Intergranular Fracture Features Research Articles

Strain-controlled creep-fatigue interaction property of Nb521 refractory alloy: Hold time effect and life prediction

Strain-controlled creep-fatigue interaction (CFI) tests were conducted for NbW alloy, i.e., Nb521 alloy, at 900 °C with a strain amplitude of ±0.3% under atmospheric conditions to study the effect of hold time on the CFI failure behaviour, with the results indicating that the creep-fatigue life of the Nb521 alloy decreased with an increase in the hold time. The decline rate gradually flattened, with no saturation effect observed. Furthermore, with an increase in the hold time, the hardening rate of the Nb521 alloy gradually decreased, but the softening rate remained high. Fatigue fractures can be divided into stable propagation, rapid propagation, and final fracture zones according to their morphological characteristics. Compared with the stable propagation zone of the pure fatigue fracture surface, the creep damage caused by the hold time resulted in numerous intergranular fracture features in the stable propagation zone of the CFI fracture surface, indicating that the crack propagation mode changed from a transgranular fracture to a mixed transgranular–intergranular fracture. The strain energy density exhaustion model under nonlinear damage summation better reflects the CFI damage behaviour, with the creep-fatigue life prediction result considered acceptable.

Transverse Stress Rupture Properties of a Nickel-Base Superalloy Bicrystal

Effects of low angle boundaries (LABs) on the stress rupture properties of bicrystals of a nickel-based third generation single crystal superalloy at 1093 °C/158 MPa were investigated. The results show that the effect of LABs on the stress rupture elongation of the alloy is higher than that of the stress rupture life at 1093 °C/158 MPa. As the misorientation angle of the LABs reaches 9.0°, the stress rupture life of the alloy with LABs can still retain nearly 50% of that with LABs of 0° at 1093 °C/158 MPa; while the stress rupture elongation of the alloy with LABs drops obviously when the misorientation angle of the LABs is larger than 6.5°. The fracture surfaces of stress ruptured alloy with LABs of 0°~2.9° are characterized by dimple features, while those with LABs of 6.5°~12.3° all exhibit intergranular fracture features. Apparent dimple features can be observed at the intergranular fracture surface of the alloy with LABs of 6.5° and the elongation of it is high. However, obvious dendrite features can be observed at the intergranular fracture surfaces of the alloy with LABs of 7.6°~12.3° and the elongations of them are relatively low.

Hydrogen-assisted decohesion associated with nanosized grain boundary κ-carbides in a high-Mn lightweight steel

While age-hardened austenitic high-Mn and high-Al lightweight steels exhibit excellent strength-ductility combinations, their properties are strongly degraded when mechanically loaded under harsh environments, e.g. with the presence of hydrogen (H). The H embrittlement in this type of materials, especially pertaining to the effect of κ-carbide precipitation, has been scarcely studied. Here we focus on this subject, using a Fe-28.4Mn-8.3Al-1.3C (wt%) steel in different microstructure conditions, namely, solute solution treated and age-hardened. Contrary to the reports that grain boundary (GB) κ-carbides precipitate only during overaging, site-specific atom probe tomography and scanning transmission electron microscopy (STEM) reveal the existence of nanosized GB κ-carbides at early stages of aging. We correlate this observation with the deterioration of H embrittlement resistance in aged samples. While H pre-charged solution-treated samples fail by intergranular fracture at depths consistent with the H ingress depth (∼20 µm), age-hardened samples show intergranular fracture features at a much larger depth of above 500 µm, despite similar amount of H introduced into the material. This difference is explained in terms of the facile H-induced decohesion of GB κ-carbides/matrix interfaces where H can be continuously supplied through internal short-distance diffusion to the propagating crack tips. The H-associated decohesion mechanisms are supported by a comparison with the fracture behavior in samples loaded under the cryogenic temperature and can be explained based on dislocation pileups and elastic misfit at the GB κ-carbide/matrix interfaces. The roles of other plasticity-associated H embrittlement mechanisms are also discussed in this work based on careful investigations of the dislocation activities near the H-induced cracks. Possible alloying and microstructure design strategies for the enhancement of the H embrittlement resistance in this alloy family are also suggested.

Effect of Low-Angle Boundaries on the Microstructures and Tensile Properties of the Third-Generation Single-Crystal Superalloy DD9

The microstructure of low-angle boundaries (LABs) of thethird-generation single-crystal superalloy DD9 and its effect on the tensile properties at 1100 °C were investigated. Double seed crystals techniques were used to obtain the specimens of DD9 alloy with LABs. The results show thatthe as-cast LABs of DD9 alloy are composed of strip-like and bulk γ′ phases with γ matrix, while no γ′ phases were foundat the LABs after the heat treatment. The LABs had little effect on the tensile strength of DD9 alloy, but hadan obvious effect on the tensile plasticity, and the fracture surfaces of tensile-ruptured DD9 alloy with LABs of 3.7°~11.4° exhibited intergranular fracture features at 1100 °C.

Fatigue Behaviour of Connectors used in Cable Harnessing through Cavity Formation Related Microstructural Degradation: A Failure Investigation Perspective

Two separately failed electrical connector pieces during a vibration test were received for failure analysis. Chemical composition, hardness values and microstructures of the each of the connector material indicate that the material of construction is a die cast aluminium-silicon type of alloy, closely matching with the standard ANSI/AA B380 alloy. Intergranular and faceted fracture features are observed and failure mechanism is found to be fatigue dominated. The connectors failed by impact fatigue arising out of the loosening of the connector assembly. This has happened by cavity formation and/or growth related microstructural degradation processes. Initial casting pores as well as microstructural degradations such as interconnected pores have developed in service and their successive growth, decohesion and interconnection of each of primary Si particles and Al-Fe-Mn precipitates (along precipitate-matrix interface) have led the initiation of the crack under fatigue loading. Brittle as-cast microstructure (as typified by the precipitate-matrix interfacial cracking), existing vibratory loading and absence of any rise in temperature in the system have assisted the initial cavity (crack) formation and/or growth. Moreover, initial fitment related looseness is an additional factor in initiating and propagating this damaging mechanism.

Microstructure and mechanical behavior of porous tungsten skeletons synthesized by selected laser melting

Because of its high ductile-brittle transition temperature, tungsten (W) is normally alloyed with other metal elements in order to obtain high fracture strength, excellent thermal and electrical properties for industrial applications. For tungsten samples sintered using the conventional powder metallurgy methods, bonding among tungsten particles is normally through sintered necking process, thus without providing a good metallurgical bonding strength. In this paper, we proposed to apply additive manufacture methodology to synthesize two types of porous tungsten skeleton structures, honeycomb (65% porosity) and square skeleton (80% porosity), using a selective laser melting (SLM) method. Results showed that for both these skeleton structures, grains in the XY plane showed an equiaxed crystal appearance, whereas those in the YZ/XZ plane showed columnar patterns parallel to the Z axis. The measured porosities for these two types of skeletons were 52 vol% and 68 vol%, and their compressive strength values were 256 MPa and 149 MPa, respectively. Both their compressive strengths and hardness showed anisotropic behaviors, with their highest values along the direction of Z axis. Results also showed that fracture morphology and mechanisms of these skeletons under compression were quite different when they were compressed along different directions, mainly due to the formed columnar crystals of the skeletons along the Z axis. Fracture morphology along the Z axis showed transgranular fracture and tearing features, whereas those along X axis showed only intergranular fracture features.

Effect of Zn addition on the microstructure and mechanical properties of as-cast BCC Mg-11Li based alloys

Effect of Zn addition on the microstructure and mechanical properties of as-cast BCC Mg-11Li based alloys were investigated. The results revealed that a certain amount of θ (MgLiZn) with coherent α-Mg phase precipitated along grain boundaries (GBs) and a high density of dispersing spherical small-sized θ′ (MgLi2Zn) precipitates distributed inside grains were contained in β-phase Mg-11Li-6Zn (in wt%, LZ116) alloy, and the volume fraction of both phases increased with the elevating Zn content under the lever of less than 6.2 wt%. After solution treatment, the θ / α-Mg phases located at the GBs and the spherical θ’ phase inside the grains were dissolved, whilst a high density of fine-laminar θ’ phase precipitated concomitantly in LZ116 alloy. The as-cast LZ116 alloy exhibited high ultimate tensile strength and ductility of 184 MPa and 30% at room temperature, respectively, which were much higher than those (72 MPa and 19%) of as-cast Mg-11Li (L11) alloy. The strength of the LZ116 alloy was further elevated to 235 MPa via solid-solution treatment at 350 °C for 4 h (the treated alloy is denoted as S-LZ116), but the elongation was dramatically reduced to 1.2%. As a result, the softening θ/α-Mg precipitates distributed at GBs enhanced the plasticity and the hardening θ′ phase elevated the strength of as-cast LZ116 alloy. Meanwhile, the intergranular fracture and dimple feature was observed in as-cast LZ116 alloy, and the fracture behavior is correlated with the softening GBs with θ/ α-Mg phases and the hardening grains reinforced by dispersed θ′ phase particles.

The effect of grain refinement and precipitation strengthening induced by Sc or Er alloying on the mechanical properties of cast Al-Li-Cu-Mg alloys at elevated temperatures

In present paper, the effect of Scandium and Erbium alloying on the microstructure evolution and mechanical performance at elevated temperature for Al-Li-Cu-Mg alloy are investigated. Furthermore, the interaction process between shell/core composite particles Al3(M, Zr, Li) (M = Sc/Er) and dislocations is described in the paper. The experimental results indicate that 0.2 wt% Sc addition has much more significant effect upon refining grain of the alloys, compared to equivalent Er addition content. The addition of the both rare elements will obviously facilitates the precipitation density of the shell/core composite Al3(M, Zr, Li) particles in the aging-treated alloys. Moreover, Sc addition will also contribute to promoting the precipitation of plate-like S/S′ phase during aging process. At 200 °C or below, the most excellent mechanical performance of Sc-containing alloy can be attributed to the synthetical effects of the grain refining, dislocation cutting (δ′-Al3Li) and Orowan (shell/core composite Al3(M, Zr, Li) particles) multiples mechanisms, although the fracture morphology of the alloy displays intergranular fracture features. While raising test temperature to 300 °C, it can be observed from the morphology of the Sc-containing alloy that the failure of the grain refining mechanism and the of grain boundary sliding (GBS) occurs that which will damage the high-temperature strength but increase deformation ability of samples. It should be noted that the large grain size and the precipitation of the composite Al3(M, Zr, Li) particles are major reasons for the improvement of elevated-temperatures mechanical properties of the Er-containing alloy.

Microstructural characterization of failed screws

Present paper reports the microstructural characterization of the failed screws. The analyzed chemical compositions of all the three screws indicate that these are fabricated from Fe-based alloy with Cr as alloying element. The analyzed chemical composition is close to AISI-51B37 standard. The microstructures of the screws in un-etched condition exhibit the presence of micro cracks and thin layer at the outer surface of threads. The micro cracks have originated from the root of the threads and propagated towards the centre. The thin layer with ∼12 μm thickness reflects the presence of mainly cadmium (Cd). This indicates that the Cd coating is applied on the outer surfaces of the threads for protection against corrosion and lubricity. The etched microstructure of the screws is very fine tempered martensite. The hydrogen content of the screws is ∼13 ppm which is significantly higher than that allowed in high strength steels (1 ppm). The fracture surfaces of the screws portray the presence of intergranular fracture features, ductile tearing and ductile dimples. The presence of high hydrogen content, integranular fracture features and ductile tearing indicate that the screws have failed due to the hydrogen embrittlement.

Microstructure and mechanical properties of hot-pressed Ti–Co–Si compounds reinforced by intermetallic phases

Monolithic TiCo and Ti5Si3 phases and Co(ss)/Ti5Si3–Ti3Co2Si nanocomposite were fabricated via mechanical alloying followed by hot-pressing (HP). The XRD patterns of bulk samples showed a general decrease in peak widths after hot-pressing owing to an increase in crystal size caused by exposure to high temperature during the HP process. Nevertheless, the nano-crystallinity of the structure was retained after HP and no new phase was formed. As a dominant fracture mode, this material exhibited an intergranular fracture feature with low dimpled ruptures. Dry-sliding wear behavior was evaluated at room and high temperatures. The mean coefficient of friction and wear rate gradually decreased with increasing temperature due to the presence of discontinuous thin tribo-oxide and tribo-nitride layers on the worn surface. The wear rate obtained in this work was ~10 times lower than that previously reported for Ti compounds.

High temperature creep behavior of an as-cast near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy

The high temperature creep behavior of a near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy prepared by induction skull melting has been studied under the conditions of 625 °C-675 °C and 150 MPa–250 MPa using uniaxial creep tests in this work. Results indicated that the incoherent secondary phase particle, silicide, mainly precipitated from the α/β lath interface, which has a positive effect on hindering the movement of dislocations and pinning α/β lamellae boundaries during creep. The stress exponent and apparent activation energy was 7.2 at 650 °C and 417.6 kJ/mol at 200 MPa, respectively. The high stress exponent and apparent activation energy indicated that the creep process was controlled by dislocation climb and slip impeded by silicides. The main failure mechanism of the as-cast alloy during creep was the initiation, growth, and coalescence of cracks at the grain boundary, which was proven from the intergranular fracture feature.

Effect of thermal compensation treatment on the microstructure and mechanical properties of IN738LC joint by electron beam welding

Thermal compensation treatment (TCT) obtained sound IN738LC joint by electron beam welding (EBW), increasing the characteristic dimension of fusion zone (FZ) and heat affected zone (HAZ) prominently. But no microstructure change occurred in the FZ of IN738LC joint subjecting to TCT, consisting of nanoscale γ′ particles, discrete MC carbide and isolated γ/γ′ eutectic. The content of γ/γ′ eutectic and γ′ size in FZ increased as the number of TCT increased, attributed to the decreased cooling rate of FZ. When adopting TCT, the dissolution of γʹ particle in HAZ was more evident, resulting in an increase of liquid film thickness. As a result, the total length of HAZ liquation cracking rapidly decreased. With the increased number of TCT, the micro-hardness of FZ and HAZ in IN738LC joint increased gradually. All the joints fractured in the BM, but obtained a decreased elongation at room temperature (RM) and 900ºC, mainly associated with the increased content of MC carbides and γ′ particles in FZ and HAZ. The fracture surfaces of IN738LC joints showed quasi-cleavage fracture mode and ductile fracture feature at RM and 900ºC, respectively. In addition, the rupture life of IN738LC joint at 900ºC/350MPa was improved significantly from 3.1h to 4.25h after TCT. All the joints fractured in FZ and showed a typical intergranular fracture feature.

Microstructural characteristics and mechanical properties of the Al–Si coating on press hardened 22MnB5 steel

The microstructural characteristics and mechanical properties of the phases in the substrate-coating interdiffusion layer of an Al–10%Si coated 22MnB5 press hardened steel (PHS) were studied by means of electron diffraction, chemical analysis, and nano-indentation. The interdiffusion layer of the Al–Si coated PHS, austenitized at 900 °C for 6.5 min, consisted of α-Fe(Al), Fe3Al, FeAl, Fe2Al5, and an Fe–Al–Si ternary intermetallic phase. The chemical analysis results indicated that the Fe3Al and Fe2Al5 phases were non-stoichiometric, and their composition ranges were not fully consistent with existing Fe–Al and Fe–Al–Si phase diagrams. These results partly explain the significant discrepancies between research groups in phase analysis of the interdiffusion layer. The observed Fe–Al or Fe–Al–Si phases were harder and more brittle compared to the martensitic matrix. With increasing austenitizing temperature and hold time, the thickness of the interdiffusion layer increased and the major constituent in the coating changed from Fe2Al5 to a multi-layered structure of FeAl, Fe3Al, and Fe(Al), leading to significant softening of the interdiffusion layer. Free bend testing was employed to investigate cracking and fracture behavior of the interdiffusion layer. The cracks formed in the thicker coating associated with the highest austenitizing temperature propagated deeper into the substrate material than in the case of lower austenitizing temperatures. Fracture of the interdiffusion layer involved transgranular cleavage and intergranular fracture features. The fracture surface appearance depended on both the phases present and the Al distribution in the coating. FeAl phase, known to be relatively “ductile”, was more susceptible to intergranular fracture than other phases (Fe2Al5, Fe3Al, and α-Fe(Al)).

Fatigue Properties of Resistance Spot Welded Dissimilar Interstitial-Free and High Strength Micro-Alloyed Steel Sheets

This study aims to investigate the influence of nugget diameter on tensile-shear and fatigue properties of the dissimilar resistance spot-welded joints of interstitial free and high strength niobium microalloyed steel sheets. The spot-weldings are done at currents of 7 kA and 9 kA at welding time of 300 ms with electrode force of 2.6 kN. Fatigue tests are done at different load amplitudes, considering the maximum tensile-shear load bearing capacity of the joints. The tests are interrupted before the final failure to understand the crack initiation and propagation paths. The study is supplemented by the microstructural and detailed fractographic analyses. The results indicated that the fatigue strength of the spot-welds increased with a decrease in nugget diameter. This could be attributed to the higher microhardness of heat affected zone (HAZ), presence of compressive residual stress and lower variation in thickness reduction on the IF steel side, welded at 7 kA. However, the tensile-shear load bearing capacity of the spot-welds increased with increase in the nugget diameter. The failure of the spot-welded joint under static loading initiates from the HAZ/base metal interface of the IF steel side. Nevertheless, under cyclic loading crack initiates from the notch root at the interface of two sheets lying in the HAZ of IF steel side. Fractographic investigations indicate intergranular fracture features in combination with striations, secondary cracks on the IF steel side under cyclic loading. Fracture surfaces of the specimens failed under static loading shows however, shear dimples on the IF steel side.

Hydrogen-assisted fracture features of a high strength ferrite-pearlite steel

Up to now, the exact reason of hydrogen-induced fracture for ferrite-pearlite (FP) steel is still not fully understood. This study presents detail observations of the feature beneath the fracture surface with the aim to reveal the hydrogen-induced cracking initiation and propagation processes. Slow strain rate tensile (SSRT) testing shows that the FP steel is sensitive to hydrogen embrittlement (HE). Focused ion beam (FIB) was used to prepare samples for TEM observations after HE fracture. The corresponding fractographic morphologies of hydrogen charged specimen exhibit intergranular (IG) and quasi-cleavage (QC) fracture feature. Pearlite colony, ferrite/pearlite (F/P) boundary and the adjacent ferrite matrix are found to be responsible for the initial HE fracture and the subsequent propagation. With increasing of the stress intensity factor, fracture mode is found to change from mixed IG and QC to entire QC feature which only occurs at the ferrite matrix. No crack is observed at the ferrite/cementite (F/C) interface. This may be mainly due to the limited pearlite lamella size and relatively low interface energy.

Failure Analysis of Cooler Fan Drive Gear System of Main Gear Box of Helicopter

This paper presents the failure analysis of cooler fan drive gear system of helicopter. The work involves failure analysis of two components of main gear box assembly i.e. intermediate gear and drive fan pinion. The materials of the gears are made up of case carburizing steel. The microstructure of both the components is tempered martensite. The hardness values measured on the intermediate gear (un-failed and failed) and drive fan pinion show that the hardness of the un-failed intermediate gear teeth is more compared to failed intermediate gear teeth and drive fan pinion. The low hardness value of failed component is likely due to coarsening of microstructure during operation. Fracture surfaces of the failed intermediate gear display the presence of rubbed features, ductile dimples and micro cracks while dive fan pinion exhibit the presence of ductile dimples, striations and intergranular fracture features. The intergranular fracture features observed can be ascribed to improper microstructure resulting from deviation in heat treatment process. Initially, a single tooth of drive fan pinion has broken due to fatigue and subsequently other teeth have broken by transferring load to its immediate neighbour

Refilled friction stir spot welding of aluminum alloy to galvanized steel sheets

Dissimilar materials lap joining of aluminum alloy to galvanized steel sheets was investigated with refilled friction stir spot welding (Refilled FSSW). For the lap joint between 1.0mm thick Novelist AC 170 PX aluminum alloy and 1.2mm thick ST06 Z galvanized steel sheets, the maximum tensile/shear fracture load is 3044N with cross-tension fracture load of 296N. For that between 1.5mm thick Aleris Superlite 200 ST aluminum alloy and 1.2mm thick ST06 Z galvanized steel sheets, the maximum tensile/shear fracture load reaches 4500N and the maximum cross-tension fracture load is 359N. All samples failed through the Al/steel interface during tensile/shear and cross-tension tests. The microstructure, composition, fractogragh and fracture mode of Al/steel joints were analyzed with electron probe microanalysis (EPMA), scanning electron microscope (SEM) and X-ray diffraction (XRD). Zinc and oxygen rich layer were observed in the interface on aluminum alloy side, and ZnO phase was detected on the fractured surface of both aluminum alloy and steel sides. The samples during tensile/shear test failed mainly through shear brittle fracture with features of cleavage and intergranular fracture feature. The spot weld can be distinguished as pin-refilled zone and sleeve-plunging zone due to the distribution of zinc coating.

Investigation of HP Turbine Blade Failure in a Military Turbofan Engine

AbstractFailure of a high pressure (HP) turbine blade in a military turbofan engine is investigated to determine the root cause of failure. Forensic and metallurgical investigations are carried out on the affected blades. The loss of coating and the presence of heavily oxidized intergranular fracture features including substrate material aging and airfoil curling in the trailing edge of a representative blade indicate that the coating is not providing adequate oxidation protection and the blade material substrate is not suitable for the application at hand. Coating spallation followed by substrate oxidation and aging leading to intergranular cracking and localized trailing edge curling is the root cause of the blade failure. The remaining portion of the blade fracture surface showed ductile overload features in the final failure. The damage observed in downstream components is due to secondary effects.

Two directional microstructure and effects of nanoscale dispersed Si particles on microhardness and tensile properties of AlSi7Mg melt-spun alloy

The two directional microstructure and multiple mechanical properties of the AlSi7Mg ribbon produced by melt-spun were investigated by optical microscopy (OM), field emission gun scanning electron microscope (FEGSEM), X-ray diffraction (XRD), microhardness and tensile tests. Both the surface and cross-sectional microstructure of the melt-spun ribbon were characterized in detail to give a clear and integrated description of the microstructure. Two kinds of nanoscale Si particles were observed, i.e., small Si particles ranging from 13 to 50nm and large Si particles ranging from 50nm to several hundreds of nanometers with clear size boundary were dispersed both in the interior and boundary of fine α-Al. XRD results revealed supersaturated solution of Si in Al matrix to be 0.62at.%. The ultimate tensile strength, yield strength, and hardness of the ribbon were 1.53, 1.75 and 1.56 times higher than that of the conventional cast ingot separately. The breaking elongation of the ribbon was 1.73% with intergranular fracture feature. The effects of nanoscale dispersed Si particles on the significant improvement of both hardness and tensile properties of the AlSi7Mg melt-spun ribbon were discussed in detail.

Strength Prediction Model of a Particle-Reinforced Shellproof Ceramic Composite

On the basis of the microstructure of particle-reinforced shellproof ceramic composite, and the intergranular fracture feature, a dislocation pile-up fracture model of the small-particle ceramic composite is developed, the mechanism of formation, growth and coalescence of microcracks. The complex effect of the small particle pull-out and large particle cracking is concerned, when constructing the crack extension fracture model. Thereafter, the influence of particles’ volume fraction and matrix grain diameter on fracture strength is studied. The experimental data shows that the proposed strength prediction model is successful and can be generally applied.