Brittle Layer Research Articles

Ultrasonic-assisted fluxless reactive bonding of Mg/Al dissimilar alloy using Zn–Al solder in air

ABSTRACTAn ultrasonic-assisted reactive brazing method, without flux and conducted in air, was developed to braze Mg/Al dissimilar alloys. By using pre-coated Zn–Al solders on the Al sheet and a short-time ultrasonic (3 s) assistance, the Mg/Al dissimilar sheets were successfully brazed at a low temperature (340°C). The effect of Al content of the solder on the shear strength was investigated based on the analysis of the microstructure and the fracture behaviour of the brazed joint. The results indicated that the microstructure and the shear strength of the ultrasonic-assisted brazed joint were significantly affected by Al content in Zn–Al solders. Increasing Al content in Zn–Al solder prevented the joint from forming the continuous brittle MgZn2 layer. The maximum shear strength of 110.5 MPa was obtained when Zn–15Al solder was used, because the joint possessed the optimum microstructure of the soft (Al, Zn) eutectoid matrix with the MgZn2 dispersed particles.

Effect of Strain Rate on Joint Strength and Failure Mode of Lead-Free Solder Joints

In surface mount technology, the Sn-3.0Ag-0.5Cu solder joint has a shorter impact lifetime than a traditional lead–tin solder joint. In order to improve the impact property of SnAgCu lead-free solder joints and identify the effect of silver content on tensile strength and impact property, impact experiments were conducted at various strain rates on three selected SnAgCu based solder joints. It was found that joint failure mainly occurred in the solder material with large plastic deformation under low strain rate, while joint failure occurred at the brittle intermetallic compound layer without any plastic deformation at a high strain rate. Joint strength increased with the silver content in SnAgCu alloys in static tensile tests, while the impact property of the solder joint decreased with increasing silver content. When the strain rate was low, plastic deformation occurred with failure and the tensile strength of the Sn-3.0Ag-0.5Cu solder joint was higher than that of Sn-0.3Ag-0.7Cu; when the strain rate was high, joint failure mainly occurred at the brittle interface layer and the Sn-0.3Ag-0.7Cu solder joint had a better impact resistance with a thinner intermetallic compound layer.

A numerical investigation on the effects of rock brittleness on the hydraulic fractures in the shale reservoir

Hydraulic fracturing is an extensively used technique for development of oil and gas resources. In the shale reservoir, rock brittleness plays an important role during hydraulic fracturing. In this paper, a numerical code known as RFPA (Rock Failure Process Analysis) is introduced and the embedded digital-image-based (DIB) technique is illustrated in detail. Based on this integration, the effects of rock brittleness on the failure mode and stress-strain characteristic of the shale specimens are numerically investigated. It is found that the brittle shale specimen is more likely to rupture with multi crossed failure planes while the ductile specimen is more likely to rupture with a penetrating failure plane, from which we deduce the brittle shale is easier to develop more natural fractures than the ductile shale. The influence of natural fractures on complex hydraulic fracture network is further investigated through numerical simulation and the positive effect of rock brittleness is indirectly verified. It is found that hydraulic fractures are preferable to propagate in brittle minerals, i.e. the hydraulic fractures always choose the brittle minerals as the favorite path to propagate or choose a thin or weak part of ductile minerals to penetrate and is blocked by the ductile minerals. Moreover, the hydraulic fractures generated in the brittle shale are tortuous and appear with multi branches, which is much beneficial to form hydraulic fracture network in contrast to the smooth hydraulic fracture generated in the ductile shale. This is probably one of the causes of that the required treatment pressure in ductile shale layer is higher than that in brittle shale layer.

Early Diagnostics of Damage and Fracture Zones in Composite Materials Using Brittle Strain Gauges and Acoustic Emission

The integrated use of the acoustic emission technique and brittle oxide strain gauges for testing the deformation of damage and fracture zones in samples of polymer composite materials (PCM) at the early phases is considered. For the realization of the proposed technique, integrated parameters were used and software including the cluster analysis and the classification of registered AE pulses was developed, which made it possible to differentiate crack formation signals in the brittle layer of a strain gauge from all the other signals that occur at the early phase of the deformation of PCM samples in the online mode. The use of acoustic emission testing and control video recording for the registration of cracks in a brittle oxide strain gauge with the threshold deformation value of 1000 μm/m provided the precise diagnostics of the forthcoming fracture zone in a sample at the early loading phase under the load level of 10–15% of the limiting one long before the beginning of the active PCM structural degradation phase and made it possible to determine the distribution fields of the highest main deformations in the region of a strain gauge and to perform their quantitative evaluation.

Corrosion mechanism of carbon brick in the blast furnace hearth by potassium

Alkali plays a significant role in the formation of brittle layer of carbon brick in the blast furnace hearth. The brittle layer in a commercial blast furnace hearth was investigated. Large amounts of potassium compounds were found in the brittle layer. Subsequently, the carbon bricks which reacted with potassium in the simulation of blast furnace hearth under different contents of potassium in the brick (0.25, 0.5, 1.0 and 2.0 wt.%) during various reaction times (0.5, 1, 2 and 4 h) were experimentally studied. Finally, the formation mechanism of the brittle layer in carbon brick was clarified. The investigation results show that a large number of cracks are present in the brittle layer. The average potassium content in the brittle layer is 1 wt.%. According to the experimental results, cracks in the carbon brick can be formed by the attack of potassium. The reason for the formation of the brittle layer in the carbon brick is that the liquid potassium permeates into the carbon brick through the pores and cracks, and then reacts with SiO2 and Al2O3 in CO atmosphere. The generated potassium compounds nepheline and leucite lead to the volume expansion and the damage of the carbon brick.

Microstructure and mechanical properties of Al-Cu joints diffusion-bonded with Ni or Ag interlayer

The heterogeneous joint constituted by Al and Cu were diffusion-bonded with Ni or Ag interlayer under different bonding parameters, which was compared with directly bonded Al-Cu joints to analyze the effects of the different intermetallic compounds (IMCs) produced at the joining interface on the joint mechanical properties. The joint microstructure was analyzed by scanning electron microscope (SEM), the compositions profiles across the joining interface were measured by energy dispersive spectroscopy (EDS), and the mechanical properties of joints were evaluated by hardness, indentation fracture toughness, and tensile test. The experiment results demonstrated that the Al-Ag-Cu and Al-Cu joints couldn't avoid the crack due to the thick, hard and brittle IMCs layer formed at the joining interface, while, the thickness of the IMCs in the Al-Ni-Cu joint is less than 3.8 μm, this thin IMCs layer ensured that the joining interface exhibited the lowest hardness and brittleness in microscales, so the Al-Ni-Cu joint was free of cracks not only after bonding but also under the load of indentation fracture toughness test. The tensile test indicated that the Al-Ni-Cu joint had the highest tensile strength (47 MPa), which was bonded with 50 μm thick Ni interlayer at 10 MPa, 520 °C, 60 min.

The emergence of asymmetric normal fault systems under symmetric boundary conditions

Many normal fault systems and, on a smaller scale, fracture boudinage often exhibit asymmetry with one fault dip direction dominating. It is a common belief that the formation of domino and shear band boudinage with a monoclinic symmetry requires a component of layer parallel shearing. Moreover, domains of parallel faults are frequently used to infer the presence of a décollement. Using Distinct Element Method (DEM) modelling we show, that asymmetric fault systems can emerge under symmetric boundary conditions. A statistical analysis of DEM models suggests that the fault dip directions and system polarities can be explained using a random process if the strength contrast between the brittle layer and the surrounding material is high. The models indicate that domino and shear band boudinage are unreliable shear-sense indicators. Moreover, the presence of a décollement should not be inferred on the basis of a domain of parallel faults alone.

Mechanical reliability of transient liquid phase bonding of Au–Sn solder with Ni(Cu) substrates

Mechanical reliability of transient liquid phase (TLP) bonded Au–Sn/Ni(Cu) joints was investigated in this study. The results show that the TLP bonded Au–Sn/Ni(Cu) joints have excellent mechanical performance, even after exposed at 400 or 450 °C, especially in Au–Sn/Ni–30Cu and Au–Sn/Ni–40Cu systems. The shear strength could reach 75 MPa in TLP bonded Au–Sn/Ni–40Cu joint and increased to 87 and 97 MPa after exposed at 400 or 450 °C for 24 h, respectively. It was found that during TLP bonding at 350 °C, ordered AuCu, Ni3Sn2, and α(Au) phases formed in the Au–Sn/Ni–Cu joints, then the ordered AuCu phase transferred to disordered (Au,Cu) phase and a new Ni3Sn phase appeared between substrate (Au,Cu) and the Ni3Sn2 layers after exposure at 400 or 450 °C. However, only Ni3Sn2 layer formed after TLP bonded and new Ni3Sn layer appeared near substrate after exposure at 400 or 450 °C in the Au–Sn/Ni bonding system. The ductile Au–Cu [ordered AuCu or disordered (Au,Cu)] layer releases local stress concentration in joint under shear stress, which improves the mechanical reliability. Moreover, the maximum stress in Au–Sn/Ni(Cu) joints decreases with the increasing thickness of Au–Cu layer. As a result, the Au–Sn/Ni joint presents lower shear strength as brittle Ni3Sn or Ni3Sn2 layer formed near substrate, whereas the shear strength of Au–Sn/Ni–Cu joint increases with thicker Au–Cu layer.

Two-stage cracking of metallic bi-layers on polymer substrates under tension

This study demonstrates two-stage cracking of an Inconel‑silver film system on a flexible Teflon substrate under uniaxial tension. In situ fragmentation experiments revealed that primary fracture of the brittle 30nm Inconel overcoat induced brittle, secondary fracture of the normally ductile 150nm silver base layer. Good correlation exists between the ratios of primary and secondary saturation crack spacing and individual layer thicknesses. Two-stage cracking was confirmed by cross-sectional analysis and explained by different steady state energy release rates of the individual layers. The results further illustrate how brittle layers are detrimental to the mechanical behaviour of polymer-supported thin film multilayer structures.

Brazing of TiB w /TC4 composite and Ti60 alloy using TiZrNiCu amorphous filler alloy

Abstract TiBw/TC4 composite was brazed to Ti60 alloy successfully using TiZrNiCu amorphous filler alloy, and the interfacial microstructures and mechanical properties were characterized by SEM, EDX, XRD and universal tensile testing machine. The typical interfacial microstructure was TiBw/TC4 composite/β-Ti + TiB whiskers/(Ti, Zr)2(Ni, Cu) intermetallic layer/β-Ti/Ti60 alloy when being brazed at 940 °C for 10 min. The interfacial microstructure evolution was influenced strongly by the diffusion and reaction between molten fillers and the substrates. Increasing brazing temperature decreased the thickness of brittle (Ti, Zr)2(Ni, Cu) intermetallic layer, which disappeared finally when the brazing temperature exceeded 1020 °C. Fracture analyses indicated that cracks were initialized in the brittle intermetallic layer when (Ti, Zr)2(Ni, Cu) phase existed in the brazing seam. The maximum average shear strength of joints reached 368.6 MPa when brazing was conducted at 1020 °C. Further increasing brazing temperature to 1060 °C, the shear strength was decreased due to the formation of coarse lamellar (α+β)-Ti structure.

Effect of disorder on shrinkage-induced fragmentation of a thin brittle layer.

We investigate the effect of the amount of disorder on the shrinkage-induced cracking of a thin brittle layer attached to a substrate. Based on a discrete element model we study how the dynamics of cracking and the size of fragments evolve when the amount of disorder is varied. In the model a thin layer is discretized on a random lattice of Voronoi polygons attached to a substrate. Two sources of disorder are considered: structural disorder captured by the local variation of the stiffness and strength disorder represented by the random strength of cohesive elements between polygons. Increasing the amount of strength disorder, our calculations reveal a transition from a cellular crack pattern, generated by the sequential branching and merging of cracks, to a disordered ensemble of cracks where the merging of randomly nucleated microcracks dominate. In the limit of low disorder, the statistics of fragment size is described by a log-normal distribution; however, in the limit of high disorder, a power-law distribution is obtained.

Characterization of carbon/carbon composite/Ti6Al4V joints brazed with graphene nanosheets strengthened AgCuTi filler

Carbon/carbon (C/C) composite and Ti6Al4V alloy (wt%) were successfully brazed with graphene nanosheets strengthened AgCuTi filler (AgCuTiG). Graphene nanosheets (GNSs) with low CTE and high strength were dispersed into AgCuTi filler by ball milling. The interfacial microstructure was systematically characterized by varieties of analytical means including transmission electron microscopy (TEM). Results show that typical interfacial microstructure of the joint brazed at 880°C for 10min is a layer structure consisting of (Ti6Al4V/diffusion layer/Ti2Cu + TiCu + Ti3Cu4 + TiCu4/GNSs + TiCu + TiC + Ag(s,s) + Cu(s,s)/TiC/C/C composite). The interfacial microstructure and mechanical properties of brazed joints changed significantly as temperature increased. High temperature promoted the growth of TiCu and TiC phases, which were attached to GNSs. Meanwhile, the diffusion layer and primary reaction layers thickened as temperature increased, while the thickness of brazing seam decreased. The maximum shear strength of 30.2MPa was obtained for the joint brazed at 900°C for 10min. GNSs decreased the thickness of brittle reaction layers and promoted the formation of TiCu and TiC phases in brazing seam, which caused the strengthening effect and decreased the CTE mismatch of brazed joints. The fracture modes are also discussed in this paper.

Degradation of hydrogenated nitrile rubber (HNBR) O-rings exposed to simulated servo system conditions

The degradation behaviors and mechanisms of hydrogenated nitrile rubber O-rings under the compression and free state were investigated in simulated hydraulic oil environments at different elevated temperatures. The chemical structures and mechanical properties of the uncompressed and compressed samples before and after exposure to oil for different days were evaluated by measuring attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, compression set, weight loss, mechanical properties and fracture morphology. The ATR-FTIR results demonstrate that the hydroxyl groups were formed under the compression and free state, and the amide groups were only generated under the free state. The crosslinking density results reveal that the crosslinking dominated during the most of aging process and the chain scission might account for more at the elevated temperature in the middle and later stages. The compression set for the aged samples increased with increasing exposure time and temperature. The mechanical properties of the uncompressed and compressed samples significantly varied with the physical and chemical changes in the matrix structure under coupling actions of stress, temperature and oil. Moreover, the fracture morphology results indicate that the formation of the hardened brittle outer layer, voids and agglomerates might be attributed to the decrease in properties of the aged samples, especially for the free state.

Effects of lateral variations of crustal rheology on the occurrence of post-orogenic normal faults: The Alto Tiberina Fault (Northern Apennines, Central Italy)

The Northern Apennines (NA) are characterized by formerly compressive structures partly overprinted by subsequent extensional structures. The area of extensional tectonics migrated eastward since the Miocene. The youngest and easternmost major expression of extension is the Alto Tiberina Fault (ATF). We estimate 2D rheological profiles across the NA, and conclude that lateral rheological crustal variations have played an important role in the formation of the ATF and similar previously active faults to the west. Lithospheric delamination and mantle degassing resulted in an easterly-migrating extension-compression boundary, coinciding at present with the ATF, where (i) the thickness of the upper crust brittle layer reaches a maximum; (ii) the critical stress difference required to initiate faulting at the base of the brittle layer is at a minimum; and (iii) the total strengths of both the brittle layer and the whole lithosphere are at a minimum.Although the location of the fault is correlated with lithospheric rheological properties, the rheology by itself does not account for the low dip (~20°) of the ATF. Two hypotheses are considered: (a) the low dip of the ATF is related to a rotation of the stress tensor at the time of initiation of the fault, caused by a basal shear stress (~100MPa) possibly related to corner flow associated with delamination; or (b) the low dip is associated to low values of the friction coefficient (≤0.5) coupled with high pore pressures related to mantle degassing.Our results establishing the correlation between crustal rheology and the location of the ATF are relatively robust, as we have examined various possible compositions and rheological parameters. They also provide possible general indications on the mechanisms of localized extension in post-orogenic extensional setting. The hypotheses to account for the low dip of the ATF, on the other hand, are intended simply to suggest possible solutions worthy of further study.

Brazing SiO2f/SiO2 with TC4 alloy with the help of coating graphene

The joining of SiO2f/SiO2 to TC4 alloys suffers from the problems of poor wettability with AgCuTi, excessive Ti dissolving from TC4 and emergence of high residual stress among the joint. To solve these problems, an effective method by coating graphene on the SiO2f/SiO2 is developed. Wetting experiments confirm that contact angle notably reduces from 131° to 42.8° after coating rGO. Further, rGO react with Ti element from the brazing alloy and TC4 alloy, inhibiting the formation of excessive brittle reaction layer and in situ forming TiC reinforcement. The highest shear strength of SiO2f/SiO2-TC4 brazed joints was up to 39.2 MPa.

Brittle ice shell thickness of Enceladus from fracture distribution analysis

We determine the depth of fracture penetration in multiple regions of Enceladus by performing self-similar clustering and length distribution analysis of fractures. The statistical characterization of fault-population attribute, such as length and clustering, provide a productive avenue for exploring deformation rate, stress transmission mode, rheology of the medium, and mechanical stratification of the ice satellite. Through this analysis, we estimate the depth of the mechanical discontinuity of Enceladus’ ice shell that is the depth to which fractures penetrate the brittle ice layer above the ductile one.In this work, we find that for the South Polar Terrain (SPT), the brittle ice shell interested by fracture penetration is about 30 km and corresponds to the total depth of the ice shell because the SPT has a very high thermal gradient and, hence, fractures likely reach the ocean-ice interface. In the other regions analyzed, the depth of fracture penetration increases from 31 to 70 km from the South Pole to northern regions up to 75°.

Squat formation and the occurrence of two distinct classes of white etching layer on the surface of rail steel

Squats are recognized as an important rail track issue related to rolling contact fatigue, and represent a serious problem affecting significant parts of railway networks across the world. The mechanisms of squat initiation and propagation are still a matter of controversy. Research efforts have included understanding of possible relationships between the narrow and brittle white etching layers (WELs) observed on rail surfaces and squat formation in rails. In the present investigation, structural and microstructural features of ex-service damaged rails containing squats and WELs were investigated in order to determine the formation mechanism of white etching layers (WELs) on the rail surface. Synchrotron XRD analysis of rail surfaces, combined with microstructural and microhardness investigations, revealed two distinguishable types of WELs; one characterised by the presence of fine martensite and the other characterised by the presence both of retained austenite and martensite. WELs containing martensite were found to be associated with a deformed subsurface pearlite structure and it was concluded that this type of WEL is induced by severe plastic deformation at wheel-rail interface. In contrast, the WELs containing both of significant amounts of retained austenite and martensite, were associated with the presence of undeformed pearlite in the underlying structure and it was concluded that this type of WEL is either thermally or thermomechanically induced. Synchrotron XRD results obtained from regions of squat defects themselves are consistent to that of WELs containing austenite and martensite in regions near the squats, which indicates a relationship between WELs and squat initiations. Occurrences of cracks extending from WELs down into the rail in squat regions were confirmed by microstructural observations, which is consistent with the premise that cracks in squats originate from an extension of cracks in WELs.

Cold Roll Bond of Al-St Strips with Wire Brush Surface Treatment

The effect of interface microstructure by wire brush treatment on the bond strength of Al-St clad strips by cold roll bonding was investigated. For the purpose, wire brush surface preparations on steel strip surface have been done before the cold roll bonding of the Al-St clad strips, and evaluate the sample bonding strength by peeling test of the rolling deformation zone. The results show that bond strength of Al-St strip by wire brush treatment increases with the total reduction increases and the threshold reduction of Al-St strip is 15%. Meanwhile the total crack area increase with the reduction. The oxide film on the surface of brittle layer hinds the metal bond and the crack between the scratch brush layer and substrate metal is the weak point and decrease the bond strength. A major feature of the bonding mechanism described in this paper is that extruded aluminum squeezed into the cracks of steel surface by normal roll pressure action, and metallic bond with underlying fresh steel metal.

Plasma nitriding without formation of compound layer for 38CrMoAl hydraulic plunger

In this study, low temperature plasma nitriding (LPN) was primarily used in 38CrMoAl hydraulic plunger to overcome the peeling problem of the compound layer in real application. The cross-sectional microstructures of the treated samples were observed by optical metallography, phase constituents were determined by X-ray diffraction (XRD), the microhardness profile and brittleness of surface were measured by microhardness tester. The results showed that no compound layer was formed while plasma nitriding at 450 °Cfor 6 h, and XRD also confirmed that there existed no γ′-Fe4N. Meanwhile, the brittleness was decreased at lower temperature plasma nitriding comparing with normal plasma nitriding (NPN), due to no brittle compound layer formed on the outmost layer.

P‐133: Optimization of Multilayer Inorganic/Organic Thin Film Structure for Foldable Barrier Films

We report the optimal structure of multilayer inorganic/organic thin film for foldable barrier films which maintain their barrier properties even after a folding test with 1 mm radius. The barrier film consists of PI film, cyclo‐aliphatic epoxy‐siloxane (CES) hybrid material, and ALD‐grown Al2O3. The optimal structure was determined by investigating mechanical properties of the film based on finite element analysis (FEA). In this structure, folding stress and strain in the brittle Al2O3 layer is minimized with values of 5.38 MPa and 0.0048% during a folding test with 1 mm radius. The optimized multilayer inorganic/organic thin film proposed by FEA results was validated by water vapor transmission rate (WVTR) measurement. Despite 10000 times folding test, the WVTR value of the barrier was comparable to that of a non‐bending sample. These results demonstrate a potential for applicability to an encapsulation technology for flexible organic light‐emitting diodes (OLEDs).