Mixed Ductile-brittle Fracture Research Articles

Fabrication of a new Al-Mg/graphene nanocomposite by multi-pass friction-stir processing: Dispersion, microstructure, stability, and strengthening

In the present research, multi-pass friction stir processing (FSP) was employed for the first time to disperse graphene in the form of graphene nano-platelets (GNPs) into an AA5052 aluminum‑magnesium alloy to fabricate a new Al-Mg/3vol% GNPs nanocomposite. After five cumulative FSP passes, the GNPs were distributed within the metal-matrix. Field emission-scanning (FE-SEM) and transmission electron microscopy (TEM) analyses were used to examine the dispersion of GNPs, and suggested negligible deterioration of the graphene planar structure following FSP. Some clusters of graphene originating from the initial powder remained due to the high surface energy of these GNPs, while grain orientations were evaluated in the nanocomposite using electron back scattering diffraction (EBSD). A fine equiaxed recrystallized grain structure with an average size of 2.1μm was formed in the stir zone (SZ) after FSP while dispersing GNPs. Indentation revealed the hardness of the nanocomposite increased by 53% compared to the processed Al-Mg alloy. Yield strength also was improved by more than three times while preserving ductility which achieved 20% strain before fracture. Fractographic studies of tensile test specimens revealed a mixed ductile-brittle fracture behavior. Based on the micromechanics theory, three models considering the microstructural parameters (i.e., aspect ratio, mean size, and volume fraction of GNPs, grain size, and clustering during process) were developed to predict the strengthening effects of GNPs in the terms of elastic modulus, yield strength, and indentation hardness. Correlation between these predicted values and experimental data are discussed in detail.

Microstructural characteristics and mechanical properties of bobbin-tool friction stir welded 2024–T3 aluminum alloy

Cold-rolled 2024–T3 sheet alloy was subjected to bobbin-tool friction stir welding (BTFSW). The microstructural characteristics and mechanical properties of the nugget zone in the as-welded state were investigated. The results show that the equiaxed grain size of BTFSW 2024–T3 alloy decreases from 7.6 to 2.8 μm as the welding speed is increased from 80 to 120 mm/min; in addition, fine grains are generated in the nugget zone and the size distribution is non-uniform. All Al2CuMg (S′) precipitates dissolve into the Al matrix, whereas Mn-rich phases confirmed as T phases (Al20Cu2Mn3, Al6Mn, or Al3Mn) remain unchanged. The optimized parameters for BTFSW are verified as the rotation speed of 350 r/min and the travel speed of 100 mm/min. The variations in precipitation and dislocation play more important roles than grain size in the nugget zone with respect to influencing the mechanical properties during the BTFSW process. After the BTFSW process, the fracture mode of base material 2024–T3 alloy transforms from ductile rupture to ductile-brittle mixed fracture.

Microstructural evolution and mechanical behavior of friction spot welded 2198-T8 Al-Li alloy during aging treatment

The microstructure and mechanical properties of friction spot welding (FSpW) 2198-T8 alloy during aging treatment were studied. Whereafter, the fracture process before and after aging treatment was observed by using the in-situ straining transmission electron microscopy method. The results show that fined equiaxed grains remained with predominant T1 phase re-precipitated in the microstrucuture of FSpW 2198-T8 Al-Li alloy after aging treatment. With the extension of holding time (12h–48h), grains gradually grew up in the range of 9.2–13.2μm and the size and volume fraction of T1 phases were also increased. Tensile strengths of thermal-mechanical coupling zone (TMCZ) of FSpW 2198-T8 alloy first increased and then decreased after aging treatment. When aging at 175°C for 36h, the strength reached the maximum (435MPa) with the elongation decreased to 11.4%. Meanwhile, the fracture mode of TMCZ zone transformed from ductile brittle mixed fracture to brittle fracture after aging treatment. And cracks initiated at and propagated along precipitate-free zones (PFZs) after aging treatment.

Effects of warm laser peening on the elevated temperature tensile properties and fracture behavior of IN718 nickel-based superalloy

IN718 nickel-based superalloy specimens were treated by laser peening (LP) and warm laser peening (WLP) respectively. Tensile tests were performed on the matrix and treated specimens at both room (25°C) and elevated temperature (700°C). Tensile properties were examined by engineering stress-strain curves. The sectional micro-structural observations were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in sequence to investigate the micro-structural evolution and precipitation strengthening mechanism. The fracture morphologies of untreated, LP and WLP treated specimens after tensile deformation at elevated temperature were also analyzed. The results show that LP process, in particular WLP process were found to have a remarkable effect on improving tensile properties of IN718 superalloy at both room and elevated temperatures. WLP treated specimens exhibited more stable high-temperature tensile properties. A serrated flow phenomenon was found in the stress-strain curves at elevated temperature. More δ precipitates dispersed in the grain boundaries after WLP treatment. Detailed TEM microstructural analysis showed that various typical microstructures including dislocation lines, dislocation tangles, dislocation arrays, mechanical twins and precipitated particles tangled together, forming the strengthening phase. A new formed substructure of submicron rhombic blocks with lengths in the range of 400–600nm was observed in the WLP treated specimens. In addition, the WLP treated specimens exhibited a ductile fracture mode, which was totally different with the ductile-brittle mixed fracture mode in untreated and LP treated specimens. WLP process is verified as an effective method to enhance the high-temperature properties of IN718 nickel-based superalloy.

Microstructure, mechanical property and corrosion resistance property of Cr26Mo3.5 super ferritic stainless joints by P-TIG and laser welding

The characteristics of microstructure, mechanical property and corrosion behavior of Cr26Mo3.5 super stainless steel joints by pulse tungsten inert gas(P-TIG)welding and laser welding were investigated. The results indicate that the widths of the center equiaxed grain zone(EGZ)and the columnar grain zone(CGZ)increase with the increase of heat input in both welding processes. The precipitates of Nb and Ti carbides and nitrides are formed in the weld metal(WM)and the heat affected zone(HAZ). The joints by laser welding show better tensile and corrosion resistance properties than those by P-TIG welding due to the heat concentration and lower heat input. The tensile strength and elongation increase with the decrease of heat input, and the fracture mode of the joints turns into ductile-brittle mixed fracture from ductile fracture when the welding method turns into P-TIG welding from laser welding. Moreover, the corrosion resistance of all joints declines slightly with the increase of heat input. Hence, laser welding is more suitable for welding Cr26Mo3.5 super stainless steel in engineering applications.

Effect of carbon equivalent on thermal and mechanical properties of compacted graphite cast iron

Abstract

Effect of aging treatment on mechanical properties and fracture behavior of friction stir processed Mg–Y–Nd alloy

In this study, precipitation behavior of Mg–Y–Nd cast alloy during friction stir processing (FSP), and the effect of subsequent artificial aging on mechanical properties and fracture behavior of the FSP alloy were investigated. It is found that the coarse α-Mg grains and large second phases are greatly refined after FSP. Moreover, due to the heat input during processing and the natural cooling, β′ and β1 precipitates are also observed in the FSP alloy. The FSP specimens were subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 150 °C for 54 h and 180 °C for 30 h. Strengths of the peak–aged specimens are further increased, which is attributed to the large quantity of β″ and β1 precipitates, respectively. Meanwhile, elongations of the peak-aged specimens are both decreased. Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of FSP and peak-aged specimens are all mixed ductile–brittle fracture mode. However, compared to the FSP specimens, different fracture paths are exhibited in peak–aged specimens.

Simulation and experimental investigation of the spall fracture of 304L stainless steel irradiated by a nanosecond relativistic high-current electron beam

The results of numerical and experimental investigations of the shock-wave induced spall fracture of bulk samples with thickness up to 10 mm made of 304L stainless steel irradiated by a nanosecond relativistic high-current electron beam with duration of \({\sim }\)45 ns, electron energy of 1.35 MeV, and peak power density of \(\hbox {34 GW/cm}^{2}\) are presented. By a mathematical model developed for numerical simulation of the shock-wave dynamics, it was found that a quasi-planar shock wave with duration of \({\sim }0.2\,\upmu \hbox {s}\), and initial amplitude of 17 GPa was formed in the irradiated samples. The effects of orientation of \(\updelta \)-ferrite interlayers in the austenitic matrix relative to the shock wave direction on the spall fracture were experimentally investigated. It was found that spallation was carried out by mixed ductile–brittle fracture. For the transversal orientation of \(\updelta \)-ferrite, the contribution of a brittle fracture mode in the spallation is higher than that for the longitudinal orientation. In both cases, the spalled layer thickness increased almost linearly with the increase of the target thickness, which was in good agreement with literature data. By the comparison of experimental data with simulation results, it was revealed that the spall strength can be estimated as 6.1 GPa at strain rate \(0.48\,\upmu \hbox {s}^{-1}\) and 3.4 GPa at strain rate 0.18 \(\upmu \hbox {s}^{-1}\), for samples with the longitudinal and transversal orientation of \(\updelta \)-ferrites, respectively. The comparison of the obtained spall strength values with literature data is considered.

Effects of Bi addition on interfacial reactions and mechanical properties of In–3Ag–xBi/Cu solder joints

In this study, the effects of Bi addition on the interfacial reactions and mechanical properties of In–3Ag–xBi/Cu joints were investigated. Better wettability was obtained with increased addition of Bi (0–2 wt%), but further additions up to 3 wt% decreased the beneficial effects. The intermetallic compounds (IMCs) formed at In–3Ag–xBi/Cu interfaces were (Ag,Cu)In2 in all cases. With the increase of Bi addition, the interfacial IMC grains coarsened and the IMC layers got thicker. The EPMA mapping revealed that Bi addition reacted with neither In or Ag to form intermetallics, it dissolved as solid solution in Indium matrix uniformly. In addition, the shear strength of the soldered joints were improved by Bi addition, with the peak shear strength occurring at 2 wt% Bi addition. The fracture mode also changed from typical ductile fracture mode to mixed ductile–brittle fracture mode with increased amount of Bi addition.

Microstructure and Mechanical Properties of Mn-Containing Maraging Steels

An attempt to the modification of the microstructure and mechanical properties of affordable, Mn-containing maraging alloys is reported. These alloys have demonstrated strong age hardening but suffered with premature intergranular brittleness despite their potential applications in tooling, dies, and machinery industries. An Fe-10Ni-6Mo-3Mn-1Ti (wt.%) alloy was prepared by vacuum melting and processed by homogenization (1250 °C/48 h), cold rolling, solution annealing (950 °C/1 h), and aging treatments (500 °C/4 h). It presented tensile strength of about 2.65 GPa, a few percent of tensile elongation and a mixed ductile-brittle fracture mode. Transmission electron microscopy (TEM) revealed the precipitation of a nearly spherical phase. Crystal symmetry of the second phase precipitates was identified hexagonal close-packed corresponding reasonably to the Fe2Mo Laves phase having lattice parameters of a = 0.4745 and c = 0.7754 nm. Precipitation of a Mo-enriched second-phase particle was occasionally found at prior austenite grain boundaries but the pronounced grain boundary precipitation was never identified. Energy-filtering transmission electron microscopy using the Mo-M4,5 post edge revealed remarkable segregation of Mo at grain boundaries.

Formation and evolution of intermetallic compounds between the In-3Ag solder and Cu substrate during soldering

The formation and growth of intermetallic compounds (IMCs) have an important effect on the reliability of the solder. In this paper, formation and evolution of IMCs between the In-3Ag solder and Cu substrate were systematically explored for different soldering time ranged from 1 to 30 min. The interface soldered for 1 or 2 min was only composed of (Ag,Cu)In2 IMCs. When the soldering time extended to 4 min, a new (Cu,Ag)11In9 IMCs layer appeared under the (Ag,Cu)In2 IMCs layer. When the soldering time exceeded 5 min, the (Ag,Cu)In2 layer was completely decomposed and replaced by the (Cu,Ag)11In9 layer. Meanwhile, the growth of (Ag,Cu)In2 and (Cu,Ag)11In9 intermetallic layers during jointing were all diffusion-controlled and the growth rates of them are 0.398 and 0.507 µm2/s, respectively. Furthermore, shear strength results showed that the shear strength of solder joints decreased with soldering time extending, and those decrease from 5.64 MPa after soldered for 1 min to 3.07 MPa when the soldering time extended to 30 min. The fracture mode also changed from typical ductile fracture mode to mixed ductile–brittle fracture mode with soldering time extending.

Effects of Silicon on Mechanical Properties and Fracture Toughness of Heavy-Section Ductile Cast Iron

The effects of silicon (Si) on the mechanical properties and fracture toughness of heavy-section ductile cast iron were investigated to develop material for spent-nuclear-fuel containers. Two castings with different Si contents of 1.78 wt.% and 2.74 wt.% were prepared. Four positions in the castings from the edge to the center, with different solidification cooling rates, were chosen for microstructure observation and mechanical properties’ testing. Results show that the tensile strength, elongation, impact toughness and fracture toughness at different positions of the two castings decrease with the decrease in cooling rate. With an increase in Si content, the graphite morphology and the mechanical properties at the same position deteriorate. Decreasing cooling rate changes the impact fracture morphology from a mixed ductile-brittle fracture to a brittle fracture. The fracture morphology of fracture toughness is changed from ductile to brittle fracture. When the Si content exceeds 1.78 wt.%, the impact and fracture toughness fracture morphology transforms from ductile to brittle fracture. The in-situ scanning electronic microscope (SEM) tensile experiments were first used to observe the dynamic tensile process. The influence of the vermicular and temper graphite on fracture formation of heavy section ductile iron was investigated.

Disintegrating fracturing ball used in ball injection sliding sleeve for staged fracturing

Abstract A disintegrating fracturing ball used for multi-stage fracturing stimulation was developed, the decomposable characteristics and mechanical properties of the decomposable material were analyzed, and the ball was tested in both ground pressure test and field operation. The decomposable material is an Mg-based alloy with the density of 1.8-2.0 g/cm3. The decomposable material can be dissolved in KCl solution and the decomposing rate increases with increasing temperature, while the decomposing rate is low in the guar gum fracturing fluid; the compressive strength of the decomposable material can reach 360 MPa, and the material breaks when the deformation reaches 20% with a mixed ductile-brittle fracture. The ground pressure test showed that the ball could hold pressure of 70 MPa and keep for 4 hours under 80 °C, which meant that the sealing performance between fracturing ball and ball seat met operation needs. Field fracturing operation showed that the disintegrating fracturing ball demonstrated high quality performance during the operation and dissolves in-situ after fracturing, which eliminated the operation risk that might be caused by fracturing ball flow-back, thereby decreased production cost and saved operation time.

Impact Behavior Characterization on Fractured Pins of AISI E52100 Steel

Two fractured pins of impact wrench were evaluated by fractography using scanning electron microscopy. This pneumatic wrench is used to the fabrication of automotive components being characterized by its power and tightening capacity. The important part of the wrench is the pin clutch impact mechanism used to generate high torque. An original pin manufactured with AISI E52100 steel adjusted 2,580 nuts before fracture and a substitute pin of E52100 modified steel with S adjusted 7,095 nuts before failure. Fractographic analysis in both fracture surfaces indicated a ductile-brittle mixed fracture mode. Original pin surface exhibited a granular appearance while substitute pin surface showed fibrous and rough morphology. Precipitation of nearly rounded, elliptical and elongate second – phase particles containing Cr, C and Mn measured as intergranular and transgranular particle density was observed, promoting some brittle failure zones and ductile fracture measured as the volume fraction of grain boundaries and microvoids, respectively. A larger amount of intergranular medium precipitates was found on the original pin which favored the brittle failure among the grain boundaries compared to that of the substitute pin. The ductile fracture by larger transgranular fine particle density which acted as nucleation sites of higher volume fraction of microvoids was found in the substitute pin.

Effect of H2S partial pressure on the tensile properties of A350LF2 steel in the absence and presence of pre-immersion

Effect of H2S partial pressure (PH2S) on the tensile properties of A350LF2 steel with and without pre-immersion was investigated by slow strain rate testing and electrochemical impedance spectroscopy. The results show that relative tensile strength and plasticity loss increased with increasing PH2S in the absence and presence of pre-immersion. Greater tensile property damage occurred in steel without pre-immersion compared to that with pre-immersion. The relative plasticity loss increment of steel without pre-immersion was larger than that with pre-immersion as PH2S was increased from 0.1MPa to 1.6MPa. Fractography exhibited flat brittle fracture on the samples without pre-immersion and mixed ductile-brittle fracture on the pre-immersed samples. Furthermore, the embrittled regions on the fracture surface of pre-immersed specimens increased with PH2S. In addition, no cracks existed in the steel after pre-immersion. The cracks on the fracture surface of steel could be attributed to the combination of tensile stress and high hydrogen concentration induced by the hydrogen diffusion and accumulation during the tensile test.

Microstructure and mechanical properties of dissimilar pure copper/1350 aluminum alloy butt joints by friction stir welding

The dissimilar friction stir welding of pure copper/1350 aluminum alloy sheet with a thickness of 3 mm was investigated. Most of the rotating pin was inserted into the aluminum alloy side through a pin-off technique, and sound welds were obtained at a rotation speed of 1000 r/min and a welding speed of 80 mm/min. Complicated microstructure was formed in the nugget, in which vortex-like pattern and lamella structure could be found. No intermetallic compounds were found in the nugget. The hardness distribution indicates that the hardness at the copper side of the nugget is higher than that at the aluminum alloy side, and the hardness at the bottom of the nugget is generally higher than that in other regions. The ultimate tensile strength and elongation of the dissimilar welds are 152 MPa and 6.3%, respectively. The fracture surface observation shows that the dissimilar joints fail with a ductile-brittle mixed fracture mode during tensile test.

A novel high-speed shear test for lead-free flip chip packages

Despite the importance of lead-free solders in modern environmentally friendly packaging, few studies have been conducted on their mechanical reliability at the wafer level. In the present study, high-speed die shear tests were conducted to investigate the effects of strain rate on the shearing resistance and fracture mode of Sn-3wt%Ag-0.5wt%Cu solder joints on electroless Ni-P/immersion Au surface finish pads. The results indicated that the solder joints underwent ductile and mixed ductile-brittle fracture at low ( 25,385 s−1) strain rates, respectively. Thus, the overall shear stress-strain curve can be divided into three areas according to Hollomon’s law, starting from low strain rates: area I, 100% ductile fracture of the solder itself; area II, mixed ductile-brittle fracture resulting in a ductile-brittle transition region; and area III, 100% brittle fracture at the interface between the intermetallic compound and the Ni-P layer.

Interrelation between Microstructure and Mechanical Properties of Heavy Section Ductile Iron Casting

The microstructures and mechanical properties of a sector ductile iron casting with a wall thickness of 500 mm were investigated. Results show that spheroidal graphite is obtained in ductile iron, and chunky graphite is avoided because of chills used during solidification. Besides, the matrix structure is nearly a full ferrite structure at the edge of casting while some pearlite forms at the center of casting. The competing effect between graphite and matrix structure keeps the strength relatively inert to the change in solidification time or cooling rate, but these two parameters both impair elongation of ductile iron as well. The samples with full ferrite matrix exhibit a typical ductile fracture mode while those with some pearlite lead to a mixed ductile-brittle fracture mode. Graphite internal fracture or graphite/matrix interfacial cracking are two crack initiation sites.

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082‐T6 aluminium alloy

ABSTRACTThe present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082‐T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD0.2) decreases in the range of ∼18 to 36% whereas the value of the non‐linear fracture mechanics parameter J0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.

FI catalyst helps produce oligomer with double bond at one end

The microstructure and mechanical properties of friction spot welding (FSpW) 2198-T8 alloy during aging treatment were studied. Whereafter, the fracture process before and after aging treatment was observed by using the in-situ straining transmission electron microscopy method. The results show that fined equiaxed grains remained with predominant T1 phase re-precipitated in the microstrucuture of FSpW 2198-T8 Al-Li alloy after aging treatment. With the extension of holding time (12 h –48 h), grains gradually grew up in the range of 9.2–13.2 μm and the size and volume fraction of T1 phases were also increased. Tensile strengths of thermal-mechanical coupling zone (TMCZ) of FSpW 2198-T8 alloy first increased and then decreased after aging treatment. When aging at 175 °C for 36 h, the strength reached the maximum (435 MPa) with the elongation decreased to 11.4%. Meanwhile, the fracture mode of TMCZ zone transformed from ductile brittle mixed fracture to brittle fracture after aging treatment. And cracks initiated at and propagated along precipitate-free zones (PFZs) after aging treatment.