Showing papers on "Fractography published in 2013"

A comparison of the tensile, fatigue, and fracture behavior of Ti–6Al–4V and 15-5 PH stainless steel parts made by selective laser melting

Abstract: In this work, Ti–6Al–4V and 15-5 PH steel samples were fabricated using selective laser melting (SLM) and their tensile, fatigue, and fracture properties were analyzed and compared. The tensile properties were compared with respect to the build orientation. The horizontally built samples showed relatively better tensile properties as compared with the vertically built samples. Fatigue performance was studied for the vertical build orientation and compared with standard wrought material data. The tensile and fatigue performance of SLM-built materials were comparable to their respective standard wrought materials. Fractography was carried out for all tensile and fatigue-tested samples. The fatigue fracture behavior of Ti–6Al–4V was different from 15-5 PH steel. For Ti–6Al–4V, the fatigue crack initiation occurred deep in the subsurface whereas for PH steel the fatigue crack was initiated from the surface.

Creep rupture behavior of Grade 91 steel

Abstract: Grade 91 steel is considered a potential structural material for advanced nuclear reactors. The creep deformation behavior of Grade 91 steel was studied in the temperature range of 600–700 °C and at stresses of 35–350 MPa. The data were analyzed in terms of Monkman-Grant relation and Larson–Miller parameter. Creep damage tolerance factor and stress exponent were used to identify the cause of creep damage. The fracture surface morphology of the ruptured specimens was studied by scanning electron microscopy to elucidate the failure mechanisms. Fracture mechanism map for Grade 91 steel was developed based on the available material parameters and experimental observations.

Grain size effect on the mechanical properties of transparent spinel ceramics

Abstract: Transparent ceramics receive increasing interest in technical window applications. The mechanical reliability of transparent MgAl 2 O 4 with different grain sizes was characterized. The investigations were based on a combination of micro- and macro-mechanical methods to assess Young's modulus, hardness, fracture toughness, crack kinetics and strength, the latter being analyzed using two- and three-parameter Weibull statistics. The effect of subcritical crack growth under ambient conditions was assessed using a strength–probability–time plot. It has been verified that increasing grain size decreases strength and increases slow crack growth sensitivity, but on the other hand appears also to increase the Weibull modulus and the impact and wear relevant hardness indentation size effect. Complementary fractography by optical, confocal and scanning electron microscopy permitted to gain insight into the failure origins.

Fatigue crack growth behaviors in hot-rolled low carbon steels: A comparison between ferrite–pearlite and ferrite–bainite microstructures

Abstract: The roles of microstructure types in fatigue crack growth behaviors in ferrite–pearlite steel and ferrite–bainite steel were investigated. The ferrite–bainite dual-phase microstructure was obtained by intermediate heat treatment, conducted on ferrite–pearlite hot-rolled low carbon steel. This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), fatigue crack growth (FCG) rate tests, and fatigue fractography analysis. Microscopy images arrested by in-situ SEM showed that the fatigue crack propagation in F–P steel could become unstable more ealier compared with that in F–B steel. The fatigue cracks in ferrite–pearlite were more tortuous and could propagate more freely than that in ferrite–bainite microstructures. However, frequent crack branching were observed in ferrite–bainite steel and it indicated that the second hard bainite phase effectively retarded the crack propagation. The variation of FCG rate ( da / dN ) with stress intensity factor range (Δ K ) for F–P and F–B steels was discussed within the Paris region. It was shown that FCG rate of F–P steel was higher than that of F–B steel. Moreover, the fatigue fracture surface analysis proved that grain boundaries could also play a role in the resistance of crack propagation.

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...

Experimental study on fatigue behavior of corroded steel

Abstract: The objective of this article is to provide datum to engineers on the fatigue behavior of corroded steel plate and the material used in this study was Q235 steel. The effect of corrosion on fatigue behavior of corroded steel was investigated by using Hydraulic Servo Universal Fatigue Testing Machine. The results indicate that the presence of corrosion pits, produced by the method of constant temperature and humidity with prior immersion in saturated NaCl solution for about 2 min, significantly reduces the fatigue life of corroded steel. The relationship between (a) fractal dimension of roughness surface and fatigue life of corroded steel plate, (b) fractal dimension of roughness surface and corrosion ratio, and (c) corrosion ratio and fatigue life of corroded steel plate was also studied. In this paper, the fractal dimension refers to the fractal dimension of roughness surface but not to the fractal dimension of the fracture surfaces. In the end of this paper, the morphology characterization and fractography were also analyzed by using 3D Profiling and stereoscopic microscope, respectively.

Influence of structural integrity on fatigue behavior of friction stir spot welded AZ31 Mg alloy

Abstract: In this paper the fatigue behavior of the friction stir spot welded (FSSW) coupons of magnesium AZ31 alloy manufactured under different welding process conditions is investigated. Two sets of lap-shear coupons were made based on variation in plunge depth, tool rotation speed and tool geometry. Metallographic analysis of the untested FSSW coupons revealed differences in weld geometry and microstructure. Interfacial hooking of the faying surface was found to vary significantly between the two sets of process conditions. Microhardness test data revealed a large gradient in hardness profile. Results from the load controlled cyclic tests showed that lower tool rotation and shallower shoulder plunge depth led to better fatigue performance. Optical fractography of the fatigued coupons revealed that fatigue cracks initiated at the weld interface in both sets of coupons. However, the fracture mode showed variability between the two process conditions. The fractography analysis suggests that the effective top sheet thickness, interfacial hook and microstructure, which are largely determined by the process conditions, significantly affect the fatigue behavior of the friction stir spot welds in magnesium alloys.

Effect of tungsten content on microstructure and mechanical properties of swaged tungsten heavy alloys

Abstract: This paper describes the effect of tungsten content on microstructure and mechanical properties of swaged Co-containing tungsten heavy alloys with varying tungsten (90W–7Ni–2Fe–1Co, 93W–4.9Ni–1.4Fe–0.7Co and 95W–3.5Ni–1Fe–0.5Co). With increasing tungsten while tensile strength goes through a maximum, both percent elongation and impact energy decrease. Microstructure and fractographic analyses have been carried out in order to explain the trends in mechanical properties. Predominant transgranular fracture of tungsten grains combined with ductile dimple failure of the matrix in 90% W alloy is responsible for superior properties of this alloy in comparison to the alloy with 95% W. The highest tensile strength attained in 93% W alloy is attributed to predominant cleavage failure of W-grains. The results clearly indicate that the matrix volume fraction, contiguity and matrix mean path greatly influence the mechanical properties of tungsten heavy alloys.

Tensile and impact behavior of swaged tungsten heavy alloys processed by liquid phase sintering

Abstract: The microstructural influence on fracture behavior of tensile and Charpy impact toughness properties were investigated for 90W 6Ni 2Fe 2Co and 90W 6Ni 2Fe 1.5Mo 0.5Co tungsten heavy alloys, which are in use of kinetic energy penetrators. The alloys were sintered, heat treated and swaged with 40% reduction in area. Both microstructure and mechanical properties were sensitively dependent on the alloy composition. The W Ni Fe Co alloy exhibited increased tensile strength, ductility and impact toughness compared to the W Ni Mo alloy. Fractographic study revealed that the mode of tensile failure changes from flat to sheared tungsten grains for low to high strength tungsten heavy alloys, whereas the fracture behavior changes from flaws/cracks to transgranular cleavage of tungsten grains for low to high Charpy impact toughness alloys. The results indicate that a strong relationship between microstructure, fracture behavior, tensile and impact properties as a function of alloy composition.

Fatigue behavior of laser-welded NiTi wires in small-strain cyclic bending

Abstract: NiTi wires and their weldments are commonly used in micro-electro-mechanical systems (MEMS), and in such applications, cyclic loading are commonly encountered. In this paper, the bending-rotation fatigue (BRF) test was used to study the bending fatigue behavior of NiTi wire laser weldment in the small-strain regime. The fracture mechanism, which includes crack initiation, crack growth and propagation of the weldment in the BRF test, was investigated with the aid of SEM fractography and discussed in terms of the microstructure. It was found that crack initiation was primarily surface-condition dependent. The cracks were found to initiate at the surface defects at the weld zone (WZ) surface, and the crack propagation was assisted by the gas inclusions in the WZ. The weldment was finally fractured in a ductile manner. The fatigue life was found to decrease with increasing surface strain and also with increasing bending frequency (controlled by the rotational speed in the BRF test). In comparison, the fatigue life of the unwelded NiTi wires was higher than their welded counterparts at all strain levels and bending frequencies. The decrease in fatigue resistance of the weldment could be attributed to the surface and microstructural defects introduced during laser welding.

Influence of nitrogen alloying on properties of Fe318Cr312Mn3XN austenitic stainless steels

Abstract: In the present work, influence of nitrogen alloying on mechanical and corrosion properties of metastable Fe318Cr312Mn3XN alloys was investigated. It was observed that stacking faults and e-martensite needles decreases with N, conversely, short range ordering and deformation twinning increases. Further, it was found that yield strength and hardness of the alloys linearly increases with N. In contrast, elongation and tensile strength of the alloys changed nonlinearly with increasing N. Influence of N on mechanical properties was mainly related to its effect on solid solution strength and deformation-induced ά-martensitic transformation. SEM fractography of the samples showed a ductile fracture for N-bearing samples. It was observed that N-alloying enhances the corrosion resistance of the alloys. In addition, it was revealed that corrosion resistance of the studied alloys is well comparable with Ni-containing austenitic stainless steels and superior to other ultrahigh strength steels.

Microstructure, hardness and tensile properties of A380 aluminum alloy with and without Li additions

Abstract: In this work, the effects of lithium (Li) on the microstructure, hardness and mechanical properties of A380 aluminum alloy have been investigated. The alloy was produced by conventional casting. Microstructures of the samples were investigated using the optical and scanning electron microscopy. The results showed that with increase of Li content up to 0.1%, the morphology of β-Al 5 FeSi and eutectic Si phases changed from intersected and branched coarse platelets into fine and independent ones. Li decreased hardness values of the alloy. Also, it was revealed from tensile tests that with addition of 0.6% Li, the ultimate tensile strength (UTS) and elongation values increased from 274 to 300 MPa and 3.8% to 6%, respectively. Fractographic examination of the fracture surfaces indicated that the alloys with Li addition had more ductile dimple and fewer brittle cleavage surfaces.

Thermoelectric and mechanical properties of melt spun and spark plasma sintered n-type Yb- and Ba-filled skutterudites

Abstract: Here we present thermoelectric and mechanical properties of n-type filled-skutterudites produced by a combination of melt spinning of pre-melted charges with subsequent consolidation by spark plasma sintering, a process we refer to as MS-SPS. This combination of processing steps leads to phase-pure n-type filled-skutterudites and obviates more energy and time intensive annealing steps. We show that both the thermoelectric properties and the tensile fracture strength compare favorably to materials made by traditional methods. The process is scalable to at least 80 g billets, such that the transport properties measured on test bars harvested from these larger billets compare favorably to those measured on lab-scale billets (5 g total billet mass). ZT values approaching 1.1 at 750 K were observed in materials made by MS-SPS. In addition, the tensile fracture strength of test bars cut from an 80 g billet is ∼128 MPa at room temperature and decreases with increasing temperature. Fractography of the test bars reveals that the majority failed due to surface and edge flaws with few failures due to volume type flaws. This indicates that the powder metallurgical methods employed to produce these samples is mature.

Observations on Mode I ductile tearing in sheet metals

Abstract: Cracked ductile sheet metals, subject to Mode I tearing, have been observed to display a variety of fracture surface morphologies depending on the material properties, and a range of studies on the fracture surface appearance have been published in the literature. Whereas classical fractures such as cup-cone, cup–cup and slanting are widely observed, the phenomenon of a slanted crack which systematically “flips” back and forth in a roughly 45° orientation has only recently been reported. The present study aims to add details and understanding to this crack growth phenomenon – through experimental testing and comparison with published results. A series of crack propagation tests have been carried out, where cracks are driven many plate thicknesses under global Mode I loading. The current study employs both the edge crack specimen (ECS) loaded in combined in-plane bending and extension, and the double edge notched tension (DENT) specimen loaded in extension. Clear evidence of the alternating slant failure is observed for a normal strength steel and details on the phenomenon are brought out by grinding and polishing cross-sections in successive steps along the crack growth direction. Moreover, these results are compared to a soft aluminum (1050A) which predominantly shows cup–cup or slant failure. In general, both materials fail by the mechanism of void growth to coalescence (fractographs are included), and evidence points toward cup–cup being favored by materials that experience extensive amounts of void growth (thus severe thinning in the process zone), whereas slanting is typical in high strength materials that rapidly nucleate large populations of smaller voids.

Influence of the microstructure on fatigue and fracture toughness properties of large heat-treated mold steels

Abstract: The standard ISO 1.2738 medium-carbon low-alloy steel has long been used to fabricate plastic molds for injection molding of large automotive components, such as bumpers and dashboards. These molds are usually machined from large pre-hardened steel blooms. Due to the bloom size, the heat treatment yields mixed microstructures, continuously varying from surface to core. Negative events (such as microcracks due to improper weld bed deposition or incomplete extraction of already formed plastic objects) or too large thermal/mechanical stresses can conceivably cause mold failure during service due to the low fracture toughness and fatigue resistance typically encountered in large slack quenched and tempered ISO 1.2738 steel blooms. Alternative steel grades, including both non-standard microalloyed steels, designed for the same production process, and precipitation hardening steels, have recently been proposed by steelworks. However, the fracture toughness and the fatigue properties of these steels, and hence their response during the service, are not well known. Results of an experimental campaign to assess the fracture toughness and fatigue properties, as well as the basic mechanical properties, of a microalloyed and a precipitation hardening plastic mold steel blooms are presented and commented, also in respect to the results previously obtained by two commercial ISO 1.2738 ones. Experimental results show that these steels generally exhibit low fracture toughness values; in the traditional quenched and tempered bloom steels the brittleness may be caused both by the presence of mixed microstructures and by grain boundaries segregation, while in the precipitation hardened one the brittleness probably stems from the precipitation phenomena. This study suggests that microalloyed and precipitation hardening steels may be used to produce large plastic mold, yet the fracture toughness still remains the most critical property.