe plastic strain-induced continuous dynamic recrystallization at the tip of microcrack propagation. Moreover, it can be found that the formation of fine grains promotes the extension and aggregation of microcracks. Based on all experimental findings, a mechanism governing crack initiation in Q450NQR1 high-strength weathering steel during VHCF loading was proposed. [ABSTRACT FROM AUTHOR]

• Microstructure inhomogeneity governs the fatigue crack initiation mechanism. • Fatigue damages prefer to concentrate in the few larger grains due to cyclic slip deformations. • Internal failures present longer fatigue lives than surface failures due to the environment effect. The VHCF behaviors of an extruded magnesium alloy are investigated. Due to the microstructure inhomogeneity, fatigue damages prefer to concentrate in the few larger grains. Basal slip deformations are supposed to be responsible for the transgranular crack initiations, and form the similar faceted morphologies. Under same stress amplitude, internal failures present longer fatigue lives than surface failures due to the requirement of larger initial crack and the deceleration rate in the vacuum-like environment at specimen interior. [ABSTRACT FROM AUTHOR]

Characterizing the transition from a crack-free to a cracked state remains a challenging topic in fatigue. Mg-rare earth alloys, containing the long-period stacking ordered (LPSO) phase, exhibit superior mechanical properties. Here, microcracks are found to nucleate at the soft α-Mg nano-layers, away from the LPSO lamellae. Notably, severe oxidation is observed along the damage bands. Based on the detailed characterizations, it is suggested that dislocation motions continuously bring the new α-Mg matrix to oxidation transformation, resulting in the thickening MgO layer. However, once the fatigue-induced oxide reaches a certain thickness, it starts acting as a barrier for the further dislocation motions. As a result, dislocation accumulation and cumulative damage occur in the region ahead of the thick oxide, causing microcrack nucleation. This fatigue-induced oxidation, assisting microcrack nucleation, is distinct from the existing fatigue mechanisms. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper investigates the high cycle fatigue behavior of industrial pure zirconium at room temperature under a stress ratio of R = 0.1, focusing on the HCP-to-FCC phase transformation and its role in fatigue crack nucleation. The results show the presence of fatigue damage in the form of the HCP-to-FCC phase transformation, which is responsible for the crack nucleation. The fatigue-induced phase transformation is triggered by the activation and slip of partial dislocations. This phase transformation results in lattice expansion, local strain redistribution, and macroscopic strain. Notably, the strain concentration along the HCP/FCC phase interfaces is identified as a critical factor contributing to fatigue crack nucleation. These findings provide new insights into the mechanisms underlying fatigue-induced phase transformations and the nucleation of fatigue crack. [ABSTRACT FROM AUTHOR]

• A sound ("defect-free") Mg-FSW joint is produced. • Unique texture and banded structure of Mg-FSW joint affect crack initiations. • A semi-quantitative mechanical model verifies the different failure mechanisms. Friction stir welding (FSW) produces heterogeneous microstructure in nugget zone of its joint, which should affect the fatigue responses. This work revealed the unique texture and banded structure of the Mg-FSW joint and their decisive roles on crack initiation behaviors. At higher stress levels, failures occur along the AS NZ/TMAZ boundary (AS: advancing side, NZ: nugget zone, TMAZ: thermo-mechanically affected zone) due to the inhomogeneous deformations near the boundary. Severe plastic deformations concentrate in the softer layers of banded structure. However, at lower stress levels, plastic deformations become weak and localized, but accumulate in the alternate layers of banded structure simultaneously due to the same soft texture orientation, and induce random failures in banded structure. The failure mechanisms are verified by semi-quantitative mechanical models. [ABSTRACT FROM AUTHOR]