Showing papers on "Fractography published in 2011"

Influence of intermetallic phases and Kirkendall-porosity on the mechanical properties of joints between steel and aluminium alloys

Abstract: The formation of intermetallic reaction layers and their influence on mechanical properties was investigated in friction stir welded joints between a low C steel and both pure Al (99.5 wt.%) and Al–5 wt.% Si. Characterisation of the steel/Al interface, tensile tests and fractography analysis were performed on samples in the as-welded state and after annealing in the range of 200–600 ◦ C for 9–64 min. Annealing was performed to obtain reaction layers of distinct thickness and composition. For both Al alloys, the reaction layers grew with parabolic kinetics with the phase (Al5Fe2) as the dominant component after annealing at 450 ◦ C and above. In joints with pure Al, the tensile strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface. The tensile strength of joints with Al–5 wt.% Si is controlled by the thickness of the phase (Al5Fe2) layer. The pre-deformation of the base materials, induced by the friction stir welding procedure, was found to have a pronounced effect on the composition and growth kinetics of the reaction layers.

A model for the strength of yarn-like carbon nanotube fibers.

Abstract: A model for the strength of pure carbon nanotube (CNT) fibers is derived and parametrized using experimental data and computational simulations. The model points to the parameters of the subunits that must be optimized in order to produce improvements in the strength of the macroscopic CNT fiber, primarily nanotube length and shear strength between CNTs. Fractography analysis of the CNT fibers reveals a fibrous fracture surface and indicates that fiber strength originates from resistance to nanotube pull-out and is thus proportional to the nanotube−nanotube interface contact area and shear strength. The contact area between adjacent nanotubes is determined by their degree of polygonization or collapse, which in turn depends on their diameter and number of layers. We show that larger diameter tubes with fewer walls have a greater degree of contact, as determined by continuum elasticity theory, molecular mechanics, and image analysis of transmission electron micrographs. According to our model, the axial st...

Improvement of fatigue property in 7050–T7451 aluminum alloy by laser peening and shot peening

Abstract: The fatigue strength for 1 × 107 cycles of 7050–T7451 aluminum alloy was determined for machined, laser-peened, and shot-peened specimens. Moreover, fatigue lives were compared under the same load conditions. Results show that the laser peening induces a deeper compressive residual stress layer and better surface finish, therefore, it improves fatigue properties more effectively. Fractographic examination and analysis shows that the fatigue cracks initiate in the subsurface layer beneath the compressive residual stress field for laser- and shot-peened specimens, whereas the fatigue cracks form at surface for as-machined ones.

Effects of particle size and distribution on the mechanical properties of SiC reinforced Al–Cu alloy composites

Abstract: This paper studied the combined effects of particle size and distribution on the mechanical properties of the SiC particle reinforced Al–Cu alloy composites. It has been shown that small ratio between matrix/reinforcement particle sizes resulted in more uniform distribution of the SiC particles in the matrix. The SiC particles distributed more uniformly in the matrix with increasing in mixing time. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength and elongation. Yield strength and ultimate tensile strength of the composite reinforced by 4.7 μm sized SiC particles are higher than those of composite reinforced by 77 μm sized SiC particles, while the elongation shows opposite trend with yield strength and ultimate tensile strength. Fracture surface observations showed that the dominant fracture mechanism of the composites with small SiC particle size (4.7 μm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, while the dominant fracture mechanism of the composites with large SiC particle size (77 μm) is ductile fracture of the matrix, accompanied by the SiC particle fracture.

Hydrogen embrittlement (HE) phenomena and mechanisms

Abstract: Mechanisms of hydrogen embrittlement in steels and other materials are described, and the evidence supporting various hypotheses, such as those based on hydride-formation, hydrogen-enhanced decohesion, hydrogen-enhanced localised plasticity, adsorption-induced dislocation- emission, and hydrogen-vacancy interactions, are summarised. The relative importance of these mechanisms for different fracture modes and materials are discussed based on detailed fractographic observations and critical experiments.

Effect of twinning, slip, and inclusions on the fatigue anisotropy of extrusion-textured AZ61 magnesium alloy

Abstract: In this study, experiments were conducted to quantify structure-property relations with respect to fatigue of an extruded AZ61 magnesium alloy using a MultiStage Fatigue (MSF) model. Experiments were conducted in the extruded and transverse directions under low and high cycle strain control fatigue conditions. The cyclic behavior of this alloy displayed varying degrees of twinning and slip depending on the strain amplitude as observed in the hysteresis loops of both directions. Under low cyclic conditions, asymmetrical stress strain response was observed for both orientations. However, systematic stabilization of the hysteresis occurred by half-life due to subsequent twinning and detwinning mechanisms. In addition, under high cycle fatigue, pseudo-elasticity was observed at the first and at half-life cycles. Structure-property relations were quantified by examining the fracture surfaces of the fatigued specimens using a scanning electron microscope. In terms of crack incubation, fatigue cracks were found to initiate from intermetallic particles (inclusions) that were typically larger than the mean size. Quantified sources of fatigue crack incubation, microstructurally small cracks, and cyclic stress–strain behavior were correlated to the MSF model. Based on the specific material parameters, the MSF model was able to predict the difference in the strain-life results of the AZ61 magnesium alloy in the extruded and extruded transverse directions including the scatter of the experimental results. Finally, the MSF model revealed that the inclusion size was more important in determining the fatigue life than the anisotropic effects from the texture, yield, and work hardening.

Mechanistic and fractographic aspects of stress corrosion cracking

Abstract: Basic aspects of stress-corrosion cracking (SCC) in metallic materials are outlined, followed by a summary of the numerous mechanisms that have been proposed for SCC. The characteristics of transgranular and intergranular SCC in model systems, e.g. pure metal and single-phase alloy single crystals and bi-crystals under testing conditions that facilitate discrimination between mechanisms, are then described. The applicability of the various proposed mechanisms, such as those based on dissolution, hydrogen embrittlement, film-induced cleavage, and adsorption, are discussed in detail for these systems. Mechanisms of SCC in complex commercial alloys are then considered in the light of these studies on model systems.

Fracture Toughness Determinations by Means of Indentation Fracture

Abstract: The assessment of fracture toughness (KIC) on fragile materials such as ceramics or composites through conventional methods can be arduous. Recently, an alternative route referred to as the Indentation Fracture technique has been widely accepted with this purpose and extensively reported in literature (Weisbrod & Rittel, 2000; Plaza, 2003; Evans & Charles, 1976; Niihara et al., 1982). Different authors have derived math equations series as to fine tune and match with KIC determination; those equations are based in the lineal mechanical fracture theory (Wang, 1996). The indentation fracture method and its application procedure are described in this chapter, whereas typical problems involved in the test are shown. Al2O3-based composites with different reinforced metals fabricated by both; liquid and solid pressureless sintering of an intensive mechanical mixture of powders were used as studied materials. Ceramic materials have properties of great interest for various structural applications, specifically those that take advantage of their high hardness, chemical and thermal stability in addition to their high stiffness. However, their great fragility has severely limited their applications, although they have developed ceramic with reinforcement materials precisely to increase the toughness of the same (Miranda et al., 2006; Konopka & Szafran, 2006; Marci & Katarzyna, 2007; Travirskya et al., 2003; Sglavo, 1997). One of the macroscopic properties that characterize the fragility of a ceramic is the fracture toughness (KIC). The fracture toughness describes the ease with which propagates a crack or defect in a material. This property can be assessed through various methods such as: Analytical solution, solution by numerical methods (finite element, boundary integral, etc.). Experimental methods such as: complianza, fotoelasticity, strain gauge, etc. and indirect methods such as: propagation of fatigue cracks, indentation, fractography, etc. The choice of method for determining the fracture toughness depends on the availability of time, resources and level of precision required for the application. In practice, measurements of KIC require certain microstructural conditions on the material to allow propagation of cracks through it in a consistent manner. The strength of materials is governed by the known theory of Griffith, which relates the strength (S) with the size of the defect or crack (c) by S = YKIC/c1/2. This expression suggests the need to reduce the grain size and processing defects in the final microstructure to optimize the mechanical performance. Moreover, with increasing KIC, resistance becomes less dependent on the size of the defect, thereby producing a more tolerant material to cracking. Due to high elastic modulus and low values of KIC in brittle materials, achieving in them a stable crack growth is complicated and sometimes it is necessary sophisticated

Grain boundary microstructure and fatigue crack growth in Allvac 718Plus superalloy

Abstract: The correlation between grain boundary microstructure and fatigue crack growth with hold-times was investigated for two conditions of the superalloy Allvac 718Plus; a Standard condition with the recommended distribution of grain boundary phases and a Clean condition with virtually no grain boundary phases. Fatigue testing was performed at 704 degrees C using 10 Hz cyclic load with intermittent hold-times of 100 s at maximum tensile load. Microstructural characterization and fractography were conducted using scanning- and transmission electron microscopy techniques. Auger electron- and X-ray photoelectron spectroscopy techniques were used for oxide analyses on fracture surfaces. It was found that in the Standard condition crack growth is mostly transgranular for 10 Hz loading and intergranular for hold-times, while for the Clean condition crack growth is intergranular in both load modes. The lower hold-time crack growth rates in the Standard condition are attributed to grain boundary delta-phase precipitates. No effect of delta-phase was observed for 10 Hz cyclic loading crack growth rates. Two different types of oxides and oxide colours were found on the fracture surfaces in the Standard condition and could be correlated to the different loading modes. For cyclic loading a bright thin Cr-enriched oxide was dominate and for hold-times a dark and slightly thicker Nb-enriched oxide was dominant These oxide types could be related to the oxidation of delta-phase and the matrix respectively. The influence of delta-phase precipitates on crack propagation is discussed.

Hot ductility behavior of a low carbon advanced high strength steel (AHSS) microalloyed with boron

Abstract: The current study analyses the influence of boron addition on the hot ductility of a low carbon advanced high strength NiCrVCu steel. For this purpose hot tensile tests were carried out at different temperatures (650, 750, 800, 900 and 1000 °C) at a constant true strain rate of 0.001 s −1 . Experimental results showed a substantial improvement in hot ductility for the low carbon advanced high strength steel when microalloyed with boron compared with that without boron addition. Nevertheless, both steels showed poor ductility when tested at the lowest temperatures (650, 750 and 800 °C), and such behavior is associated to the precipitation of vanadium carbides/nitrides and inclusions, particularly MnS and CuS particles. The fracture mode of the low carbon advanced high strength steel microalloyed with boron seems to be more ductile than the steel without boron addition. Furthermore, the fracture surfaces of specimens tested at temperatures showing the highest ductility (900 and 1000 °C) indicate that the fracture mode is a result of ductile failure, while in the region of poor ductility the fracture mode is of the ductile–brittle type failure. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism which in turn causes a hot ductility loss. Likewise, dynamic recrystallization (DRX) which always results in restoration of ductility only occurs in the range from 900 to 1000 °C. Results are discussed in terms of boron segregation towards austenitic grain boundaries and second phase particles precipitation during plastic deformation and cooling.

Evaluation of interface microstructure and mechanical properties of the diffusion bonded joints of Ti–6Al–4V alloy to micro-duplex stainless steel

Abstract: In the present investigation, Ti–6Al–4V and micro-duplex stainless steel was diffusion bonded in vacuum. The layer wise σ phase and λ + FeTi phase mixture were observed at the bond interface when bonded joints was processed at and above 850 °C for 90 min and at 800 °C for 120 min and longer bonding time. Effect of bonding temperature and time on the strength properties at room temperature were evaluated. The maximum tensile strength of ∼510.1 MPa and shear strength of ∼397.5 MPa along with 6.5% elongation were obtained for the diffusion couple processed at 850 °C for 90 min. Fracture surface observation in SEM using EDS demonstrates that, failure takes place through λ + FeTi phase when bonding was processed at 850 °C for 90 min and at 800 °C for 120 min, however, failure takes place through σ phase for the diffusion couples processed at and above 900 °C for 90 min and at 800 °C for 120 min and longer bonding time.

Compressive fracture behavior of 3D needle-punched carbon/carbon composites

Abstract: Compressive fracture behavior under transverse and longitudinal compressive loading are determined for 3D needle-punched carbon/carbon (C/C) composites with single rough laminar (RL) pyrocarbon matrix or dual matrix of RL pyrocarbon and resin carbon. The results of Weibull statistics analysis indicate that scale parameter σ 0 of transverse and longitudinal compression of the composites with single matrix are 153.41 and 94.26 MPa, and σ 0 of the composites with dual matrix are 205.16 and 105.33 MPa, respectively. The mean compressive strength of both composites is nearly equal to σ 0 under each experimental condition. Failure modes of both composites under transverse and longitudinal compressive loading are shear and extension, respectively. Both composites exhibit quasi-ductile fracture behavior under transverse compression. Many small fragments of fibers and matrix carbon on the fracture surface of the composites are observed for single matrix composites. And the fiber bundle breakage with extensive debonding occurs for dual matrix composites. Under longitudinal loading, the composites with single matrix show quasi-ductile fracture behavior and delamination and splitting of non-woven long carbon fiber cloth layers are observed. The composites with dual matrix exhibit catastrophic failure behavior and crack runs through the composites along compressive loading direction.

Fatigue crack growth behavior of titanium foams for medical applications

Abstract: There is an urgent need to understand the failure behavior of titanium foams because of their promising application as load-bearing implant materials in biomedical applications. Following our recent study on fracture toughness of titanium foams [1] , this paper investigates the mode I fatigue crack propagation in 60% porous open pore titanium foams both with and without solid coated surface. Fatigue crack propagation tests were performed on compact tension specimens at load ratios of R = 0.1 and R = 0.5 and the fracture surfaces were examined using scanning electron microscopy. The crack growth rate, d a /d N , versus the stress intensity factor range, Δ K , curves were measured and compared using two different techniques; image processing and compliance methods. The crack extension rates were well described by Δ K , using the Paris-power law approach. Coated and non-coated titanium foams with 60% porosity had a significantly higher Paris exponent than solid titanium, which can be explained by crack closure and crack bridging. It was also shown that the fatigue crack grows along the centerline, following the weakest path throughout the foam. The results obtained from this work provide important information for evaluating the structural integrity of porous titanium components in the future biomedical applications.

Thermal Fatigue and Failure Analysis of SnAgCu Solder Alloys With Minor Pb Additions

Abstract: The thermal fatigue performances of Sn98.5Ag1.0Cu0.5 (SAC105), Sn97.5Ag2.0Cu0.5 (SAC205), Sn96.5Ag3.0Cu0.5 (SAC305) and Sn95.5Ag4.0Cu0.5 (SAC405) solder alloys with Pb terminations were investigated by accelerated temperature cycling with and without thermal preconditioning. The performance of the SAC alloys was compared to eutectic SnPb and aged SAC alloys. The test vehicle consists of commercial 2512 ceramic chip resistors soldered to printed wiring boards using the different solder alloy compositions. The solder joints were monitored continuously during a thermal cycle of 0°C -100°C with a ramp rate of 9°C/min and a 30 min dwell between temperature extremes. Failures were defined in accordance with the IPC-9701 A industry test guidelines and failure data are reported as characteristic life η (number of cycles to 63.2% failure) from a two-parameter Weibull distribution. The microstructural evolution was characterized using metallographic techniques and back-scattered scanning electron microscopy. The findings show that the lifetime of the alloys can be ranked as follows: SAC 305 ~ SAC 405 >; SAC 205 >; SAC 105 >; SAC305 aged >; SAC 105 aged >; SnPb and to determine mechanisms of failure, electron microscopy analysis and fractography were performed on post-cycled components.

Thermo-mechanical fatigue behavior of a single crystal nickel-based superalloy

Abstract: Thermo-mechanical fatigue (TMF) behavior in a oriented nickel-based single crystal superalloy was investigated under different cycles of strain and temperature. Fracture surface and microstructural evolution were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively. It was found that the fatigue lives under in-phase (IP) TMF were longer than those of out-of-phase (OP) TMF, and the maximum tensile stress level was concluded to be the lifetime-limiting factor. Compared to isothermal low-cycle fatigue (LCF) lives obtained under the maximum temperature 900 C, thermo-mechanical fatigue lifetime was much shorter. This result indicates that varying temperature superimposed mechanical strain greatly reduces the fatigue lifetime of superalloys. Based on observation of fracture surface and microstructure evolution, it was concluded that creep is the dominant damage mechanism under IP-IMF condition and oxidation causes shorter lifetime for OP-TMF tests. The similarities and differences in the changes of gamma' morphology during in-phase (IP) and out-of-phase (OP) TMF tests were also discussed. (C) 2011 Elsevier B.V. All rights reserved.