Intergranular Fracture Mode Research Articles

Effect of different microstructural features on the hydrogen embrittlement susceptibility of alloy 718

Due to its high strength and acceptable corrosion resistance, Alloy 718 is a viable material for the use in subsea applications. The alloy's susceptibility to hydrogen embrittlement is however limiting its viability. In this work the effect of microstructure on the hydrogen embrittlement susceptibility of Alloy 718 was examined by the use of slow strain rate testing on three different heat treatments. Cathodic pre-charging and polarization during testing were used to introduce hydrogen into the samples. A severe reduction in the ductility of the alloy due to the precipitation phases γ″ and γ′ were found. In addition, one of the heats had a continuous film of a Nb and C rich phase on the grain boundary giving an intergranular fracture mechanism. This intergranular fracture mode was further enhanced by the presence of hydrogen. In the solution annealed condition, the presence of hydrogen reduced the dimple size on the fracture surface.

Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties

AbstractAl2O3/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al2O3 and TiH2 powders. During sintering, TiH2 decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al2O3 matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al2O3. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm1/2, which was 37% greater than that of monolithic Al2O3. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10−1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al2O3 increased the fracture toughness and improved the conductivity of Al2O3.

Effect of strain rate, soaking time and alloying elements on hot ductility and hot shortness of low alloy steels

Susceptibility to hot shortness and hot cracking of Cu bearing steels is generally minimized with the addition of Ni. However, in this study, attempts have been made to reduce the hot working problems in such steels by the addition of Si, P and either very low content of Ni or no Ni, which will be understood after evaluating their hot ductility and hot shortness properties. The main techniques employed for this study are hot tensile test, analysis of fractography and characterization of scale/steel interface after long exposure at high temperature. The steels having 0.80–0.86% Si exhibited adequate hot ductility at all deformation temperatures ranging from 850 °C to 1200 °C during slow strain rate (0.005 s−1) testing conditions and the fracture morphology exhibited both ductile conical dimples and transgranular fracture. On the other hand, ~0.40% Si containing steels exhibited significantly reduced hot ductility at 850 °C and failed in intergranular brittle fracture mode. The improved hot ductility of high Si containing steels is explained using Fe-P phase diagram along with the proposition of inverse segregation of Si and repulsive interaction between P and Si. On increasing strain rate to 6.5 s−1, ductile failure was observed in all steels. The long duration of soaking at 1200 °C lead to intergranular decohesion and hot tearing during tensile testing in ~0.12% and ~0.19% P containing steels, respectively. A separate study of oxidation in air at 1200 °C indicated that presence of ~ 0.12% P in steel is beneficial in reducing hot shortness by keeping Cu enriched phase away from the scale/steel interface. In contrary to this, oxidation of ~0.19%P containing steel is detrimental to hot working as the grain boundaries of this steel were severely oxidized to the depth of few grains. Other alloying elements such as Ni and Cr in experimental steels had no effect on hot ductility and hot shortness properties.

Effect of vanadium-alloying on hydrogen embrittlement of austenitic high-nitrogen steels

The effect of hydrogen on tensile behavior and fracture mechanisms of V-alloying and V-free high-nitrogen austenitic steels was evaluated. Two steels with the chemical compositions of Fe-23Cr–17Mn–0.1C–0.6N (0V-HNS) and Fe-19Cr–22Mn–1.5V–0.3C–0.9N (1.5V-HNS) were electrochemically hydrogen-charged in NaCl water-solution for 100 hours. According to X-ray diffraction analysis and TEM researches, V-alloying promotes particle strengthening of the 1.5V-HNS. Despite differences in chemical compositions, namely, carbon and nitrogen concentrations, a solid solution hardening is similar for both steels because of precipitate-assisted depletion of austenite by interstitial atoms (carbon and nitrogen) in 1.5V-HNS. For hydrogen-free state, the values of the yield stress and the tensile strength are higher for particle-strengthened 1.5V-HNS as compared to 0V-HNS. Hydrogen-charging increases both the yield stress and the tensile strength of the steels, but hydrogen-assisted fracture micromechanisms are different for 0V-HNS and 1.5V-HNS. Hydrogen-charging drastically reduces a total elongation in 0V-HNS but provides insufficient embrittlement in 1.5V-HNS. Hydrogen-assisted brittle layers form on lateral surfaces of the specimens, and the widths and fracture micromechanisms in them are different for two steels. For 0V-HNS, surface layers of 84 μm in width possess transgranular brittle fracture mechanism (quasi-cleavage mode). For 1.5V-HNS, the brittle surface layers (31 μm width) destroy in intergranular brittle fracture mode. The central parts of steel specimens show dimple fracture similar to hydrogen-free steels. The possible reasons for different hydrogen-induced effects in 0V-HNS and 1.5V-HNS are discussed.

Creep-fatigue crack growth behavior in GH4169 superalloy

This study aims to examine the crack growth behavior of turbine disc GH4169 superalloy under creep-fatigue loading. Crack growth experiments were performed on compact tension specimens using trapezoidal waveform with dwell time at the maximum load at 650 °C. The crack growth rate of GH4169 superalloy significantly increased with dwell time. The grain boundaries oxidize during the dwell process, thereby inducing an intergranular creep-fatigue fracture mode. In addition, testing data under the same dwell time showed scattering at the crack growth rate. Consequently, a modified model based on the Saxena equation was proposed by introducing a distribution factor for the crack growth rate. Microstructural observation confirmed that the small grain size and high volume fraction of the δ phase led to a fast creep-fatigue crack growth rate at 650 °C, thus indicating that two factors, namely, fine grain and presence of the δ phase at the grain boundary, increased the amount of weakened interface at high temperature, in which intergranular cracks may form and propagate.

The effect of long-term service exposure on the stability of carbides in Co–Cr–Ni–W (X-45) superalloy

Abstract The present study was conducted to investigate the effect of long-term thermal exposure during service on the microstructure and tensile properties of Co–Cr–W alloy (known as X-45 alloy) used as the first stage nozzle in a gas turbine engine. Samples for the microstructure and tensile properties were cut from the side wall and trailing edge exposed to 700 °C and 800 °C, respectively, for various durations, i.e. 12,000 h, 16,000 h, and 18,000 h. At temperature around 800 °C, the irregular coarse M 23 C 6 carbides precipitate at the grain boundaries and interior of the grains from the decomposition of M 6 C → M 23 C 6 whereas M 23 C 6 carbides precipitation occurs directly on the stacking faults and grain boundaries at 700 °C due to the reaction of 23 M + 6C → M 23 C 6 . The tensile results indicate the correlation of the exposure time to morphology and dispersion of carbides. Small and shallow dimples were the main fracture mode of the exposed samples to 800 °C while intergranular fracture mode was observed in the samples exposed to 700 °C.

Failure analysis of an aluminum alloy material framework component induced by casting defects

Failure analysis on a fractured radome framework component was carried out through visual observations, metallographic examination using optical microscope, fractog-raphy inspections using scanning electron microscope and chemical composition analysis. The failed frame was made of casting Al–Si7–Mg0.4 aluminum alloy. It had suffered a former vi-bration performance tests. It was indicated that the fractures were attributed to fatigue cracks which were induced by casting porosities at the outer surfaces of frame. Failure analysis was carefully conducted for the semi-penetrating crack appearing on the framework. According to the fractography inspected by scanning electron microscope, it was indicated that numerous casting porosities at the outer surface of the framework played the role of multiple fracture sources due to some applied stresses. Optical microstructure observations suggested that the dendrite-shaped casting porosities largely contributed to the crack-initiation. The groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosities supplied the preferred paths of the crack-propagation. Besides, the brittle silicon eutectic particles distrib-uting along grain boundaries induced the intergranular fracture mode in the region of the over-load final fracture surface.

Diffusion-coupled cohesive interface simulations of stress corrosion intergranular cracking in polycrystalline materials

To study the stress corrosion intergranular cracking mechanism, a diffusion-coupled cohesive zone model (CZM) is proposed for the simulation of the stress-assisted diffusional process along grain boundaries and the mechanical response of grain boundary sliding and separation. This simulation methodology considers the synergistic effects of impurity diffusion driven by pressure gradient and degradation of grain boundary strength by impurity concentration. The diffusion-coupled CZM is combined with crystal plasticity finite element model (CPFEM) to simulate intergranular fracture of polycrystalline material under corrosive environment. Significant heterogeneity of the stress field and extensive impurity accumulation is observed at grain boundaries and junction points. Deformation mechanism maps are constructed with respect to the grain boundary degradation factor and applied strain rate, which dictate the transition from internal to near-surface intergranular fracture modes under various strain amplitudes and grain sizes.

Crack Formation Mechanism of High Silicon Steel during Twin-Roll Strip Casting

High silicon steel was fabricated by twin-roll strip casting. The cracks on the surfaces of the processed strips were obtained and analyzed by digital camera after series of surface treatment. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to observe and characterize the microstructure nearby crack and fracture surface along the normal direction, respectively, and the crack formation mechanism was further analyzed in conjunction with processing parameters utilized during twin-roll strip casting process. The results indicated that morelongitudinal cracks along the rolling direction were observed in comparison with transverse cracks along the transverse direction on the strip surfaces. Trans granular and intergranular fracture modes both worked during the formations of longitudinal and transverse cracks on the processed strips. The dominant factor causing the formation of crack on the surface of the processed strips was the inhomogeneous transfer of heat during casting and rolling. The inhomogeneous transfer of heat induced by gas gap during casting resulted in variations of dendrite length and secondary dendrite spacing (SDAS). Meanwhile, the casting velocity influenced the formation of gas gap, which further influenced the thermal contraction. So the control of velocity of casting above a certain level proved beneficial to enhancing the performance of strip casting and to improving the quality of strip products.

Effects of tempering temperature on fatigue crack propagation behavior of high-strength low-alloy D6AC steel plasma arc welds

The objective in this study was to elucidate fatigue crack propagation in D6AC steel plasma arc weldments (PAWs) that underwent post-weld (PW) tempering at 300°C, 450°C, or 600°C. PW tempering at 600°C was shown to enhance the refinement of the microstructure and thereby increase resistance to the growth of fatigue cracks. Surfaces with fatigue fracturing presented quasi-cleavage planes and intergranular fracture modes. The zigzag nature of the crack path (i.e., crack deflection) may contribute to the retardation of fatigue crack propagation.

High temperature mechanical properties of TZM alloys under different lanthanum doping treatments

La(NO3)3 and La2O3 doped titanium-zirconium-molybdenum (TZM) alloys were fabricated by using powder metallurgy method, and their room and high temperature mechanical properties were studied in this paper. Results show that the mechanical properties of La(NO3)3-TZM alloy sheet is better than that of La2O3-TZM alloy sheet in room temperature and the high temperature mechanical properties of both alloys have similar change trend but different sensitivity of temperature. The tiny grains in La(NO3)3-TZM alloy sheet is more sensitive to temperature and the lanthanum doping significantly influences the temperature sensitivity of the alloy. When the temperature is lower than 1400 °C, both TZM alloy sheets present a transgranular fracture mode. It is ductile intergranular fracture mode when the temperature is higher than 1400 °C, and the temperature of fracture mode transition is 1400 °C.

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 evolution and bonding mechanism of Ti2SnC-Ti6Al4V joint by using Cu pure foil interlayer

Ti2SnC, as one of functional ceramics with self-healing ability, was studied in welding with Ti6Al4V (TC4) through Cu interlayer under an applied mechanical pressure 10MPa in Ar atmosphere. Electron probe microanalyses indicated that the outward diffusion of Sn from Ti2SnC played a critical role in the chemical composition of the joining interface. After 60min, the reaction layers consisted of five zones: an interleaved zone (V) of β-Cu(Sn) and α-Cu(Sn), an enriched zone (IV) of Sn and intermetallic CuTi0.5Sn0.5 phase, the intermetallic zones (III) and (II) composed of TiCu4 and Ti3Cu4, a zone (I) consisting of β-Ti (Cu) phase. After shear tests, the corresponding fractographs indicated that the cracks mainly propagated in the Ti2SnC matrix with an intergranular fracture mode.

Effects of Y2O3 on the densification and fracture toughness of SPS-sintered TiC

In this work, we report a novel approach to fabricate titanium carbide (TiC) ceramics toughened by Y2O3 introduced via chemical precipitation method. TiC ceramics with and without Y2O3 addition were consolidated by SPS, and the densification, microstructure and fracture toughness were investigated. Compared to the additive-free counterpart, improved density and finer grain microstructure were found for TiC ceramics with Y2O3 sintered at 1600 °C, indicating that Y2O3 additive homogeneously scattered in TiC matrix could not only improve the density but also inhibit grain growth. In addition, the grain boundary was strengthened due to Y2O3 pinning in grain boundary, leading to a fracture behaviour transformation from intergranular type to the mixed mode of intergranular and transgranular fracture. Because of the improved density and strengthened grain boundary, the fracture toughness increased from 4.3 to 5.3 MPa·m1/2, which is improved by ~23% compared with that of additive-free TiC sample.

Investigation on compression behavior of TZM and La2O3 doped TZM Alloys at high temperature

Mechanical properties of Titanium-zirconium-molybdenum (TZM) and La2O3 doped TZM alloys under compression were tested at 1000°C and 1200°C. Microstructure of TZM and La2O3 doped TZM alloys after compressing was characterized by scanning electron microscopy. The effects on La2O3 doping on the high temperature deformation behavior and microstructure evolution of the TZM alloy were analyzed. Results show that La2O3 doping can refine the grain size of TZM alloy. La2O3 doping changes fracture model of TZM alloy. TZM alloy exhibits mainly intergranular fracture, while the La2O3 doped TZM alloy exhibits both intergranular and transgranular fracture mode.

The influences of Al content on the microstructure and mechanical properties of as-cast Mg-6Zn magnesium alloys

Mg-6Zn magnesium alloys microalloyed with varying Al content (0, 1, 3, 5 and 7wt%) were prepared by permanent mould casting. The effects of Al on the microstructure and mechanical properties of as-cast Mg-6Zn alloy were characterized with an optical microscope (OM), a scanning electron microscope (SEM) equipped with energy dispersive spectroscope (EDS), an X-ray diffractometer (XRD) and mechanical tests at room temperature, respectively. The experimental results indicate that the grain sizes do not decline obviously while the amount of eutectic phase and the secondary dendrite arm spacing (SDAS) of the alloys gradually increase when the Al content exceeds 3%. The main phases α-Mg, MgZn2, Mg2Zn3, Mg7Zn3, MgZn, Mg32(Al, Zn)49 and Mg17Al12 are found in these alloys. A higher addition of Al (≥5wt%) causes the formation of the Mg17Al12, meanwhile, the partial morphology of some eutectic phases is modified into lamellar formation, which has an adverse effect on mechanical properties of the Al-containing alloys. The mechanical testing reveals that, the tensile properties are gradually improved within the range of 0 ~ 3%Al, and the maximum values of ultimate tensile strength (UTS, 214MPa) and elongation (EL, 8.7%) are simultaneously obtained from the alloy with 3% Al, which increases by 21MPa and 16.0% compared with that of the ZA60 alloy, respectively. Fracture analysis demonstrates that quasi-cleavage fracture, inter-granular and trans-granular fracture are dominant modes in the alloy with additions of 0, 1, 3wt% Al. In contrast, the rupture mechanisms of the other investigated alloys belong to cleavage and inter-granular fracture modes.

Study on the microstructure and toughness of dissimilarly welded joints of advanced 9Cr/CrMoV

Abstract

Cyclic deformation and fatigue of rolled AZ80 magnesium alloy along different material orientations

The effect of material orientation on cyclic deformation and fatigue behavior of rolled AZ80 magnesium (Mg) alloy was experimentally investigated under fully reversed strain-controlled loading in ambient. The testing specimens were taken from a rolled AZ80 Mg plate at four orientations with respect to rolled plane: 0°(ND, normal direction), 30°(ND30), 60°(ND60), and 90°(RD, rolled direction). Fatigue fracture morphologies of specimens along different orientation were analyzed by scanning electron microscopy (SEM). Overall cyclic hardening was observed for the material loaded in different directions. For a given strain amplitude, the ND specimens had the lowest fatigue resistance among the specimens of all material orientations. The fatigue life of an ND30 specimens is similar to that of an ND60 specimen at a given strain amplitude and both are higher than that of an RD specimen when the strain amplitude is higher than 0.4%, whereas an RD specimen exhibits a better fatigue resistance when the strain amplitude is lower than 0.4%. A mixed fracture mode with transgranular and intergranular cracking related to lamellar-like features occurred during stable crack growth, and an intergranular fracture mode related to dimple-like features exhibited in the fast fracture region. A multiaxial fatigue model based on the strain energy density can correlate all the fatigue experiments of the material at different material orientations.

Microstructure and performance of rare earth element-strengthened plasma-facing tungsten material.

Pure W and W-(2%, 5%, 10%) Lu alloys were manufactured via mechanical alloying for 20 h and a spark plasma sintering process at 1,873 K for 2 min. The effects of Lu doping on the microstructure and performance of W were investigated using various techniques. For irradiation performance analysis, thermal desorption spectroscopy (TDS) measurements were performed from room temperature to 1,000 K via infrared irradiation with a heating rate of 1 K/s after implantations of He+ and D+ ions. TDS measurements were conducted to investigate D retention behavior. Microhardness was dramatically enhanced, and the density initially increased and then decreased with Lu content. The D retention performance followed the same trend as the density. Second-phase particles identified as Lu2O3 particles were completely distributed over the W grain boundaries and generated an effective grain refinement. Transgranular and intergranular fracture modes were observed on the fracture surface of the sintered W-Lu samples, indicating some improvement of strength and toughness. The amount and distribution of Lu substantially affected the properties of W. Among the investigated alloy compositions, W-5%Lu exhibited the best overall performance.

Fracture Behavior of 316 Stainless Steel under Creep at 600 and 800°C

The fracture characteristics of two AISI 316 austenitic stainless steels, creep-tested at constant load and temperatures of 600 and 800°C, were studied by scanning electron microscopy. The morphological aspects of the fracture were analyzed and correlated to the ductility level attained under creep. In one of the 316 steels, a marked change from ductile to intergranular mode of fracture was observed in going from 600 to 800°C. It is proposed that the condition for crack nucleation at precipitates as well as grain boundary oxidation are responsible for these microstructural changes.