Effect of Directional Solidification on the Structure and Properties of Ni3Al-Based Alloy Single Crystals Alloyed with Cr, Mo, W, Ti, Co, Re, and REM

The effect of the solidification rate (R = 2, 5, 10, 20 mm/min) at the same solidification gradient (G = 150 degrees C/cm) on the structural parameters of single-crystal blade workpieces made of an alloy based on the gamma'(Ni3Al) intermetallic compound and alloyed with cobalt and rhenium apart from chromium, molybdenum, titanium, and rare-earth metal microadditions is studied. The single crystals have a dendritic-cellular structure. Primary gamma'-phase precipitates are observed in the interdendritic space of heterophase gamma' + gamma dendrites. An increase in the solidification rate from 2 to 20 mm/min at a solidification gradient of 150 degrees C/min leads to refinement of all structural constituents by a factor of 1.5-2, with the morphology and the mutual position of the structural constituents being independent of the solidification rate. In experiments with moderate additional alloying with cobalt and rhenium, the yield strength increases by 10-20% and the long-term strength increases by at least 20-25% at a temperature of 900 and 1100 degrees C upon holding for 100 and 500 h. The VKNA-25 alloy single crystals have moderate plasticity (delta = 6-20%) over the entire temperature range (20-1200 degrees C) and have no sharp increase in the plasticity characteristic of a VKNA-1V alloy without cobalt and rhenium. During long-term tests, local raft structure regions misoriented with respect to the tension direction form in gamma' + gamma dendrites. gamma'-Phase nanoparticles precipitate in the gamma layers. During tests, refractorymetal-rich nanoparticles of a predominantly acicular-lamellar shape precipitate in dendrite arms.