Change In Fracture Mode Research Articles

The development of microstructural damage during high temperature creep–fatigue of a nickel alloy

Alloy 617 is the leading candidate material for an Intermediate Heat Exchanger (IHX) of the Very High Temperature Reactor (VHTR). To evaluate the behavior of this material in the expected service conditions, strain-controlled cyclic tests that include hold times up to 9000s at maximum tensile strain were conducted at 950°C. The fatigue resistance decreased when a hold time was added at peak tensile strain, owing to the mechanisms resulting in a change in fracture mode from transgranular in pure fatigue to intergranular in creep–fatigue. Increases in the tensile hold duration beyond an initial value were not detrimental to the creep–fatigue resistance. An analysis of the evolving failure modes was facilitated by interrupting tests during cycling for ex situ microstructural investigation.

Poly(vinylidene fluoride)/halloysite nanotubes nanocomposites: The structures, properties, and tensile fracture behaviors

AbstractPoly(vinylidene fluoride)/halloysite nanotubes (HNTs) nanocomposites with good dispersion were prepared. The effect of HNTs on the crystalline structure, dynamic rheological properties, dynamic mechanical properties, and tensile properties was investigated. Results showed that at low concentration, the effect of HNTs on the modulus was minor. However, at high concentration, improvement of the modulus can be observed. The tensile fracture behaviors were investigated, which showed that the addition of HNTs changed the fracture mode. When the concentration of HNTs was 20 phr, a fracture mode change from microductility/brittle to brittle/fibrillation occurred. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Influences of Hydrogenation on Tensile Properties of Ti40 Alloy

The influence of hydrogenation on microstructure and tensile properties of Ti40 alloy has been investigated. The microstructure observation reveals that a new phase called η phase with FCC structure appears at the grainboundaries when the hydrogen content above 0.3 wt.%. With increasing hydrogen contents, the strength first increases and then decreases, and the ductility decreases as the specimens tensioned at both room temperature and 700oC. These phenomenons are resulted from the solution strengthening of hydrogen addition in Ti40 alloy with single β phase. As the hydrogen content increases, the fracture mode changes from dimple to cleavage fracture for specimens tensioned at room temperature. When tested at 700oC, the morphology of fracture surfaces in the specimens are all characterized by polygonal grains and the fractograph exhibits typically “sugar candy” brittle fracture for the specimen with 0.5wt% H.

Impact Toughness of Ultrafine-Grained Interstitial-Free Steel

Impact toughness of an ultrafine-grained (UFG) interstitial-free (IF) steel produced by equal-channel angular extrusion/pressing (ECAE/P) at room temperature was investigated using Charpy impact tests. The UFG IF steel shows an improved combination of strength and impact toughness compared with the corresponding coarse-grained (CG) one. The CG IF steel samples underwent a transition in fracture toughness values with decreasing temperature because of a sudden change in fracture mode from microvoid coalescence (ductile) to cleavage (brittle) fracture. Grain refinement down to the submicron (≈320 nm) levels increased the impact energies in the upper shelf and lower shelf regions, and it considerably decreased the ductile-to-brittle transition temperature (DBTT) from 233 K (−40 °C) for the CG steel to approximately 183 K (−90 °C) for the UFG steel. Also, the sudden drop in DBTT with a small transition range for the CG sample changed to a more gradual decrease in energy for the UFG sample. The improvement in toughness after UFG formation was attributed to the combined effects of grain refinement and delamination and/or separation in the heavily deformed microstructure. Although an obvious change from the ductile fracture by dimples to the brittle fracture by cleavage was recognized at 233 K (−40 °C) for the CG steel, no fully brittle fracture occurred even at 103 K (−170 °C) in the UFG steel.

A Correlation Between Failure Angle and Constituent for Al-AlN Composites Under Uniaxial Tensile Conditions

Al-AlN composites with different AlN reinforcement fractions and porosity were fabricated through nitridation of laser-sintered AA 6061 powder followed by infiltration with AA 6061. Their failure behaviors were investigated under uniaxial tensile loading conditions. Tensile testing and fractography indicate that the fracture mode changes gradually from ductile to brittle fracture with increasing AlN reinforcement or porosity. An analysis of the fractured Al-AlN tensile samples reveals that the failure surface angle, θ, is dictated by the volume fraction of the matrix, fm, in a form of tan θ = fm5.5.

Mechanical Properties and Microstructure of Boron Carbide-Cerium Boride Composite

The mechanical properties of B4C-CeB6 composite prepared by hot pressed sintering method were tested. The study shows: the hardness of B4C-CeB6 composite increases with the content of cerium boride. When the content of the cerium boride is 4wt%, the hardness reaches its supreme value of 31.98Gpa，its hardness is improved nearly 21.09% compared to monolithic boron carbide. The content of the cerium boride does not affect greatly on flexibility strength. However，it gives much effect on fracture toughness. When the content of the cerium boride is 4wt%, the fracture toughness reaches its supreme value of 5.06MPa.m1/2, which is improved nearly 37.5% compared to monolithic boron carbide materials. The main fracture way of B4C-CeB6 composite is intercrystalline rupture, while the transcrystalline rupture is minor. It appears that this change of fracture mode gives rise to the improvement of the fracture toughness.

Thermal shock behavior of nano-sized SiC particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.

Mechanical properties and toughening mechanisms of silicon carbide nano-particulate reinforced Alon composites

Abstract Aluminum oxynitride (Alon) has been considered as a potential ceramic material due to excellent stability, chemical and mechanical properties such as high rigidity and good chemical stability, but it has relatively low strength and poor fracture toughness. The aim of this study was to investigate a type of silicon carbide (SiC) nano-particulate reinforced Alon composites with improved mechanical properties and fracture resistance via hot-press sintering. The addition of SiC nano-particles resulted in a reduction of both porosity and grain size, and a change of fracture mode from intergranular cracking in the monolithic Alon to intragranular cracking in the composites due to the pinning effect of SiC nano-particles positioned at grain boundaries or triple junctions of micro-sized Alon particles. With 8 wt% SiC nano-particles addition, the relative density, microhardness, Young's modulus, flexural strength, and fracture toughness all increased. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in the flexural strength and fracture toughness of the composites.

Effect of sintering temperature and siliconcarbide fraction on density, mechanical properties and fracture mode of alumina–silicon carbide micro/nanocomposites

In this study Al2O3–SiC micro/nanocomposites have been fabricated by mixing alumina nanopowders and silicon carbide micro/nanopowders, followed by hot pressing at 1550, 1600, 1650 and 1700°C. The density, mechanical properties and fracture mode of Al2O3–SiC composites containing different volume fractions (2.5%, 5%, 7.5%, 10% and 15%) of micro/nanoscale SiC particles were investigated and compared with those of alumina. The relative density of composites could reach values very close to theoretical density, especially after sintering at 1700°C. However, relative density declined by increasing the SiC fraction at the same sintering temperature. The flexural strength of composites was best for sintering temperature of 1700°C and showed a maximum of 545MPa for Alumina–10%SiC sintered at 1700°C. Hardness showed a remarkable increase by adding SiC and reached a pick of 22.6GPa for Alumina–15%SiC. Fracture toughness and fracture mode of alumina and composites with 5%, 10% and 15% SiC sintered at 1700°C were also investigated. Composites were tougher than alumina and the scanning electron microscopy observations showed that fracture mode changes from intergranular for alumina to transgranular for composites. Finally X-ray diffraction analysis could not detect any chemical reactions between Al2O3 and SiC particles.

STUDY OF MECHANICAL PROPERTIES AND FRACTURE MODE OF ALUMINA-SILICON CARBIDE NANOCOMPOSITES

In this study Al 2 O 3- SiC nanocomposites have been fabricated by mixing of alumina and silicon carbide nano powders, followed by hot pressing at 1700°C. The mechanical properties and fracture mode of Al 2 O 3- SiC nanocomposites containing different volume fractions (5, 10 and 15%) of nano scale SiC particles were investigated and compared with those of alumina. Al 2 O 3- SiC powders were prepared by planetary milling in isopropanol. Fracture mode of specimens was investigated by means of scanning electron microscopy. Nanocomposites were tougher than alumina when they were hot pressed at the same temperature, and the values of nanocomposite's flexural strength and hardness were higher than those of alumina. Flexural strength, hardness and fracture toughness of the nanocomposites increase by increasing the volume percent of SiC up to 10% and then decrease slightly. The Scanning electron microscopy observations showed that fracture mode changes from intergranular for alumina to transgranular for nanocomposites. Finally X-ray diffraction analysis couldn't detect any chemical reactions between Al 2 O 3 and SiC particles.

접착제 경화시점에 따른 하이브리드 접합 파단모드 및 접합강도 평가

Abstract With the development of pre-painted steel sheets for automotive body application, a new joining method is required such as hybrid joining with combination of adhesive bonding and mechanical joining. The objective of this study is to investigate the effect of pre- and post-baking of adhesive bonding on failure mode and strength of hybrid joining of automotive steel sheets. Experiments show that the hybrid joining exhibits better bonding strength and displacement than conventional adhesive joining and mechanical fastening each. Comparison of pre- and post-baked hybrid joining results suggested that baking at 160℃ after mechanical joining was found to have higher joining properties than pre-baking condition. The pre- baking condition changed its fracture mode from interfacial to button fracture. The changes in fracture mode with post-baking of hybrid joining was attributed to variation in neck thickness and undercut of joint.Key Words : Hybrid joining, Adhesive bonding, Clinching joining, Baking, Tensile Strength

Microstructure and mechanical properties of TiB2–TiC–WC composite ceramic tool materials

TiB2–TiC–WC composites with Ni as a sintering aid were fabricated by a hot-press technique at 1700°C and 1650°C for 1h, respectively. The microstructure and mechanical properties were investigated. The composites were analyzed by means of scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The matrix phases consisted of TiB2 and TiC. No severe chemical reactions happened between the additive and matrix. The microstructure consisted of the fine WC grains and uniform matrix grains. When the proper WC content added to TiB2–TiC composites, the growth of matrix grains was inhibited and the mechanical properties of the composites were improved. The interface energy was strengthened by Ni that dispersed in the interfaces among WC grains and matrix grains, which made the fracture mode change from intergranular fracture to transgranular fracture. The transgranular fracture and the pulling out of WC grains played a predominant role in the propagating of cracks when WC content was 20wt.% in TiB2–TiC–WC composites. The optimal mechanical properties of TiB2–TiC–20wt.%WC composite were 955.71MPa of flexural strength, 7.5MPam1/2 of fracture toughness and 23.5GPa of Vickers hardness.

Microstructure and mechanical properties of hot rolled Fe–40 at-%Al intermetallic alloys with Zr and B addition

The microstructure and mechanical properties of hot rolled Fe–40 at-%Al based intermetallic alloys, with 0·1 at-%Zr and different additions of B (varying from 0·01 to 0·1 at-%), are characterised. The additions of Zr and B improve tensile properties at room and elevated temperatures. Increasing B content is also associated with a number of other effects. First, the fracture mode changes from intergranular decohesion to cleavage, which correlates with significant increases in the fracture toughness. Second, there is a certain stabilisation of dislocations arranged in parallel systems of slip bands, as shown by transmission electron microscopy. Numerous complex stacking faults on {100} planes are also observed in the alloy with the highest B content. Third, B is found to modify the formation of second phase particles; such particles (coarse and fine) are analysed by energy dispersive X-ray spectroscopy and electron energy loss spectrometry to obtain compositional information.

Study on Intermediate Frequency Spot Welding Process of Hot Stamping High Strength Steel

The joints of RSW for HSHSS were prepared with IF spot weld equipment. The weld nugget, fracture mode, microstructure and micro-hardness of the joints were investigated. The weld nugget increases and the fracture mode changes from interfacial fracture to nugget plug fracture with the increasing welding current. There are three distinct zones in the weld joint: weld nugget, heat affected zone (HAZ) and base metal. The microstructure of HAZ includes tempered zone (HAZ-1), incomplete quenching zone (HAZ-2) and complete quenching zone (HAZ-3). The micro-hardness of weld nugget and HAZ-3 is same with the base metal. The micro-hardness of HAZ-1 and HAZ-2 is smaller than that of base metal.

Bulk WC–Al 2O 3 composites prepared by spark plasma sintering

WC and WC–Al 2O 3 materials without metallic binder addition were densified by spark plasma sintering in the range of 1800–1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC–Al 2O 3 composite were investigated. The addition of Al 2O 3 facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al 2O 3 additive helps to limit the formation of W 2C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV 10) of 25.71 GPa, fracture toughness of 4.54 MPa·m 1/2, and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al 2O 3 composites are 24.48 GPa, 6.01 MPa·m 1/2, and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al 2O 3 are thought to result from the reduction of W 2C phase, the crack-bridging by Al 2O 3 particles and the local change in fracture mode from intergranular to transgranular.

Evaluation of mechanical properties of tubular LSFCO perovskite membranes at elevated temperatures in reactive environments

This paper reports the mechanical properties of tubular membranes of mixed conducting La 0.2Sr 0.8Fe 0.8Cr 0.2O 3-δ (LSFCO) perovskites at elevated temperatures. Strength, measured in a C-ring test over a temperature range of 22°–1000 °C in three different environments – air, 100% N 2, and 90% CO 2/10% CO mixtures – showed a decreasing behavior with increasing temperature and severity of reducing conditions. A maximum strength value of 340 MPa and a minimum of 105 MPa were observed at room temperature and at 1000 °C, respectively, in CO 2/CO environment. Fractography was used to characterize the flaws and changes in fracture mode. Results indicate that the observed changes in strength and fracture morphology are associated with non-equilibrium segregation of elements when exposed to the reducing environment.

The ductile to brittle transition for C–Mn steel with an ultrafine grain ferrite/cementite microstructure

The Charpy impact energy was measured as a function of temperature for plain C–Mn steel with an ultrafine grain ferrite/cementite (UGF/C) microstructure and with a coarse-grain ferrite/pearlite (CGF/P) microstructure. Instrumented Charpy impact tests were carried out at 273 K and 77 K for the UGF/C microstructure. The steel with the UGF/C microstructure exhibited an upper shelf energy lower than the steel with the CGF/P microstructure, a higher lower shelf energy and the transition in impact energy occurred more gradually at a lower temperature. The ductile to brittle transition in the steel with the UGF/C microstructure was caused by a decrease in energy absorbed by ductile fracture rather than a change of fracture mode from ductile fracture to cleavage as occurred for the CGF/P microstructure. The fracture surface of the UGF/C steel in the upper shelf region contained large dimples and numerous small dimples whereas the fracture surface in the lower shelf region contained small dimples and cleavage facets. A lower amount of energy was absorbed in the transition region and the lower shelf region due to the decrease of dimple diameter and depth. Nevertheless, some energy was absorbed in the lower shelf because there were some small dimples even at the liquid helium temperature. In contrast, essentially no energy was absorbed in the lower shelf for the CGF/P steel because of typical cleavage fracture. The UGF/C microstructure has a high cleavage fracture stress.

Creep and Environmental Effects on the High Temperature Creep-Fatigue Behavior of Alloy 617

Abstract Alloy 617 is the leading candidate material for an intermediate heat exchanger (IHX) of the very high temperature reactor (VHTR), expected to have an outlet temperature as high as 950°C. Acceptance of Alloy 617 in Section III of the ASME Code for nuclear construction requires a detailed understanding of the creep-fatigue behavior. Strain-controlled low cycle fatigue (LCF) tests including hold times up to 1800 s at maximum tensile strain were conducted at 950°C and a total strain range of 0.3 % in air and in a simulated VHTR impure helium. For similar test conditions, the creep-fatigue behavior was globally similar in both environments. The fatigue resistance decreased when a hold time was added at peak tensile strain, consistent with the observed change in fracture mode from transgranular to intergranular. Further increases in the tensile hold time, beyond 180 s, were not detrimental to the creep-fatigue resistance. The influence of oxidation on the fatigue crack initiation and propagation is discussed. Bulk damage in the form of grain boundary cracking was present in the interior of the creep-fatigue specimens. This bulk cracking was quantified and found to be similar for hold times of up to 1800 s consistent with the saturation in failure lives.

Grain refinement of AZ91D magnesium alloy by Al–Ti–B master alloy and its effect on mechanical properties

The effects of grain-refining parameters on grain size of AZ91D alloy have been investigated using an Al–Ti–B master alloy as refiner and an adequate refining technique has been developed. Simultaneously, the ultimate tensile strength (UTS) and hardness of the as-cast, solutionized and solution-aged alloys have also been examined at room temperature. The results indicate that the Al–Ti–B master alloy is an effective grain refiner for AZ91D alloy. Increasing pouring temperature or cooling rate from holding temperature to pouring temperature is beneficial for obtaining small grains. But for addition amount, holding temperature or holding time, there is an optimal value. The grain size can be decreased from 422 μm to 79 μm after being refined according to the obtained technique. Accordingly, both the UTS and hardness are obviously enhanced. Solution treatment can further improve the UTS but decrease the hardness while solution-age treatment leads these two properties to increase. Compared with the un-refined as-cast alloy, the UTS and hardness of the solution-aged alloy are increased by 71% and 53% respectively. Correspondingly, the fracture mode during tensile testing changes with the treatments. These changes in mechanical properties and fracture mode are attributed to the different contributions of dissolution or precipitation of β phase to grain bonding strength, solution strengthening and precipitation strengthening. In addition, the microstructure sensitivity of the alloy refined by this refiner to casting thickness is quite low compared with that of the alloy refined by SiC particles.

Effects of specimen size on fracture toughness of phosphorous added F82H steels

Reduced-activation ferritic (RAF) steels are promising candidate structural materials for fusion reactor blanket components. However, a large amount of He on grain boundaries over 550–700 appm may cause non-hardening embrittlement (NHE) on the RAF materials. Therefore, understanding the fracture toughness on the RAF steels are critical in design and operation of the reactor. In the present study, constitutive data and widely obtained fracture toughness data sets at transition temperatures of F82H are investigated to establish the master curve method for RAF steels from viewpoints of fracture mechanics. For effective production of irradiation database, small specimen tests is applied to evaluate fracture toughness properties for F82H steels with different levels of phosphorous addition so that it can simulate the effect of fracture mode change from cleavage to intergranular fracture on the shift of transition curve. In this study, effects of specimen size and phosphorous on fracture toughness are also evaluated using the different size specimen (1 CT, 1/2 CT and 1/4 CT). As a result, phosphorous added steels show intergranular fracture in a brittle manner even in temperatures of transition region. The master curve approach is used here for evaluation of fracture toughness change by the degradation of grain boundary strength based on the ASTM E1921.