The Application of Directional Solidification to a NASA Nickel-Base Alloy (TAZ-8B) 680449
A nickel-base alloy, TAZ-8B, has been developed which compares favorably in high temperature strength with known high strength nickel-base alloys. Although basically a cast material, the alloy also possesses workability potential.
By applying directional solidification techniques, test specimens were produced with a preferred columnar grain orientation. Grain boundaries transverse to the major stress axis were largely eliminated. Substantial increases in ductility, ultimate tensile strength, and stress-rupture life were obtained with the alloy in the directional polycrystalline condition as compared to the random polycrystalline condition. For example, the 1400 F ductility, which was 3% in the random polycrystalline form, was increased to 6% in the directional polycrystalline form. Ultimate tensile strength was similarly raised from 144,000 to 172,000 psi at this temperature. The 1000, 100, and 10 hr use temperatures at 15,-000 psi are 1785, 1925 and 2025 F in the conventional random polycrystalline form and 1830, 1940 and 2040 F in the directional polycrystalline form.
On the basis of calculated electron vacancy number, TAZ-8B would not be expected to form sigma phase. Only minor decreases in 1400 F ductility were observed after exposure for 1000 hr at 1600 F. These were 3.3-2.0% for the random polycrystalline material and 6.5-6.0%for the directional polycrystalline material.
The application of directional solidification to TAZ-8B has resulted in alloy properties which are of interest for potential advanced gas turbine engine applications.