Microstructural characteristics of spray formed and heat treated Al–(Y, La)–Ni–Co system

• Al–(La, Y)–Ni–Co based alloys are spray formed to thickness 10–12 mm.
• XRD and DSC confirms the presence of large fraction of amorphous phase.
• Optical, SEM and TEM studies corroborated the observations made. Mechanism of microstructural evolution brought out.
• Heat treatment of spray deposited materials showed increased hardness which decreased at high temperature annealing.
• La containing system showed better thermal stability than that without La.

Recent studies on the synthesis of bulk Al–RE (Rare Earth)-TM (Transition Metal) based materials, from melt spun ribbons and gas atomized powders, have shown that partially amorphous or nano-crystalline structures lead to a high specific strength. In the present study, therefore, spray atomization and deposition process has been used to produce plates of Al85Y8Ni5Co2 (deposit D1) and Al83Y5La5Ni5Co2 (deposit D2) systems so as to synthesize bulk deposit of nano-crystalline and/or partially amorphous materials in a single step. The rapid solidification and high undercooling of droplets during atomization and the chilling effect on undercooled liquid upon deposition give rise to the above microstructural features. The microstructural features of deposits as well as overspray powders were studied using optical, scanning and transmission electron microscope. The alloys invariably showed a large fraction of nano-crystalline structure and amorphous features, characterized by featureless regions at optical resolution, along with distribution of primary equilibrium phases. The differential scanning calorimetric (DSC) analysis of the deposits showed similar crystallization features as observed during crystallization of fully amorphous melt spun ribbons of respective compositions. The transmission electron microscopy of deposit D1 showed the presence of 50–100 nm size fcc-Al precipitates in an amorphous matrix decorated with 5–20 nm fcc-Al crystallites. The annealing treatment of deposits at different temperatures, determined from the crystallization peak temperatures of the deposits, showed precipitation of nanoscale fcc-Al and intermetallic phases giving rise to a remarkable increase in hardness values. The bulk hardness of the deposits D1 and D2 was 391 and 427 HV, respectively. Whereas, the annealed deposits showed a bulk hardness value of 476 HV for deposit D1 and 583 HV for deposit D2 at annealing temperature of 298 and 380 °C, respectively. An attempt has been made to bring out the possible mechanism of microstructural evolution during spray deposition of the above alloy systems, and the effect of microstructural features on the hardness values has been discussed.