Interaction between refractory crucible materials and the melted NiTi shape-memory alloy

Abstract

Attempts have been made to quantify the amount of contaminants absorbed by liquid metal from commercial ZrO 2 -, Al 2 O 3 -, and SiC-base crucibles used for vacuum melting of Ni-45 wt pct Ti alloy. The molten alloy was held under vacuum for 90 minutes at 1450 °C to become homogenized. Reactions between the liquid metal and the crucible were investigated by visual observation, chemical analysis, scanning electron microscopy (SEM) image processing, and X-ray mapping. The relative degree of contamination declined in the following sequence: commercially pure SiC Ͼ SiC-5 wt pct Al 2 O 3 -5 wt pct SiO 2 Ͼ slurry cast alumina Ͼ recrystallized alumina Ͼ zircon type A Ͼ oxygen deficient high-purity zirconia. Thermodynamic calculations showed a difference between the equilibrium and the experimental data, indicating that except for commercially pure SiC crucible, the amount of the crucible elements entering the melt is greater than the calculated equilibrium values. This discrepancy seems to be due to the immersion into the melt of the undissolved chemical compounds formed due to the reactions between the crucible and the liquid phase.

Alloy Design of Ti Alloys Using Ubiquitous Alloying Elements and Characteristics of Their Levitation-Melted Alloys

“…15) Commercial production process usually involves induction melting of alloys under heavy vacuum. A major source of contaminants is refractory melting crucibles, which needs to be carefully chosen.…”

“…19) In contrast, interaction between ZrO 2 -, Al 2 O 3 -or SiC-crucibles and molten titanium was investigated and discussed in the point of thermodynamic calculation. 15) Moreover, utilization of crucibles made of CaO is useful for melting Ti and its alloys, because of their refining effects such as de-oxidation, de-sulfurization and de-nitrification. 20) CaO crucibles are very expensive and the handling is difficult due to their hydration.…”

Effect of magnesium addition on the structural homogeneity of NiTi alloy produced by self-propagating high-temperature synthesis

“…Ni-Ti shape memory alloys are usually produced by melting metallurgy, which has a problem with high reactivity of Ni-Ti melt. Therefore it is necessary to use inert atmosphere (e.g., vacuum or argon atmosphere) and to choose a crucible material which cannot contaminate the Ni-Ti melt during Vacuum Induction Melting (VIM) [5][6][7]. Vacuum Arc Re-melting produces a melt with high purity, but melting must be repeated to get a homogeneous chemical composition of the product [8].…”

Formation of Ni–Ti intermetallics during reactive sintering at 500–650 °C

“…9,10 In the VIM of Ti-containing alloys there is a serious danger of a strong contamination of the melt due to the high reactivity of molten titanium. 11 The VAR technique makes it possible to prepare alloys of higher purity, but there is a problem with homogeneity. To obtain a sufficiently homogenous product, the VAR process has to be repeated even more than 4 times.…”