Mechanism Of Cleavage Fracture Research Articles

Unravelling crack tip damage mechanisms: In-situ tensile assessment of Al-6Zn-2.1 Mg-2Cu alloy strengthened by Ti, Zr, and Sc micro-alloying

This study investigates the effect of micro-alloying elements (Ti, Zr and Sc) on the crack tip damage behaviour of cast Al-6Zn-2.1Mg-2Cu-X alloys to understand the effect of grain boundary interfaces, matrix, intermetallics and dispersoids on deformation and fracture behaviour. Notched tensile specimens were prepared and subjected to in-situ tensile deformation under FESEM to investigate the crack initiation, propagation, and eventual failure processes under uniaxial tensile loading. The experimental results reveal that fracture in the four Al-6Zn-2.1Mg-2Cu-X alloys is quasi-brittle due to strain accumulation and cleavage fracture mechanisms. The crack mainly initiates from the grain boundary lath S-Al2CuMg and η-MgZn2 intermetallic phases. Adding 0.25 % Sc forms small coherent Al3(Sc, Zr) dispersoids, increasing crack resistance. The ductility of the matrix is due to void nucleation, crack meandering, and crack tip-blunting phenomena. At high additions of microalloying elements (1 wt%), large Al3(Zr, Ti) intermetallics form and agglomerate, contributing to premature fracture. These findings from materials characterization and in-situ tensile testing indicate potential methods to enhance the fracture toughness of Al-6Zn-2.1Mg-2Cu alloys with additions of Zr, Ti, and Sc.

Investigations on cleavage fracture mechanism and surface damage of indium phosphide by molecular dynamics simulation

The quality and undamaged size in the resonant cavity surface directly affect the operational stability and output power of semiconductor lasers. To meet the demand for large-size and high-quality mirror facets, a deep understanding of fracture mechanisms of mechanical cleavage is essential. Nevertheless, 3D models of cleavage fracture are rarely investigated. In this work, a molecular dynamics study on the cleavage mechanisms of indium phosphide is reported through examining the effects of the cleavage gap, scratch depth, and cleavage rate on the cleavage damage morphology. The proposed molecular dynamics approach provides insights into the details of cleavage fracture at the atomic level. The simulation results reveal that the cleavage fracture mechanism includes the deviation of cracks between adjacent cleavage surfaces. The predominant damage manifestations on the cleavage surfaces are mainly characterized by cleavage terraces that begin at the bottom of the scratched groove. The cleavage terraces propagate along the [001] crystal direction on the (110) cleavage surface and deviate from their path. This phenomenon is essentially related to the stress released during the crack propagation process. Compared with the cleavage gap and scratch depth, the cleavage rate significantly affects the critical cleavage load. Additionally, a good correlation between the density of the cleavage terraces and the maximum undamaged length is achieved and reasonably explained. Mastering the correlation between various cleavage conditions and cleavage surface damage will facilitate and guide the fabrication of nanoscale resonant cavity mirrors and advance the development of mechanical cleaving processes.

Fracture surface microstructure and new fracture mechanism in the pearlite structure

The fracture behaviours of pearlite structures in several typical carbon steels were investigated by scanning electron microscopy (SEM). Microstructural analysis indicated that the fracture of pearlite is a mixed ductile-brittle fracture, and brittle fracture has cleavage fracture characteristics, which often occur inside the pearlite structure. The fracture surface can exhibit any orientation relationship with the ferrite/cementite interface plane in the pearlite structure. The two phases in the pearlite structure (ferrite (α-Fe) and cementite (θ-Fe3C)) are fine grains rather than single-crystal platelets. The cleavage fracture mechanism in any direction of pearlite structure occurs due to the fine grain substructure.

Preparation of Al-Ti-Sc master alloys and refining effects on the 6016 aluminum alloy

The 6016 aluminum alloy has good plasticity but low strength. In this work, Al-Ti-Sc master alloy was prepared by thermite reduction method in order to obtain an effective grain refiner with a simplified preparation process and a reduced cost. The refining effects of the ternary master alloy on the 6016 aluminum alloy were checked to improve the strength and plasticity simultaneously. The chemical reactions during the preparation process of the Al-Ti-Sc master alloy was studied by thermodynamic software of HSC 6.0. The content of Sc and Ti in the master alloy reaches 2.48 wt% and 1.4 wt%, respectively, on proper experimental conditions, and the main phases of the master alloys are α-Al and Al3(Sc,Ti) phase. In the refined alloys, the average grain size can reach 27 μm, and the ultimate tensile strength and the elongation after fracture reaches 280 MPa and 35% in cast state, respectively. Compared to the pristine 6016 aluminium alloy, the significantly enhanced strength and good plasticity of the refined alloys contribute to the combined effects of grain refinement and precipitated phase strengthening. For the fracture of low doped alloys, originates from cleavage fracture mechanisms, and for the ones with high dosages, ductile fracture becomes dominant.

Effects of Pb and Bi on mechanical properties and fracture modes of bcc iron symmetrical tilt grain boundaries

The internal structural material T91 in the accelerator driven subcritical reactor system (ADS) has encountered the challenge of liquid metal embrittlement caused by Pb and Bi. Therefore, it is of utmost importance to gain a comprehensive understanding of their embrittlement mechanism. The effects of Pb and Bi on the surface energy of surfaces and separation work and fracture toughness of grain boundaries in body center cubic (BCC) Fe are examined using first-principles calculation. Pb and Bi atoms reduce the surface energy and grain boundary cohesion strength of BCC Fe, with a more significant decrease in surface energy. Meanwhile, the segregation of Pb and Bi in the grain boundary reduces the fracture toughness of the grain boundary. By comparing different surface energy and separation work, it is found that BCC pure Fe is more prone to intergranular fracture, while the presence of Pb and Bi causes cleavage fracture. This elucidates the low-temperature fracture mode of BCC-Fe and reveals the potential mechanism of brittle cleavage fracture caused by Pb and Bi in BCC-Fe. The derived results are discussed in terms of electronic structure and show good agreement with experimental results in the literature, ultimately contributing to a thorough understanding of the intrinsic mechanism of liquid metal embrittlement of T91 steel.

Fatigue crack initiation and competitive crack propagation behavior in 500 MPa grade automobile beam steel

The fatigue crack initiation and propagation in Nb–Ti micro-alloyed 500 MPa grade steels that usually be used in the automobile beam were investigated. Reducing the content of C and Mn elements based on the reference steel and adding microalloying elements Nb and Ti to obtain the modified steel. The fatigue strengths of the reference steel and the modified steel are 201 MPa and 225 MPa, respectively. Fatigue cracks in the experimental steel were initiated at surface defects and persistent slip bands (PSBs). The modified steel is more susceptible to initiating cracks at the PSBs than the reference steel. There were three kinds of fracture modes in the experimental steels, which were determined by the competition between carbide and inclusion. As the loading stress and the stress intensity factor (Δk) increase, the fracture mode transforms from cleavage (brittle) to dimple (ductile) fracture. Under the circumstance of low loading stress and low Δk, the fracture mode is dominated by the carbide-induced cleavage fracture mechanism. Under the situation of high loading stress and Δk, the fracture mode is dominated by the inclusion-induced dimple fracture mechanism. In addition, the mixed cleavage-dimple region is formed when the two fracture mechanisms compete in the steel.

Microstructure inducement of different tensile fracture mechanisms at 800 ℃ of directional solidified Ti-45Al-5Nb alloys produced by electromagnetic confinement

Directional solidified TiAl alloys have low density and good comprehensive high temperature performance, which is an ideal substitute material for high temperature structural parts. In this paper, Φ 16.5 mm directional solidified Ti-45Al-5Nb sample with full-lamellar microstructure was prepared successfully by electromagnetic confinement technique at 10 µm/s and relative tensile tests at 800 ℃ were also performed. The tensile strength of Ti-45Al-5Nb alloy at 800 ℃ is 765 MPa and the elongation is 16%. Moreover, results of tensile process characteristics and fracture analysis show that this material has two distinct fracture mechanisms at 800 ℃, which are cleavage fracture mechanism and microporous aggregation fracture mechanism. Two fracture mechanisms do not exist at the same time, and for a particular specimen, the fracture process is dominated by only one of them. Different fracture mechanisms are determined by the lamellar orientation difference and element segregation. When the orientation of adjacent grains is similar and the segregation is light, the fracture process is dominated by the cleavage fracture mechanism. Otherwise, proportion and scale of the γ phase at the grain boundary increase and the B2 phase is generated. In this case, the microporous aggregation fracture mechanism will dominate the fracture process.

A new method for the toughness assessment of mobile crane components based on damage mechanics

The characterization of toughness properties in standard Charpy or fracture mechanics tests calls for thickness requirements to be met. Therefore, the characterization of toughness properties is a problem for thin-walled structures. Replacing Charpy impact toughness testing by impact notch tensile testing can solve this problem. However, the toughness requirements are still expressed in terms of standard test results. Therefore, a framework is proposed here for translating these standard test requirements into impact notch tensile test requirements. The proposed framework relies on numerical simulations with a phenomenological damage mechanics model, which uses state-of-stress-dependent, strain-based criteria for the prediction of local damage and global fracture. This model takes the effects of non-proportional strain paths into account and applies different criteria for cleavage and ductile fracture in order to predict correctly the activation of cleavage and ductile fracture mechanisms in the corresponding numerical simulations.

Cleavage Fracture of the Air Cooled Medium Carbon Microalloyed Forging Steels with Heterogeneous Microstructures.

Cleavage fracture of the V and Ti-V microalloyed forging steels was investigated by the four-point bending testing of the notched specimens of Griffith-Owen’s type at −196 °C, in conjunction with the finite element analysis and the fractographic examination by scanning electron microscopy. To assess the mixed microstructure consisting mostly of the acicular ferrite, alongside proeutectoid ferrite grains and pearlite, the samples were held at 1250 °C for 30 min and subsequently cooled instill air. Cleavage fracture was initiated in the matrix under the high plastic strains near the notch root of the four-point bending specimens without the participation of the second phase particles in the process. Estimated values of the effective surface energy for the V and the Ti-V microalloyed steel of 37 Jm−2 and 74 Jm−2, respectively, and the related increase of local critical fracture stress were attributed to the increased content of the acicular ferrite. It was concluded that the observed increase of the local stress for cleavage crack propagation through the matrix was due to the increased number of the high angle boundaries, but also that the acicular ferrite affects the cleavage fracture mechanism by its characteristic stress–strain response with relatively low yield strength and considerable ductility at −196 °C.

Two new 3d transition metals AlCrCuFeTi and AlCrCuFeV high-entropy alloys: phase components, microstructures, and compressive properties

Two 3d transition metal high-entropy alloys, AlCrCuFeX (X = Ti, V), were prepared by a vacuum arc-melting, and their phase components, microstructures, and compressive properties were investigated. The phase components of the alloy with X = V were the BCC + FCC dual-phases, while that of the X = Ti alloy was composed of a multi-phase structure (2BCC + FCC + Laves + L21). Core–shell structural dendrites, together with discontinuous inter-dendrites formed in the X = Ti alloy, while netlike inter-dendrites surrounded the cellular dendrites in the X = V alloy. Moreover, the X = V alloy possessed a good synergy in strength and plasticity (ultimate strength: 2138 MPa, plastic strain: 7.1%) due to the ameliorating role of the netlike Cu-rich FCC inter-dendrites on the crack extension, while cleavage fracture mechanism for the X = Ti alloy caused by CrTi-rich Laves phase deteriorated its strength and ductility. The high hardness of the AlCrCuFeX alloys with X = Ti and V reaches 714 ± 27 and 565 ± 22 HV, significantly larger than 495 ± 15 HV of the alloy with X = Ni.

PM versus IM Ti-5Al-5V-5Mo-3Cr Alloy in Mechanical Properties and Fracture Behaviour

The comparisons of mechanical properties and fracture behaviour between as-consolidated PM Ti-5553 alloy and as-cast IM Ti-5553 alloy were investigated through tensile, fracture toughness and impact toughness tests in this research. The slightly higher strength but much higher ductility and toughness can be identified in IM alloy specimens, which is also confirmed by the fracture behaviour of the specimens after mechanical tests. IM alloy specimens always exhibit the ductile dimple fracture mechanism in the different tests, while the fracture mechanism of PM alloy specimens indicates a high loading rate sensitivity, changing from the mixed ductile-brittle quasi-cleavage fracture into the brittle cleavage fracture mechanism accompanied by the remarkable decrease of impact toughness. The relatively low mechanical properties, especially the ductility and the brittle fracture behaviour of as-consolidated PM Ti-5553 alloy, are mainly explained by the differences in the initial microstructures between these two alloys.

Novel high entropy alloys of FexCo1-xNiMnGa with excellent soft magnetic properties

The FexCo1-xNiMnGa (FeNiMnGa, Fe0.5Co0.5NiMnGa, and CoNiMnGa) alloys are designed based on the high entropy effect and the model of the Ni2MnGa alloy in searching for the potential functional magnetic alloys with improved mechanical properties. Although the substitution of Ni by Fe or Co can reduce the ordering of the phases due to the high mixing of entropy, these three alloys are composed of ordered phases, which lead the alloys to show the brittle deformation behavior with the combined intergranular and cleavage fracture mechanisms. These alloys exhibit excellent soft magnetic properties with very low saturation magnetostriction coefficients. For CoNiMnGa alloy, the saturation magnetization can be 115.92 emu/g, and the Curie temperature is about 657.3 K. The addition of Co enhances the exchange interaction and makes the alloy demonstrate potential applications at higher temperatures. In addition, the single disordered solid-solution phase-formation rule of high entropy alloys is evaluated, which can shed light on the HEAs design.

The Cellular Automata Theory in Fracture Mechanisms Predictions

In the presentation the cellular automata theory is applied to model the evolution of cleavage and ductile fracture mechanisms in front of the crack in three-point-bent specimens containing crack, made of Hardox-400 steel. The experimental and numerical results were used to define: dead elements, weakened elements and healthy elements. One element in the cellular automata model represents one grain. In some of elements large inclusions/interstitials are present. They are randomly distributed; however, to some degree the distribution of these inclusions depend on the strain level. Each element of the cellular automata can change the actual state due to the primary stress distribution, orientation of the cleavage planes {100} and the neighborhood of the fractured grain. The computer program allows for reproduction of the fractured surface as well as to simulate fracture mechanisms in various geometrical and temperature conditions.

Fracture Analysis of an Aluminum Alloy Gear Pump Housing

The gear pump housing in bulldozer, which was made of cast Al-Si alloy, was found to break in two halves during running. In order to determine fracture reason of the gear pump housing failure analysis in detail were conducted by visual inspection, optical microscope and scanning electron microscope (SEM), chemical analysis, hardness inspection. Visual inspection showed that the crack source has a white semicircle on the interior side of the section and propagated toward the exterior surface of the housing with obvious radial state. Meanwhile, the inner wall has the wear trace in depth of 0.1 mm due to boring phenomenon. SEM observation indicated brittle cleavage fracture mechanism with many casting defects such as dendritic shrinkage and inclusion as well as initiated crack in gear pump housing. The casting defects probably promoted the initiation and propagation of the crack and decreased the loading capacity of the gear to impacting force from the inside shaft. The analysis results show that the failure of gear pump housing mainly resulted from serious boring phenomenon, dendritic shrinkage and the lower material performance.

Mg-ZrO2 Nanocomposite: Relative Effect of Reinforcement Incorporation Technique

Abstract In this study, ingot metallurgy and powder metallurgy processes were used to incorporate 0.2 and 0.7 vol% of nano-size ZrO2 particles reinforcement to develop magnesium nanocomposite. Both of the processing methods led to reasonably fair reinforcement distribution, substantial grain refinement and minimum porosity in the nanocomposites. Strengthening effect of nano-ZrO2 reinforcement in the magnesium matrix was greater when incorporated using the ingot metallurgy process. The matrix ductility and resistance to fracture were significantly improved due to the presence of nano-ZrO2 reinforcement when incorporated using the powder metallurgy process. Tensile fracture surfaces revealed that the less-ductile cleavage fracture mechanism of hexagonal close packed magnesium matrix has changed to more ductile mode due to the presence of nano-ZrO2 particles in all the processed nanocomposites.

Impact toughness of a gradient hardened layer of Cr5Mo1V steel treated by laser shock peening

Laser shock peening (LSP) is a widely used surface treatment technique that can effectively improve the fatigue life and impact toughness of metal parts. Cr5Mo1V steel exhibits a gradient hardened layer after a LSP process. A new method is proposed to estimate the impact toughness that considers the changing mechanical properties in the gradient hardened layer. Assuming a linearly gradient distribution of impact toughness, the parameters controlling the impact toughness of the gradient hardened layer were given. The influences of laser power densities and the number of laser shots on the impact toughness were investigated. The impact toughness of the laser peened layer improves compared with an untreated specimen, and the impact toughness increases with the laser power densities and decreases with the number of laser shots. Through the fracture morphology analysis by a scanning electron microscope, we established that the Cr5Mo1V steel was fractured by the cleavage fracture mechanism combined with a few dimples. The increase in the impact toughness of the material after LSP is observed because of the decreased dimension and increased fraction of the cleavage fracture in the gradient hardened layer.

Phenomenological Modelling of Impact Toughness Transition Behaviour

The toughness transition behavior of ferritic steel results from the fact that two competing fracture mechanisms can be activated independently or progressively. Temperature, strain rate and the material ́s hardening properties are the major influences affecting the result of this competition between cleavage and ductile fracture mechanisms. An elastic visco-plastic plasticity model with stress-state dependent yielding and isotropic hardening forms the basis of a model to predict the Charpy impact toughness properties of steels with bcc crystal structure for transition behavior. A scalar damage variable is coupled into the yield potential in order to capture the effects of damage induced softening. The corresponding damage evolution law considers damage initiation criteria for both mentioned fracture mechanisms. Material parameter identifications and successful model application in terms of Charpy impact toughness tests are demonstrated.

Effect of Li on microstructure, mechanical properties and fracture mechanism of as-cast Mg–5Sn alloy

This work explores trace addition of Li on the magnesium alloy Mg–5Sn, with an emphasis laid on the microstructure, mechanical properties and fracture mechanism. The results reveal that with trace addition of Li, a new Li2MgSn phase forms distributed along the grain boundaries. When the concentration of Li increases, the secondary dendrite arm spacing of the primary α-Mg phase can be refined and the formation of Li2MgSn becomes more favored, and even totally substitute Mg2Sn phase finally. Trace addition of Li can significantly increase the yield stress and elongation to failure of Mg–5Sn alloy during both tensile and compressive tests, and manifests the highest ultimate strengthening effect during compression. The optimal comprehensive mechanical property is achieved in Mg–5Sn–0.3Li, with an ultimate compressive strength of 334MPa. With the increment of Li, the fracture mechanism shifts from mixture of cleavage fracture and dimple fracture in Mg–5Sn alloy to the mechanism of cleavage fracture in Mg–5Sn–0.3Li and Mg–5Sn–1Li alloy.

EBSD characterization of secondary microcracks in the heat affected zone of a X100 pipeline steel weld joint

In order to get better understanding of the mechanism of cleavage fracture in the heat affected zone (HAZ) of X100 pipeline steel, secondary microcracks underneath the brittle fracture surface of a Charpy impacted sample with the notch located in the HAZ were characterized using electron backscattered diffraction. Since the coarse grained (CG) HAZ and intercritically reheated coarse grained (ICCG) HAZ are well accepted as the weakest region in the HAZ, the cleavage secondary microcracks in these two regions were observed respectively. Initiation and propagation of cleavage microcracks were discussed. The results show that the fracture behavior is obviously influenced by local microstructure. There are more secondary microcracks in the ICCGHAZ than in the CGHAZ which shows different probability for microcrack nucleation. Fracture mechanism changes from nucleation control in the CGHAZ to propagation control in the ICCGHAZ. The main reason for the increased possibility of secondary microcracks formation and the change in fracture mechanism is due to the formation of coarse necklacing martensite–austenite constituent in the ICCGHAZ. The results also show that high angle boundary, with the misorientation larger than $$45^{\,\circ }$$ , is effective in deflecting or arresting brittle cracks, while low angle boundary ( $$15^{\,\circ }{-}45^{\,\circ }$$ ) seems not. Most preferred crack planes are {100}, with decreasing probability of {110}, {112}, {123}.

Effect of crystallographic texture on the cleavage fracture mechanism and effective grain size of ferritic steel

The effect of crystallographic texture on impact transition behavior has been studied in a low-carbon steel. Crystallographic texture was found to influence the general yield temperature through its effect on the plastic constraint factor. The effective grain size depends on the angle between the {001} cleavage planes of the neighbouring crystals, rather than the grain boundary misorientation angle as determined from electron backscattered diffraction analysis considering the angle–axis pair.