V.C. Srivastava

Synthesis of single phase i-AlCuFe bulk quasicrystal by spray forming

Abstract In the present study, an icosahedral single phase bulk quasicrystalline material based on Al62Cu25.5Fe12.5 has been synthesised by a spray forming route. Microstructural characterization showed an average grain size of 10 μm. The oversprayed fine powder showed the presence of β- and λ-phases, whereas, the deposit consited of the fully single phase bulk quasicrystalline material with compositional homogeneity. The hardness and fracture toughness measurements were carried out at different indentation loads of 50–500 g. The hardness values varied in the range 10.4–8.1 GPa and fracture toughness was seen to decrease with increasing load. The varionan of hardness with load, which is known as indentation size effect (ISE), has been established clearly. Fracture toughness value was constant in the load range from 200 to 500 g at 1.2 MPa m1/2. The cracking pattern after indentation at higher load has been observed to be intergranular as well as transgranular. The evolution of the single phase bulk quasicrystalline material has been discussed in light of the unique combination of atomization and deposition process elements in spray forming technique.

Grain size softening effect in Al62.5Cu25Fe12.5 nanoquasicrystals

Inverse Hall-Petch (IHP) behavior in nano-quasicrystalline Al62.5Cu25Fe12.5 is reported. Powders with varying grain sizes were produced by mechanical milling of spray-formed quasicrystals. The hardness of the milled powders increased with decreasing grain size down to about 40 nm and decreased with further refinement, demonstrating the IHP behavior. This critical grain size was found to be larger compared to other metallic nanocrystalline alloys. This IHP behaviour has been attributed to the structural complexity in quasicrystals and to thermally activated shearing events of atoms at the grain boundaries.

Cooling conditions for the generation of bulk metallic glasses by droplet deposition

Abstract The cooling rate during material processing until glass transition rate is the key parameter for the production of bulk metallic glasses. But in the past, little attention has been paid to advanced production techniques such as deposition of molten metal sprays or spray forming, which offer elevated cooling rates. In this work, cooling conditions during spray forming were investigated due to its utmost importance for producing amorphous structures. Spray forming is treated in this work as a three step cooling process consisting of droplet flight phase, splat phase and deposit phase. All cooling steps were simulated for different droplet sizes. The surface temperature of the deposit was found to play an important role in the production of metallic glasses via spray forming. The simulation model can be used to find suitable spray conditions for the generation of bulk metallic glasses.

Production of high-strength Al85Y8Ni5Co2 bulk alloy by spark plasma sintering

Highly dense bulk samples were produced by spark plasma sintering (SPS) through combined devitrification and consolidation of partially amorphous Al85Y8Ni5Co2 gas atomized powders. The microstructure of the consolidated samples shows a mixed structure containing crystalline, ultrafine-grained and amorphous/nanocrystalline particles. The sintered sample exhibits a remarkable high strength of about 1050 MPa combined with 3.7 % fracture strain.

Characterization of Sn-reinforced Al–Cu–Fe quasicrystalline matrix nanocomposite

Quasicrystals are a relatively new class of materials, which belong to a family of aperiodic intermetallics, and exhibit high hardness, excellent wear resistance, low coefficient of friction, low thermal and electrical conductivity accompanied with good corrosion resistance. However, these materials are inherently brittle and show low fracture toughness [1]. It has been observed that the presence of soft phases overcomes this limitation of quasicrystals [2]. In the present investigation, Al62.5Cu25Fe12.5 quasicrystalline material was prepared by induction melting followed by heat treatment. This quasicrystalline material was reinforced with 10, 20 and 30 vol% of Sn through mechanical alloying using high energy planetary ball mill. The milling was carried out in a PM 400 Retsch ball mill at a speed of 200 rpm up to 40 h with a ball to powder ratio of 10:1 with toluene as process control reagent. The nanocomposite (NC) powders were characterised by electron microscopy and X – Ray diffraction techniques. The mechanical properties of these NC powders were evaluated by microindentation of individual NC powder particles. The hardness of NC was found to decrease with increasing volume fraction of Sn. Inverse Hall – Petch relationship was also evident for mechanically alloyed NC powder. Further, these milled NC powders were sintered and annealed thereafter. The consolidated pellets were characterised by optical microscopy, scanning and transmission electron microscopy, x-ray diffraction. The mechanical properties of sintered samples were evaluated through micro-indentation. The hardness and fracture toughness of as-annealed Al62.5Cu25Fe12.5 quasicrystalline matrix were found to be 10.87 GPa and 1.58 ± 0.27 MPa.√m respectively. The hardness of the Sn reinforced Al62.5Cu25Fe12.5 quasicrystalline matrix composite decreased with increasing volume fraction of Sn. The fracture toughness was found to increase appreciably by 22 pct. The increase in fracture toughness was attributed to inhibition of crack propagation by Sn particles. The present study provides an insight into the mechanisms of phase and microstructural evolutions during MA of the Sn reinforced Al62.5Cu25Fe12.5 quasicrystalline nanocomposite and their consequent effects on mechanical properties. It is anticipated that the interface will be highly coherent in this class of intermetallics based composites. Attempts are made to discuss the stability of microstructure and the prospects of Sn reinforcement nanocomposite in order to improve the fracture toughness. [1] Saarivirta, E. H. (2004). J. Alloys Comp., 363, 150 – 174. [2] Srivastava, V. C. et al. (2014). J. Alloys Comp., 597, 258 – 268.