Dimple Rupture Research Articles

Substantially improved room-temperature tensile ductility in lightweight refractory Ti-V-Zr-Nb medium entropy alloys by tuning Ti and V content

Lightweight high/medium-entropy alloys (H/MEAs) possess attractive properties such as high strength-to-weight ratios, however, their limited room-temperature tensile ductility hinders their widespread engineering implementation, for instance in aerospace structural components. This work achieved a transformative improvement of room-temperature tensile ductility in Ti-V-Zr-Nb MEAs with densities of 5.4–6.5 g/cm3, via ingenious composition modulation. Through the systematic co-adjustment of Ti and V contents, an intrinsic ductility mechanism was unveiled, manifested by a transition from predominant intergranular brittle fracture to pervasive ductile dimpled rupture. Notably, the modulated deformation mechanisms evolved from solitary slip toward collaborative multiple slip modes, without significantly compromising strength. Compared to equimolar Ti-V-Zr-Nb, a (Ti1.5V)3ZrNb composition demonstrated an impressive 360% improvement in elongation while sustaining a high yield strength of around 800 MPa. Increasing Ti and V not only purified the grain boundaries by reducing detrimental phases, but also tailored the deformation dislocation configurations. These insights expanded the applicability of lightweight HEAs to areas demanding combined high strength and ductility.

Strengthening phase and mechanical property analysis of artificially aged Al7075 – Ni coated Al2024 composites

This technical paper demonstrates the possibilities of nickel (Ni) coated Al2024 powder reinforcement in an Al7075 matrix using the liquid stir casting technique. Additionally, the paper focuses on achieving stable properties by implementing artificial aging heat treatment. To apply the Ni coating, the electroless nickel plating technique was utilized, and a minimum coating thickness of 8 μm was determined to effectively prevent the dissolution of Al2024 powder reinforcements within the Al7075 matrix. Stir casting facilitated the uniform dispersion of the coated Al2024 powder up to a weight percentage of 7%. Subsequently, the Al7075 alloy and composites underwent artificial aging through solution heat treatment (SHT) at 450 °C for 4 h, followed by water quenching and aging at temperatures of 120, 150, and 180 °C. Aging at 120 °C was found to yield superior results compared to aging at 150 and 180 °C, thus identified as the optimum aging temperature. When the Ni coating thickness was increased beyond the optimal 8 μm, the resulting enhancements in hardness for both as-cast and peak-aged specimens, as well as the tensile strength, were not significant. The improvements observed were only marginal, ranging between 2 to 3%. Fracture surface analysis revealed that the predominant fracture mode in the Al7075 alloy was ductile, characterized by dimple rupture. In the as-cast Al7075-(7%, 8 μm) Al2024 composite, a mixed fracture mode comprising both brittle and ductile characteristics was observed. In the peak-aged (120 °C) Al7075-(7%, 8 μm)Al2024 composite, the overall fracture mode exhibited a dominant brittle nature. Analytical techniques including XRD, TEM, and EDS confirmed the presence of Mg2Si, MgZn2, CuAl2, and CuAl2Mg phases in the peak-aged (120 °C) Al7075-(7%, 8 μm) Al2024 composite. These phases contributed to the enhancement of the properties of both the Al7075 alloy and its composites. The developed composites can be used in automobile parts and aerospace applications.

An experimental fracture mechanics study of the combined effect of hydrogen embrittlement and loss of constraint

This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen assisted transgranular dimple rupture.

A non-local GTN model with two length scales – Application to ductile failure in a wide range of stress triaxiality

A non-local extension of the shear-modified Gurson model for porous plasticity is proposed, which is capable of preventing spurious strain localization and suitable for predicting crack initiation and growth under general loading conditions. In the extended model, the progression of shear failure is separated from flat dimple rupture by the assumption that these failure mechanisms are characterized by a deviatoric and a dilatational material length parameter, respectively. The two length parameters are introduced through a delocalization process based on the non-local integral approach evaluated on the current configuration. A comprehensive numerical investigation is conducted to disclose the influence of these length parameters. For this purpose, five different specimen geometries are considered covering a wide range of stress triaxiality including pure shear. It is observed that separation of the length parameters is essential for predicting crack branching driven by shear localization, e.g., the flat dimple-to-shear failure transition of a crack that approaches and penetrates a free surface.

Evaluation of mechanical properties of Ti–6Al–4V BCC lattice structure with different density gradient variations prepared by L-PBF

Based on the optimized pore size distribution and relative density, the functionally graded lattice structure is capable of achieving better mechanical properties than a uniform lattice structure with similar porosity. In this study, in order to investigate the effect of the strategy of density gradient variation on the lattice structure with similar porosity, three different density gradients were designed for the BCC-based lattice structure in parallel and perpendicular to the loading direction based on Ti–6Al–4V alloy powder and laser beam powder bed fusion technology, respectively. The results showed that the elastic modulus, yield strength and compressive strength of the lattice structure with a density varying as a logarithmic function perpendicular to the loading direction were 39.91%, 51.85% and 51.72% higher than the uniform lattice structure respectively. Moreover, the layer-by-layer collapse phenomenon was the primary aspect impacting the energy absorption in the lattice structures with density gradient parallel to the loading direction. By observing the fracture morphology of the lattice structures, it was found that all specimens exhibited dimple and smooth plane morphological features for a mixed failure mechanism of cleavage and dimple rupture, indicating that the failure mode of the lattice structures are a combination of brittle and ductile deformation. Therefore, a reasonable density gradient design can significantly improve the mechanical properties of the lattice structure with similar porosity. This work provides valuable inspiration for the design of lattice structures in engineering applications.

Individual and Combined Effects of Reinforcements on Fractured Surface of Artificially Aged Al6061 Hybrid Composites

The present work mainly focuses on a comparative study of the individual and combined effect of reinforcements on tensile strength and fracture surface analysis of Al6061 alloy and its composites during artificial aging. SiC and B4C are the two reinforcements used in the present work for the preparation of Al6061 composites by the stir casting process, and the reinforcement percentage from 2, 4, and 6 wt.% varied. Both Al6061 alloy and its composites are solution-treated at 558 °C/2 h and artificially aged at 100 and 200 °C for different time intervals to achieve peak aging. The results show substantial improvement in ultimate tensile strength during low temperature aging at 100 °C. Approximately 80–110% increase in UTS value is observed in both individual and hybrid composites compared to Al6061 alloy. The mechanism of failure governing the tensile strength for both alloy and its composites is thoroughly analyzed and discussed using a scanning electron microscope. The morphology of crack propagation is also studied to determine the mechanism of failure. Al6061 alloy shows ductile failure due to coarser dimples. Al6061-SiC composites show particle-matrix interface cracking and shear failure. Al6061-B4C composites show elongated dimple rupture mode of failure, whereas Al6061-SiC + B4C hybrid composites fail due to nucleation growth and mixed fracture mode.

Hydrogen embrittlement susceptibility of dual-phase DP980 steel and its prevention with the help of coatings

ABSTRACTAHSS DP980 is widely used in automobiles and is susceptible to hydrogen embrittlement (HE), which negatively impacts the mechanical properties of the material. The aim of this work was to prevent the DP980 steel against the HE with the help of various surface coatings. Hydrogen charging of bare and various coated samples was accomplished by electrochemical reactions at a current density of 40 mA/cm2 for 24 hours. After charging, SSRT testing was performed at a strain rate of 10−5/s. Coating characterisation was completed by Coating Porosity Test, EDX and Microhardness test. Test results stated that the Ni-Cr and Gr ACS coating effectively reduces the hydrogen diffusion inside the material with improved mechanical properties, and the HE ratio decreases from 43.3% for the bare sample to 9.6% and 7.3% for those coated samples, respectively. The microhardness test also gave a high microhardness value for these coatings. Fracture surface examination shows that the bare hydrogen charged specimen failed in a mixed-mode manner, whereas all regions of Ni-Cr and Gr ACS-coated specimen fracture surface comprise dimple rupture.

Experimental Investigation to Confirm the Presence of TiB2 Reinforcements in the Matrix and Effect of Artificial Aging on Hardness and Tensile Properties of Stir-Cast LM4-TiB2 Composite

The present work focuses on the effect of multistage solution heat treatment (MSHT) and artificial aging on two-stage stir-cast LM4 + TiB2 (1, 2, and 3 wt.%) composites on the mechanical properties as compared to as-cast and single-stage solution heat-treated (SSHT) composites. Two novel tests, viz. the confirmation hardness test and the chemical analysis test, were performed to ensure the soundness of the casting and uniform distribution of TiB2 within the matrix. Samples subjected to MSHT + aging at 100–200 °C displayed the highest hardness and UTS values compared to as-cast and SSHT + aging at 100–200 °C samples. Compared to as-cast alloy, peak-aged samples of 1–3 wt.% (MSHT + aging at 100 °C), hardness values improved from 107–150%, and UTS values improved from 47–68%. The presence of metastable phases (θ′-Al2Cu and θ″-Al3Cu) and of hard TiB2 particles are the reason for the improvement in the properties. Peak aged LM4 + 3 wt.% TiB2 composite displayed the highest hardness of 175 VHN and UTS of 251 MPa. Fracture analysis of the LM4 alloy showed dimple rupture, and its composites revealed quasi-cleavage fracture. Based on the overall results, the inclusion of TiB2, MSHT, and artificial aging treatment on the LM4 alloy significantly influenced the composites’ mechanical properties.

Tensile properties on dissimilar welds between 11Cr-ferritic/martensitic steel and 316 stainless steel after thermal aging

The aim of this study was to evaluate the tensile properties and microstructures of dissimilar welds between 11Cr-ferritic/martensitic steel and 316 stainless steel after thermal aging at temperatures between 400 and 600 °C up to 30,000 h. Characterization of microstructure was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microstructural analysis showed that the microstructure in the weld metals (WMs) consisted of lath martensite containing a small amount of residual austenite. Thermal aging hardening of WMs occurred at 400 and 450 °C due to the effects of both α-α’ phase separation and G-phase precipitation. However, there was no significant change in the total elongation, and fracture surfaces indicated that very fine dimpled rupture was predominant rather than the cleavage rupture. It was suggested that lath martensite phases enhanced the tensile strength due to phase separation, while residual austenite played a role in keeping elongation as a soft phase.

Investigation on shear strength of AISI 431 martensitic stainless steel subjected to high temperature heating

Martensitic stainless steels (MSS) are often used to manufacture fasteners such as bolts that are used to secure critical components operating at high temperature environments. Particularly, it is often observed that AISI 431 MSS bolts fractured by shear when force is applied to untighten it during maintenance. This experimental study aims to investigate the shear strength and hardness of AISI 431 MSS after subjected to high temperature heating and deduce the failure mechanism of the material when shear load is applied. Double shear test and hardness test were conducted on numerous AISI 431 MSS rod samples after subjected to heating that ranged from room temperature (RT) to 900 °C while fracture morphology was studied by means of Field Emission Scanning Electron Microscope (FESEM). Results showed that shear strength after heating was almost consistent with little changes up to 500 °C followed by a slight degradation of strength between 500 °C and 700 °C. As for the heating above 700 °C, a significant increase in strength was observed. Adding on, the notable decrease of hardness between 500 °C and 700 °C suggested that this is the tempering temperature region where ductility is enhanced. Fracture morphology showed that failure is contributed by a mixed mode of brittle and ductile failure with ductile failure by dimple rupture being the final failure mechanism.

An investigation on fracture toughness of the heat‐affected zone in the welded joints of 13% Cr‐4% Ni martensitic stainless steels

AbstractFracture behavior of the welded joints of 13% Cr‐4% Ni stainless steels deposited using multipass flux‐cored arc welding process was examined in both as‐welded and heat‐treated conditions, and JIc tests were performed. A crack starter notch was placed one millimeter far from the fusion line to evaluate the heat‐affected zone (HAZ). Unstable crack propagations toward the weld metal occurred during stable tearing for both as‐welded and heat‐treated HAZ. Only single‐point fracture toughness Ju values could be determined before fracture instability. The fracture toughness of the as‐welded HAZ was about 60% lower than that of the substrate steel but get significantly improved by heat‐treatment, reaching a value even slightly higher than that of the substrate steel. The fracture mode of the stable tearing was transformed from a mix of cleavage and dimpled rupture occurring by micro‐void coalescence mechanism for the as‐welded condition to a totally dimpled rupture in the case of the heat‐treated material.

Compressive response and microstructural evolution of in-situ TiB2 particle-reinforced 7075 aluminum matrix composite

The hot forming behavior, failure mechanism, and microstructure evolution of in-situ TiB2 particle-reinforced 7075 aluminum matrix composite were investigated by isothermal compression test under different deformation conditions of deformation temperatures of 300–450 °C and strain rates of 0.001-1 s−1. The results demonstrate that the failure behavior of the composite exhibits both particle fracture and interface debonding at low temperature and high strain rate, and dimple rupture of the matrix at high temperature and low strain rate. Full dynamic recrystallization, which improves the composite formability, occurs under conditions of high temperature (450 °C) and low strain rate (0.001 s−1); the grain size of the matrix after hot compression was significantly smaller than that of traditional 7075Al and ex-situ particle reinforced 7075Al matrix composite. Based on the flow stress curves, a constitutive model describing the relationship of the flow stress, true strain, strain rate and temperature was proposed. Furthermore, the processing maps based on both the dynamic material modeling (DMM) and modified DMM (MDMM) were established to analyze flow instability domain of the composite and optimize hot forming processing parameters. The optimum processing domain was determined at temperatures of 425-450 °C and strain rates of 0.001-0.01 s−1, in which the fine grain microstructure can be gained and particle crack and interface debonding can be avoided.

Microstructure characterization and mechanical properties of Al-matrix composites reinforced by artificially-cultured diatom frustules

In this study, to enhance the understanding of characteristics and interaction of Al-SiO2 composites manufactured with hot extrusion, artificially-cultured diatom frustules (DFs) reinforced aluminum matrix composites have been manufactured by isostatic cool pressing and hot extrusion process. The effect of DFs content on composite particle characteristics, microstructures, phase distribution, fracture surfaces and mechanical properties of the hot-extruded composites were investigated in details. For hot-extruded Al-DFs composite samples, two typical morphologies of DFs were identified: (a) those with apparent matrix-DFs voids were formed by deformability inconsistency between matrix and reinforcement; (b) those with strong matrix-DFs interfacial integration originated from aggregated nano-sized DFs particles and potential reduction reaction between Al and SiO2. Uniform distribution of nano-sized DFs particles surrounding matrix grain boundaries significantly improved mechanical properties of Al-DFs composites. When DFs increased to 3 wt%, the fractography were transformed from dimple rupture of ductile failure into brittle-dominant fracture mode. Meanwhile, aluminum composites with 3 wt% DFs illustrated optimum tensile strength (185.2 MPa), yield strength (117.6 MPa) with elongation (13.5%), which exhibited strength increase of 20.7% and 32.0% respectively, compared with that of pure aluminum.

Rotary Friction Welding of Inconel 718 to Inconel 600

Nickel-based superalloys exhibit excellent high temperature strength, high temperature corrosion and oxidation resistance and creep resistance. They are widely used in high temperature applications in aerospace, power and petrochemical industries. The need for economical and efficient usage of materials often necessitates the joining of dissimilar metals. In this study, dissimilar welding between two different nickel-based superalloys, Inconel 718 and Inconel 600, was attempted using rotary friction welding. Sound metallurgical joints were produced without any unwanted Laves or delta phases at the weld region, which invariably appear in fusion welds. The weld thermal cycle was found to result in significant grain coarsening in the heat effected zone (HAZ) on either side of the dissimilar weld interface due to the prevailing thermal cycles during the welding. However, fine equiaxed grains were observed at the weld interface due to dynamic recrystallization caused by severe plastic deformation at high temperatures. In room temperature tensile tests, the joints were found to fail in the HAZ of Inconel 718 exhibiting good ultimate tensile strength (759 MPa) without a significant loss of tensile ductility (21%). A scanning electron microscopic examination of the fracture surfaces revealed fine dimpled rupture features, suggesting a fracture in a ductile mode.

Microstructure and mechanical properties of hot-pressed Ti–Co–Si compounds reinforced by intermetallic phases

Monolithic TiCo and Ti5Si3 phases and Co(ss)/Ti5Si3–Ti3Co2Si nanocomposite were fabricated via mechanical alloying followed by hot-pressing (HP). The XRD patterns of bulk samples showed a general decrease in peak widths after hot-pressing owing to an increase in crystal size caused by exposure to high temperature during the HP process. Nevertheless, the nano-crystallinity of the structure was retained after HP and no new phase was formed. As a dominant fracture mode, this material exhibited an intergranular fracture feature with low dimpled ruptures. Dry-sliding wear behavior was evaluated at room and high temperatures. The mean coefficient of friction and wear rate gradually decreased with increasing temperature due to the presence of discontinuous thin tribo-oxide and tribo-nitride layers on the worn surface. The wear rate obtained in this work was ~10 times lower than that previously reported for Ti compounds.

Evaluation of synergistic effect and failure characterization for Ni-based nanostructured coatings and 17-4PH SS under cavitation exposure in 3.5 wt % NaCl solution

The Ni/SiC nanocomposite and Ni nanocrystalline coatings were electrodeposited on a 17-4 precipitation hardening stainless steel substrate. The failure characterization of specimens as well as synergistic effect of cavitation and corrosion, and the influence of the mechanical, electrochemical, and microstructural attributions of the Ni-based nanostructured and 17-4 PH stainless steel substrate on their cavitation resistance were investigated. The surface morphology of 17-4PH stainless demonstrated the quasi-cleavage failure mechanism, and both coatings had the dimpled rupture morphology after the cavitation deterioration in both mediums. The results also represented that the synergism contribution of cavitation and corrosion was 29.5, 22.6, and 6.3% for 17-4 PH stainless steel substrate, Ni nanocrystalline, and Ni/SiC nanocomposite coatings, correspondingly.

Microstructure and mechanical properties of Al−Mg−Si alloy U-shaped profile

Abstract To get a full understanding of hot extrusion, solid solution treatment and aging process on the Al−0.56Mg− 0.63Si alloy, the microstructure and mechanical properties of a U-shaped profile were studied through optical microscopy, scanning electrical microscopy, transmission electrical microscopy, hardness, and tensile tests. The coarse equiaxed grains existed near the profile edge as a result of the dynamic recrystallization nucleation and exceeding growth during hot extrusion. The fibrous deformed and sub-structured grains located between the two coarse grain layers, due to the occurrence of work-hardening and dynamic recovery. Perpendicular needle β′′ precipitates were distributed inside the grain, and obvious precipitates-free zone appeared after aging treatment. The tensile strength, yield strength and elongation of the aged Al−Mg−Si alloy U-shaped profile were no less than 279.4 MPa, 258.6 MPa, and 21.6%, respectively. The fracture morphology showed dimple rupture characteristics. The precipitates and grain boundaries played key role in the strengthening contribution.

Hard Cladding by Supersolidus Liquid Phase Sintering: An Experimental and Simulation Study on Martensitic Stainless Steels

Martensitic stainless steels are suitable for diverse structural applications but degrade when subjected to wear-prone activities in service. To enhance their service life, the densification of high Cr, martensitic, X190CrVMo20-4-1 tool steel powder on two different martensitic stainless steel substrates via supersolidus liquid-phase sinter (SLPS) cladding was investigated. The objective was to assess the influence of the difference in compositions of the martensitic stainless steels employed as substrates on the interfacial diffusion, microstructure, hardness and bonding strength of the steel-to-steel claddings. Computational thermodynamics and diffusion simulations were employed to supplement experimental findings. Owing to interdiffusion, a M7C3 carbide-free, banded region exists in the X190 adjacent to the interface with the width dictated by chemical potential gradient of carbon. The hardness of the substrate was lower near the interface region because of carbon enrichment, which promoted the presence of retained austenite. An interfacial strength of 798 MPa was achieved with fairly ductile X190 matrix near the cladding interface as the fracture surface was characterized by mixed fracture modes of dimple rupture and cleavage with localized quasi-cleavage features. Experimental observations and computational simulations are in agreement. The implications of the SLPS cladding technique are discussed in the context of tool development.

Experimental investigation of fatigue failure in an ASTM A401 compression spring of a passenger car

Abstract This work outlines the fatigue failure in a failed helical coiled compression spring of a passenger car used as a commercial vehicle. As a failure investigation case study, the fractured left rear axle suspension spring of the car is thoroughly examined. The fractographic investigations included the visual examination, spectroscopy, microstructure analysis, SEM and micro-hardness testing. The foremost causes of failure have been identified. From the SEM fractograph, microvoid coalescence is observed at the grain boundaries on of the fractured surface which eventually resulted in intergranular dimple rupture. Residual stress is also measured on the fractured surface through two-dimensional (2-D) imaging using a Debye Scherrer ring, with the conclusion that the spring under investigation experiences micro-stress due to the variation of theintergranular spacing resulting from the vibrations induced by overloading, road conditions and prolonged use which eventually results in a rupturing of the compression spring.

Experimental investigation of fatigue failure in an ASTM A401 compression spring of a passenger car

Abstract This work outlines the fatigue failure in a failed helical coiled compression spring of a passenger car used as a commercial vehicle. As a failure investigation case study, the fractured left rear axle suspension spring of the car is thoroughly examined. The fractographic investigations included the visual examination, spectroscopy, microstructure analysis, SEM and micro-hardness testing. The foremost causes of failure have been identified. From the SEM fractograph, microvoid coalescence is observed at the grain boundaries on of the fractured surface which eventually resulted in intergranular dimple rupture. Residual stress is also measured on the fractured surface through two-dimensional (2-D) imaging using a Debye Scherrer ring, with the conclusion that the spring under investigation experiences micro-stress due to the variation of theintergranular spacing resulting from the vibrations induced by overloading, road conditions and prolonged use which eventually results in a rupturing of the compression spring.