Effect of a lower oxygen level in chromium-nickel alloy doped with refractory metals on the mechanical properties and the microstructure

Abstract

VKh4Sh (Kh65NVFT) stands for a chromium-nickel alloy that is currently used to make combustion chambers for smaller spacecrafts. Due to a high level of oxygen, this alloy has low plasticity at room temperature, which affects its processability. At the same time, the alloy demonstrated high plasticity at a test temperature of 1,100 oC, which constrains its use as a refractory material. Some researchers attempted to make the alloy more heat-resistant by doping it with such refractory metals as Ta, Nb, Hf, Zr. But the chemical analysis showed no presence of Hf or Zr or they were found at minimum detectable concentration. Later, a process was described when different deoxidizers had been used for a VKh4Sh type alloy, and there are data that show that the concentration of oxygen decreased from 300–500 to 40–50 ppm while hafnium and zirconium were successfully absorbed. Therefore, the aim of this research is to understand how a lower level of oxygen and doping with refractory metals (Ta, Nb, Hf, Zr) change the mechanical properties and the microstructure of this heatresistant chromium-nickel alloy. The structure of the following chromiumnickel alloys was studied with the help of a scanning electron microscope and an attachment designed for energy dispersive microanalysis: Alloy I (base) produced using the conventional process without doping; Alloy II doped with refractory metals: 0.15 Ta – 0.15 Nb – 0.05 Hf – 0.05 Zr; Alloy III produced using a process that involves double vacuum remelting with magnesium deoxidation and doping with 0.15 Ta – 0.15 Nb – 0.1 Hf – 0.1 Zr. A series of mechanical tests was carried out with the alloys at the temperatures of 20, 900 and 1,100 oC. Through experiments, it was established that at 1,100 oC the tensile strength of Alloy III increased by 42% compared with Alloy I and by 20% compared with Alloy II, while the elongation of Alloy III dropped by 86% compared with Alloy I and by 25% compared with Alloy II. A microstructural study of the specimens after the tests showed an enhanced structural stability of Alloy III, which is associated with a lower coagulation rate of the phase particles.