Fractography

About: Fractography is a research topic. Over the lifetime, 5043 publications have been published within this topic receiving 86068 citations.

High cycle fatigue behavior of a single crystal superalloy at elevated temperatures

Abstract: Smooth and notched specimens of single crystal (SC) superalloy SRR99 with [0 0 1] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C, 760 °C, 850 °C and 900 °C in ambient atmosphere. The results demonstrate that the fatigue strength of smooth specimens reached the maximum at 760 °C and decreased with increasing temperature. The alloy became more notching sensitive with increase of temperature while the notch sensitivity declined at 900 °C. Analysis on fracture surfaces of both smooth and notched specimen shows that a transition from ductile fracture at lower temperatures to cleavage mode at higher temperatures were observed. Evolution of the microstructure was investigated by SEM and TEM observation. With the process of cyclic plastic deformation at elevated temperatures, the primary cuboidal γ′ precipitates tended to dissolve into the matrix channels, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix. In addition, different types of dislocation structures were developed during the cyclic deformation, which would have a significant impact on the fatigue life of the material.

Investigating the effects of hybrid reinforcement particles on the microstructural, mechanical and tribological properties of friction stir processed copper surface composites

Abstract: Friction Stir Processing (FSP) technique was employed in this research to fabricate copper surface composites through incorporating a hybrid mixture of reinforcement particles. Hybrid combination of advanced nitride based ceramic particles (25% AlN + 75% BN) was dispersed onto the surface of copper matrix at varying volume fractions through FSP technique. Results demonstrated an increase in mechanical properties with respect to increase in the amount of particle dispersion. Mechanism of fracture of developed set of surface composites was studied with the aid of the fractography. A tremendous increase in wear resistance was observed with respect to increase in the hybrid particle dispersion owing the hardness and the self-lubricating characteristics of dispersed ceramic particles.

Metallographic and fracture characteristics of resistance spot welded TWIP steels

Abstract: In this study, the microstructural characterisation, mechanical testing and fractography investigation were performed on twinning induced plasticity (TWIP) steels, fabricated with resistance spot welding. Failure mode during the cross-tensile test was found to follow the sequences of strain localisation of both sheets, crack initiation at notch tip, crack following along the fusion boundary and, finally, ductile shear fracture along the sheet thickness direction. On the other hand, failure in the tensile shear test was always directed along the sheet/sheet (s/s) interface; the interfacial failure and shear deformation were observed at the weld centreline. Solidification occurred as a primary austenitic solidification mode, and no martensitic transformations were detected through electron backscatter diffraction analysis. The fusion zone was mainly composed of austenite with directional solidification towards the centreline; the columnar dendritic and equiaxed structures were identified. Interdendr...

Effects of microstructure on fatigue crack growth behavior in cold-rolled dual phase steels

Abstract: Fatigue crack growth behaviors of cold-rolled dual phase steels with different microstructures were investigated at room temperature. The ferrite–martensite dual-phase microstructure was obtained by intercritical annealing. Fatigue crack growth (FCG) behaviors were described by both the Paris model and a new exponential model; fatigue fractography and surface morphology near the fracture were arrested by scanning electron microscopy (SEM); the relationship between macroscopic and microcosmic FCG rate was analyzed quantificationally. The results showed that both the models can be used to describe the fatigue crack growth rate of the samples rather well; fatigue striations and secondary cracks were observed in the fracture surface at stable expanding region (II), while the fracture at rapid expanding region (III) combined dimple and quasi-cleavage morphology; the roughness of fracture surface and the degree of secondary cracking increased with an increase in martensite content, leading to a higher threshold value. Moreover, the changes of microcosmic FCG rate were smoother than that of the macroscopic FCG rate.

Fracture and toughening mechanisms in an α2 titanium aluminide alloy

Abstract: The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α2 + β microstructure have been characterized as a function of temperature by performing uniaxial tension andJIC fracture toughness tests. The micromechanisms of crack initiation and growth have been studied bypost mortem fractographic and metallographic examinations of fractured specimens, as well as byin situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage. The results indicate that quasistatic crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have been identified: (1) decohesion of planar slipbands in the α2 matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α2 + β interface due to strain incompatibility resulting from impinging planar slip originated in α2. The sources of fracture toughness in the 25 °C to 450 °C range have been attributed to crack tip blunting, crack deflection, and a bridging mechanism provided by the ductile β phase. At 600 °C, a change of toughening mechanisms leads to a lowering of the initiation toughness (theKIC value) but a drastic increase in the crack growth toughness and the tearing modulus.