Quasi-cleavage Fracture Mode Research Articles

Coupling between phase transition and spallation in hierarchically structured high-strength martensitic steels under shock loading

This work presents a comprehensive and in-depth investigation of spall damage behaviors of bainitic and martensitic steels involving phase transitions. Plate-impact experiments, postmortem characterizations, molecular dynamics simulations considering realistic microstructures of bainitic and martensitic steels, and one-dimensional hydrodynamics simulations incorporating phase transition and spall damage are conducted and investigated. Origins of the multiple spall planes and their characteristics within steels are elucidated. Both steels exhibit a quasi-cleavage fracture mode involving phase transitions, with damage morphologies dependent on the microstructures and validated by molecular dynamics simulations. One-dimensional hydrodynamics simulations successfully replicate the dynamic spallation evolutions in steels considering phase transitions.

Thermomechanical fatigue behavior and failure mechanism of a nickel-based directional solidification column crystal superalloy

Thermomechanical fatigue (TMF) is a primary cause of turbine blade failure. TMF tests of a nickel-based directional solidification superalloy, which is a typically used material for turbine blades, are performed out in the surface temperature range of 400 °C–700 °C, two phases (in-phase and out-of-phase, i.e. IP and OP, respectively), three stresses, and two stress ratios. The test results idicate that the TMF life of the OP exceeds that of the IP. An increase in the stress ratio prolongs the TMF life. Scanning electron microscopy and transmission electron microscopy are performed to investigate the failure mechanism. The TMF failure behavior is complex and mainly includes fatigue, creep, and oxidation damage, with a quasi-cleavage fracture mode. Based on the theory of crystal plasticity as well as comprehensively considering the failure mechanism and focusing on the statistical characteristics of the thermal–stress interaction, a TMF constitutive model is established, which explains the differences in TMF behavior between the IP and OP conditions from a theoretical perspective.

Interfacial microstructure, element diffusion, mechanical properties and metallurgical bonding mechanism of 316L-AlSi10Mg multi-material parts fabricated by laser powder bed fusion

This work explored the interfacial microstructure, element diffusion, mechanical properties and metallurgical bonding mechanism of 316L-AlSi10Mg multi-material parts fabricated by laser powder bed fusion (LPBF). Experimental results revealed that insufficient volumetric energy density (VED) of the laser caused lack-fusion porosity in the 316L-AlSi10Mg transition zone, while too high VED produced keyhole-induced porosity defects. Using the optimal process parameters, multi-material parts can be produced with a good interface metallurgical bonding without significant defects. The partial Fe-FCC phase in 316L stainless steel changed into the Fe-BCC structure, and this shift has also changed the preferred orientation of the grains. The intermetallic compound Al5Fe2 and AlFe phases were found in the transition zone. In addition, Al–Fe icosahedral quasicrystals with five-fold symmetry were found at the boundary of the molten pool, which was caused by an extremely high cooling rate. The tensile strength of 316L-AlSi10Mg specimens is higher than that of AlSi10Mg but lower than that of 316L. In contrast to the 316L and AlSi10Mg regions, the fracture mechanism of multi-material fusion zone exhibits a quasi-cleavage fracture mode. The Vickers microhardness of the Al–Fe interface zone was higher than that of 316L with an average value of 235.57 HV0.2 and AlSi10Mg with 124.59 HV0.2, and the interfacial maximum hardness reached 526.68 HV0.2, which was caused by the very hard intermetallic compound Al5Fe2 and AlFe. The metallurgical bonding mechanism of multi-materials was that the dissimilar metals were mixed and in-situ alloyed in the molten pool by the Marangoni convection-induced strong circular flow during LPBF processing.

Effect of bimodal microstructure on the tensile properties and fracture toughness of extruded Mg-9Gd-4Y-0.5Zr alloy

The effects of bimodal microstructure on the tensile properties and fracture toughness of Mg-9Gd-4Y-0.5Zr alloys were investigated. The Mg-9Gd-4Y-0.5Zr alloy was extruded at different temperatures to produce samples with bimodal and fully dynamic recrystallized (DRXed) grain structures. The results show that the bimodal grain structure exhibits a better combination of yield strength (YS of 277 MPa) and plane strain fracture toughness (KIc = 21.5 MPa m1/2), compared to the fully DRXed microstructure (YS of 207 MPa, KIc = 16.3 MPa m1/2). The fracture mechanisms for the bimodal and fully DRXed microstructures are the ductile fracture mode and quasi-cleavage fracture mode, respectively. Both hetero-deformation induced (HDI) strengthening effect and strong basal texture can contribute to high tensile strength in the bimodal grain structured sample. As compared to the sample with fully DRXed microstructure, the stronger strain hardening capacity and larger hardened region around the crack tip in the bimodal grain structured sample can bring about the improvement in the fracture toughness. The tortuous crack propagation path and the hindrance of propagating crack by the un-DRXed grains result in larger fracture resistance and more energy dissipation. The increase of geometrically necessary dislocation (GND) density and the activation of non-basal dislocations also have positive effects on the fracture toughness of the samples with bimodal grain structure. The fracture toughness is effectively improved via intrinsic and extrinsic toughening mechanisms associated with the bimodal microstructure.

Crack initiation mechanisms and life prediction of GH4169 superalloy in the high cycle and very high cycle fatigue regime

In this study, the high cycle and very high cycle fatigue behavior of the GH4169 superalloy was investigated. The results of the fatigue tests demonstrated that failures in the high cycle fatigue regime were initiated from the material surface, whereas failures in the very high cycle fatigue regime had two competing modes of crack initiation: surface and internal. The fatigue fracture analysis revealed a quasi-cleavage fracture mode with facets for internal crack initiation. Additionally, the electron backscatter diffraction (EBSD) results indicated that the dislocations located at the internal twin boundaries of the material were preferentially activated under fatigue cyclic loading, resulting in significant strain localization at the twin boundaries. The {111} <110> slip systems with high Schmid factors had a higher probability of activation. The stress intensity factor (SIF) estimated based on rough area (RA) and facet sizes correspond to the fatigue long crack and short crack growth thresholds of the material, respectively. Finally, a modified FIP fatigue life prediction model was proposed based on the microstructure of crystallographic facets, and the results from experiment and the model prediction were found to be in good agreement.

Mechanical properties and deformation behaviors of Ti-5Al-5Mo-1Fe-1Cr alloy with duplex and single-phase microstructures

To clarify the differences of mechanical properties and deformation behaviors of a Ti−5Al−5Mo−1Fe−1Cr (wt.%) alloy with duplex and single-phase microstructures, the tensile properties and deformation modes of the alloy with these two different microstructures were studied systematically. The results show that the sample with the duplex microstructure has a tensile yield strength of 886 MPa, an ultimate tensile strength of 1075 MPa and an elongation of 21.5%. The sample with the single β phase has a relatively lower strength and a similar elongation. The sample with the duplex microstructure has a ductile fracture mode and deforms via dislocation slipping in the β matrix and spherical or plate-like α phases. The slip systems are {112}β<111>>β in β matrix, and {1122}α <1123>α and {1010}α <1210>α in α phase. In contrast, the single-phase sample has a quasi-cleavage fracture mode and deforms via dislocation slipping and stress- induced α″ martensite transformation, and the slip systems are {110}β <111>β and {112}β <111>β.

Kinetics, stages and fracture modes of stress-corrosion cracking in ZK60 and AZ31 alloys

The kinetics and fractographic features of stress corrosion cracking (SCC) in ZK60 and AZ31 alloys was investigated. It is established that ZK60 alloy possesses higher SCC resistance and fracture toughness compared to those of AZ31 alloy as is evidenced by the higher threshold stress intensity factors K1SCC and Kc. However, the lower average velocity of the stable SCC is observed in AZ31 alloy than in ZK60 one. Stable SCC occurs in two sub-stages, the first of which is featured by the lower crack growth rate and brittle fracture mode, while the higher crack velocity and quasi-cleavage fracture mode herald the second sub-stage. The crack growth velocities at specific SCC stages as well as stress intensity factor Kf associated with the alternation of the fracture mode are evaluated.

The effect of charging conditions on hydrogen embrittlement behavior of ultra-high-strength steel 22MnB5

The effect of charging conditions on hydrogen embrittlement (HE) of the hot-stamping boron steel was investigated with its hydrogen distributions and reversibility properties. With low hydrogen concentration, no noticeable hydrogen-induced cracking was detected, coupled with the uniform distributed diffusible hydrogen, realizing reversible HE. The quasi-cleavage fracture modes suggested the hydrogen-enhanced localized plasticity (HELP) mechanism. At high hydrogen concentration, the diffusible hydrogen distributed at the martensitic block and prior austenite grain boundaries which facilitated hydrogen-induced cracking initiation and propagated along the {110} M plane, leading to irreversible HE. The intergranular fracture modes suggested the transition of HELP to hydrogen-enhanced decohesion mechanism (HEDE). • The HE mechanism transits from HELP to HEDE as hydrogen concentration increases. • The distribution of hydrogen atoms is visualized by hydrogen microprint technology. • The reversibility of HE depends on the formation of hydrogen-induced cracking.

Effect of interlayer temperature on microstructure evolution and mechanical performance of wire arc additive manufactured 300M steel

The 300M steel straight wall parts (SWPs) are fabricated by wire arc additive manufacturing based on cold metal transfer technology. The effects of interlayer temperature on microstructure evolution and mechanical performance in the different regions of the SWPs are investigated. The results show that the surface waviness of the SWP first decreases and then increases with the increase of interlayer temperature, and the lowest surface waviness is controlled in 0.47 mm at the interlayer temperature of 200 °C. In the top region, the microstructure mainly consists of untempered martensite and slightly changes with the increase of interlayer temperature. In the middle and bottom regions, the microstructure is mainly composed of tempered martensite as the interlayer temperature is 100 or 200 °C. However, when the interlayer temperature is increased above 400 °C, the needle-like bainite and feather-like bainite are gradually formed due to the slow cooling rate and long dwell time in the bainite transformation zone. The microhardness and ultimate tensile strength in the top region are significantly higher than those in the middle and bottom regions, which are attributed to the enhanced solid solution strengthening effect of untempered martensite. The tensile fracture morphology is transformed from the quasi-cleavage fracture mode to the ductile mode with the increase of interlayer temperature or the decrease of distance from the substrate.

Revealing ductile/quasi-cleavage fracture and DRX-affected grain size evolution of AA7075 alloy during hot stamping process

The microstructural evolution, including ductile fracture modes and grain size at elevated temperature, exhibited significant effects on the macroscopic rheological behaviour and restricted the formability of the high-strength AA7075 alloy employed in the hot stamping process. In this study, AA7075 alloy was subjected to uniaxial hot tensile tests at deformation temperatures of 350–450 ℃ and strain rates of 0.01–1 s−1 to analyse the thermal rheological behaviour, damage and grain size evolution, in tandem with the preferred grain orientations affected by intergranular and intracrystalline evolution. A set of improved viscoplastic damage constitutive models was established, which consider the combined interactions of dislocation density and grain size as well as the ductile and quasi-cleavage fracture modes, to elaborate and reveal the fracture modes and grain size evolution of AA7075 alloy during the hot stamping process. Particularly, the evolution of dislocations accumulation was established incorporated with DRX-affected grain refinement. Subsequently, the material parameters were determined and calibrated based on a genetic algorithm, and the feasibility of the constitutive model was verified by simulations of uniaxial tension experiments. Finally, a finite element (FE) simulation of a hot formed T-shaped specimen was carried out. The mesoscopic and macroscopic deformation behaviours, including the evolution of grain size, damage and thickness distribution, were predicted, and the influences of grain size and damage on the formability were explained. Especially it was found that the grain size of the T-shaped components at the punch fillet was the largest, while the grain size at the bottom of the T-parts was the smallest. In addition, severe thinning region of T-shaped parts possess the largest normalized grain size. This study provides theoretical guidance and a scientific basis for the application and forming control of AA7075 alloy in hot stamping processes.

Effect of thermal compensation treatment on the microstructure and mechanical properties of IN738LC joint by electron beam welding

Thermal compensation treatment (TCT) obtained sound IN738LC joint by electron beam welding (EBW), increasing the characteristic dimension of fusion zone (FZ) and heat affected zone (HAZ) prominently. But no microstructure change occurred in the FZ of IN738LC joint subjecting to TCT, consisting of nanoscale γ′ particles, discrete MC carbide and isolated γ/γ′ eutectic. The content of γ/γ′ eutectic and γ′ size in FZ increased as the number of TCT increased, attributed to the decreased cooling rate of FZ. When adopting TCT, the dissolution of γʹ particle in HAZ was more evident, resulting in an increase of liquid film thickness. As a result, the total length of HAZ liquation cracking rapidly decreased. With the increased number of TCT, the micro-hardness of FZ and HAZ in IN738LC joint increased gradually. All the joints fractured in the BM, but obtained a decreased elongation at room temperature (RM) and 900ºC, mainly associated with the increased content of MC carbides and γ′ particles in FZ and HAZ. The fracture surfaces of IN738LC joints showed quasi-cleavage fracture mode and ductile fracture feature at RM and 900ºC, respectively. In addition, the rupture life of IN738LC joint at 900ºC/350MPa was improved significantly from 3.1h to 4.25h after TCT. All the joints fractured in FZ and showed a typical intergranular fracture feature.

Effects of Filler Wires on the Microstructure and Mechanical Properties of 2195-T6 Al-Li Alloy Spray Formed by TIG Welding.

The main purpose of this work was to investigate the microstructure and mechanical properties of spray-formed 2195-T6 Al-Li alloy welding joints produced by tungsten inert gas (TIG) with Al-Cu and Al-Si-Cu filler wires, so that they can be better used in space vehicle tanks. The porosity analysis indicates that the porosity area of the weld seam with the Al-Si-Cu filler wire is approximately 7.989 times larger than that of the Al-Cu filler wire. Furthermore, the microstructure and microhardness results indicate that the Al/Cu eutectic near the fusion line distributes more at the grain boundaries, while more dispersed Al2Cu phase is found inside the grain, which improves the strength of the joint when using Al-Cu filler wire. However, when using the Al-Si-Cu filler wire, more Si, Cu, and Ti elements are segregated at the grain boundaries, forming a brittle-hard network Al/Cu/Ti eutectic, which reduces the performance of the joint. Additionally, the tensile strength and elongation of the weld joint are about 68.6% and 89.9% of the base metal (BM) when using the Al-Cu filler wire, and can approach the level of friction stir welding (FSW). However, the tensile strength and elongation are only about 56.8% and 39.9%, respectively, of the BM in the weld joint when using the Al-Si-Cu filler wire. Lastly, the fractures both occur on the fusion line and the fracture morphology of the weld joint shows that it is a mixed fracture mode dominated by plastic fracture when using Al-Cu filler wire, while it is mainly a quasi-cleavage fracture mode when using Al-Si-Cu filler wire. Therefore, the joint strength when using Al-Si-Cu filler wire with high strength matching is not as good as that of Al-Cu filler wire with low strength matching.

Metallurgical investigation of failure analysis in industrial machine components

Bearings and chains are important industrial machine components used in wide range of heavy cyclical load applications. Main aim of this research work is to conduct a metallurgical investigation of failure analysis in failed bearing and chain parts. The failure analysis is carried out by chemical analysis, optical metallography, hardness analysis and scanning electron microscopy (SEM) analysis. In failed bearing inner race ways ridged marks were perceived due to electrical pitting and outer race ball groove worn-out on one side due to axial thrust load. Bearing balls have surface spalling due to contact fatigue. In chain failed bush multiple crack fronts are observed in longitudinal and circumferential. Crack geometry and fracture surface indicates failure through impact fatigue mode. The fracture surface indicated quasi cleavage fracture mode near the crack origins. The results in both investigations suggest that the major common causes of failures are due to over load, fatigue, wear and unsuitable material selection. MoS2coating, thin dense chromium coating and thin hard titanium nitride coating in all bearing components has been suggested to further improve the wear resistance and fatigue strength. Material SAE 8620 alloy steel for chain bush has been suggested to further improve life to withstand in the impact load conditions.

Effect of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC joint

Influence of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC superalloy was investigated systematically. The microstructure of as-cast base metal (BM) mainly consisted of “ogdoadically” diced cube γ′ particles, fanlike γ-γ′ eutectics, MC carbides, Cr–Mo borides and Ni–Zr intermetallics. Grain-boundary liquation cracking was observed in the HAZ of as-cast joint, resulting from the segregation of elemental boron. Pre-weld heat treatment had a significant influence on the size, volume fraction and distribution of γ′ particles. It also decreased the non-equilibrium segregation of boron during cooling, which inhibited the formation of grain-boundary liquid film and liquation cracking. The precipitation of MC carbides and nanoscaled γ′ particles in the fusion zone (FZ) improved the micro-hardness of the FZ. The micro-hardness of the BM and the HAZ were influenced by the volume fraction and the size of γ′ particles. All the joints fractured in the BM at RM and 900 °C. The as-cast and water quenched (WQ) joints failed with a quasi-cleavage fracture mode at RM and 900 °C while the tensile fracture of furnace cooled (FC) joint showed a ductile rupture at RM and 900 °C.

Welding parameter selection and short fatigue crack growth of dissimilar aluminum alloy friction stir welded joint

The welding process parameters of dissimilar 2024-T4 and 7075-T6 aluminum alloy friction stir welded (FSWy) joints are selected by the metallographic microstructure, the mechanical properties, and the hardness distribution. The joints with rotational speed of 900 rpm and advancing speed of 100 mm/min that have the highest tensile strength are used to carry out the experimental fatigue research. The short crack growth behavior of dissimilar FSW joints is analyzed through the replica technique. The stage of the crack initiation and stable growth accounts for about 60~80% of total fatigue life. The short crack grows rapidly once it reaches the crack size of approximately 350~450 μm. The cracks are prone to initiate at the defects of subsurface in the weld nugget zone and propagate in a quasi-cleavage fracture mode.

Investigation of microstructure and mechanical performance in IN738LC joint by vacuum electron beam welding

A study of microstructure evolution and mechanical performance of vacuum electron beam welded IN738LC joint was performed. The joint obtained a typical bowl-like weld profile. No macro-cracks occurred in the fusion zone (FZ). The results of EPMA showed that obvious elemental segregation formed in the FZ. Microstructure characterization of FZ exhibited the existence of secondary micro-constituents, comprised of the ordered γ′ phase, MC carbide, γ/γ′ and γ/MC eutectic. The solidification processing of the precipitations in the FZ was adequately expounded. In the heat affected zone (HAZ), the γ′ particles were dissolved partially. The distance to fusion line and the size of γ′ particles affect the extent of dissolution of γ′ particles. liquation cracking was also observed, mainly resulting from the constitutional liquation of γ′ precipitates. The micro-hardness of the joint from base metal (BM) to FZ increased gradually, related to the formation of more MC carbides in the FZ and high-density reprecipitated γ′ phase in the FZ and HAZ. All the joints fractured in the BM at room temperature (RM) and 900 °C. Fracture analysis indicated that the joint tested at RM failed with a quasi-cleavage fracture mode, and the tensile fracture at 900 °C showed a dominant ductile rupture.

Microstructure and Mechanical Properties of Mg2Si/AZ91 Composites In Situ Synthesized by Using Silica Fume as the Si Source

Microstructure and mechanical properties of in situ Mg2Si/AZ91 composites fabricated using silica fume as the Si source were investigated. The composites contained α-Mg, network Mg17Al12, lamellar Mg17Al12, coarse polygonal Mg2Si and needle-type Mg2Si. Mischmetal modification eliminated the coarse polygonal Mg2Si and resulted in a decreased amount of lamellar Mg17Al12 and refined matrix grain size. Solution and aging treatment made the network Mg17Al12 transform into lamellar Mg17Al12. As the volume fraction of Mg2Si increased, the matrix grain size decreased, the amount of Mg17Al12 increased, and more Mg17Al12 precipitated in grain interiors as a result of the increased Al/Mg ratio caused by increasing the amount of Mg2Si. The tensile strength and elastic modulus were much higher for the modified composites than for the unmodified one and increased by both increasing the volume fraction of Mg2Si and performing aging treatment. The composites failed in a quasi-cleavage fracture mode, and coarse polygonal Mg2Si phases were responsible for the lowest ductility of the unmodified composite. The highest UTS of 291.55 MPa obtained is much higher than those reported so far by using fly ash as the Si source, indicating that silica fume is more suitable as the Si source to synthesize Mg2Si/AZ91 composites.

Initial microstructure effect on the mechanical properties of Ti-6Al-4V ELI alloy processed by high-pressure torsion

effect of initial microstructure on the mechanical properties and fracture mode of the Ti-6Al-4V ELI alloy subjected to the high-pressure torsion (HPT) processing at room temperature and 500 °C was investigated. In this purpose, the four different microstructures (fully lamellar, martensitic, equiaxed and globular microstructures) were developed by the proper heat treatments. The results showed that the application of the HPT processing provided substantial microstructural refinement, independent on the alloy initial microstructure. As a consequence, the alloy hardness, tensile strength and ductility significantly increased. At the same time, the elastic modulus of the HPT-processed alloy is almost 2 times lower as compared to that of the alloy in the initial heat treated state. The obtained improvements strongly depend on the alloy initial microstructure and HPT processing temperature. It was found that the initial martensitic microstructure is beneficial to improve the hardness, tensile and fracture properties. The HPT processing at 500 °C enhanced them more effectively. After the warm HPT processing, alloy with the initial martensitic microstructure exhibited exceptional hardness (455 MPa), high ultimate tensile strength (1546 MPa), high elongation to failure (18.8%) and low elastic modulus (78.6 GPa). Although the alloy with initial fully lamellar microstructure displayed the lowest elastic modulus (68 GPa), the ultrafine and homogeneous (α + β) two-phase microstructure produced by the HPT processing of the alloy with initial martensitic microstructure offered the best combination of the strength-ductility balance and strength-to-elastic modulus ratio (19.8 × 10−3). The complex fracture process, involving transgranular quasi-cleavage and ductile dimple fracture mode, corresponded to the changes in ductility.

Inhomogeneous interface structure and mechanical properties of rotary friction welded TC4 titanium alloy/316L stainless steel joints

The inhomogeneous interface structure and mechanical properties along the radius direction of rotary friction welded TC4 titanium alloy/316 L stainless steel dissimilar joints were investigated under as-welded and post-weld heat treated conditions. The interface in the joint is convex-shaped on TC4 titanium alloy side and concave-shaped on 316 L stainless steel side. The mechanical properties of joints are highly correlated with the inhomogeneous interface structure and the dimension of tensile samples. The tensile strength of the as-welded entire joint was 117 MPa, while it dramatically increased to 419 MPa after post-weld heat treatment at 600 °C for 2 h due to homogenization of aggregated atoms. However, the sliced samples was relatively weak even after post-weld heat treatment, because the internal defects became surface crack sources after machining, and caused stress concentration and sharply reduced the strength of samples. All tensile samples failed through the interface. The X–ray diffraction patterns of TiC, Cr23C6, Fe2Ti, FeTi, FeNi3, AlTi3, CrTi4, NiTi and TiCr phases and the river-like features on fracture surfaces indicate a brittle quasi-cleavage fracture mode. C and Cr aggregated around the interface under as-welded condition. Nonetheless, elemental homogenization occurred in the joint after post-weld heat treatment.

Investigations on the solidification microstructures and room temperature compression properties of Nb-doped NiAl-32Cr-6Mo hypereutectic alloy

Solidification microstructures and compression properties of NiAl-32Cr-6Mo hypereutectic alloys containing different contents of Nb were investigated. The addition of Nb did not change the basic phase constituents, though a new phase of Laves Nb(Cr1-x-yNixAly)2 was observed. However, the primary Cr(Mo) phase no longer showed obvious dendritic morphology, the formation of lamellae was disturbed and the eutectic cellular microstructure degenerated. When the Nb content was 5%, eutectic lamellae coarsened and became spherical and poorly aligned. Fully eutectic microstructures were obtained in the directionally solidified hypereutectic alloys grown at 6 µm/s regardless of the addition amount of Nb. With the increasing content of Nb addition, the volume fraction of Laves phase located at the NiAl/Cr(Mo) boundary became more. The yield strength of directionally solidified NiAl-32Cr-6Mo-3Nb alloy reached to 2072 MPa, which is the highest value in the NiAl-based eutectic alloy at present. Well-aligned microstructure and more strengthening phase are conducive to the improvement of properties. All the alloys exhibited brittle quasi-cleavage fracture mode, cleavage in the NiAl phase and debonding of the NiAl/Cr(Mo) interface accompanied by Cr(Mo) lamellae and Cr2Nb phase pulled out were observed in the fracture surface.