Ductile Transgranular Fracture Research Articles

High temperature embrittlement of Inconel 625 alloy manufactured by laser powder bed fusion

The comprehensive mechanical behavior of Inconel 625 alloy fabricated by laser powder bed fusion (LPBF) and hot isostatic pressing (HIP) was studied. Tensile experiments were performed at six temperatures ranging from room temperature to 1000 °C to explore the effect of temperature on the mechanical properties, and the microstructure and fracture mechanism was investigated by combining electron backscatter diffraction, transmission electron microscopy, and atom probe tomography. In comparison with the properties of conventionally fabricated Inconel 625 alloys, the mechanical properties at room temperature are essentially the same. However, the plasticity of the Inconel 625 alloys fabricated by LPBF is drastically decreased with increasing temperature, especially when the temperature exceeds 700 °C due to the transformation from transgranular ductile fracture to intergranular fracture. Detailed characterization of cracks and grain boundaries (GBs) reveals significant segregation of non-metallic elements at GBs, which impedes the movement of GBs and decreases their strength, thus enhancing the intergranular embrittlement and resulting in the drastic reduction of ductility. Moreover, the irregular grain morphologies result in numerous GBs directional sharp changes and steps, as well as GBs triple junctions, in which the stress concentration is further enlarged, thus promoting the nucleation of voids and cracks and its associated high temperature embrittlement. The underlying mechanism of ductility deterioration of LPBF-fabricated Inconel 625 alloy at elevated temperatures would provide important insights into its performance at high temperatures.

Carbon and Titanium Effect on Tensile Behavior of Aged A286 Nickel-Iron Based Superalloy

A286 nickel-iron based superalloy used in high temperature applications. Age hardening is done to enhance the creep behavior which is much affected by TiC and eta (ⴄ (Ni3Ti)) phases. Effect of carbon and titanium (0.02C-2.46Ti, 0.04C-2.54Ti, 0.05C-2.58Ti, and 0.06C-2.62Ti) on tensile behavior of aged A286 superalloy is systematically investigated via TiC and ⴄ (Ni3Ti) phases. It has been revealed that carbon and titanium contents are in proportional to nucleation of TiC and eta phases in the austenitic matrix of this alloy. Precipitation of these phases enhanced yield strength from 354MPa to 501MPa and ultimate tensile strength (UTS) 543MPa to 651MPa. However, plasticity decreased nearly 4%. Fracture topography showed that the ductile transgranular fracture in low C-Ti alloys are due to TiC particles, whereas in high C-Ti alloys fracture nature is found brittle intergranular due to eta phases.

Study on stress corrosion properties of aviation aluminum alloy friction stir welded joint at the different strain rate

Aviation aluminum alloy welded structures are subjected to stresses at different strain rates in corrosive environments such as the ocean. Studying the stress corrosion properties of aviation aluminum alloy Friction Stir Welding (FSW) structures is essential. Therefore, this paper focuses on the study of 12 mm thick 7085-T7452 high-strength aviation aluminum alloy FSW joints under different strain rates of stress corrosion properties changes and the impact of welding process parameters. The main conclusions of this thesis are as follows: For the base material (BM) and the aluminum alloy FSW joints, when the strain rate increased from 10−7 s−1 to 10−5 s−1, the stress corrosion strength increased, and the stress corrosion cracking (SCC) sensitivity index ISSRT decreases. The SCC sensitivity at the same strain rate: SS-400-120 > SS-300-60 > SS-300-120 > BM. After the Slow strain rate tensile (SSRT) test, the FSW joint fracture in the air is a transgranular ductile fracture. Meanwhile, the fracture is a ductile-brittle hybrid fracture in 3.5% NaCl solution, and the number of secondary cracks along the grain boundary increased significantly, showing obvious SCC sensitivity. The BM fracture after SSRT is ductile, showing lower SCC sensitivity.

Effect of humid atmosphere on the microstructure and mechanical properties of a PM Ni-based superalloy: From powders to bulk alloys

Understanding the effects of powder storage condition is helpful to improve the mechanical properties of powder metallurgy (PM) and additive manufacturing (AM) superalloys. In this work, nickel-based FGH96 superalloy powders were stored under ambient air (relatively dry) and humid (25 °C/95%RH and 90 °C/95%RH) atmospheres and then consolidated by hot isostatic pressing (HIP). The powders and corresponding bulk alloys were characterized by XPS, OM, FESEM, EPMA, EBSD, and TEM. The results indicate that the humid atmosphere mainly affects the gas content, chemical state, and oxide/hydroxide layer thickness of powders, leading to highest values of oxygen content (~450 ppm), hydrogen content (~25 ppm), and NiO/Ni(OH)2 thickness (~13.9 nm) for 90 °C/95%RH powders, and further forms much severe prior particle boundaries (PPBs) defects in HIPed alloy. The detrimental PPBs are composed of large-sized γ', MC carbides-(Ti, Nb)C, Al2O3 and ZrO2 oxides. In comparison with ambient air powders compact, the strength of 90 °C/95%RH powders compact is unexpectedly increased, which is due to the internal grains pinned inside the original particle boundaries by higher severity degree of PPBs, and leads to the increase of grain boundary strength. Meanwhile, the ductility is significantly decreased, because the fracture mode transited from ductile transgranular fracture to brittle inter-particle debonding. The study reveals the relationship between humid atmosphere and properties of both powders and bulk alloys, and intends to obtain good combination of strength and ductility for PM Ni-based superalloys.

Study on Microstructure and Mechanical Properties at Constant Electromigration Temperature of Sn2.5Ag0.7Cu0.1RE0.05Ni-GNSs/Cu Solder Joints

To solve the electromigration problem of micro-electronic connection solder joints, an ideal electromigration tester was designed, and the thickness of the intermetallic compounds (IMCs), average void diameter, grain orientation, failure, shear strength, and fracture path of Sn2.5Ag0.7Cu0.1RE0.05Ni-GNSs/Cu solder joints under constant-temperature electromigration were studied. The results indicate that the solder joints show evidence of typical electromigration polarity in the asymmetric growth of interfacial IMCs on the anode and cathode sides under the conditions of a current density ≥7 × 103 A/cm2 and an included angle between the c-axis of the β-Sn grains and the current direction θ ≤ 53.2°. The anode-side interfacial IMC is dominated by a Cu6Sn5 phase with a gradually increasing thickness, forming a Cu3Sn phase and showing evidence of microcracks. The Cu6Sn5 phase of the cathode-side interfacial IMC is gradually completely dissolved, and the growth of the Cu3Sn phase is accompanied by the formation of Kirkendall voids. The anisotropic diffusion of Cu atoms in the β-Sn of the micro-solder joints causes increased solder joint resistance and reduced shear strength. The shear fracture path of the solder joints moves from the cathode side near the IMC solder seam to the Cu3Sn interface. The shear fracture mechanism changes from ductile transgranular fracture dominated by β-Sn dimples to brittle fracture dominated by interfacial IMC cleavage and slip steps.

Effect of Aging on Impact Toughness and Fracture Behavior of Three Wrought Aluminum Alloys

Herein, the effect of the aging state on the impact toughness of three typical wrought Al alloys (7075, 2024, and 6A01) is investigated. At room temperature, the fracture mode of the three aluminum alloys at different aging states is a ductile transgranular fracture. At low temperatures, high‐strength aluminum alloys exhibit a mixture fracture of transgranular and intergranular, while medium‐strength aluminum alloys are still dominated by transgranular fractures. The effect of the aging state on the impact toughness of aluminum alloys was analyzed based on the deformation mechanism and morphology near grain boundaries. The underaged state with planar slip dislocations exhibits good plasticity and work‐hardening ability, which reduces the strain localization caused by dislocation plugging and thus improves the impact toughness of the alloy. Furthermore, the fine grain boundary precipitates and the narrow precipitation‐free zone of the underaged increase the uniform deformation capacity near the grain boundaries and inhibit the initiation and propagation of cracks. Compared with high‐strength aluminum alloys, medium‐strength aluminum alloys exhibit higher impact toughness due to the formation of a larger plastic zone and shear lip area during impact.

Effect of heat treatment on microstructures and mechanical properties of Inconel 718 additively manufactured using gradient laser power

To achieve high-performance nickel-based superalloys by laser powder deposition for applications in the aviation industry, post-heat treatments are always indispensable. We prepared two typical as-deposited microstructures by constant laser power deposition (CLP) and gradient laser power deposition (GLP), respectively. A systematic research on the room-temperature microstructural evolution, hardness, tensile property and fracture morphology of Inconel 718 after three kinds of heat treatments is performed. The as-deposited microstructure and segregation ratio (SR) can significantly influence the microstructure evolution. The as-deposited GLP samples with fine discrete Laves phases and a low SR can achieve a relatively uniform element distribution and γ′′ and γ′ phase precipitation after standard solution treatment plus aging (STA), while CLP samples with coarse long-chain Laves phases and high SR require a high-temperature homogenization plus STA (HSTA) heat treatment to achieve the uniform element distribution. Differences in heat treatments have a greater impact on the fracture mechanism. With the transformation of heat treatments from direct aging, STA to HSTA, the fracture mode of both CLP and GLP samples transforms from a ductile transgranular fracture mode to a mixed mode of transgranular and intergranular fractures.

Effect of grain size on deformation and fracture of Inconel718: An in-situ SEM-EBSD-DIC investigation

The effects of grain size on tensile behavior of Inconel718 superalloy have been characterized by an in-situ tensile stage inside a scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) and digital image correlation (DIC) at room temperature. The microstructure evolution and mechanical properties of specimens under different heat treatments were performed and compared. The in-situ tensile test showed that the yield strength, ultimate tensile strength and reduction of cross section decrease with the increasing grain size. An in-situ EBSD study showed that with increasing stress the local plastic deformation become more inhomogeneous. An analysis combined in-situ SEM morphology evolution, DIC strain distribution and EBSD results indicated that the grains deformed coordinately. Geometrically necessary dislocation (GND) density depends on the grain size and the orientation of individual grains. The coordinated deformation ability decreased with increasing grain size. The fracture morphology showed that with the increase of grain size, the fracture mechanism transformed from the ductile transgranular fracture to the brittle intergranular-transgranular mixed fracture.

A novel damage constitutive model for creep deformation and damage evolution prediction

AbstractIn order to accurately predict creep deformation and damage evolution of nickel‐based superalloy GH4169, a novel damage constitutive model, that is, TTC CDM‐based model, was proposed based on TTC relations and continuum damage mechanics (CDM). This model combined the best features of the TTC relations and CDM. The recently developed TTC relations were intrinsic relations between threshold stress, tensile properties, and creep behavior and exhibited better prediction ability than Wilshire equations. It was determined that the model accurately predicted the minimum creep rate, rupture time, creep deformation, and damage evolution process of GH4169. Microstructural study has revealed that the creep fracture mode gradually converts from intergranular brittle fracture to transgranular ductile fracture as the stress decreases. Furthermore, the nonlinear creep damage accumulation effect was also revealed by the novel model.

Interlaminar Microstructure and Mechanical Properties of Narrow Gap Laser Welding of 40-mm-Thick Ti-6Al-4V Alloy.

Narrow gap laser welding (NGLW) is a common solution for the welding of thick structures. NGLW was carried out on narrow-gap butt joints of 40 mm-thick Ti-6Al-4V alloy plates with a U-shaped groove. The distribution characteristics of the interlaminar microstructure in different height ranges of the joint were investigated, and the evolution behavior and formation mechanism of the interlaminar microstructure of the joint were also revealed. This showed that a large amount of short needle martensite nucleated and grew up near the fusion line and the upper boundary of the remelting zone. The "softening" phenomenon occurred in all welds except the cover layer weld. The microstructure evolution and defect migration, induced by multiple welding thermal cycles in the upper weld forming process, were the main reasons for the "softening" of the lower weld. The tensile strength of each sample changed in the range of 920~990 MPa; the fracture mode of the sample belongs to a transgranular ductile fracture. In addition, compared with the upper part of the joint, the plasticity and toughness of the weld area in the lower part of the joint was improved.

Ambient, elevated temperature tensile properties and origin of strengthening in friction stir welded 6 mm thick reduced activation ferritic-martensitic steel plates in as-welded and post-weld normalised conditions

Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.

High-temperature creep rupture behavior of dissimilar welded joints in martensitic heat resistant steels

The creep rupture behavior and microstructure changes of dissimilar welded joints of two types of martensitic heat resistant steel F92 and Co3W2, under a creep testing of 115–200 MPa and 873–893 K, were investigated. Fracture position and mode change from transgranular ductile fracture in parent F92 to intergranular brittle fracture in the fine grain heat affected zone (type IV cracking) near F92, with the drop of applied stress. Microstructural degradation resulted from creep with respect to dislocation density decrease, precipitates coarsening, dislocation cells, microvoids, macroscopic secondary cracks, and fracture surface morphology are researched based on detailed experimental results. The creep rupture data are analyzed employing Norton’s power law, Monkman–Grant relation, creep damage tolerance, and Larson–Miller parameter. Main creep damage micromechanisms are analyzed with the aid of calculated and experimental data. The extrapolating creep rupture stress for a creep life of 100,000 h are obtained and the influence of stress change and temperature change on creep life are compared in terms of calculated normalized creep life growth rate.

Microstructure and mechanical properties of Nb-alloyed austenitic CrNi steel fabricated by wire-feed electron beam additive manufacturing

We investigated the phase composition, microstructure, mechanical properties, and fracture mechanisms of Nb-alloyed austenitic stainless steel, produced by electron beam additive manufacturing (EBAM), after different regimes of the post-EBAM solid-solution treatments. As-build EBAM-produced steel has a predominantly austenitic structure, contains vermicular δ-ferrite (Vδ = 5–7%) and Nb-enriched precipitates (Laves phase Fe2Nb, FeNb and NbC). Niobium-alloying assists the stabilization of austenitic phase but does not suppress the columnar grain growth during EBAM-fabrication of austenitic stainless steels. Solid-solution treatments provide a change in morphology of δ-ferrite, a decrease in the ferrite volume fraction and the dissolution of intermetallic phases (Fe2Nb, FeNb), which all affect the tensile properties. Post-EBAM treatments contribute a decrease in the yield strength of the steel and improve the elongation-to-failure. All EBAM-produced specimens have good mechanical properties, which are comparable with those for steels produced by conventional methods. Irrespective of treatment regime, EBAM-fabricated specimens exhibit ductile transgranular fracture. For the EBAM-produced Nb-alloyed steel, the phase composition is described from the point of view solidification mode, and the effects of the precipitate hardening and solid-solution strengthening on the mechanical properties and fracture mechanisms were discussed.

Studying crack propagation along symmetric tilt grain boundary with H segregation in Ni by MD simulation

Hydrogen ingression in metals generally causes catastrophic failure. The H-induced fracture surface mostly exhibits intergranular feature, suggesting that grain boundary (GB) is the dominant crack propagation path. In this paper, the grand canonical Monte Carlo method was first performed to obtain equilibrated H distribution around various symmetric tilt grain boundaries (STGBs) in nickel. Then, atomistic simulations were conducted to study the influence of H segregation on the Mode I crack propagation behavior. In the absence of hydrogen, significant crack tip blunting and transgranular fracture are observed, implyinga tendency toward ductile transgranular fracture. However, in the case of H segregation, it is found that dislocation emission from the crack tip is enhanced but the crack tip still remains highly sharp. Moreover, crack propagation is dominated by alternate GB dislocation emission and GB cleavage, implying a tendency of ductile–brittle alternating intergranular fracture behavior. The H-inhibited GB migration and H-enhanced GB dislocation emission are believed to be the key mechanisms governing the H-assisted crack propagation along the GB.

Dynamic response of wrought and additively manufactured nickel-based alloys to high velocity impacts of laser-launched flyers

Although nickel-based superalloys are widely used in the industry, their response to shock loading is still rarely investigated. Here, the impacts of laser-launched flyers were used to study the dynamic behavior of a Rene 65 superalloy under shock pressures of about 10 GPa at very high strain rates of about 106 s−1. Three types of samples, wrought or additively manufactured (laser powder bed fusion) and subjected to different heat treatment conditions, were investigated. These experiments allowed the measurement of the Hugoniot elastic limit (compressive yield strength) and the spall (tensile) strength, both in upper ranges compared with the most common metals. Post-recovery analyses involving various techniques provided insight into dynamic failure with a combination of transgranular ductile fracture and intergranular cracks with preferential nucleation sites, strongly dependent on the different microstructures related to fabrication routes and thermal treatments.

Influence of precipitate evolution on the impact fracture behavior in Nb-containing 16Cr–25Ni superaustenitic stainless steel weld metal

In this work, the precipitate evolution and its influence on the impact fracture behavior of Nb-containing 16Cr–25Ni superaustenitic stainless steel weld metal were systematically studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results show completely transgranular ductile fracture mode of as-welded condition, of which the M6C carbides and Nb(C, N) carbonitrides contributed to the crack initiation, but the interface between M6C carbide and matrix was more prone to crack under impact load. In contrast, due to the semi-continuous distributed M23C6 carbides on the grain boundary, an intergranular/transgranular mixed mode was observed in the post-weld heat treatment (PWHT) sample. Additionally, the crack sources in grain interior were not limited to primary precipitation phases, whereas secondary M2(C, N) carbonitrides and M23C6 carbides also acted as preferential sites for crack initiation. The massive M2(C, N)/matrix and M23C6/matrix interfaces led to easier crack propagation path simultaneously. Moreover, the deformation mechanism were governed by dislocation slip and twinning in the impact fracture of as-welded sample, but the mechanical twins disappeared in the fractured sample at PWHT condition, which was attributed to the local stress concentration in the matrix could not exceed the critical twinning stress. Therefore, the combination of secondary precipitates induced embrittlement and weakened plastic deformation capacity of matrix, leading to the impact toughness declined from 91.0 ± 3.6 J to 34.3 ± 4.6 J after PWHT.

Mechanism of Microscopic Fracture of Typical Shell Steel under Impact Loading

The aim of the current study was to investigate the effects of strain rate and metallographic structure on the fracture mode and fracture pattern of a typical shell steel material under impact loading. A ballistic gun was used to launch a spherical tungsten alloy projectile to impact target 50SiMnVB and 60Si2Mn steel plates. The morphological characteristics of the cracks on different target plates were observed under a metallurgical microscope, and the effects of the strain rate and metallographic structure on the fracture mode and fracture pattern were analyzed. The results showed that when the strain rate was relatively low, the material mainly produced ductile fracture and brittle trans-granular fracture under impact loading; when the strain rate was relatively high, intergranular fracture and cleavage fracture were the main modes of fracture under impact loading. In addition, at higher strain rates, the metallurgical form mainly influenced the pattern of fracture of the material, with tempered troostite being more likely to produce a mixture of shear and tensile fractures than tempered sorbite. The results obtained provide an experimental basis for the mechanism of microscopic fracture of shell steel materials and, to a certain extent, reveal the correlations between fragmentation and the strain rate and microstructure of the material.

Tensile Fracture Behavior of 2A14 Aluminum Alloy Produced by Extrusion Process

In this study, the tensile mechanical properties of a 2A14 aluminum alloy produced by extrusion were tested at room temperature to investigate the tensile fracture behavior. The results showed that the tensile fracture of the alloy was mixed intergranular and transgranular ductile fracture, as numerous coarse second-phase particles were distributed in a band along the loading direction, making it prone to microcracking. This was determined to be the main cause of fracture failure of the alloy. In addition, we observed large α-AlFeMnSi(Cu), Al(Fe,Mn)Cu, AlCuMgSi, and Al2Cu phases in the microstructure of the 2A14 aluminum alloy, and both Al2Cu second phase and precipitation-free phase zone (PFZ) at the grain boundaries were observed, which made the alloy susceptible to fracture failure and reduced the mechanical properties of the alloy.

Comparative analysis of cold and warm rolling on tensile properties and microstructure of additive manufactured Inconel 718

Despite the high efficiency and low cost of wire + arc additive manufacture (WAAM), the epitaxial grown columnar dendrites of WAAM deposited Inconel 718 cause inferior properties and severe anisotropy compared to the wrought components. Fundamental studies on the influence of one-pass cold and warm rolling on hardness and microstructure were investigated. Then the interpass cold and warm rolling on tensile properties were also analyzed. The results show that the one-pass rolling increases the hardness and displays a heterogeneous hardness distribution compared to the as-deposited material, and the warm rolling exhibits a larger and deeper strain compared to cold rolling. The columnar dendrites gradually change to cell dendrites under the rolling process and then change to equiaxed grains with the subsequent new layer deposition. The average grain size is 16.8 μm and 23.5 μm for the warm and cold rolling, respectively. The strongly textured columnar dendrites with preferred < 001 > orientation transform to equiaxed grains with random orientation after rolling process. The grain refinement contributes to the dispersive distributed strengthening phases and the increase in its fraction with heat treatment. The as-deposited samples show superior tensile properties compared to the cast material but inferior compared to the wrought components, while the warm-rolled samples show superior tensile properties to wrought material. Isotropic tensile properties are obtained in warm rolling compared to cold rolling. The rolling process and heat treatment both decrease the elongation and lead to a transgranular ductile fracture mode. Finally, the rolling-induced strengthening mechanism was discussed.

Optimization of continuous drive friction welding process for AA6061-T6 circular pipe and conical head

Aluminum alloy double-layer grid structure with bolt-ball joint has the advantages of light weight, corrosion resistance, convenient construction, etc. However, when circular pipe and conical head are connected by fusion welding, its strength is low. In order to solve this problem, the continuous drive friction welding (CDFW) process of aluminum alloy circular pipe and conical head and its strength of joint were studied in this paper. Based on the orthogonal experiment scheme, the CDFW experiments were conducted on AA6061-T6 circular pipe, then the tensile tests were completed on the standard samples to obtain joint efficiency under different welding parameters. The optimum welding parameters were determined by the method of variance and range analysis. The microstructure of joints was characterized by hardness test, OM, XRD, SEM, etc. The test results show that parameters affecting joint efficiency are friction pressure, rotational speed and upsetting rotational speed in descending order. Under the optimal welding process, the joint efficiency can reach 87.0%, which is superior to fusion welding. The fracture form of the welded joint is a mixture of intergranular and ductile transgranular fracture. WNZ is the crucial region of the welded joint, where the grain growth is greatly influenced by friction pressure. The friction pressure of the optimal welded joint is moderate, so the grains in WNZ are fine and grow adequately. The hardness softening of the welded joint also has certain influence on its mechanical properties. The solid solubility of the optimal welding sample is high and alloy elements are stable in the Al matrix, so it exhibits good mechanical properties. Finally, the CDFW experiments and tensile tests of circular pipe and conical head were conducted, which verifies the effectiveness of the optimal welding parameters.