T. Srinivasa Rao

Sliding Wear Behavior of Al-Si/Graphite Composite

In the present investigation, wear behavior of aluminum silicon alloy and its composites reinforced with graphite particulates (3 wt. %) fabricated using a stir casting route were examined. In order to improve the wettability of graphite with matrix alloy, graphite particulates were separately ball milled with 2% Cu and 2% Ni by weight percentage, preheated to 400°C, and incorporated into the molten metal. The uniform distribution of graphite particulates was observed in the microstructural investigation of developed composites, which resulted in more wear resistance than that of the matrix alloy. Scanning electron microscopic analysis of wear surface and wear debris supported the wear behavior of both the matrix alloy and the composites.

Structure – property correlation of precipitation hardened AZ91 magnesium alloy

Abstract The microstructure and mechanical properties of AZ91 magnesium alloy with composition Mg – 8.8%Al – 0.78%Zn (wt-%) has been investigated to assess the possibility of improving the properties through precipitation hardening. Initially, both solutionising temperature and soaking time were optimised at 693 K and 8 h respectively. After solutionising, the AZ91 samples were subjected to artificial aging at 423 K, 448 K, and 473 K for a period of 12 to 120 h. The sample age hardened at 473 K attained peak hardness after a shorter time than the other two aging temperatures. Structural changes and corresponding mechanical properties were studied. As cast, solutionised and age hardened AZ91 samples were also subjected to corrosion studies to observe potentiodynamic polarisation behaviour. Fine precipitate formation throughout the matrix was observed following solutionising and then artificial aging. Microstructural modifications improved both tensile strength and elongation, however, the corrosion rate was not significantly influenced.

Nanocrystalline and amorphous structure formation in Ti-Al system during high energy ball milling

Abstract The aim of the present work was to study the structural evolution of Ti–48Al (at.-%) powder blend during mechanical alloying. Because milling parameters play a vital role in achieving the desired structure/phase, milling was carried out with optimised parameters. Both powder handling and milling operations were performed under high pure argon atmosphere to prevent oxidation of the powder blend. Mechanically alloyed powder blend was then characterised by scanning electron microscope (SEM), X-ray diffraction (XRD) and differential thermal analyzer (DTA). Mechanical alloying induced severe plastic deformation resulting in cold welding, powder particle refinement and narrowing of powder particle size distribution as is evidenced from SEM micrographs. XRD analysis indicated complete dissolution of aluminium in titanium at 20 h of milling and achieving nanostructure before amorphous phase formation. DTA scan indicated the disappearance of one of the exothermic peaks, and a gradual drop in crystallisation temperature with increasing milling time.

Microstructure and mechanical properties of duplex stainless steels sintered in different atmospheres

Abstract Present investigation is aimed at developing duplex stainless steels through powder metallurgy route and study the effect of sintering atmospheres on density, mechanical properties and microstructures. Duplex stainless steel composition was prepared from the mixture of 316L and 430L alloyed powders. The powders were mixed in a pot mill for 12 h and compacted at a pressure of 560 MPa. The green compacts were sintered at 1350°C in four different atmospheres such as nitrogen, argon, hydrogen and partial vacuum. Sintered duplex stainless steels were subjected to density measurement, metallography examinations and tensile testing. Duplex stainless steel sintered in partial vacuum showed highest densification of 96% theoretical, tensile strength and ferrite content, when compared with stainless steels sintered in other three atmospheres. Microstructure of stainless steels sintered in argon, hydrogen and partial vacuum showed the bi-phase structure. SEM fractographs of the stainless steels sintered in partial vacuum and hydrogen revealed completely ductile mode of failure.

Fabrication of Al–Si/graphite composites and their structure–property correlation

In this study, mechanical and wear behaviors of aluminum silicon alloy and its composites reinforced with graphite (3 wt.%) particulates have been investigated. In order to improve the wettability of graphite with matrix alloy, graphite particulates were ball milled with Cu and Ni by weight percent (1% each) and preheated to 400°C and then incorporated into the molten metal. Tensile properties of composites were better than those of the matrix alloys. Wear rate of composites was lower than that of matrix alloys for given applied stress and varying sliding distance. The wear mechanism was understood from the scanning electron microscopic images of worn-out surfaces and wear debris.

Effect of Milling on Sintering Behavior of γ-TiAl by Spark Plasma Sintering

In this study, densification behavior of mechanically alloyed TiAl powders by spark plasma sintering has been analyzed. TiAl alloy with a composition of TiAl-xNb-1Cr-0.4Mo-0.1B (x = 0, 3.5, 6, 8.5) was prepared by mechanical milling for 20 hrs and was consolidated by spark plasma sintering. Average particle size of 20 hrs milled powders was 5 µm and morphology of these milled powders was flaky. Density of these sintered samples varied from 99.1% to 99.8% of the theoretical density. The addition of Nb increases the densification of TiAl intermetallic compounds. The increase in densification of these milled TiAl powders has been related to the effects of milling such as decrease in particle size and morphological changes.

Microstructural refinement and mechanical properties of direct extruded ZM21 magnesium alloys

Cast ZM21 magnesium alloys were subjected to symmetric extrusion at four different temperatures (200, 250, 300 and 350 °C) with three extrusion ratios of 4:1, 9:1 and 16:1, respectively. The effects of extrusion parameters such as temperature and extrusion ratio were studied by optical microscopy, X-ray diffraction (XRD) and tensile test. The optical micrographs exhibited various stages of recrystallization, i.e., partial to full recrystallization influencing mechanical properties to good extent. Higher extrusion temperature resulted in coarse grains, whereas finer grains were obtained at higher extrusion ratios. Ultimate tensile strength of this alloy was increased from 160 MPa to 316 MPa after extrusion at 250 °C with an extrusion ratio of 9:1.

Development of amorphous and TiAl-Nb2Al intermetallic nanocomposite powders by mechanical alloying

Present investigation was focused on to synthesize TiAl-Nb2Al nanocomposite powders by high energy ball milling from a mixture of prealloyed TiAl, niobium, aluminium and SiC powders. Systems chosen with different Nb and Al concentrations were processed at optimized ball milling parameters. The synthesized powders were characterized with the help of X-ray diffraction (XRD), electron microscopy, differential thermal analysis (DTA) to understand the milling behaviour of TiAl / TiAl-Nb-Al-SiC systems. High energy ball milling of prealloyed TiAl powder resulted nanocrystalline structure at early time intervals (10hrs) and sustained up to 50hrs. TiAl-Nb-Al-SiC systems exhibited amorphous structure in lower Nb content and formation of Nb2Al nanocrystalline compound with increasing Nb and Al additions. Stability of TiAl covalent bonded intermetallic compound was weakened by dissolution of Nb in the matrix and resulted amorphous structure. The final product contained nanocrystalline TiAl, amorphous structure and TiAl-Nb2Al intermetallic nanocomposite powders with varying Nb and Al concentrations.

Influence of Scandium on Magnesium and its Structure-Property Correlation

Magnesium alloys are the most demanded lightweight structural materials for different engineering applications such as aerospace, automobile, electronics, etc. However, the high temperature properties of Mg alloys are not comparable with its competitor, the Al alloys. Mg alloys are not recommended beyond 120°C due to their poor creep and oxidation resistance. In order to improve the high temperature properties of the magnesium alloys, rare earth containing Mg alloys were developed. Among these alloys, Mg-Sc alloys were found to be very interesting which exhibits better high temperature properties. In the present work, magnesium-scandium alloy was fabricated through liquid metallurgy route under inert cover. The alloy was characterized by optical microscopy, X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and hardness testing. The microstructural analysis reveals the α-Mg phase and the distribution of fine Mg-Sc intermetallic. It is observed from the DTA that the melting point of the base alloy has got enhanced by the addition of Sc. There is also an appreciable improvement in the hardness by the addition of Sc.

Structure-Property Correlation of Rolled ZM21/Al Magnesium Macrocomposite

Cast magnesium alloys are resulting coarse grained structure and micro porosity. In order to improve the formability and ductility in magnesium alloy, it is planned to incorporate aluminum wires as reinforcement through extrusion. In the present paper, ZM21/Al macrocomposites were fabricated by direct co-extrusion process. The extrusion was carried at 450°C with the extrusion ratio of 25:1. Metallurgical bonding of ZM21 magnesium alloy and aluminum was studied with the help of optical metallography and hardness testing. It was observed a white interface layer (intermetallic) formed in between magnesium alloy and aluminum. The same was confirmed by hardness test and SEM-EDS test. The co-extruded macrocomposite was again processed with unidirectional rolling and cross rolling to improve the bonding and grain refinement of the alloy. The composite was rolled at 250°C for 75% reduction in thickness. The microstructure and hardness of the rolled composite were characterized.