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

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 depo-
sition process has been used to produce plates of Al 85 Y 8 Ni 5 Co 2 (deposit D1) and Al 83 Y 5 La 5 Ni 5 Co 2 (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 chill-
ing 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 inter-
metallic 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.