Microstructure evolution,mechanical properties and fracture behavior of Al-xSi/AZ91D bimetallic composites prepared by a compound casting

In this paper,the effect of the Si content on microstructure evolution,mechanical properties,and fracture behavior of the Al-xSi/AZ91D bimetallic composites prepared by compound casting was investigated systematically.The obtained results showed that all the Al-xSi/AZ91D bimetallic composites had a metallurgical reaction layer(MRL),whose thickness increased with increasing Si content for the hypoeutectic Al-Si/AZ91D composites,while the hypereutectic Al-Si/AZ91D composites were opposite.The MRL included eutectic layer(E layer),intermetallic compound layer(IMC layer)and transition region layer(T layer).In the IMC layer,the hypereutectic Al-Si/AZ91D composites contained some Si solid solution and flocculent Mg_(2)Si+Al-Mg IMCs phases not presented in the hypoeutectic Al-Si/AZ91D composites.Besides,increasing Si content,the thickness proportion of the T layer increased,forming an inconsistent preferred orientation of the MRL.The shear strengths of the Al-xSi/AZ91D bimetallic composites enhanced with increasing Si content,and the Al-15Si/AZ91D composite obtained a maximum shear strength of 58.6 MPa,which was 73.4% higher than the Al-6Si/AZ91D composite.The fractures of the Al-xSi/AZ91D bimetallic composites transformed from the T layer into the E layer with the increase of the Si content.The improvement of the shear strength of the Al-xSi/AZ91D bimetallic composites was attributed to the synergistic action of the Mg_(2)Si particle reinforcement,the reduction of oxidizing inclusions and the ratio of Al-Mg IMCs as well as the orientation change of the MRL.

Regulation of Pore Structure and Hightemperature Fracture Behavior of CACbonded Alumina-Spinel Castables Based on Hydration Design

The lamellar hydrates of CAC were designed with the introduction of nano CaCO_(3)or Mg-Al hydrotalcite(M-A-H),and the effects on the green strength,pore structures,and high-temperature fracture behavior of alumina-spinel castables were investigated.The results show that nano CaCO_(3)or M-A-H stimulates rapidly the hydration of CAC and the formation of lamellar C_(4)AcH_(11)or coexistence of C_(2)AH_(8)and C_(4)AcH_(11)at 25℃.The formation of lamellar hydrates can contribute to a more complicated pore structure,especially in the range of 400-2000 nm.Meanwhile,the incorporation of well-distributed CaO or MgO sources from nano CaCO_(3)or M-A-H also regulates the distribution of CA_(6)and spinel(pre-formed and in-situ).Consequently,the optimized microstructure and complicated pore structure can induce the deflection and bridging of cracks,thus facilitating the consumption of fracture energy and enhancing the resistance to thermal stress damage.

Effects of temperature on fracture behavior of Al-based in-situ composites reinforced with Mg_2Si and Si particles fabricated by centrifugal casting

An aluminum-based in-situ composites reinforced with Mg2Si and Si particles were produced by centrifugal casting A1-20Si-5Mg alloy. The microstructure of the composites was examined, and the effects of temperature on fracture behavior of the composite were investigated. The results show that the average fraction of primary Si and Mg2Si particles in the composites is as high as 38%, and ultimate tensile strengths （UTS） of the composites first increase then decrease with the increase of test temperature. Microstructures of broken specimens show that both the particle fracture and the interface debonding affect the fracture behavior of the composites, and the interface debonding becomes the dominant fracture mechanism with increasing test temperature. Comparative results indicate that rich particles in the composites and excellent interface strength play great roles in enhancing tensile property by preventing the movement of dislocations.

Effects of plastic deformation on precipitation behavior and tensile fracture behavior of Mg-Gd-Y-Zr alloy

The effects of plastic deformation on precipitation behavior and tensile fracture behavior of Mg-10Gd-3Y-0.6Zr alloy were investigated.The results indicate that more precipitation cores can be provided by the crystal defects caused by the plastic deformation,as well as increasing the amount of β＇ phases,and the formation of precipitations at grain boundaries and interfaces between the twins and matrix.Because of an increase in precipitations,the dislocation slipping during deformation process is effectively hindered and the matrix is strengthened,especially for the 2% deformed alloy which can achieve a good combination of strength and ductility.With increasing the plastic deformation,the microcracks occur at the interface between grain boundary precipitations and matrix,and then propagate intergranularly.When intergranular fracture combines with the formation of smoothing facets on the fracture surface,the tensile properties decrease.

Fracture behavior of double-pass TIG welded 2219-T8 aluminum alloy joints under transverse tensile test

2219-T8 aluminum alloys were butt welded by the double-pass tungsten inert gas （TIG） arc welding process. The transverse tensile test of the joint showed that the fracture mainly occurred in the partially melted zone （PMZ）. Effects of the PMZ on the fracture behavior were systematically studied. Continuous intergranular eutectics were observed in the PMZ close to the fusion line. Away from the fusion line, the intergranular eutectics in the PMZ became discontinuous. The fracture morphology and the microhardness distribution of the joint showed that the PMZ was gradient material with different mechanical properties, which strongly affected the fracture process. It was observed that the crack initiated in the PMZ near the front weld toe, and propagated in the PMZ away from the fusion line. Then, the crack tip was blunt when it propagated into the PMZ with higher plasticity. Finally, the rest part of the joint was shear fractured.

Strength and fatigue fracture behavior of Al-Zn-Mg-Cu-Zr(-Sn) alloys

The strength and fatigue fracture behavior of A1-Zn-Mg-Cu-Zr（-Sn） alloys were studied by performing tensile tests and fatigue crack propagation （FCP） tests. The microstructures of the experimental alloys were further analyzed using optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）; phase analysis of these alloys was conducted with an X-ray diffraction （XRD）. The results show that when Sn is included, growth of the recrystallization grains in the solution-treated A1-Zn-Mg-Cu-Zr alloy is obstructed, the precipitation-free zone （PFZ） of the overaged A1-Zn-Mg-Cu-Zr-Sn alloy becomes narrow, and the grain boundary precipitates are smaller. Consequently, the FCP resistance is higher. In addition, the overaged Sn-containing alloy has considerably higher tensile strength than the alloy without Sn.

Fracture behavior and microstructure of as-cast NiTi shape memory alloy

The as-cast ingot of equiatomic nickel-titanium shape memory alloy （NiTi SMA） was prepared by vacuum consumable arc melting. The tensile tests and the compressive tests with respect to as-cast NiTi SMA were performed to study its mechanical properties of fracture. The microanalysis of as-cast NiTi SMA as well as its fractured samples was performed so as to better understand microstructure evolution and fracture behavior of NiTi SMA. Under tensile loading, the as-cast NiTi SMA shows higher plasticity and is characterized by ductile fracture at 750℃, but it demonstrates poorer plasticity and is characterized by cleavage fracture as well as transcrystalline fracture at room temperature and -100 ℃. Under compressive loading at -100 ~C, the as-cast NiTi SMA is characterized by shear fracture where the normal to the shearing fracture surface inclines about 45° to the compressive axis, and belongs to cleavage fracture where the cracks exoand via transcrvstalline fracture.

Variation of the second-phase morphology and its influence on fracture behavior of Al-Si alloy

Using an optical microscope and scanning electron microscope (SEM), the variation of eutectic Si morphology of Al-Si alloy in solution treatment was observed to study its influence on mechanical properties and fracture behavior. The results show that eutectic Si undergoes stubbing, necking, fragmentation, and growth in the initial stage (250 min); in the middle solution stage (250 to 400 min), the eutectic Si morphology has no significant change, only the degree of spheroidizing becomes higher; after 600 min, the growth of eutectic Si is a coarsening process controlled by diffusion and follows the Liftshitz-Slyozov-Wangner (LSW) model, and the eutectic Si morphology deteriorates due to the occurrence of facets and lap. Based on the quantitative measure and regression analysis, the eutectic Si morphology has a remarkable influence on mechanical properties and fracture behavior.

Fracture behavior of high strength Mg-Gd-Y-Zr magnesium alloy

The fracture behavior of a permanent mould casting Mg-8.57Gd-3.72Y-0.54Zr(mass fraction,%)(GW94) alloy was investigated under different thermal conditions,including as-cast,solution-treated,peak-aged,and over-aged states.Scanning electron microscopy(SEM) and optical microscopy(OM) were employed to examine the crack nucleation and fracture model.The results indicate that the GW94 alloy shows different behaviors of crack initiation and fracture under different thermal conditions. During tensile test at room temperature,the fracture model of the as-cast GW94 alloy is quasi-cleavage,while that of the solution-treated alloy is transgranular cleavage.It is a mixed pattern of transgranular and intergranular fracture for both the aged conditions.Large cavities formed at grain boundaries are observed in the peak-aged sample tested at 300℃,corresponding to the intergranular fracture.Localized plastic deformation at grain boundaries is also observed and corresponds to the high elongation at 300℃.

The transient fracture behavior for a functionally graded layered structure subjected to an in-plane impact load

The transient fracture behavior of a functionally graded layered structure subjected to an in-plane impact load is investigated. The studied structure is composed of two homogeneous layers and a functionally gradedinterlayer with a crack perpendicular to the boundaries. The impact load is applied on the face of the crack. Fourier transform and Laplace transform methods are used to formulate the present problem in terms of a singular integral equation in Laplace transform domain. Considering variations of parameters such as the nonhomogeneity constant, the thickness ratio and the crack length, the dynamic stress intensity factors （DSIFs） in time domain are studied and some meaningful conclusions are obtained.

Effect of tool plunge depth on the microstructure and fracture behavior of refill friction stir spot welded AZ91 magnesium alloy joints

We used refill friction stir spot welding(RFSSW)to join 2-mm-thick AZ91D-H24 magnesium alloy sheets,and we investigated in detail the effect of tool plunge depth on the microstructure and fracture behavior of the joints.A sound joint surface can be obtained using plunge depths of 2.0 and 2.5 mm.Plunge depth was found to significantly affect the height of the hook,with greater plunge depths corresponding to more severe upward bending of the hook,which compromised the tensile-shear properties of the joints.The hardness reached a minimum at the thermo-mechanically affected zone due to the precipitation phases of this zone as it dissolved into theα-matrix during the welding process.The fracture modes of RFSSW joints can be divided into three types:shear fracture,plug fracture,and shear–plug fracture.Of these,the joint with a shear–plug fracture exhibited the best tensile-shear load of 6400 N.

Influence of silicide on fracture behavior of a fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar binary γ-TiAI alloys is extremely anisotropic with respect to the lamellar orientation. For the fully lamellar Ti-46Al-0.5W-0.5Si alloy, the existence of silicide clusters plays a critical role on the fracture behavior. In the present study, tensile test and three point bending test were performed at room temperature with the loading axis parallel and perpendicular to the lamellar orientation, respectively. To investigate the influence of silicide clusters on the initiation and propagation of cracks, the fracture surface and the cracks adjacent to the fracture zone of the specimens have been analyzed. Results show that the fracture process is related to the morphology and distribution of the silicide clusters. Crack preferentially initiates at and propagates along the interface of silicide and a2/7 lamellar with the loading axis perpendicular to the length direction of silicide. While the silicide can prevent the propagation of cracks from running across with the crack growth direction perpendicular to the length direction of silicide.

A sophisticated simulation for the fracture behavior of concrete material using XFEM

The development of a powerful numerical model to simulate the fracture behavior of concrete material has long been one of the dominant research areas in earthquake engineering. A reliable model should be able to adequately represent the discontinuous characteristics of cracks and simulate various failure behaviors under complicated loading conditions. In this paper, a numerical formulation, which incorporates a sophisticated rigid-plastic interface constitutive model coupling cohesion softening, contact, friction and shear dilatation into the XFEM, is proposed to describe various crack behaviors of concrete material. An effective numerical integration scheme for accurately assembling the contribution to the weak form on both sides of the discontinuity is introduced. The effectiveness of the proposed method has been assessed by simulating several well-known experimental tests. It is concluded that the numerical method can successfully capture the crack paths and accurately predict the fracture behavior of concrete structures. The influence ofmode-Ⅱ parameters on the mixed-mode fracture behavior is further investigated to better determine these parameters.

Influence of columnar grain boundary on fracture behavior of as-cast fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar γ-TiAl alloys depends on the angle between the lamellar orientation and loading axis,but the role of the presentation of grain boundary cannot be ignored.To investigate the influence of the grain boundary on the initiation and propagation of cracks,the tensile test of the alloy was conducted at room temperature with loading axis parallel and perpendicular to the lamellar orientation,respectively.The cracks adjacent to the fracture zone of the tensile specimens have been investigated to analyze the fracture behavior.Results show that the grain boundary has dual influences on the fracture behavior.When the loading axis is parallel to the lamellar orientation,cracks are preferentially initiated at and propagate along the grain boundaries.When the loading axis is perpendicular to the lamellar orientation,the grain boundaries can prevent the propagation of cracks from running across.Additionally,serrated-shape grain boundaries have a better inhibiting effect on the propagation of cracks than planar boundaries.

Fracture toughness and fracture behavior of SA508-Ⅲ steel at different temperatures

The fracture toughness of SA508-Ⅲ steel was studied in the temperature range from room temperature to 320℃ using the J-integral method. The fracture behavior of the steel was also investigated. It was found that the conditional fracture toughness （JQ） of the steel first decreased and then increased with increasing test temperature. The maximum and minimum values of do were 517.4 kJ/m^2 at 25℃ and 304.5 kJ/m^2 at 180℃, respectively. Dynamic strain aging （DSA） was also observed to occur when the temperature exceeded 260℃ with a certain strain rate. Both the dislocation density and the number of small dislocation cells effectively increased because of the occurrence of DSA; as a consequence, crack propagation was more strongly inhibited in the steel. Simultaneously, an increasing number of fine carbides precipitated under high stress at temperatures greater than 260℃. Thus, the deformation resistance of the steel was improved and the Jo was enhanced.

Effect of rapid cold stamping on fracture behavior of long strip S′phase in Al−Cu−Mg alloy

High-angle annular dark-field scanning transmission electron microscopy and selected area electron diffraction techniques were used to study the mechanism that underlies the influence of rapid cold-stamping deformation on the fracture behavior of the elongated nanoprecipitated phase in extruded Al−Cu−Mg alloy.Results show that the interface between the long strip-shaped S′phase and the aluminum matrix in the extruded Al−Cu−Mg alloy is flat and breaks during rapid cold-stamping deformation.The breaking mechanisms are distortion and brittle failure,redissolution,and necking.The breakage of the long strip S′phase increases the contact surface between the S′phase and the aluminum matrix and improves the interfacial distortion energy.This effect accounts for the higher free energy of the S′phase than that of the matrix and creates conditions for the redissolution of solute atoms back into the aluminum matrix.The brittle S′phase produces a resolved step during rapid cold-stamping deformation.This step further accelerates the diffusion of solute atoms and promotes the redissolution of the S′phase.Thus,the S′phase necks and separates,and the long strip-shaped S′phase in the extruded Al−Cu−Mg alloy is broken into a short and thin S′phase.

Effect of Zn content on microstructure, mechanical properties and fracture behavior of Mg-Mn alloy

The optical microscope, scanning electron microscope and universal testing machine are used to investigate the effect of Zn content on the microstructure, mechanical properties and fracture behavior of Mg- Mn-Zn alloy. The results indicate that fine （Mg, Mn, AI）-containing phases are distributed uniformly in the Mg-Mn alloy matrix, while small amount of （Mg, Zn）-containing phases are formed in the matrix and the grain boundary becomes coarse when 1wt.% Zn is added. As the Zn content increases, the amount of （Mg, Zn）-containing phases increases, and the grain boundary becomes coarser. When the Zn content is between 3wt.%-5wt.%, slender （Mg, Zn）-containing phases precipitate at the grain boundary. The addition of Zn could reduce the grain size and enhance the mechanical properties of the alloy matrix, and both of the effects can be enhanced by increasing the Zn content further more. When the Zn content is more than 3wt.%, grain size stops decrease, the strength cannot be improved any more and elongation decreases significantly. The fracture behavior of Mg-Mn alloy appears to be cleavage fracture, and transforms into quasi-cleavage fracture as Zn is added. When Zn content exceeded 3wt.%, large amount of （Mg, Zn）-containing phases appear on the fracture face, and act as the crack sources.

Densification,microstructure,and fracture behavior of TiC/Si_3N_4 composites by spark plasma sintering

TiC/Si3N4 composites were prepared using the β-Si3N4 powder synthesized by self-propagating high-temperature synthesis （SHS） and 35 wt.% TiC by spark plasma sintering. Y2O3 and Al2O3 were added as sintering additives. The almost full sintered density and the highest fracture toughness （8.48 MPa·m1/2） values of Si3N4-based ceramics could be achieved at 1550℃. No interfacial interactions were noticeable between TiC and Si3N4. The toughening mechanisms in TiC/Si3N4 composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si3N4 matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.

Fracture behaviors of columnar jointed rock mass using interface mechanics theorem

For a special geological structure of columnar jointed rock mass(CJRM),its mechanical properties are strongly affected by the columnar joints.To describe the fracture behaviors of CJRM using the basic theories of interface mechanics for composite materials,the interface stresses of the vertical and horizontal joints,which are the two primary joints in the CJRM under triaxial compression,are studied,and their mathematical expressions are derived based on the superposition principle.Based on the obtained interface stresses of the vertical and horizontal joints in the CJRM,the crack initiation of the joint interface in the CJRM is studied using the maximum circumferential stress theory in fracture mechanics.Moreover,based on this investigation,the fracture behaviors of CJRM are analyzed.According to the results of similar material physical model tests for the CJRM,the theoretical study is verified.Finally,the influence of the mechanical parameters of the CJRM on the joint interface stress is discussed comprehensively.

Fracture behavior and microstructure analysis of Al2O3–MgO–CaO castables for steel-ladle purging plugs

Three different castables based on the Al_2O_3–MgO –CaO system were prepared as steel-ladle purging plug refractories： corundum-based low-cement castable（C-LCC）, corundum-spinel-based low-cement castable（C-S-LCC）, and corundum-spinel no-cement castable（C-S-NCC）（hydratable alumina（ρ-Al_2O_3） bonded）. The fracture behavior at room temperature was tested by the method of ＂wedge-splitting＂ on samples pre-fired at different temperatures; the specific fracture energy G′f and notched tensile strength σNT were obtained from these tests. In addition, the Young＇s modulus E was measured by the method of resonance frequency of damping analysis（RFDA）. The thermal stress resistance parameter R′′′′ calculated using the values of G′f, σNT, and E was used to evaluate the thermal shock resistance of the materials. According to the microstructure analysis results, the sintering effect and the bonding type of the matrix material were different among these three castables, which explains their different fracture behaviors.