Change In Fracture Mode Research Articles

Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air, dry air, and vacuum

AbstractEnvironmental influences on near‐threshold fatigue crack growth in wrought magnesium alloy AZ61 were investigated. Fatigue tests were performed in ambient (humid) air, dry air, vacuum, and dry nitrogen gas at 19 kHz cycling frequency and load ratio R = −1. Threshold stress intensity factor amplitudes, Kth, determined for limiting growth rates below 5 × 10−13 m/cycle were 1.1 MPam1/2 in ambient air and 1.2 MPam1/2 in dry air. A much higher Kth of 1.9 MPam1/2 was measured in vacuum and dry nitrogen gas. This suggests oxygen to be the most detrimental constituent of ambient air that increases near‐threshold crack propagation rates and decreases Kth. The deleterious effect of humidity is comparatively small. Corrosive influences are effective at ultrasonic cycling frequency for growth rates below approximately 3 × 10−9 m/cycle. The crack propagation curves in ambient and dry air show a plateau‐like regime where the fracture mode changes from purely ductile to a mixed ductile and brittle mode. In vacuum and dry nitrogen gas, a ductile crack path is found for all investigated crack growth rates.

Interfacial microstructure and mechanical reliability of the Sn-Ag-Cu/Au/Pd(xP)/Ni(P) reactive system: P content effects

Electroless deposition of a palladium film, e.g., pure Pd or Pd(P), between Au and Ni(P) films has received a great deal of attention from the microelectronic industry over the past decade. We investigated the effect of P on the solderability and the corresponding mechanical reliability of the Sn-3Ag-0.5Cu/Au/Pd(xP)/Ni(P)/Cu joint system, where x = 0, 3, and 4.5 wt%. Analyses of electron probe X-ray microanalysis and field-emission transmission electron microscopy showed that a trilayer of (Cu,Ni)6Sn5/(Ni,Cu)3Sn4/Ni3P intermetallic structure grew at the interface at x = 0 wt%, and the growth of (Ni,Cu)3Sn4 was replaced by that of Ni2SnP at x = 3 and 4.5 wt%. In high-speed ball shear testing, the microstructure transition induced by P addition would cause a fracture mode change from brittle to ductile, and increased the shear resistance of solder joints. These findings indicated that the P content in the Pd(P) film is a crucial factor in interfacial strength, and an appropriate amount of P is beneficial to the solderability of the Au/Pd(xP)/Ni(P) surface finish.

Role of Sc on the glass forming ability and mechanical properties of Al-Y-Ni-Sc bulk metallic glass produced with different cooling rates

In the present study, the role of Sc in the glass forming ability (GFA) and mechanical properties of (Al87Y8Ni5)99Sc1 alloy ribbon produced by single roller melt-spinning with various rotation speeds has been investigated in an attempt to understand the influence of cooling rate in the transition element of the Al-TM-RE (TM: transition metal, RE: rare earth metal) alloy system. The crystallinities of the two alloy ribbons formed at various cooling rates were evaluated by X-ray diffraction (XRD) in order to examine whether the amorphous structure, α-Al particles and precipitation compounds could be formed at any cooling rate. Results revealed α-Al in the Al87Y8Ni5 alloy, but both phases (i.e., α-Al and Al3Sc) were indexed for (Al87Y8Ni5)99Sc1 alloy at a low cooling rate (1000–2000 rpm). Moreover, larger full width at half maximum (FWHM) of characteristic peaks (corresponding to α-Al) were observed for both alloy ribbons, indicating a lower degree of structural relaxation as the cooling rate was increased. Therefore, better short-range ordering of the atomic configuration can be achieved when the alloy ribbons are synthesized at a lower cooling rate, which allows sufficient time for the atoms to reach their local ordered equilibrium positions. On the other hand, the maximum hardness was obtained for both non-scandium and scandium-doped alloys produced at the intermediate cooling rate (2000 rpm) due to the random and homogeneous distribution of numerous tiny precipitates in the amorphous matrix. In addition, compared to Al87Y8Ni5 alloy ribbon, the occurrence of glass transition temperature (Tg), higher crystallization temperature (Tx, Tp1, Tp2), and smaller amount of crystallization fraction in the (Al87Y8Ni5)99Sc1 during continuous heating proves the advantageous effects, namely higher glass forming ability and better thermal stability, of the scandium addition. The mechanism underlying this advantage should be associated with the enhancement of the viscosity of the alloy melt, which results from the increased packing density of the alloy melt when scandium is added.The shear-banded structures that characteristically appear around the indents were observed to be completely absent around indents made at a low cooling rate, revealing that inhomogeneous deformation is not mediated by shear bands below a certain cooling rate. In addition, observation of the fracture surfaces revealed that the fracture surfaces of both alloy ribbons produced at high and low cooling rates transformed from vein patterns to isotropic dimple structures, revealing a change in fracture mode from brittle to ductile as the cooling rate decreased.

Effect of physio-chemically functionalized graphene nanoplatelet reinforcement on tensile properties of aluminum nanocomposite synthesized via spark plasma sintering

The present study aims to understand the effect of physio-chemically functionalized graphene nanoplatelet (GNP) reinforcement on tensile properties of aluminum nanocomposite synthesized via spark plasma sintering (SPS). Aluminum matrix was reinforced with different weight percentage of functionalized GNPs (0.25, 0.5 and 1.0 wt%). The novel GNP dispersion technique, i.e. the physio-chemical functionalization of GNPs along with improvised Al powder – GNP particle mixing method resulted in homogeneous distribution of GNPs throughout the Al matrix. The optimization of SPS parameters assisted in achieving a very good densification (up to 99%) in the nanocomposite. Tensile test revealed 98% increment in yield strength of Al-0.5 wt% GNP nanocomposite. Further increase in GNP content (1.0 wt%) resulted in deterioration in the tensile properties attributed to agglomeration of GNPs in the Al matrix. A theoretical prediction of the yield strength of the Al-GNP nanocomposites was carried out considering the various strengthening mechanisms. Fractographs of pure Al and Al-GNP nano composites indicate change of fracture mode from ductile to brittle with increase in the GNP amount.

Tensile behavior of C/SiC composites plate after hypervelocity penetration: Residual strength and fracture mechanism

The damage and tensile mechanical behavior of C/SiC composites plate after hypervelocity penetration are studied in this paper. Hypervelocity penetration is applied along the out-of-plane direction by an electrical gun at 3.5 km s-1, and the target is recovered for damage observation and tension test. Quasi-static tensions on the recovered targets and the slices cut from damage zone are examined along with high-speed photography. Moreover, corresponding numerical simulations are carried out. The damage zone in the vicinity of the penetration hole is small (∼10 mm), and the damage mode involves SiC matrix fracture, delamination, fiber bundle splitting and fiber breaking. The simulation results suggest that the degree of damage decreases exponentially with the increasing distance from the penetration hole center of the target plate. As the degree of damage increases, the tensile strength of C/SiC composites decreases and the corresponding fracture mode changes from shear failure to tensile breakage. Additionally the fracture surface becomes smoother, the quantity and length of fiber pullout also increases. Since the decrease of elastic modulus induced by damage weakens the stress concentration effect of the penetration hole, the residual strength of recovered target plate is only reduced from 241 MPa to 175 MPa.

How grain boundary chemistry controls the fracture mode of molybdenum

The design of metallic alloys with superior properties requires a deep understanding of the atomistic processes governing the materials behavior. In this paper, we present results providing a close link between grain boundary (GB) chemistry and fracture behavior in a Mo-Hf alloy. By combining atom probe tomography and ab-initio simulations of GBs, we unravel the origin for the transition between intergranular and transgranular fracture in technological Mo-Hf alloys. The main agent affecting GB strength is not the primary alloying element Hf, but rather the impurities O, C and B. With larger Hf additions, an intricate interplay between segregation and precipitation leads to a strong GB enrichment of C and B and to a depletion of O and Hf resulting in a higher cohesion of the GBs and thus, leading to a change in fracture mode. Our investigation exemplarily demonstrates that smallest additions of solutes can be decisive for understanding fracture behavior on the macroscale.

Impact Tensile Properties of Notched Titanium Alloy Bolt for Fairing Separation of Launch Vehicle

The payload fairing in Japan is fixed by a lot of notched bolts. These notched bolts were fractured by axial impact tensile using the explosive devices to separate the fairing. In this case, the stress waves and the oscillations propagate, which may seriously damage the satellites. In this study, the impact deformation and the fracture behavior of notched titanium alloy bolt was investigated using a split Hopkinson pressure bar method. The notched bolt specimen was made of commercial Ti-6Al-4V alloy. The maximum load value was increased with increasing the displacement rate. It can be said that the strain rate dependence of strength for Ti-6Al-4V alloy appeared. From the observation of fracture surface using a scanning electron microscope, compared with the quasi-static test, it was clear that the irregularities of the fractured surface at the impact tensile test became rough. Therefore, it was found that the brittle fracture was mainly observed due to the increase in displacement rate, which may mean that the mode of fracture changes from the transgranular to the intergranular. It was surmised that this change of fracture mode was caused by the high strain rate due to stress concentration of the notched part.

Novel parameters in load capacity and failure of coaxial steel tubes jointed by wrapped GFRP sleeve

In this paper, the uniaxial load capacity of two coaxial steel tubes joined by glass fiber reinforced plastic (GFRP) was investigated. The axial connection was made by hoop winding of textile woven fabrics on circular hollow steel tubes. A simple efficient theory was selected to estimate the load capacity of sleeve joint. To define novel parameters in load capacity and failure, tensile tests were conducted on several specimens. Based on fracture mechanics experimental observations showed that three kinds of failure modes occurred including GFRP rupture, adhesive failure and mixed mode. The wetted cross-ply textile of woven glass was used in order to study the effect of the number of plies. One novel parameter named lamination cohesion rate was introduced to explore the GFRP rupture. The results revealed that increasing the number of plies leads to maximize the allowable axial tensile load. A second novel parameter was introduced for Adhesive failure. By employing more plies, fracture mode changes from GFRP rupture to adhesive failure resulting in a considerable increase in the load capacity. However, in this fracture mode, by increasing the number of plies the joint average shear stress increases but the maximum shear stress approximated from theories decreases. In the case of load capacity, a good agreement was found between theoretical and experimental results. By using this joining method, steel tubes can be toughly connected and withstand stresses up to steel ultimate tensile stress.

Effect of Scandium addition on evolution of microstructure, texture and mechanical properties of thermo-mechanically processed Al-Li alloy AA2195

In the present study, influence of Scandium (Sc) content (0.025 and 0.25 wt%) on microstructure, texture and mechanical properties of a thermo-mechanically processed AA2195 alloy has been investigated. Addition of Sc to AA2195 alloy results in grain refinement and enhanced precipitation kinetics. TEM analysis revealed the presence of Al3(Sc,Zr) dispersoids in the Sc containing AA2195 alloys. Solution treatment and peak aging of the Sc containing alloys revealed partially recrystallized microstructure. Texture of Sc containing alloys displayed weaker Brass component compared to the Sc free material. A clear improvement in hardness and strength was observed which could be attributed to grain refinement as well as the presence of fine Al3(Sc,Zr) dispersoids. Ductility decreased slightly on Sc addition and there was an associated change in fracture mode as a result of Sc addition.

Microstructure and Interfacial Shear Strength in W/(Zr55Cu30Al10Ni5)100−x Nb x Composites

In the present study, (Zr55Cu30Al10Ni5)100−x Nb(x=0,1,2,3) bulk metallic glass matrix/tungsten wire composites were fabricated by a gas pressure infiltration process at temperature 950 °C for 5 min. Microstructural studies and mechanical behaviors of the materials have been investigated by scanning electron microscopy, transmission electron microscopy and pullout tests. The mechanical results showed that the interface shear strength in the composite sample with X = 2 increased more than twice compared to the composite sample with X = 0. Based on the microstructural results, the addition of two atomic percent Nb in the matrix composite causes an increase in the diffusion band thickness during the melt infiltration and change in the interface fracture mode as a result of pullout test.

Mechanical properties and fracture behavior of B4C-nano/micro SiC composites produced by pressureless sintering

In the present work, boron carbide (B4C) powder was mixed separately with various quantities (2.5, 5, 7.5 and 10vol%) of nano/micro silicon carbide (nano/micro SiC) in a high-energy mill and in the presence of isopropanol solution. The resulting compounds were then dried and formed into test specimens at a pressure of 80MPa. Next, using the pressureless sintering technique, the obtained pieces were sintered at temperatures of 2100, 2150 and 2200°C. The effects of additives and sintering temperature on the grain size and mechanical properties of the produced specimens (e.g. flexural strength and fracture toughness (KIC)) were studied, and the fracture behavior of these samples was evaluated through microstructural analysis. By adding nano/micro SiC, the results indicate a considerable increase in the flexural strength (up to 860MPa) and KIC value (up to 6MPam1/2) of specimens. Also, it was found that by increasing the vol% of nano/microSiC additive, not only the produced composites become fine-grained, but also their fracture mode changes mostly from intergranular to transgranular.

The creep deformation and fracture behaviors of nickel-base superalloy M951G at 900 °C

The creep behaviors of M951G alloy were carried out under stress ranging from 240MPa to 400MPa at 900°C, and the corresponding deformation mechanisms and fracture behaviors after rupture had been investigated by various techniques. Results showed that both the deformation mechanisms and fracture behaviors were dependent on the applied stress. According to the transmission electron microscope (TEM) observations, the dominant deformation mechanism changed from a combined process of slip and climb of dislocations in matrix channel to shearing of dislocations in γ′ precipitates and cross-slip of dislocations in matrix channel with the applied stress increasing. Fracture behaviors of M951G alloy were characterized using optical microscope (OM) and scanning electron microscope (SEM), which changed from intergranular to transgranular with the increase of applied stress. At low applied stress, M951G alloy was failure in the form of intergranular owing to coalescence of micropores along the grain boundaries. However, at higher applied stress the microcracks initiated at broken carbides in the grain interior, and finally resulted in transgranular fracture. Additionally, creep strain rate also played a key role in determining the transition of creep fracture modes by effect the corresponding temperature of equal strength for grain boundary and grain interior. The values of apparent stress exponent at low and high stress regions were calculated to be 5.14 and 11.13 respectively, which was due to the change of deformation mechanisms and fracture modes with the increase of applied stress.

Texture, microstructures, and mechanical properties of AlN‐based ceramics with Si3N4–Y2O3 additives

AbstractTextured AlN‐based ceramics with improved mechanical properties were prepared by hot pressing using Si3N4 and Y2O3 as additives. The introduction of Si3N4–Y2O3 into AlN matrix led to the formation of secondary Y3AlSi2O7N2 and fiber‐like 2Hδ AlN‐polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN‐based ceramics by the replacement of Y2O3 by Si3N4–Y2O3 increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m½ and 389.5±45.5 MPa, respectively.

Twisting cracks in Bouligand structures.

The Bouligand structure, which is found in many biological materials, is a hierarchical architecture that features uniaxial fiber layers assembled periodically into a helicoidal pattern. Many studies have highlighted the high damage-resistant performance of natural and biomimetic Bouligand structures. One particular species that utilizes the Bouligand structure to achieve outstanding mechanical performance is the smashing Mantis Shrimp, Odontodactylus Scyllarus (or stomatopod). The mantis shrimp generates high speed, high acceleration blows using its raptorial appendage to defeat highly armored preys. The load-bearing part of this appendage, the dactyl club, contains an interior region [16] that consists of a Bouligand structure. This region is capable of developing a significant amount of nested twisting microcracks without exhibiting catastrophic failure. The development and propagation of these microcracks are a source of energy dissipation and stress relaxation that ultimately contributes to the remarkable damage tolerance properties of the dactyl club. We develop a theoretical model to provide additional insights into the local stress intensity factors at the crack front of twisting cracks formed within the Bouligand structure. Our results reveal that changes in the local fracture mode at the crack front leads to a reduction of the local strain energy release rate, hence, increasing the necessary applied energy release rate to propagate the crack, which is quantified by the local toughening factor. Ancillary 3D simulations of the asymptotic crack front field were carried out using a J-integral to validate the theoretical values of the energy release rate and the local stress intensity factors.

Comparative study of the behavior of different highly alloyed steels in liquid sodium

By small punch tests performed in air and in liquid sodium at different temperatures and strain rates, the sensitivity of different structural materials to liquid sodium embrittlement was studied: martensitic 9Cr steel, ferrito-martensitic 14Cr steel, ferritic 18Cr steel, austenitic 316LN and 15-15Ti steels, martensitic 9Cr oxide-dispersion strengthened (ODS) steel, ferritic 14CrODS and 18CrODS steels. All materials present ductile mechanical behavior in air. No effect of liquid sodium has been detected for the two 9Cr martensitic steels and the ferritic 18Cr steel. A minor effect of sodium without change in fracture mode or with local brittle fractures has been observed for the 15-15Ti and 316LN austenitic steels, for the ferritic 18CrODS steel and for the ferrito-martensitic 14Cr steel. The ferritic14CrODS steel exhibited a marked liquid metal embrittlement.

Study of Aging-Induced Degradation of Fracture Resistance of Alloy 617 Toward High-Temperature Applications

For the Alloy 617, the effect of aging on the fracture energy degradation has been investigated after aging for different time periods at 1023 K (750 °C). A sharp reduction in impact energy (by ~55 pct vis-a-vis the as-received material) after 1000 hours of aging, as evaluated from room-temperature Charpy impact tests, has been observed. Further aging up to 10,000 hours has led to a degradation of fracture energy up to ~78 pct. Fractographic examinations using scanning electron microscopy (SEM) have revealed a change in fracture mode from fibrous-ductile for the un-aged material to intergranular mode for the aged one. The extent of intergranular fracture increases with the increasing aging time, indicating a tendency of the material to undergo grain boundary embrittlement over long-term aging. Analysis of the transmission electron microscopy (TEM) micrographs along with selected area diffraction (SAD) patterns for the samples aged at 10,000 hours revealed finely dispersed γ′ precipitates of size 30 to 40 nm, rich in Al and Ti, along with extensive precipitation of M23C6 at the grain boundaries. In addition, the presence of Ni3Si of size in the range of 110 to 120 nm also has been noticed. The extensive precipitation of M23C6 at the grain boundaries have been considered as a major reason for aging-induced embrittlement of this material.

Changes in mechanical property and microstructure of lithium metatitanate tritium breeder caused by thermal annealing

Changes in mechanical property and microstructure of sintered pebbles of lithium metatitanate (Li2TiO3) and Li excessive lithium metatitanate (Li2+xTiO3, x>0) as tritium breeder pebbles for thermal fusion reactors, which were caused by thermal annealing, were investigated. The compressive strength was measured by using a universal tester. The Li2TiO3 pebble was stable during the thermal annealing at 900°C, and its compressive strength was not decreased. In contrast, the microstructure and Li/Ti ratio of the Li2+xTiO3 pebble were markedly changed during the thermal annealing; the grain growth was severe and the excessive Li was disappeared in a 1000h annealing. As a result, its compressive strength was significantly changed; it was drastically decreased by a 100h annealing but it was once increased again in the additional annealing by a change in fracture mode.

Study of process/structure/property relationships in probeless friction stir spot welded AA2198 Al-Li alloy

Probeless friction stir spot welding (probeless FSSW) is a useful variant of conventional friction stir spot welding (FSSW), where there is no probe at the end of the tool shoulder. In this study, AA2198-T8 aluminum-lithium alloy has been successfully welded by probeless FSSW. The variations of geometric features of probeless FSSWed joints: stir zone width, stir zone edge angle and hook angle, and their effects on the joint mechanical properties were studied quantitatively. Results show that the width of stir zone is constrained by shoulder diameter, and the ratio of the width to shoulder diameter approaches 0.9. With the increase of stir zone depth, the stir zone edge angle approaches 45° eventually during the welding process. Additionally, the hook defect can be found among all of the probeless FSSWed joints due to the upward material flow of the lower sheet. When the hook angle is around 90°, the tensile/shear strength reaches a relatively higher value. The fracture mode changes from shear fracture to plug fracture when hook angle transforms from obtuse to acute.

Preparation and Properties of β/α SiC Composite Ceramics

In this paper, the production of high-performance β/α SiC composite ceramics technology as the goal, by measuring density, we can optimize and determine the best technical ceramic formulations. Because a small amount of β-SiC in α-SiC,the sintered ceramic appeared long axis-like crystals, indicating fracture mode changes from the original transgranular fracture to transgranular fracture mainly and intergranular fracture supplement. The results show that, when added in an amount of 10% β-SiC, SiC ceramics having the best performance.

Bonding Characteristics of Sn-57Bi-1Ag Low-Temperature Lead-Free Solder to Gold-Plated Copper

To fabricate a higher melting point joint by bonding at a low temperature, bonding characteristics of Au/Cu joints bonded at various bonding conditions using Sn-57Bi-1Ag solder were investigated. They were compared with those of the Cu/Cu joint. In the Au/Cu joint bonded at 170 oC for 1min, a large scallop-shaped AuSn4 layer formed in the joint interface of Au and solder. A Au-diffused layer formed in the tip of the AuSn4 layer. The Sn-Cu-Au layer including approximately 20 mol% Au formed in the joint interface of Cu and solder. With increasing the bonding time, both the eutectic microstructure with β-Sn and Bi and the Au layer disappeared. As the result, the solder area changed from the eutectic microstructure to Bi phases including AuSn4 and Ag3Sn phases in the bonding time over 30min. In contrast, the eutectic microstructure formed in the Cu/Cu joint bonded at 170 oC regardless of the bonding time. The bond strength of the Au/Cu joint was approximately one-third of that of the Cu/Cu joint. In both joints, the bond strength reduced with increasing the bonding time due to the fracture mode change.