Showing papers on "Fractography published in 2015"

TiB2 reinforced aluminum based in situ composites fabricated by stir casting

Abstract: In this study, a new technique involving mechanical stirring at the salts/aluminum interface was developed to fabricate TiB 2 particulate reinforced aluminum based in situ composites with improved particle distribution. Processing parameters in terms of stirring intensity, stirring duration and stirring start time were optimized according to the microstructure and mechanical properties evaluation. The results show that, the first and last 15 min of the entire 60 min holding are of prime importance to the particle distribution of the final composites. When applying 180 rpm (revolutions per minute) stirring at the salts/aluminum interface in these two intervals, a more uniform microstructure can be achieved and the Al-4 wt% TiB 2 composite thus produced exhibits superior mechanical performance. Synchrotron radiation X-ray computed tomography (SR-CT) was used to give a full-scale imaging of the particle distribution. From the SR-CT results, the in situ Al– x TiB 2 composites ( x =1, 4 and 7, all in wt%) fabricated by the present technique are characterized by fine and clean TiB 2 particles distributed uniformly throughout the Al matrix. These composites not only have higher yield strength ( σ 0.2 ) and ultimate tensile strength (UTS), but also exhibit superior ductility, with respect to the Al–TiB 2 composites fabricated by the conventional process. The σ 0.2 and UTS of the Al–7TiB 2 composite in the present work, are 260% and 180% higher than those of the matrix. A combined mechanism was also presented to interpret the improvements in yield strength of the composites as influenced by their microstructures and processing history. The predicted values are in good agreement with the experimental results, strongly supporting the strengthening mechanism we proposed. Fractography reveals that the composites thus fabricated, follow ductile fracture mechanism in spite of the presence of stiff reinforcements.

Effect of alloying addition and microstructural parameters on mechanical properties of 93% tungsten heavy alloys

Abstract: Liquid phase sintering, heat treatment and swaging studies on three tungsten heavy alloys, 93W–4.9Ni–2.1Fe (wt%), 93W–4.2Ni–1.2Fe–1.6Co (wt%) and 93W–4.9Ni–1.9Fe–0.2Re (wt%) were carried out in detail with respect to microstructure, tensile and impact properties. All the alloys were sintered and swaged to 40% deformation. The results indicate that Re addition reduces the grain size of the alloy compared to W–Ni–Fe and W-Ni-Fe-Co alloys. W–Ni–Fe–Re alloy shows superior tensile properties in heat treated condition as compared to W–Ni–Fe and W–Ni–Fe–Co alloys. SEM study of fractured specimens clearly indicates that the failure in case of W–Ni–Fe–Re was due to transgranular cleavage of tungsten grains and W–W de-cohesion. W–Ni–Fe and W–Ni–Fe–Co alloys also failed by mixed mode failure. However, in these cases, ductile dimples corresponding the failure of the matrix phase was rarely seen. Thermo-mechanical processing resulted in significant changes in mechanical properties. While W–Ni–Fe–Re alloy showed the highest tensile strength (1380 MPa), W–Ni–Fe–Co exhibited the highest elongation (12%) to failure. A detailed analysis involving microstructure, mechanical properties and failure behavior was undertaken in order to understand the property trends.

Prediction of grain size and mechanical properties in friction stir welded pure copper joints using a thermal model

Abstract: In this study, a thermal model was developed and applied to simulate the friction stir welding of pure copper plates with the thickness of 2 mm. The different traverse speeds of 100, 200, 300, and 400 mm min−1 and rotational speeds of 400, 700, 900 rev min−1 were considered as welding parameters. Microstructural characterization, hardness measurement, tensile test, and fractography were conducted experimentally. The comparison between the numerical and experimental results showed that the developed model was practically accurate. In addition, the results confirmed that the peak temperature was the dominant factor controlling the grain size and mechanical properties, where the fine grains could be achieved at low rotational speed as well as high traverse speed. Consequently, lower peak temperature leads to the high ultimate tensile strength and hardness and the low elongation values.

Microstructure and Mechanical Properties of Inconel 718 Produced by SLM and Subsequent Heat Treatment

Abstract: The article presents results of selective laser melting of Inconel 718 superalloy. It was studied phase microstructure of the material obtained by selective laser melting and also the material after heat treatment. The phase composition of the initial powder material, the specimens after selective laser melting before and after heat treatment was studied. The effect of heat treatment on microstructure and mechanical properties of the specimens was shown. It was studied the mechanical behavior of the manufactured specimens before and after heat treatment at room and elevated temperatures as well. The results of impact tests and fractography of the specimens are presented. Mechanical tests showed that the specimens after heat treatment have decent mechanical properties comparable to hot-rolled material. Fractography showed that the obtained material is characterized by ductile failure mode with local elements of brittle fracture.

Microstructure and Mechanical Properties of Ti-6Al-4V Manufactured by SLM

Abstract: The article presents results of a study of phase composition and microstructure of initial material and samples obtained by selective laser melting of titanium-based alloy, as well as samples after heat treatment. The effect of heat treatment on microstructure and mechanical properties of specimens was shown. It was studied mechanical behavior of manufactured specimens before and after heat treatment at room and elevated temperatures as well. The heat treatment allows obtaining sufficient mechanical properties of material at room and elevated temperatures such as increase in ductility of material. The fractography of samples showed that they feature ductile fracture with brittle elements.

Fracture and fatigue behavior of WC-Co and WC-CoNi cemented carbides

Abstract: The fracture and fatigue characteristics of several cemented carbide grades are investigated as a function of their microstructure. In doing so, the influence of binder chemical nature and content (Co and 76 wt.% Co–24 wt.% Ni), as well as carbide grain size on hardness, flexural strength, fracture toughness and fatigue crack growth (FCG) behavior is evaluated. Mechanical testing is combined with a detailed inspection of crack–microstructure interaction, by means of scanning electron microscopy, in order to evaluate crack-path tortuosity and to discern fracture and fatigue micromechanisms within the metallic binder. Results show that CoNi-base hardmetals exhibit slightly lower hardness but higher toughness values than Co-base grades. Meanwhile, flexural strength is found to be rather independent of the binder chemical nature. Regarding FCG behavior, experimental results indicate that: (1) FCG threshold (Kth) values for coarse-grained grades are higher than those measured for the medium-grained ones; and (2) fatigue sensitivity levels exhibited by CoNi- and Co-base cemented carbides, for a given binder mean free path, are similar. These findings are rationalized on the basis of the increasing relevance of crack deflection mechanisms as microstructure gets coarser and the evidence of similar fatigue degradation phenomena within the binder independent of its chemical nature, respectively.

Effect of microstructure and defects on fatigue behaviour of directed energy deposited Ti-6Al-4V

Abstract: The fatigue behaviour of laser engineered net shaping (LENS) deposited Ti–6Al–4V is studied in the as built condition. The fatigue properties are measured, and the influence of microstructure and physical defects on fatigue performance is analysed through microscopy and fractography. Fine basketweave microstructure is obtained using a substrate with large equiaxed grains. The presence and location of unmelted particles are observed to significantly affect fatigue life. On the other hand, smaller gas porosity is not found to have major effect on the fatigue life. Further, the fatigue properties in a ‘repair’ condition are evaluated through samples consisting of LENS deposited parts joined to a preexisting wrought part. The properties are compared with the as built LENS parts.

Tensile Properties of Friction Stir Welds of AA 7020 Aluminum Alloy

Abstract: In this investigation, response surface methodology was used to predict and optimize the tensile properties of friction stir welded AA 7020 aluminum alloy. Tensile properties, microstructural features and fractography of the joints were measured and investigated using tensile test, optical and scanning electron microscopes, respectively. In addition, the influences of friction stir welding parameters on tensile properties of the joints were examined thoroughly. The results revealed that with increasing the heat input, the tensile strength of the joints increased up to a maximum value and then decreased, where the elongation of the joints increased continuously. Moreover, the optimal condition to obtain a maximum of tensile strength was 1,055 rpm, 97 mm/min and 7.4 kN, where as for tensile elongation was 1,320 rpm, 72 mm/min and 7 kN.

Tensile mechanical properties and fracture behavior of tungsten heavy alloys at 25–1100 °C

Abstract: The tensile mechanical properties and fracture behavior of three tungsten heavy alloys (fine-grained 93W–4.9Ni–2.1Fe–0.03Y; coarse-grained 93W–4.9Ni–2.1Fe; coarse-grained 95W–3.5Fe–1.5Ni, wt%) have been investigated in the temperature range from 25 to 1100 °C. The results show that ultimate tensile strength, yield strength, fracture strength and nominal total elongation of the three tungsten heavy alloys are strongly temperature-dependent and in most cases decrease with increasing temperature. Fractographic observations using scanning electron microscopy (SEM) show that the fracture modes of the three alloys evolve from a mixture of W cleavage and matrix phase ductile rupture at low temperatures to a mixture of tungsten/matrix and tungsten/tungsten interfacial debonding failure at elevated temperatures.

Synergistic toughening effects of dispersed components in PP/PA6/EPDM ternary blends; quantitative analysis of the fracture toughness via the essential work of fracture (EWF) methodology

Abstract: The present work reports and investigates the significant rigid-toughening, obtained by the incorporation of rigid polyamide 6 (PA6) phase into toughened polypropylene/ethylene–propylene–diene terpolymer (PP/EPDM) blend, to avoid substantial softening associated with the rubber-toughening. The effects of dispersed phase composition and compatibilization using PP-g-MA, on the microstructure, quasi-static fracture toughness, failure mechanisms, and tensile properties were investigated. The fracture properties were characterized in detail by the essential work of fracture (EWF) method. While addition of PA6 into neat PP reduced the fracture toughness, a remarkable monotonic increase in fracture toughness was observed upon the progressive replacement of EPDM with PA6 in PP/EPDM blends. A synergistic toughening effect of dispersed soft EPDM particles and rigid PA6 phase domains was observed, and compared to PP/EPDM binary blend the uncompatibilized ternary blends showed significantly higher fracture toughness values (we) at much lower rubber contents. The deformation micro-mechanisms and the critical role of rubber particles in achieving such a synergistic effect was highlighted and discussed by post-mortem fractography. Based on the finding that compatibilization reduced the fracture toughness of ternary blends, effective contribution of PP/PA6 interface in activation of different energy-absorbing micromechanical deformations was demonstrated. The energy partitioning approach was also employed to provide more insight into energies dissipated for yielding and subsequent tearing of the samples. The results of fracture analysis in conjunction with the tensile data showed a simultaneous toughening and stiffening effect achieved via incorporation of PA6 into PP/EPDM blends. This work could provide a new and deep understanding of rigid-toughening effect observed in multiphase systems.

High frequency fatigue test of IN 718 alloy – microstructure and fractography evaluation

Abstract: J. Belan, Faculty of Mechanical Engineering, University of Žilina, Žilina, Slovak Republic INCONEL alloy 718 is a high-strength; corrosion-resistant nickel chromium material used at -253 °C to 705 °C for production of heat resistant parts of aero jet engine mostly. The fatigue test provided on this kind materials were done via low frequency loading up to this time. Nowadays, needs of results at higher volume of loading cycles leads to high frequency loading with aim to shorten testing time. Fatigue test of experimental material was carried out at frequency 20 kHz with stress ration R = 1 (push – pull) at room temperature. It was found that this superalloy can still fracture after exceeding 108 cycles. Besides fatigue test were microstructural characterisation and scanning electron microscopy (SEM) fractography evaluation done.