Quasi-cleavage Fracture Research Articles

Optimization of bobbin tool friction stir welding parameters of AZ31 magnesium alloy based on FEM and its influence mechanism on mechanical properties

Based on the 3D transient heat transfer theory, the finite element model of bobbin tool friction stir welding (BT-FSW) of AZ31 magnesium alloy sheet is established. The effects of different process parameters on the quality of BT-FSW joint of magnesium alloy were discussed using temperature distribution, mechanical properties, fracture morphology analysis, and EDS element distribution analysis. The results show that in the process of BT-FSW, the cross-section temperature of the plate is symmetrically distributed in an hourglass shape, the AS is slightly lower than that of RS, and the transverse temperature is distributed in an ‘M’ shape. When the rotating speed is 700 rpm and the welding speed is 180 mm/min, the weld surface is smooth, and its ultimate tensile strength can reach 81.82% of the base metal strength. The tensile specimen is quasi-cleavage fracture. When the heat input is insufficient, there are a lot of Al8Mn5 Precipitated phases in the TMAZ fracture zone, which increases the fracture brittleness of the joint. When the heat input is enough, the precipitated phase dissolves evenly, the river-like fracture surface of the joint retains the dimple characteristics, and the mechanical properties are improved.

Performance of the GH4169 Joint Using a Novel Ni-Based Amorphous Brazing Filler Metal

A novel Ni-Cr-Si-B filler metal (JNi-5) was designed and further fabricated into the amorphous brazing filler metal for joining the GH4169 alloy. The effect of brazing temperature on the microstructure and mechanical properties of GH4169 joints was investigated. The typical microstructure of the joint at 1030 °C is composed of four specific zones: the base metal (BM), heat-affected zone (HAZ), isothermal solidification zone (ISZ), and athermal solidification zone (ASZ). The typical microstructure of the joint is GH4169/(Nb, Mo)-rich boride+(Cr, Nb, Mo)-rich boride/γ(Ni)/Ni-rich boride+γ(Ni)/γ(Ni)/(Cr, Nb, Mo)-rich boride+(Nb, Mo)-rich boride/GH4169. As the temperature increased, the HAZ continued to widen and the ASZ depleted at 1090 °C and 1120 °C. Additionally, the borides within the HAZ coarsened at temperatures of 1090 °C and 1120 °C. At 1030 °C, the fracture path is in the ASZ, and the existence of the brittle phase in the ASZ provides the potential origin for crack growth. The fracture mode is a quasi-cleavage fracture. At 1060 °C, 1090 °C, and 1120 °C, the fracture behavior mainly happened in the HAZ, and the existence of borides in the HAZ provides the potential origin for crack growth. Namely, the shear strength of joints was principally dominated by the brittle precipitations in the HAZ. The fracture mode of these joints is the hybrid ductile. At 1060 °C, the shear strength of the obtained joint is the highest value (693.78 MPa) due to the volume fraction increase in the Ni-based solid solution. Finally, the optimized brazing parameter of 1060 °C/10 min was determined, and the corresponding highest shear strength of 693.78 MPa was obtained owing to the increased content of the Ni-based solid solution in the joint.

Effects of hydrogen blending ratios and CO2 on hydrogen embrittlement of X65 steel in high-pressure offshore hydrogen-blended natural gas pipelines

In this study, the effects of hydrogen blending ratios and high CO2 content on hydrogen embrittlement (HE) of offshore natural gas X65 pipeline steel were investigated. In high-pressure hydrogen gas, the impact of hydrogen content on yield strength and tensile strength was minimal. However, the effect on elongation was more significant, with the area decreasing and the HE index increasing until the hydrogen-blended ratio approached 30%, at which point the HE index reached 8.94%. Introducing CO2 into high-pressure hydrogen-rich gas further increased the HE index with high CO2 content. At 40% CO2, the HE index was 2.42 times higher than in high-pressure hydrogen-blended mixtures (30% H2). Additionally, the local fracture morphology transitions from ductile to quasi-cleavage fracture with the addition of CO2 and H2.

Ratcheting-fatigue behavior and fracture mechanism of 316H ASS under cyclic random loading block

In this study, a set of programmed random factors with non-zero mean were designed. Then various stress levels (15, 18 and 22 KN) were multiple superimposed to factors to form one random loading block (RLB), the blocks were repeated to failure to investigate the synergistic damage of 316H ASS under low-cycle fatigue (LCF), high-cycle fatigue (HCF) and ratcheting effect. The lifetime of cyclic RLB tests decreased with the increase of block-mean stresses σmBlock (208、255 and 311.5 MPa). The normalized strain amplitudes indicate that when the σmBlock amplitude below the yield strength (208 and 255 MPa), a stable ratchet accumulation phase allows the specimens to exhibit cyclic hardening behavior. When σmBlock exceeds the yield strength (311.5 MPa), the ratcheting strain increases significantly and the specimens exhibit cyclic softening behavior. Especially, the transgranular cleavage fracture, quasi-cleavage fracture and intergranular secondary cracks were identified when the failure of cyclic RLB tests were induced by LCF, HCF and ratcheting. With the increase of σmBlock amplitude, the decrease of LAGB proportion and the increase of dislocation density further reduce the fatigue resistance. In addition to dislocation motion, the α’-martensite phase transformation induced by ratcheting-fatigue has been further demonstrated as a mechanism for coordinated deformation. The percentage of stresses (within one block) that exceeds the diverge critical stress (375.6 MPa) of stacking faults (SFs) determines the α’-martensite nucleation mechanism.

Influence of Heat Treatment on Microstructure and Mechanical Properties of Laser Cladding Coatings

This study investigates the influence of various heat treatment processes on the microstructure and properties of laser cladding Fe314 coatings. The microstructure, phases, and impact fracture morphology of the cladding layer were observed using X-ray diffraction and scanning electron microscopy, among other methods. The hardness and impact performance of the cladding layer were also tested. The results indicated that there was compositional segregation and non-equilibrium microstructure in the untreated cladding layer, with an average microhardness of 368.67 HV and an impact toughness of 27 J, exhibiting quasi-cleavage fracture. The stress-relief annealing treatment resulted in a uniform distribution of M23C6 carbides inside the cladding layer. The pinning effect generated by M23C6 reduced the microhardness by 16.26% and increased the impact toughness to 54 J. The impact fracture surface exhibited ductile fracture. After secondary normalizing and annealing, the microstructure of the cladding layer transformed into a fine single-phase austenite structure, and fine M7C3 carbides precipitated at the grain boundaries. Under the effects of fine grain strengthening and dispersion strengthening, the microhardness of the cladding layer decreased by 38.14%, and the average impact absorbed energy of the specimen was 64 J, showing complete ductile fracture.

Bonding behavior of Ti-6Al-3Nb-2Zr-1Mo/WC composite coating on titanium alloy by gas tungsten arc welding cladding

In this study, tungsten carbide (WC) ceramic particles were introduced into the molten pool of gas tungsten arc welding (GTAW) to successfully prepare a composite coating without solidification cracking and with lower dilution. The formation mechanism and properties of the bonding interface are deeply analyzed. Results demonstrate that metallurgical bonding was achieved. The dissolution behavior of WC particles and the diffusion of W element into the heat-affected zone promoted the formation of a special β(Ti, W) diffusion layer below the fusion line. (Nb, Ti)C was mainly found distributed close to the diffusion layer. Nanoindentation test results show remarkable inhomogeneity in the interface area. The fracture surface of the broken coating revealed that the titanium matrix exhibited quasi-cleavage fracture, while the particles displayed brittle fractures. The fracture surface of coatings that experienced decohesive rupture in the shear test underwent plastic deformation in the shear direction.

Microstructure, deformation and fracture mechanism of Mg-Gd-Y-Zn-Zr alloy with different rolling routes during tension

The microstructure, deformation and fracture mechanism of unidirectional rolling (UR) and cross rolling (CR) Mg-4.7Gd-3.4Y-1.2Zn-0.5Zr alloy during tension were studied. Both rolled alloys exhibit typical bimodal microstructures, with the difference being that UR alloy has elongated grains containing rod LPSO phase, while CR alloy has relatively rounded deformed grains containing lamellar LPSO phase. After tension until fracture, the LPSO phase morphology of the two alloys remains unchanged. CR alloy exhibits a higher yield strength and lower elongation than UR alloy. Although the higher average basal <a> slip Schmid factor result in the lower yield strength of UR alloy, the activation of the high proportion of non-basal slip is responsible for its higher elongation. UR alloy and CR alloy exhibit ductile fracture and quasi cleavage fracture characteristics, respectively. For UR alloy, a variety of dislocations accumulated near grain boundary and the weak slip transfer effects between grains both contributes to the propagation of grain boundary cracks. However, the effects of weak interfacial bonding between metastable lamellar LPSO phase and matrix, along with the similar orientation and higher geometric compatibility of CR alloy, lead to the rapid spread of transgranular cracks.

Origin of Serrated Markings on the Hydrogen Related Quasi-cleavage Fracture in Low-carbon Steel with Ferrite Microstructure

Origin of Serrated Markings on the Hydrogen Related Quasi-cleavage Fracture in Low-carbon Steel with Ferrite Microstructure

Low cycle fatigue behavior and failure mechanism of 34CrNiMo6 steel used as connecting rod for diesel engine

Abstract In this paper, for the 34CrNiMo6 steel material used in diesel engine connecting rods, the low-cycle fatigue properties and failure mechanism were studied. The metallographic structure of 34CrNiMo6 has been investigated by using optical microscopes, scanning electron microscopes, and transmission electron microscopes. Tensile tests and low-cycle fatigue experiments were performed at different levels of strain amplitudes, and the microstructural changes were monitored. The examination indicates that the structure of 34CrNiMo6 steel consisted primarily of a ferrite base and small spherical and rod-shaped carbides, with a mean particle size measuring 2 μm. It had exceptional mechanical characteristics of high strength and moderate plasticity. The total strain amplitude (TSA) of 34CrNiMo6 steel affected the cycling behavior, which was stable at low TSA (0.2% ~ 0.3%). At high TSA (0.4% ~ 0.7%), a large number of dislocations were annihilated through the dynamic recovery process, showing a typical cyclic softening phenomenon. The low-cycle fatigue failure behavior was mainly a combined mode of quasi-cleavage fracture and cross-grain fracture.

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.

Study on low cycle fatigue properties of Cr-Mo steel in 50 MPa gaseous hydrogen

At present, a lack of fatigue design curves for Cr-Mo steel in high-pressure gaseous hydrogen worldwide impedes the use of the design fatigue curve (S-N curve) evaluation for vessel fatigue design, necessitating reliance on fracture mechanics methods related to the initial crack size. In this study, strain-controlled fatigue tests with a strain ratio of −1 were carried out in 50 MPa gaseous hydrogen to study the low cycle fatigue properties of Cr-Mo steel. Results indicate that hydrogen reduces the fatigue life of 4130X under high strain amplitudes. Under 0.5 % strain amplitude, different from characteristics of ductile fracture in air, the fracture in 50 MPa gaseous hydrogen showed quasi-cleavage and intergranular fracture characteristics. Based on the test results, the S-N curve in 50 MPa gaseous hydrogen was obtained with fatigue life safety factor 20 and stress amplitude safety factor 2. The S-N curve in 50 MPa gaseous hydrogen is under the curve in KD-320.2 M of the 2023 ASME Boiler & Pressure Vessel Code VIII-3, which does not consider the effect of hydrogen, when the number of cycles to failure is less than 1×106. This indicates that the fatigue life calculated by the S-N curve in KD-320.2 M cannot ensure that 50 MPa high-pressure hydrogen storage vessels avoid fatigue failure under high stress amplitudes.

Improving the Mechanical Properties of Al-Si Composites through the Synergistic Strengthening of TiB2 Particles and BN Nanosheets

The size and distribution of the silicon phase and intermetallic phase are important factors affecting the properties of Al11Si3Cu2NiMg alloy (M142). In this study, BNNS and micro-TiB2 were used to synergistically refine and reinforce M142 composites (M142-BNNS-TiB2). After T6 heat treatment, the comprehensive mechanical properties of M142-BN-TiB2 composites were excellent, with an ultimate tensile strength of 463 MPa and an elongation of 2.6%. In addition, the introduction of BNNS and micro-TiB2 changed the fracture mode of M142 from brittle fracture to quasi-cleavage fracture, and the introduction of BNNS and micro-TiB2 refined the Si phase and intermetallic phase, which could change the origin of the crack in the composite, thus improving the ductility of the composite.

Microstructure Regulation and Mechanical Properties of Mg–6xZn–xY Alloys Containing Icosahedral Quasicrystalline Phases

Mg–6xZn–xY (x = 0.8, 5, and 7 wt%) alloys containing an icosahedral quasicrystal phase (I‐phase) are synthesized, and the effects of Y doping on the microstructure and mechanical properties of the Mg matrix and I‐phase are investigated. The results show that as the Y content increases, the I‐phase content gradually increases, with the distribution of the I‐phase changing from a discontinuous to a continuous network and lamellar structures. Additionally, the eutectic structure (α‐Mg + I‐phase) in the alloys gradually increases, while the Mg matrix area decreases. The tensile strength of the as‐cast alloy initially increases and then decreases, reaching a maximum value at Y = 5. Heat treatment of the Mg–30Zn–5Y alloy reveals an optimal solid solution temperature of 320 °C, resulting in a tensile strength of 175.09 MPa and quasicleavage fracture characteristics. The maximum tensile strength of the alloy is achieved when the I‐phase distribution is continuous and the porosity of the Mg matrix is ≈66%. These results provide guidance for the microstructural design of I‐phase‐reinforced Mg alloys with rare earth elements to enhance their mechanical properties.

Effect of Swing Amplitude on Microstructure and Properties of TC4 Titanium Alloy in Laser Welding

The welding of TC4 titanium alloy sheets with a thickness of 1 mm was successfully accomplished by a swinging laser. The microstructure and mechanical properties of the welding seam under different swing amplitudes were studied. In this paper, the microstructure, phase composition, mechanical properties, and fracture morphology of the weld with swing frequency of 50 Hz and different swing amplitudes (0.2 mm, 1 mm, 2 mm, and 3 mm) were tested and analyzed. The results show that basket-weave microstructures are present in the fusion zone of welds under different oscillation amplitudes, but the morphology of martensite within the basket-weave differs. The weld microstructure is mainly composed of acicular α′ martensite, initial α phase, secondary α phase, and residual β phase. The hardness of the weld is higher than that of the base metal, and the overall hardness decreases from the weld center to the base metal. When the oscillation amplitude A = 1 mm, the weld microstructure has the smallest average grain size, the highest microhardness (388.86 HV), the largest tensile strength (1115.4 MPa), and quasi-cleavage fracture occurs. At an oscillation amplitude of A = 2 mm, the tensile specimen achieves the maximum elongation of 14%, with ductile fracture as the dominant mechanism.

Carbon distribution in lath martensite and quench embrittlement

Effect of water quenching and low-temperature tempering (LTT) at T≤280 °C on fracture mechanisms upon tension and during the Charpy V-notch (CVN) impact test of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was studied. The steel exhibits quench embrittlement (QE). Tensile specimens fractured in elastic region after quenching due to easy crack initiation by intergranular fracture mechanisms. CVN impact toughness was ∼5 J/cm2. Low fracture toughness is attributed to superposition of intergranular fracture and transgranular quasi-cleavage fracture within packets during crack propagation; crack initiation occurs locally at the notch surface. QE is attributed to carbon segregations on high-angle boundaries if a sufficient portion of bulk carbon could not be consumed for the formation of Cottrell clouds on dislocations. LTT provides precipitation of transition carbides that leads to depletion of carbon from martensite and boundary segregations that decreases severity of QE due to suppression of intergranular fracture, increase of fracture brittle stress and contribution of dimple fracture to overall fracture process. The onset of fracture takes place after yielding in tension and fracture toughness increases by a factor of ∼2 after LTT.

Relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel

V-N microalloying treatment is an important way to improve the service performance of non-quenched and tempered ship plate steel. Herein, the influence of V(C, N) on the evolution of microstructure and improvement of mechanical properties was studied. In addition, the relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel was revealed. The results showed that the composite addition of V and N not only formed a fine dispersed precipitated phase, but more importantly, significantly refined the ferrite/pearlite microstructure, promoted the formation of intragranular acicular ferrite, increased the proportion of high angle grain boundaries, and decreased the kernel average misorientation value. The optimization of microstructure brought about by V-N microalloying achieved synchronous improvement of strength and cryogenic toughness. The impact energy of V-N microalloying ship plate steel increased from 97 J of V-N-free ship plate steel to 239 J at −40 °C, and the impact fracture mode changed from brittle quasi-cleavage fracture to microvoid coalescence fracture with a large number of equiaxial dimples.

Effect of the hot forming with synchronous quenching process on high cycle fatigue properties of the 2A97 Al-Li alloy

The 2A97 Al-Li alloys have been used in aircraft to achieve lightweight, its high cycle fatigue performance is one of the important factors that determines aircraft safety. The 2A97-T8 Al-Li alloy is obtained through the 2A97-T3 Al-Li alloy undergoes the hot forming with synchronous quenching process. The 2A97-T3 and 2A97-T8 Al-Li alloys are tested for high cycle fatigue under different stress amplitudes. The fatigue crack propagation path of the 2A97-T3 Al-Li alloy is not only along the direction of maximum shear stress at 45° to the stress direction but also along the direction perpendicular to the stress axis. The fatigue crack propagation path of the 2A97-T8 Al-Li alloy propagates along the direction perpendicular to the stress axis. Under high amplitude stress, the characteristics of quasi-cleavage fracture are found in the fracture morphology of 2A97-T3 Al-Li alloy. However, the characteristics of cleavage fracture are found on the fracture morphology of 2A97-T3 Al-Li alloy under low amplitude stress. Under high amplitude stress, the quasi-cleavage characteristics and the dimples are found on the fracture morphology of 2A97-T8 Al-Li alloy. Under low amplitude stress, the small voids and the dimples are found on the fracture morphology of 2A97-T8 Al-Li alloy. The precipitated phases of the 2A97-T3 Al-Li alloy are multitudinous, fine, and dispersed δ’ phases and a few T1 phases. The precipitates of the 2A97-T8 Al-Li alloy are large, fine, and acicular T1 phases and a few δ’ phases. The hot forming with synchronous quenching process significantly improves the high cycle fatigue properties of the 2A97 Al-Li alloy. In addition, the fatigue cycles, the fatigue life rise and fall plots, the S-N curves, and the fatigue crack growth behaviors of the 2A97-T3 and 2A97-T8 Al-Li alloys are obtained.

A comparative study on microstructures of a high-Zn AlZnMgCuZr alloy fabricated by casting and melt spinning

A high-Zn Al–27Zn-1.5Mg-1.2Cu-0.08Zr alloy was prepared by traditional casting and melt spinning. The as-casted alloy consists of well-developed α-Al dendrites with many coarse η-phase and low solute concentration, coarse network precipitates at grain boundaries and the α-Al/η-phase eutectic structures at triple junctions. Due to these featured structures, the as-cast alloy exhibits a low tensile strength of 101 MPa with room temperature and high temperature (300 °C) damping capacity of 0.0058 and 0.027 respectively. Comparatively, the as-spun alloy reveals gradient cross-sectional microstructures after solidification: an ultrafine grained region near the wheel surface, a transition region and an equiaxed-grained region near the ribbon free surface, with changes of precipitate morphologies from granular shape to network shape via vermicular. High-density nano-sized η′-phase/GP-zones within α-Al grains result in a higher tensile strength (121 MPa) of as-spun alloy with room temperature and high temperature (300 °C) damping capacity of 0.0049 and 0.048 respectively. Fracture analysis shows that the as-casted alloy fails in a mixed-mode fracture, comprised predominantly of transgranular fracture, quasi-cleavage fracture, cleavage fracture and dimple fracture, whereas the as-spun alloy failed in cleavage fracture and dimple fracture.

Semi-solid rheo-diecast Mg-xGd-3Y–1Zn-0.4Zr (wt%) alloys and their comparison with conventional HPDC alloys

This work systematically investigated the microstructure and mechanical properties of Mg-xGd-3Y–1Zn-0.4Zr (wt%) alloys prepared via rheo-diecasting (RDC) in comparison to those of conventional high pressure die casting (HPDC) alloys, and revealed their differences in microstructure, casting defects, and mechanical properties. The microstructure of the as-cast RDC Mg-xGd-3Y–1Zn-0.4Zr (wt%) alloys is primarily composed of α-Mg and Mg3(Gd,Y,Zn) eutectic phases. Of the four alloys studied, the GWZ831K and GWZ1431K alloys exhibit the best elongation (EL: 6.4±1.3 %) and the highest strengths (UTS: 270±9 MPa; YS: 201±5.7 MPa), respectively. The GWZ831K alloy exhibits a quasi-cleavage fracture during room-temperature tensile tests, whereas the GWZ1431K alloy demonstrates a brittle fracture. The phase composition of conventional HPDC alloys is fundamentally the same as that of RDC alloys, but their grains and eutectic phases differ in morphology and are smaller in size. Oxide films are frequently found in conventional HPDC castings, but neither oxide films nor pore defects are observed in RDC alloys. Compared to conventional HPDC alloys, RDC alloys have better elongation but lower strength. The lower strength of RDC alloys compared to HPDC alloys can be attributed to larger grains and eutectic phases, along with fewer stacking faults (SFs), resulting in limited strengthening effects of grain boundaries, eutectic second phases and SFs. In contrast, RDC alloys exhibit superior elongation compared to HPDC alloys due to the elimination of oxide films and pore defects, as well as the excellent ductility of the α-Mg matrix that can effectively coordinate deformation.

Tailoring the strength and low-temperature toughness of HSLA structural steel by adding trace Ce

In this study, the trade-off between strength and low-temperature toughness of high-strength low-alloy (HSLA) structural steel was solved by adding trace Ce, which increased yield strength by 5.4 % and Charpy V-notch (CVN) impact energy at −40 °C by 57.9 %. The strengthening and toughening mechanism was revealed by multi-scale characterization. The results show that the strain hardening ability is improved after adding Ce. The grain size is refined and the geometrically necessary dislocation (GND) density increases to ensure the strength improvement. The CVN impact specimens change from cleavage fracture to quasi-cleavage fracture. The grain size, inclusions and high angle grain boundaries (HAGBs) are refined and optimized, which enhances the resistance to impact crack propagation and improves the low-temperature impact toughness.