Dimple Fracture Research Articles

Flow and fracture behavior of aluminum alloy 6082-T6 at different tensile strain rates and triaxialities.

This study aims to investigate the flow and fracture behavior of aluminum alloy 6082-T6 (AA6082-T6) at different strain rates and triaxialities. Two groups of Charpy impact tests were carried out to further investigate its dynamic impact fracture property. A series of tensile tests and numerical simulations based on finite element analysis (FEA) were performed. Experimental data on smooth specimens under various strain rates ranging from 0.0001~3400 s-1 shows that AA6082-T6 is rather insensitive to strain rates in general. However, clear rate sensitivity was observed in the range of 0.001~1 s-1 while such a characteristic is counteracted by the adiabatic heating of specimens under high strain rates. A Johnson-Cook constitutive model was proposed based on tensile tests at different strain rates. In this study, the average stress triaxiality and equivalent plastic strain at facture obtained from numerical simulations were used for the calibration of J-C fracture model. Both of the J-C constitutive model and fracture model were employed in numerical simulations and the results was compared with experimental results. The calibrated J-C fracture model exhibits higher accuracy than the J-C fracture model obtained by the common method in predicting the fracture behavior of AA6082-T6. Finally, the Scanning Electron Microscope (SEM) of fractured specimens with different initial stress triaxialities were analyzed. The magnified fractographs indicate that high initial stress triaxiality likely results in dimple fracture.

Effects of Trace Si Addition on the Microstructures and Tensile Properties of Ti-3Al-8V-6Cr-4Mo-4Zr Alloy

The microstructural evolution and tensile properties of Ti-3Al-8V-6Cr-4Mo-4Zr titanium alloys with various Si contents were investigated. The results revealed that the addition of trace Si and the presence of Zr induced the formation of (TiZr)6Si3 silicides, in the size range from 100 nm to 300 nm. The fine silicide precipitates refined β grains. The tensile strength increased about 40 MPa due to precipitation strengthening and grain refinement, and the ductility of the two alloys was similar. The tensile fracture mode of the alloys was dimple ductile fracture.

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

ABSTRACTThe effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100 kJ cm-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.

Infrared Dissimilar Joining of Ti50Ni50 and 316L Stainless Steel with Copper Barrier Layer in between Two Silver-Based Fillers

Infrared dissimilar joining Ti50Ni50 and 316L stainless steel using Cu foil in between Cusil-ABA and BAg-8 filler metals has been studied. The Cu foil serves as a barrier layer with thicknesses of 70 μm and 50 μm, and it successfully isolates the interfacial reaction between Ti and Fe at the 316L SS (stainless steel) substrate side. In contrast, the Cu foil with 25 μm in thickness is completely dissolved into the braze melt during brazing and fails to be a barrier layer. A layer of (CuxNi1−x)2Ti intermetallic is formed at the Ti50Ni50 substrate side, and the Cu interlayer is dissolved into the Cusil-ABA melt to from a few proeutectic Cu particles for all specimens. For the 316L SS substrate side, no interfacial layer is observed and (Ag, Cu) eutectic dominates the brazed joint for 70 μm/50 μm Cu foil. The average shear strength of the bond with Cu barrier layer is greatly increased compared with that without Cu. The brazed joints with a 50 μm Cu layer demonstrate the highest average shear strengths of 354 MPa and 349 MPa for samples joined at 820 °C and 850 °C, respectively. Cracks are initiated/propagated in (Ag, Cu) eutectic next to the 316L substrate side featured with ductile dimple fracture. It shows great potential for industrial application.

Creep deformation and fracture behaviour of modified 9Cr-1Mo steel in flowing liquid sodium environment

The creep deformation and rupture behaviour of modified 9Cr-1Mo ferritic steel in flowing liquid sodium has been studied at 873K over a stress range of 130–160MPa and compared with those tested in air environment. In the flowing sodium environment, the steel exhibited better creep deformation and rupture strength than those in air environment. The onset of tertiary stage of creep deformation has delayed in sodium environment over that in air environment, which led to the decrease in minimum creep rate. Also the rate of tertiary creep strain accumulation was lower in sodium environment than that in air, leading to the increase in rupture life. The steel failed in ductile dimple fracture mode in both the testing environments. The steel exhibits relatively higher creep damage tolerance in sodium environment than in the air environment. Minimal oxidation was observed in the specimens creep tested in sodium environment. The near absence of oxidation of the steel in sodium environment delayed on onset of tertiary stage of creep deformation and decrease the rate of tertiary creep strain accumulation, leading to the increase in creep deformation and rupture strength.

Evaluation of the microstructure, secondary dendrite arm spacing, and mechanical properties of Al–Si alloy castings made in sand and Fe–Cr slag molds

The microstructure and mechanical properties of as-cast A356 (Al–Si) alloy castings were investigated. A356 alloy was cast into three different molds composed of sand, ferrochrome (Fe–Cr) slag, and a mixture of sand and Fe–Cr. A sodium silicate–CO2 process was used to make the necessary molds. Cylindrical-shaped castings were prepared. Cast products with no porosity and a good surface finish were achieved in all of the molds. These castings were evaluated for their metallography, secondary dendrite arm spacing (SDAS), and mechanical properties, including hardness, compression, tensile, and impact properties. Furthermore, the tensile and impact samples were analyzed by fractography. The results show that faster heat transfer in the Fe–Cr slag molds than in either the silica sand or mixed molds led to lower SDAS values with a refined microstructure in the products cast in Fe–Cr slag molds. Consistent and enhanced mechanical properties were observed in the slag mold products than in the castings obtained from either sand or mixed molds. The fracture surface of the slag mold castings shows a dimple fracture morphology with a transgranular fracture nature. However, the fracture surfaces of the sand mold castings display brittle fracture. In conclusion, products cast in Fe–Cr slag molds exhibit an improved surface finish and enhanced mechanical properties compared to those of products cast in sand and mixed molds.

Abnormal temperature dependence of impact toughness in AlxCoCrFeNi system high entropy alloys

In this work, the effect of Al contents and the associated phase transformations on the impact toughness of AlxCoCrFeNi high-entropy alloys (HEAs) (x denotes Al molar ratio; x = 0, 0.1, 0.75, and 1.5) in the as-cast state were investigated at T = 77 K, 200 K, and 298 K. For the alloys with x = 0 and 0.1, the alloys have the FCC structure, and an inverse temperature dependence of impact toughness between 298 K and 77 K was observed. The enhanced impact toughness (397.87 J) at 77 K is attributed to the capability of extensive nano-twinnings as well as ductile dimple fracture. Increasing Al contents to x = 0.75 and 1.5 induces the formation of brittle BCC phase and ordered B2 compound and accordingly embrittlement, leading to tremendously reduced Charpy-impact energies. The present study demonstrates that FCC AlxCoCrFeNi system HEAs with low Al contents (x < 0.5) may be well suitable for cryogenic applications.

Brazing Inconel 600 to Manufacture the Plate Heat Exchanger

The purpose of this investigation is concentrated on brazing Inconel 600 (IN-600) using pure Cu and Cu-6Sn (wt%) filler foils in order to replace the current plate heat exchanger made by austenitic stainless steel. Both Cu and Cu-6Sn brazed joints consist of interfacial chromium carbides in the Cu/Ni-rich matrix. The application of Cu-6Sn filler foil to braze IN-600 alloy demonstrates average shear strengths of above 300 MPa, which is much better than those of Cu brazed ones (217 ~ 290 MPa). Ductile dimple fracture is observed from all fractographs of Cu-6Sn brazed joints after shear tests. Because the brazing temperature of Cu-6Sn filler is lower than pure Cu, dissolution of IN-600 substrate into the brazed joint is significantly decreased. Isothermal solidification of Cu-6Sn brazed joint becomes less prominent than that of Cu brazed one. Better Cu-6Sn brazed joint is obtained due to less isothermal solidification defects in the joint. The Cu-6Sn filler foil shows great potential in brazing the IN-600 plate heat exchanger for industrial application.

Mechanical bonding properties and interfacial morphologies of austenitic stainless steel clad plates

The current paper focuses on the correlation between microstructure and mechanical properties in low-carbon steel/austenitic stainless steel clad composite fabricated by hot-roll bonding. For this reason, the morphology and interfacial characteristics, tensile properties shear strength and fracture toughness of the cladded material were evaluated.From the main results, it was found that carbon element diffusion caused the forming of a decarburized ferrite zone (DFZ) of the parent metal and a carburized austenite zone (CAZ) of the clad layer, and between these two area, a thin diffusion layer with rapid element component change are formed in the hot-roll cladding process. Stress-strain curves obtained from tensile testing of parent metal and clad layer can predict the bi-material tensile behavior. The shear test proved that the stainless steel clad plate presents an acceptable shear bond strength at the interface joint. Impact test toughness results confirm that fracture took place only in the parent metal side of cladded specimens; the clad layer was bent but without obvious fracture. Fractography was carried out using scanning electron microscope in the tensile, shear bond test specimens and Charpy impact ones. It reveals the presence of predominantly dimpled fracture. Charpy impact specimens of the interface failed in mixed mode while impact specimens of the base plate failed in ductile mode.

The Effects of Prestrain and Subsequent Annealing on Tensile Properties of CP-Ti

The aim of the present work is to investigate the effects of prestrain and subsequent annealing on tensile properties of commercial pure titanium (CP-Ti). According to tensile test results, yield strength and ultimate tensile strength increase with the increase of prestrain. Elongation and uniform strain decrease linearly with prestrain. In the case of prestrain that is higher than 3.5%, the macro-yield of specimens changes from gradual yielding to discontinuous yielding. It is supposed that considerable numbers of dislocations introduced into the material lead to the appearance of yield plateau. The quantitative analysis of the contribution of dislocation hardening to the strain hardening shows that dislocation-associated mechanisms play an important role in strain hardening. Moreover, a modified Fields-Backofen model is proposed to predict the flow stress of prestrained CP-Ti at different strain rates. Both strain rate sensitivity and strain hardening exponent decrease with prestrain. Fracture surfaces of the specimens show that fracture mechanism of all tested specimens is dimple fracture. The more ductile deformation in prestrained CP-Ti after annealing indicates that its ductility is improved by annealing.

Formation mechanism of the α variant and its influence on the tensile properties of laser solid formed Ti-6Al-4V titanium alloy

Laser solid forming (LSF) is a newly developed additive manufacturing which offers a less material waste and reduction in lead-time for fabricating aerospace titanium alloys components. In this paper, two types of block with different build dimension (section geometries) were fabricated by LSF with same processing parameters. The corresponding microstructure, texture, and tensile properties were investigated systematically. The results show that the samples exhibits similar columnar β grains morphology and 〈100〉 fiber texture, but very different α variant characterizations (morphology and texture) due to the different thermal history they experienced respectively. The fine basket-weave microstructure with weak texture can be obtained under the fast cooling conditions, while the colony microstructure shows a strong transformation texture as a result of variant selection in the relative slow cooling rate. The α characterizations depend strongly on the competition growth mechanism between the αWGB (grin boundary Widmanstatten structure) and αI (intragranular α nuclei) during cooling process. The presence of αGB (grain boundary α layers) enhances the nucleation of certain variants in β→α phase transformation. Tensile results reveal that fine basket-weave microstructure has relative high strength and ductility with dimple fracture mode. The colony microstructure shows a feature of dominant brittle fracture appearance and results in low tensile ductility.

Comparison of single-beam and dual-beam laser welding of Ti–22Al–25Nb/TA15 dissimilar titanium alloys

Laser beam welding (LBW) was used to join Ti–22Al–25Nb/TA15 dissimilar titanium alloys. The microstructure and mechanical properties of the welded joints under single and dual beam welding were analyzed and compared. In the mode of single laser beam, the fusion zone only consisted of B2 phase because of existence of β-phase stabilizer and rapid cooling rate of LBW. However, O phase was formed in the fusion zone while applying dual-beam laser welding due to decrease of the cooling rate. The microhardness distribution of the welded joint in dual-beam welding mode was consistent with that in single mode, but the hardness of the weld under dual laser beam was higher than that of single laser beam. In room-temperature tensile tests, the fractures all occurred in the weld, but the morphology exhibited a quasi-cleavage feature in single mode while the morphology was dimple fracture in the mode of dual laser beam. The tensile strength and elongation were both increased under dual-beam laser welding compared with those under single-beam laser welding.

Microstructure, SDAS and Mechanical Properties of A356 alloy Castings Made in Sand and Granulated Blast Furnace Slag Moulds

Abstract Investigations were carried out to ensure the granulated blast furnace (GBF) slag as an alternative mould material in foundry industry by assessing the cast products structure property correlations. Sodium silicate-CO2 process was adopted for preparing the moulds. Three types of moulds were made with slag, silica sand individually and combination of these two with 10% sodium silicate and 20 seconds CO2 gassing time. A356 alloy castings were performed on these newly developed slag moulds. The cast products were investigated for its metallography and mechanical properties. Results reveal that cast products with good surface finish and without any defects were produced. Faster heat transfers in slag moulds enabled the cast products with fine and refined grain structured; and also, lower Secondary Dendrite Arm Spacing (SDAS) values were observed than sand mould. Slag mould casting shows improved mechanical properties like hardness, compression, tensile and impact strength compared to sand mould castings. Two types of tensile fracture modes, namely cleavage pattern with flat surfaces representing Al−Si eutectic zone and the areas of broken Fe-rich intermetallic compounds which appear as flower-like morphology was observed in sand mould castings. In contrast, GBF slag mould castings exhibit majority in dimple fracture morphology with traces of cleavage fracture. Charpy impact fractured surfaces of sand mould castings shows both transgranular and intergranular fracture modes. Only intergranular fracture mode was noticed in both GBF slag and mixed mould castings.

Microstructure and Mechanical Properties of As-cast 42CrMo Ring Blank During Hot Rolling and Subsequent Quenching and Tempering

The hot rolling of as-cast 42CrMo ring blank and its subsequent quenching and tempering were conducted based on the casting-rolling compound forming technique. The effects of feed rate and tempering temperature on the microstructure were studied by optical microscopy and scanning electron microscopy. The mechanical properties of the rolled rings were examined. The results show that when the feed rate of the idle roll increases, the degree of grain refinement becomes slightly smaller and the average grain size is approximately 44 μm through the whole thickness of the rolled ring. The microstructure is inhomogeneous near the center-layer and minimum spread region, which is characterized by a small amount of irregular and coarse grain. The strength and hardness of the hot-rolled rings are high, and the plasticity and toughness are relatively low. The depth and diameter of the dimples in the fracture of the ring fabricated with a low feed rate are larger than those of the ring fabricated with a high feed rate. The carbide particles cannot be observed in the rolled rings after the rings are quenched and tempered at 803 K, but the fine and dispersed particles are precipitated by tempering at 863 K. As a result, the mechanical properties are significantly improved and satisfy the technical demands after quenching and tempering. The fractures of both tensile and impact specimens are characterized by regular and fine dimples at a higher tempering temperature, which indicates that a dimple fracture and an excellent combination of strength, plasticity and toughness are obtained.

Effects of Hydrogen on Tensile Properties at Slow Strain Rate of Ultra High-Strength TRIP-aided Bainitic Ferrite Steels

Abstract To clarify the effects of hydrogen on the deformation properties of TBF steel at low strain rate, the tensile properties of hydrogen-charged TBF steel were investigated by using slow strain rate technique (SSRT). Total elongation of TBF steel decreased due to the hydrogen absorption. The hydrogen absorption changed fracture surfaces of TBF steel from dimple fracture to quasi cleavage fracture. In addition, hydrogen-assisted cracks perpendicular to the tensile direction were observed at the prior austenitic grain, the packet and the bainitic ferrite lath boundaries although voids occurred at the lath boundary in non-hydrogen-charged TBF steel. In the case of the hydrogen-assisted cracks in hydrogen-charged TBF steel, transformed martensite is located near cracks. Transformation of retained austenite to martensite in the TBF steel was slightly accelerated during tensile deformation when the TBF steel was charged with hydrogen. When tensile strain was applied to the hydrogen-charged TBF steel, hydrogen was evolved at the high temperature region of peak corresponding to vacancy clusters generated by the intersection of dislocation in the hydrogen evolution curve. Thus, deterioration of ductility of hydrogen-charged TBF steel was caused by the acceleration of transformation of retained austenite because of the hydrogen absorption and the promotion of crack initiation at the prior austenitic grain, packet and lath boundaries due to the hydrogen diffusion from transformed martensite.

Infrared Brazing of Ti50Ni50 Shape Memory Alloy and Inconel 600 Alloy with Two Ag-Cu-Ti Active Braze Alloys

Infrared brazing of Ti50Ni50 SMA and Inconel 600 alloy using Cusil-ABA and Ticusil filler metals has been investigated. The joints were dominated by Ag-Cu eutectic with proeutectic Cu in the Cusil-ABA brazed joint and with proeutectic Ag in the Ticusil one. A continuous curved belt composed of a Ni3Ti layer and a (Cu x Ni1−x )2Ti layer formed in the brazed Ti50Ni50/Ticusil/Inconel 600 joint. On the Ti50Ni50 SMA side, an intermetallic layer of (Cu x Ni1−x )2Ti formed in all joints, with x values around 0.81 and 0.47. Layers of (Cu x Ni1−x )2Ti, Ni3Ti, and mixed Ni3Ti and Ni2Cr intermetallics were observed next to the Inconel 600 substrate in the brazed Ti50Ni50/Cusil-ABA/Inconel 600 joint. The maximum shear strengths of the joints using the Cusil-ABA filler metal and the Ticusil filler metal were 324 and 300 MPa, respectively. In the Cusil-ABA brazed joint, cracks with cleavage-dominated fracture propagated along the (Cu x Ni1−x )2Ti interfacial layer next to the Ti50Ni50 SMA substrate. In the Ticusil brazed joint, ductile dimple fracture occurred in the Ag-rich matrix near the Inconel 600 alloy substrate. The absence of a detrimental Ti-Fe-(Cu) layer on the Inconel 600 substrate side can effectively improve the shear strength of the joint.

THE INFLUENCE OF HEAT TREATMENT ON MICROSTRUCTURE AND MECHANICAL STRENGTH OF LATERITIC NICKEL STEEL

Indonesia has abundant resources of lateritic nickel ore and coal. However, this resource has not been considered as a main raw material in the Indonesian steel industry . Therefore, this study are aimed to investigate several aspects of lateritic nickel intrinsic propertiesand its potency through heat treatment process. Nickel laterite steels are usually used for bridge and building construction steel. Their properties can be improved in various ways, such as by conducting temper heat treatment. This research reported the influences of heat treatment on microstructure and mechanical strength in nickel laterite steel. Samples were austenitized at 900oC and subsquently tempered at 100 oC, 200 oC, 300 oC, 400 oC, 500 oC, and 600oC. Metallographic observation was conducted by using an optical microscope meanwhile mechanical strength were conducted by tensile test and surface hardness. The results showed evolution of mictrostructure in nickel laterite steel after heat treatment. The microstructure formed consisted of martensite temper. The presence of inclusion can affect brittle behavior of the lateritic nickel. The optimum tempering temperature was 200 oC. The result of Scanning Electron Microscope (SEM) fractography showed the dimple and intergranular fracture has caused a more ductile and mechanical behavior which is compatible with construction steels standard .

Influence of rolling temperature on interface properties of the cross wedge rolling of 42CrMo/Q235 laminated shaft

The cross wedge rolling (CWR) method was used to produce laminated shafts of chromium-molybdenum steel (42CrMo). The effect of the rolling temperature on interface properties of composite 42CrMo/Q235 laminated shaft were explored, for example, the interface micro-structure, chromium diffusion, micro-hardness, bonding strength, and tensile fracture morphology. Results indicate that after rolling, the core material (Q235) and the clad material (42CrMo) combined effectively and formed a notable interface. Under high tem-perature rolling and composite binding, the chromium dif-fused near the interface. By rising the rolling temperature, the diffusion depth of chromium from the clad material to the core material increased. Near the interface of 42CrMo/Q235, the micro-hardness changes gradient. By rising the rolling temperature, the micro-hardness of same location decreased. The maximum interface bonding strength is 1100 °C, the maximum tensile strength is 532.0 MPa, and the maxi-mum shear strength is 422.3 MPa. Scanning electron microscope (SEM) images display the tensile fracture with a typical ductile dimple fracture pattern. Raising the rolling tempera-ture, the size of the dimple increased and the oxide at the bottom of the dimple decreased. At a rolling temperature of 1100 °C, the interface bonding strength should be the highest, which correlated with the interface bonding strength test results.

Cold Cracking Sensitivity of a Newly Developed 9Cr2WVTa Steel

Cold cracking susceptibility of the newly developed 9Cr2WVTa steel plate welded with the corresponding gas tungsten arc welding wire has been evaluated using the implant test and the Y-groove cracking test. Measurement of diffusible hydrogen content shows that the welding wire is ultra-low welding consumables (0.40 mL/100 g). Results of the implant test show that the fracture critical stress without pre-heating is only 300 MPa and increases up to the yield strength with pre-heating to 150 °C. The fracture critical stress under high heat input decreases to 250 MPa owing to the formation of quenched microstructure. Fractographies show ductile dimple fracture under higher loads and quasi-cleavage fracture under lower loads. Results of Y-groove cracking tests show that the minimum pre-heating temperature is 150 °C and the maximum transverse residual stress is located at the root of the weld metal. Therefore, the heat input should be chosen in a proper range and the pre-heating temperature is no less than 150 °C.

Microstructure and mechanical properties of TiB2-based composites with high volume fraction of Fe-Ni additives prepared by vacuum pressureless sintering

TiB2-based composites with high volume fraction of Fe-Ni as additive were fabricated by vacuum pressureless sintering. The effect of Fe-Ni addition (Fe:Ni=6:4) at different volume fractions from 20 to 35vol% on the microstructure evolution and fracture behaviour of the TiB2-(Fe-Ni) composites was investigated. It was found that the TiB2-(Fe-Ni) composites consisted of only TiB2 and γ(Fe, Ni) phases without any brittle phases. The relative density, flexural strength and fracture toughness of the composites increased as the Fe-Ni content increased. The results indicated that the TiB2–35vol% (Fe-Ni) composites exhibited the optimum combination of physical and mechanical properties, i.e., the relative density of 98.32%, the Vickers hardness of 8.56GPa, the flexural strength of 1050.92MPa and the fracture toughness of 17.75MPam1/2. The high fracture toughness of the TiB2–35vol% (Fe-Ni) composite was due to an intensive coupled mechanism of the near-full density, dimple fracture of Fe-Ni phase, crack deflection, crack bridging, grain's pull-out and transgranular fracture of the TiB2 particles.