Intergranular Mode Research Articles

The combined effect of corrosive media and its temperature on the fatigue performance of AA5086-H321 weld joints

This work investigates the fatigue behavior and damage mechanisms in the friction stir weld joints of AA-5086-H321 alloy under the combined effect of media: 3.5 % NaCl and air and its temperature: 24°C and ∼0°C. Sinusoidal loading tests were performed at different load ratios: 0.1 and 0.5, in load-controlled mode at 1 Hz in 3.5 % NaCl. The fatigue lives of the weld joints deteriorated notably under 3.5 % NaCl by selectively attacking its vulnerable nugget zone and heat-affected zone, while the effect of low temperature (∼0°C) was beneficial in both air and 3.5 % NaCl. Two types of crack initiation mechanisms were identified irrespective of the other test conditions: pit-to-pit type, favored at low-stress amplitudes, and pit-to-crack type, favored at high-stress amplitudes. A few cases of crystallographic corrosion pitting were also observed and were accredited to the inherited crystal defects within the Al-matrix. The effect of surrounding temperature was apparent on the crack growth mechanism with limited or no impact on the crack initiation mechanism. The fatigue cracks grew in the intergranular mode under 3.5 % NaCl, while its quantitative presence was decreased at ∼0°C, irrespective of the media, which was attributed as one of the key factors for the increased fatigue performance at ∼0°C.

In-situ synthesized multi-component particle reinforced TiAl composite with excellent mechanical properties

In order to overcome the inherent trade-off between strength and ductility of TiAl alloys, the BN nanosheets were added in Ti-48Al-2Cr-2Nb alloy. Ti-48Al-2Cr-2Nb-xBN (x = 0, 0.1, 0.5, 1, 2 wt%) alloys were prepared by vacuum arc melting (VAR), and the microstructures displayed a fully lamellar structure accompanied by simultaneous precipitation of Ti2AlN and TiB2 phases. The reaction of BN nanosheets with the Ti and Al elements resulted in formation of Ti2AlN and TiB2 phases. The lamellar colony size of TiAl composites was refined from 276.7 to 29.4 µm with BN content increasing from 0 to 2 wt%. While the lamellar spacing displayed an initial decrease followed by an increase, which was attributed to the synergistic effect of Ti2AlN and TiB2. Room temperature compression results showed that the compressive strength surged from 1653 to 2272 MPa as BN content rose from 0 to 0.5 wt%, with the highest compression strain of 34.7 % at 0.5 wt% BN. The high temperature compression results showed that the compressive strength increased from 887 to 1399 MPa as the BN content increasing from 0 to 1 wt%. Crack extension took the form of both trans-granular and inter-granular modes. The strengthening effect of TiAl composites was attributed to grain refinement strengthening and secondary phase strengthening via Ti2AlN and TiB2.

Simulation of Microscopic Fracture Behavior in Nanocomposite Ceramic Tool Materials

In this paper, the microstructures of nanocomposite ceramic tool materials are represented through Voronoi tessellation. A cohesive element model is established to perform the crack propagation simulation by introducing cohesive elements with fracture criteria into microstructure models. Both intergranular and transgranular cracking are considered in this work. The influences of nanoparticle size, microstructure type, nanoparticle volume content and interface fracture energy are analyzed, respectively. The simulation results show that the nanoparticles have changed the fracture pattern from intergranular mode in single-phase materials to intergranular–transgranular–mixed mode. It is mainly the nanoparticles along grain boundaries that have an impact on the fracture pattern change in nanocomposite ceramic tool materials. Microstructures with smaller nanoparticles, in which there are more nanoparticles dispersed along matrix grain boundaries, have higher fracture toughness. Microstructures with a nanoparticle volume content of 15% have the most obvious transgranular fracture phenomenon and the highest critical fracture energy release rate. A strong interface is useful for enhancing the fracture toughness of nanocomposite ceramic tool materials.

Micro-mechanical behavior of lamellar structured Ti6Al4V alloy upon compression via experimental and crystal plasticity study

The micro-mechanical behavior of lamellar structured Ti6Al4V alloy to a small compressional strain of 4.5% was studied. Strain and stress intends to localize along grain boundaries (GBs), triple points (TPs) as well as grain interiors (GIs). The maximum strain and stress is found at some GB-TPs with soft and grain clustering mostly owing to their strong strain incompatibility. Their heterogeneous distribution is highly related to individual crystallographic orientation, their size, shape and grain-grain interactions. The local dislocation behavior forms ledges along with Geometrically necessary dislocations (GNDs) to accomodate strain incompatibility. EBSD and crystal plasticity (CP) simulation disclosed some long range meso-bands (about 45° to loading direction) which crosse multi-grains and some mini-bands within grain interiors (GIs) reflecting a concentrated strain behavior and ductile nature of the Ti6Al4V material. Prismatic slip dominates Ti6Al4V lattice (mainly from surface dislocation activation) determined by its low critical resolved shear stress (CRSS) which is away from some findings of literature. Dislocation is found commenced from some high angle GBs (HAGBs) and extened into GI which can pass through GBs of both low (LA) and high angles given their slip plane coherency. The substructures (α-lamellae) separated by LAGBs within α-colonies, have no significant effect to dislocation movement. Fatigue failure can be inferred from the early stage micro-mechanical behavior through accumulated plastic strain and strong strain incompatability. Fractography reaveals fatigue failure from surface, sub-surface, shearing, cleavage and intergranular modes are presumed highly related to surface defects, compressional residual stress gradient, easy prismatic slip and soft and hard GBs. Multiple crack origins from such combinations are deemed for reduced fatigue life of shot peened samples regardless of the surface compressional residual stress being generated.

Enhancement of thermal shock resistance of Al2O3–MgAl2O4 composites by controlling the content and distribution of spinel phase

Herein, the Al2O3–MgAl2O4 composites were prepared using magnesite as variate and bauxite and alumina as main raw materials through a solid oxide reaction at 1500 °C. The content and distribution of MgAl2O4 on Al2O3–MgAl2O4 composites were adjusted with the aim of altering the microstructure and mechanical property, thereby enhancing their thermal shock resistance. The studies observed that by adding 5 wt% magnesite, a limited amount of MgAl2O4 particles can be generated, partially distributing on the microstructure; thus, the crack's propagation cannot be hinder with ease, which leads to a more limited residual strength ratio (68.97%). When 10 wt% of magnesite is added, the solid-solution (Mg1-xAl2x/3)Al2O4 is formed, and the apparent growth and wide distribution of spinel grains is induced, which pins the propagated front crack and enhances the toughening mechanism. The essential fracture path is altered from the predominantly transgranular mode to the mostly intergranular mode, and the residual strength ratio attains the highest value (94.36%).

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.

Microstructure, mechanical properties and fatigue crack growth behavior of an Al-Zn-Mg-Cu-Si-Zr-Er alloy fabricated by laser powder bed fusion

An Al-Zn-Mg-Cu alloy modified by Si-Zr-Er was prepared by laser powder bed fusion (LPBF), the tensile strength of as-printed alloy reached 445 MPa. Precipitates observed in the alloy included Al2CuMg, MgZn2, Mg2Si and Al3(Zr,Er). The accumulated dislocations near the crack tip could combined with the second phases to hinder the propagation of fatigue crack. Transgranular and intergranular modes of fatigue crack growth were both observed in the alloy, and deflection was prone to occur when the crack encountered grains with high misorientation. The fatigue life (N = 1.02 × 106 cycles) reached highest under the condition of r = 0.5 and Pmax = 0.3kN.

Effect of tool pin geometry and multi-pass intermittent friction stir processing on the surface properties of aerospace grade aluminium 7075 alloy

Multiple pass intermittent friction stir processing was conducted on the surface of an aerospace grade AA7075 alloy using pentagonal and hexagonal profiled pins and its microstructural evolution and mechanical properties were investigated. Results showed a defect-free cross-section with better material mixing for the specimens processed using pentagonal pin. Application of 6 intermittent friction stir processing passes reduced average grain size to 2.32 μm as a result of dynamic recrystallization. Texture index improved after 6 intermittent friction stir processing passes with a higher volume of deformation textures. Microhardness and tensile strength of the alloy increased to 125.5 HV, and 476 ± 10 MPa after 6 intermittent friction stir processing passes due to precipitation strengthening, grain refinement, and dislocation density. Fracture surface investigation revealed an intergranular mode of failure.

The influence of rotation angle between layers in laser-based powder bed fusion on the hydrogen embrittlement of Ni

This work investigated the relation between hydrogen embrittlement and the scanning rotation angle between adjacent layers in laser-based powder bed fusion (L-PBF) fabricated Ni. The microstructure analysis indicated that all samples failed with an intergranular mode, and all intergranular facets were propagated along the fine-grain region. However, the rotation angle during manufacturing changed the grain shape and size distribution in Ni, which in turn modified the pathway for hydrogen-induced cracking. The sensitivity of LPBFed Ni to hydrogen embrittlement is influenced by the scanning strategy, and the samples with a rotation angle of 0° present the highest resistance to hydrogen embrittlement.

Hydrogen-induced degradation behavior of nickel alloy studied using acoustic emission technique

Hydrogen embrittlement susceptibility of nickel alloy (Inconel 625) is examined by a conventional strain rate tensile test (10−3 s−1) equipped with an acoustic emission (AE) setup. From the spectral analysis of AE signals coupled with the clustering procedure followed by the electron microscopic characterization, we have demonstrated the capability of the AE technique in identifying the hydrogen-dislocation interactions, crack initiation and propagation in various hydrogen-charged samples. The enhanced hardening behavior of hydrogen-charged samples is investigated from the viewpoint on the elementary dislocation theory, using a phenomenological relationship between the acoustic emission power and the experimentally measurable strain hardening parameters derived from the monotonic tensile test. In this way, we have shown that the mobile dislocation density increases with hydrogen content. In addition, the average velocity of dislocations is assessed from the AE data, and it is found to be reduced with the hydrogen concentration. The statistical crack analysis reveals the crack transition from the transgranular to intergranular mode in the samples failed under the hydrogen environment. The competition between these two fracture modes is explained by relating the hydrogen concentration profile with loading conditions.

Microstructure development and mechanical properties of a C-Mn-Si-Al-Cr cold rolled steel subjected to quenching and partitioning treatment

The microstructural evolution and mechanical behavior of Fe-0.2C-2.7Mn-1.2Si-0.8Al-0.3Cr (wt.%) quenching and partitioning steel were studied. Partitioning was performed at 400, 440, and 480 °C for various durations from 30 to 300 s to obtain different fractions of retained austenite. Scanning electron microscopy, dilatometry, and X-ray diffraction analysis as well as uniaxial tensile testing were employed to characterize the microstructural evolutions and mechanical properties. The resulting microstructures were primarily composed of tempered lath martensite, bainitic ferrite, retained austenite, and blocky fresh martensite. The lower partitioning temperature of 400 °C was appropriate for efficient carbon partitioning and stabilization of retained austenite at room temperature. However, the retained austenite fraction decreased with increasing partitioning temperature, due to accelerated bainite formation and carbide precipitation at higher partitioning temperatures. The tensile results revealed that all specimens had tensile strength values of more than 1128 MPa and yield strength values of over 830 MPa. Additionally, prolonging the isothermal holding time had a negative effect on the yield strength due to bainite formation, while it enhanced the elongation. Fractography analysis showed that specimens treated at the partitioning temperature of 400 °C were fractured in a relatively ductile mode, but the fracture nature altered to an intergranular mode after partitioning at higher temperatures.

Compressive vs. tensile yield and fracture toughness behavior of a body-centered cubic refractory high-entropy superalloy Al0.5Nb1.25Ta1.25TiZr at temperatures from ambient to 1200°C

The microstructure of high-entropy alloys with refractory elements and Al as constituents can be considered to be analogous to superalloys. These so-termed refractory high-entropy superalloys (RHSAs) can show remarkable compressive strength up to temperatures exceeding 1200 °C. Here, we examine the microstructure and properties - compressive, tensile, and fracture toughness - of a precipitation-hardened, body-centered cubic, RHSA, Al0.5Nb1.25Ta1.25TiZr, at ambient temperature (RT) to 1200 °C. Two dual-phase microstructures comprising ordered B2 (brittle) and disordered A2 (ductile) phases were produced in this alloy - one with B2 as the matrix, the other with A2 - for evaluation of the mechanical properties. Under compression, both microstructures display RT compressive strengths above 1.5 GPa and considerable ductility exceeding 40% at elevated temperatures; the alloy with the A2 matrix has ∼15% compressive ductility even at RT. However, properties are very different under tensile loading; at all temperatures, both microstructures fail predominately in an intergranular mode in the elastic regime at a fracture stress less than 200 MPa and ductility below 0.15%. The microstructure with the A2 matrix has a KIc fracture toughness of ∼15 MPa√m at RT, although at all temperatures above 800 °C, measured KIc values for both dual-phase microstructures are less than 5 MPa√m. In this study, we investigate the microstructural origin of these mechanical properties, and emphasize the importance of evaluating these alloys in tension.

Fracture behavior of Hastelloy X elaborated by laser powder bed fusion: Influence of microstructure and building direction

Laser Power Bed Fusion (LPBF) results in complex meso and microstructures, which significantly change the fracture mechanisms of Hastelloy X superalloys. This article aims to investigate the role played by original crack orientation (perpendicular or co-linear to sample lasing planes) on the fracture toughness values of LPBF HX. Single notched three-point bending samples were manufactured vertically (V) or horizontally (H) to estimate the fracture toughness for these two original crack directions. The highest value (about 1360 kJ/m²) is computed in V specimens with an original crack parallel to the lasing plane whereas a value of 1042 kJ/m² is estimated in H specimens with the second crack orientation. Compared to the values obtained in cast specimens, the fracture toughness of LPBF specimens is virtually 5–6 times higher. A non-self-similar cracking is specific to V specimens revealing a more tortuous crack path than in H specimens with self-similar cracking. In both cases, ductile failure is driven by both transgranular and intergranular modes. Nevertheless, the strong morphological texture contributes to promote intragranular fracture mode in V specimens which requires more energy than the one dissipated by an intergranular failure associated to H samples. Consequently, the fracture toughness is significantly higher in V specimens. • Fracture tests were conducted on Hastelloy X specimens elaborated by Laser Power Bed Fusion with two build directions. • In H specimens, intergranular fracture is dominant due to the grain boundaries aligned with the initial notch. • In V specimens, transgranular fracture occurs along with grain boundaries perpendicular to the initial notch. • The morphological texture is responsible for the fracture toughness anisotropy.

Enhanced mechanical properties of as-cast rare earth bearing magnesium alloy via elevated-temperature homogenization

The effects of elevated-temperature homogenization on the microstructure and mechanical properties of a magnesium alloy containing mischmetal rare earth (RE) were investigated. It was revealed that the homogenization treatment leads to spheroidization and partial dissolution of the Mg12RE phase in the eutectic constituent. For the optimum homogenized alloy (20 h at 535 ºC), the elimination of the deleterious eutectics and the solid solution strengthening effect of RE elements were the main contributing factors for improving the ultimate tensile strength and ductility. Accordingly, a 157 % increase in tensile toughness compared to the as-cast condition was recorded. Moreover, as a direct consequence of eliminating the eutectic constituent, homogenization led to a change in the fracture mechanism from a brittle intergranular mode to an intragranular cleavage with the incorporation of some ductile dimples. Accordingly, homogenization heat treatment was found to be an alternative enhancing route of strength-ductility trade off, besides thermomechanical processing (by hot deformation and dynamic recrystallization) and severe plastic deformation.

Numerical investigation on the temperature effect in nanometric cutting of polycrystalline silicon

In nanometric cutting, the deformation mechanism is significantly affected by the cutting temperature. In this paper, molecular dynamics simulation was conducted to investigate the cutting mechanism of polycrystalline silicon with increasing temperature. The surface generation, plastic deformation, and phase transition mechanism were discussed with consideration of the effect of grain boundaries and non-homogeneity of crystal orientation in workpiece. The results indicate that the surface generation and subsurface damage mechanism of polycrystalline silicon is greatly influenced by the cutting temperature. Squeeze of amorphous atoms into grain boundaries is observed at room temperature while sliding of grains becomes more apparent when the cutting temperature is increased. Besides, intra-granular and inter-granular fracture can be detected near the triple junction, leading to voids and cracks on the machined surface. Furthermore, the plastic deformation is promoted with increasing cutting temperature, and the deformation depth for polycrystalline silicon is larger than that for single-crystal silicon. Moreover, the amorphization of crystal phase in workpiece involves intra-granular and inter-granular mode. When the cutting temperature is increased, the inter-granular amorphization can be effectively promoted. In addition, the recrystallization is observed when cutting polycrystalline silicon at high temperature while the crystal purity of the machined surface is less than that in single-crystal silicon.

Corrosion fatigue crack propagation of 7XXX series aluminum alloys from key components of high-speed train

The corrosion fatigue cracks propagation behavior of A7N01P-T4 and A7N01S-T5 aluminum alloys in 3.5 wt.% NaCl solution and air were studied by single side notch corrosion fatigue tests (SNET). The crack growth rate (da/dN) was measured and the crack propagation mechanism was analyzed by scanning electron microscopic analysis. The results showed that the corrosion fatigue cracks growth rate of A7N01S-T5 alloy in air and 3.5 wt.% NaCl solution is faster than that of A7N01P-T4 alloy. The crack propagation follows a mixed intergranular and transgranular mode. The anodic dissolution and hydrogen embrittlement accelerate the propagation of corrosion fatigue crack of 7-series Al alloy in 3.5 wt.% NaCl solution. The corrosion resistance and fatigue crack propagation are related to the strength of the alloys and the density of grain boundaries.

Notch tensile and dry sliding wear studies on Mg-Nd-Gd-Zn alloy

This main purpose of the present research is to investigate the properties of wear and tensile in smooth and notch conditions of the heat-treated Mg-Nd-Gd-Zn alloy. The material applications such as complex helicopter gear box casing, piston and brake actuating components demands tensile data in notch conditions and wear characterizations. The alloy was characterized for microstructure, hardness, tensile properties in smooth and notch conditions and tribological properties such as wear resistance and friction coefficient. The microstructure examination revealed equiaxed grain structure with the size of about 42 μm. The XRD results confirms the presence of Mg3 (Nd) precipitates. Smooth and notched samples were evaluated to determine the effect of notch radii and ductile-brittle transition. Notched samples show 25.31% improvements in strength compared to the smooth sample with the marginal reduction in ductility (1.3%–1.8%). The fractography results for notched specimens exhibited cleavage, intergranular mode whereas the smooth specimens exhibited quasi cleavage, inter and transgranular modes. At 10N, the wear rate and friction coefficient change from 8.83 × 10–5 mm3 min−1 to 7.11 × 10–5 mm3 min−1 (24.2%) and to 0.29 and 0.26 (11.5%) respectively when the sliding velocity increases from 1 to 5 m s−1. Similarly, at 20N, the wear rate and friction coefficient change from 9.11 × 10–5 mm3 min−1 to 0.75 × 10–6 mm3 min−1 (12.7%) and 0.29 to 0.23 (26.1%) respectively when the sliding velocity increases from 1 to 5 m s−1. With the increase of load, plastic deformation is dominant controlling the wear rate. With the increase of sliding speed, oxidation is dominant controlling the wear rate and friction coefficient. Mixed modes of wear mechanisms such as abrasion, oxidation, delamination, third body assisted wear and melt wear were observed.

Stress Corrosion Cracking Behavior of Fine-Grained Al5083 Alloys Processed by Equal-Channel Angular Pressing (ECAP).

In the present study, the stress corrosion cracking (SCC) behavior of ECAP Al5083 alloy was investigated in air as well as in 3.5 % NaCl solution using the slow strain rate tensile test (SSRT). The characteristics of grain boundary precipitates (GBPs), specifically the microchemistry of the SCC behavior of Al5083 alloys, both in “as-received” condition and when deformed by the ECAP process, were examined. The correlations between the SCC resistance and GBP microchemistry were examined. A microstructural evaluation was performed using an optical microscope. SCC tests were carried out using a universal tensile testing machine and the fracture surfaces were studied using scanning electron microscopy (SEM). A strain rate of 1×10−6 s−1 was applied for the SSRT. As the passes increased, the SCC susceptibility of the fine-grained ECAP Al5083 alloy also increased. Moreover, higher ultimate tensile strength and greater elongation were observed. This was due to grain refinement, high-density separations, and the expanded extent of high-density dislocations instigated by severe plastic deformation. Due to the high strength and elongation, the failure analysis showed a ductile mode of fracture. Electron backscattering diffraction (EBSD) analysis was performed to determine more clearly the nature of cracking. EBSD analysis showed that the crack propagation occurred in both transgranular and intergranular modes.

Characterization of externally solidified crystals in a high-pressure die-cast AlSi10MnMg alloy and their effect on porosities and mechanical properties

The characterization of externally solidified crystals (ESCs) and porosities in a high-pressure die-cast AlSi10MnMg alloy was investigated using optical microscope (OM) and computed tomography (CT) with particular attention on the effect of shot speeds on the ESCs and fracture behavior. Results showed that ESCs tended to agglomerate at the center region and changed from fine globular grains into large size dendrites from surface to center. The formation of the ESCs is found to be coarser as the "slow shot speed" in the first stage of high-pressure die casting (HPDC) or the "fast shot speed" in the second stage of HPDC is reduced, consequently, the corresponding ESC area fraction increased. From the fractured morphology, the plate specimen with a higher ESC content exhibited a rough fracture and a large volume fraction of shrinkage porosities was discovered beneath the fracture surface especially when a lower fast shot speed was applied. In in-situ tensile test, the large size shrinkage porosity served as a crack source and it further expanded in an inter-granular mode along ESC boundaries/other porosities (which caused a tortuous crack route) or in a trans-granular mode across the ESC grains/fine (α-Al) II grains (which caused a flat crack route). In addition, the gas porosities in the specimen promoted the crack propagation as a crack concentration area and experienced almost no deformation during the tensile process.

Multiaxial low cycle fatigue behavior and life prediction method of 316LN stainless steel at 550 °C

This work studied the uniaxial, torsional and multiaxial low cycle fatigue behavior of 316LN stainless steel at 550 °C. Results show that the normalized kernel average misorientation value can be used to describe the loading path non-proportionality quantitatively. The formation of equiaxed and small-sized dislocation cells leads to the occurrence of continuously cyclic hardening under circular loading path. The crack initiation and early crack growth is dominated by intergranular mode in uniaxial test, whereas the transgranular cracking is observed in multiaxial test. Moreover, a weighting factor, evaluating the contribution of shear fatigue damage to the total multiaxial damage is proposed to modify the damage parameter of Chen-Xu-Huang (CXH) life prediction model, which enables all life data falling into the scatter band of 1.5.