Rajendra U. Vaidya

Dynamic mechanical response and thermal expansion of ceramic particle reinforced aluminium 6061 matrix composites

Abstract The mechanical response of silicon carbide (SiC) particle and boron carbide (B4C) particle reinforced aluminium 6061 alloy was studied under quasi-static and dynamic loading conditions, using an Instron universal testing machine and split Hopkinson pressure bar respectively. The stiffness and yield strength of the B4C and SiC particle composites were significantly enhanced as compared to the unreinforced alloy. The strain hardening behaviour of the SiC particle reinforced composites was not significantly different as compared to that of the unreinforced alloy, at either low or high strain rates. On the other hand, the strength and strain hardening of the B4C particle reinforced composites was significantly greater than that of the unreinforced alloy, at both low and high strain rates. Differences in the mechanical behaviour of the two composites was attributed to the differences in the strength of the reinforcing particles and bonding between the B4C and SiC particles and the matrix alloy. Therma...

Effect of microbiologically influenced corrosion on the tensile stress-strain response of aluminum and alumina-particle reinforced aluminum composite

Abstract The tensile stress-strain responses of Al 6061 (UNS A96061) and alumina (Al2O3)-particle reinforced Al 6061 composite were investigated while exposed to a marine bacteria, Pseudomonas species strain NCMB 2021. Tensile responses of the unreinforced alloy and composite samples were altered by the microbial exposures. The strain to failure of the unreinforced alloy was lowered significantly as a result of microbiologically influenced corrosion (MIC). The effect of MIC on composite samples was more drastic. The ultimate tensile strength (UTS) and strain to failure were reduced significantly in the presence of the microbial species. Changes in the stress-strain responses of both materials were attributed to preferential formation of surface voids as a result of biofilm formation and the MIC process.

High-Temperature Oxidation of Ti-48Al-2Nb-2Cr and Ti-25Al-10Nb-3V-1Mo

The short-term oxidation behavior of aγ-TiAl alloy (Ti-48Al-2Nb-2Cr) was compared andcontrasted to that of anα2-Ti3Al base(Ti-25Al-19Nb-3V 1Mo) alloy. Oxidation ofTi-25Al-10Nb-3V-1Mo was found to occur at a moderate rate at 800°C, in aN2 + 20% O2 environment. A largeincrease in the oxidation rate occurred above thistemperature. This large weight increase was attributedto a breakdown in the protective oxide scale on the surface of theα2 intermetallic alloy, therebypermitting rapid diffusion of oxygen and nitrogen to thesurface of the intermetallic. The oxidation rate of thisalloy at 1200°C was not significantly higher thanthe oxidation rate at 1000°C. In contrast, theoxidation rate of Ti-48Al-2Nb-2Cr remained low up to1200°C. At this temperature, a significant increasein oxidation was observed and was attributed to acceleratedoxygen diffusion through the α2 phaseand increased solubility of oxygen in the gamma phase ofthe intermetallic microstructure. This weight increaseoccurred despite the fact that at 1200°C, theintegrity of the oxide layer formed on the surface ofthis alloy was maintained. The results of this studyillustrate the need for developing protectiveenvironmental coatings tailored to the individualintermetallic alloy.

A comparative study of the strain rate and temperature dependent compression behavior of Ti-46.5Al-3Nb-2Cr-0.2W and Ti-25Al-10Nb-3V-1Mo intermetallic alloys

Extensive research over the past two decades has led to the development of alloys such as Ti-46.5Al-3Nb-2Cr-0.2W ({gamma}-TiAl) and Ti-25Al-10Nb-3V-1Mo (super {alpha}{sup 2}-Ti{sub 3}Al), which offer a good balance of formability and high temperature strength. Although the mechanical behavior of these intermetallic alloys at low strain rates has been extensively studied, fewer studies have probed the systematic mechanical behavior of these alloys over a wide range of strain rates and temperatures. The purpose of this study is to compare and contrast the behavior of these two intermetallic alloys over a wide range of strain rates and temperatures. Variations in the mechanical responses of these two structurally different Ti-Al based intermetallics are documented.

Stacking faults in sic particles and their effect on the fracture behavior of a 15 Vol Pct SiC/6061-AI matrix composite

Mechanical tests and microstructural examinations performed on a SiC-particle-reinforced 6061-A1 matrix composite indicate that particle cracking during mechanical testing significantly affects the failure mechanisms of the composite. Microcracks were observed to nucleate and propagate on stacking faults and interfaces between the 4H (or 3C) and 6H hexagonal phases within the SiC particle reinforcements. These planar defects were the predominant defects seen in the SiC particles. Partial dislocations, having a 1/3 (10άrc10) Burgers vector, were also observed bounding the stacking faults in the reinforcement phase.

Effect of plasma-sprayed alumina on the strength, elastic modulus, and damping of Ti-25Al-10Nb-3V-1Mo intermetallic

The effect of a plasma-sprayed Al2O3 coating on the bend strength, elastic modulus, and damping of Ti-25Al-10Nb-3V-1Mo intermetallic substrate was measured. Two coating thicknesses of 0.1 and 1.0 mm were used in the study. The average strength and Weibull coefficients of the intermetallic samples coated with the 0.1 mm Al2O3 coating were very similar to those of the uncoated intermetallic samples. On the other hand, the average strength of the samples coated with 1.0 mm Al2O3 was significantly lower than the strength of the uncoated intermetallic substrate. The lower strength of the 1.0 mm coated samples was attributed to the higher volume fraction of the Al2O3 coating (which has a lower strength than the Ti-25Al-10Nb-3V-1Mo substrate) and higher porosity in the 1.0 mm coating. The Young’s modulus and damping values of the 0.1 mm Al2O3-coated intermetallics did not vary significantly from those of the uncoated substrate. However, the damping values of the 1.0 mm Al2O3-coated intermetallics were significantly larger than those of the uncoated substrate. The higher damping values measured for the 1.0 mm Al2O3-coated samples were attributed to the higher porosity in the thicker coating and to defects in the coating as a result of the spraying process.

Use of plasma spraying in the manufacture of continuously graded and layered/graded molybdenum disilicide/alumina composites

Plasma spraying was used to produce continuously graded and graded/layered structures of molybdenum disilicide (MoSi2) and alumina (Al2O3). These functionally graded materials (FGMs) were achieved by manipulating the powder hoppers and plasma torch translation via in-house created computer software. The resultant microstructures sprayed uniformly and were crack free. The interface between MoSi2 and Al2O3 was continuous and no evidence of debonding or cracking at the interface was found. The mechanical strength of these sprayed materials was evaluated using C-ring samples (in diametrical compression). Weibull analysis conducted on the C-ring data indicated that the continuously graded samples were slightly stronger and had a significantly narrower strength distribution than the graded/layered samples. Although the average strength values of both types of functionally graded samples were closer to those of monolithic MoSi2, the fracture energy of the graded samples was significantly larger (∼2–3 times) compared with the monolithic materials. Scanning electron microscopy (SEM) conducted on the fracture surfaces of the FGMs illustrated a wavy and tortuous crack path through the composite cross section of the sample, with extensive crack kinking. This study has two important results. First, we demonstrated the ability to produce such functionally graded composite ceramic microstructures using a conventional plasma spraying process. Second, we quantified the improvements in mechanical performance provided by these FGMs over conventional monolithic materials.

Effect of the matrix ageing conditions on the low strain rate response of a B4C particle reinforced 6061 aluminium matrix composite

Ceramic particle reinforced aluminium matrix-based composites have been studied extensively because of their technologically interesting properties. These include an enhancement in the strength, stiffness, wear resistance, and creep resistance, over unreinforced aluminium alloys. Ceramic particle reinforced aluminium composites have densities comparable to the aluminium alloys from which they are derived and hence are being pursued for use in a variety of engineering applications. The two most commonly used reinforcements for aluminium alloys are SiC and A1203. A number of studies have documented that improvements in the properties of the alloys can be achieved by incorporating relatively small volume fractions (1025 vol %) of SiC or A1203 into the aluminium alloys. Very few studies have been done on the use of BgC particles as reinforcements for conventional aluminium alloys. Boron carbide is an interesting candidate reinforcement material, and possesses physical and mechanical properties that are comparable with or better than those of SiC or A1203. A recent study [1] compared and contrasted the mechanical behaviour of a B4C particle reinforced 6061 aluminium matrix composite with that of a SiC particle reinforced 6061 aluminium matrix composite. Both composites studied were manufactured under identical conditions and incorporated identical volume fractions of the reinforcing particles. The compression strength of the B4C/6061 composite was significantly better than the strength of the SiC/ 6061 composite tested under identical conditions. It was also found that the tensile strength of the B4C particle composite was superior to that of the SiC particle composite. Scanning and transmission electron microscopy studies on these composites revealed that particle/ matrix debonding and particle cracking were the i~rimary reasons for the decreased strength of the SiC particle composites. No evidence of particle/ matrix debonding or particle fracture was found in the B4C/6061 composites. The difference in the interfacial particle/matrix strengths in the two composites was attributed to the differences in the particle/matrix wetting characteristics in the two composite systems [2, 3]. The lower strength of the SiC particles was primarily due to the presence of stacking faults within the ceramic particles themselves [4, 5]. In a previous study [1], matrix contributions to the composite strength were intentionally minimized. This was achieved by testing the unreinforced altoy

Evaluation of the elastic modulus and vibrational damping of Al2O3 coated Ti-25Al-10Nb after exposure to repeated thermal cycling

Ceramic coated intermetallic alloys offer good strength as well as resistance to high temperatures. The study reported here involved Ti-25Al-10Nb samples plasma-spray coated with A1203 for possible high temperature applications. Titanium alloys such as Ti-25Al-10Nb have replaced iron and nickel based alloys in many of these applications due to their low density, high strength and comparatively low cost [1]. However, at temperatures above 650 °C oxidation of the intermetallic is a serious problem. Alumina coatings can be used to enhance the materials tolerance to high temperatures by providing a protective layer. Alumina has been shown to be thermally, mechanically and chemically compatible with the Ti-25Al-10Nb substrate. The bonding mechanism between the Ti-25Al-10Nb and A1203 is purely mechanical. The A1203 coating is brittle but has an attractively high elastic modulus (which varies with density and purity). Elastic modulus and vibrational damping measurements are being considered as a measure of determining the mechanical performance of the alumina coated Ti-25Al-10Nb in high temperature applications. By examining the changes in the elastic modulus between thermal cycles, the tolerance of the material to high temperatures can be revealed. A change in elastic modulus could indicate changes at the coating-substrate interface. The interface between the intermetallic and the coating is vital, and is where spalling is most likely to occur. Furthermore, a significant decrease in the vibrational damping of the material would result in instability and failure in applications such as turbine blades. The Ti-25Al-10Nb used in this study was a polycrystalline material with a grain size of approximately 0.125 mm and had no significant texture. The crystal structure was DO19. The chemical composition is listed in Table I. Ti-25Al-10Nb has a density of 4.3 gcm -3, an elastic modulus of 125 GPa and a tensile strength between 800 and 1140 MPa [2]. Alumina has an

The effect of structural defects in SiC particles on the static & dynamic mechanical response of a 15 volume percent SiC/6061-Al matrix composite

Static and Dynamic mechanical tests, and microstructural examinations performed on a SiC particle reinforced 6061-Al matrix composite indicated that particle cracking significantly affected the strength, strain hardening, and failure mechanism of the composite. Cracks were observed to nucleate and propagate on stacking faults and interfaces between the various phases within the reinforcing SiC particles. Planar defects were the predominant artifacts seen in the SiC particles. Partial dislocations were also observed bounding the stacking faults within the reinforcement phase.