Phase equilibrium between bcc solid solution matrix and B2 aluminides in Nb-based alloy
- PDF / 905,650 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 6 Downloads / 190 Views
Phase equilibrium between bcc solid solution matrix and B2 aluminides in Nb-based alloy Takuya Yamanouchi*1; Seiji Miura1; 1
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University; Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Hokkaido, Japan *Graduate student, Graduate School of Engineering, Hokkaido University ABSTRACT B2 aluminides have a role of Al reservoir for Al2O3 surface and it is expected to increase oxidation resistance of (Nb,Mo)-bccss substrate. For the accumulation of the basic information to design the alloy composed of B2 coating on bccss matrix, bccss - B2 two-phase field was investigated in the Nb-Mo-Ni-Pd-Al system at 1273 K. It is found that Pd-rich B2-(Ni,Pd)Al phase is in equilibrium with Nb-rich bccss phase, while without Pd, the composition range of bccss coexisting with B2-NiAl phase is limited to be low Nb. INTRODUCTION Alloys based on refractory element, Nb, are expected to replace Ni based superalloy because they have advantages such as high melting point and low density. However, it is required to improve their oxidation resistance because pure Nb rapidly oxidizes in a high temperature atmosphere. Protective coatings for Nb alloys from oxidation are one of the solutions. NiAl has been applied as a coating on Ni-based superalloys because of their superior oxidation resistance[1-3]. Fukumoto et al. aimed to realize NiAl coating on Nb substrate by electrodeposition of Ni and Al but it was revealed that after oxidation test at 1273 K intermetallics are formed between the coating and the substrate, resulting in spallation of the coating from the substrate[4]. It is due to the fact that niobium solid solution, Nbss(bccss), phase and β-NiAl(B2) phase never coexist in equilibrium in Nb-Ni-Al ternary system[5]. As just described, to keep good bonding between the substrate and the coating, phase equilibrium between bccss phase and B2 aluminide phase is required. The coexistence relationships of Nbss phase with PdAl(B2) phase in Nb-Pd-Al system[6] and also Moss(bccss) phase with β-NiAl phase in Mo-Ni-Al system[7] have been reported. It is known that NiAl and PdAl show continuous solid solution by substituting Ni for Pd in B2 structure[8]. Therefore, by adding Mo and/or Pd to Nb-Ni-Al system, two-phase equilibrium between bccss phase and B2 phase is expected by replacing Nb to Mo in bccss phase and Ni to Pd in B2 phase. Miura et al.
has already reported phase equilibrium between B2-NiAl and (Nb,Mo)-bccss in the Nb-Mo-Ni-Al quaternary system with an atomic fraction of Mo in bccss of 0.59 or higher[9]. However, little information was found on the phase equilibrium in the Nb-Ni-Pd-Al quaternary system or quinary Nb-Mo-Ni-Pd-Al system and composition where bccss and B2 are in equilibrium is unknown. In this study, we intended to find composition range of bccss - B2 two phase field in the Nb-Mo-Ni-Pd-Al system. EXPERIMENTS Nb-Mo-Ni-Pd-Al alloy ingots were arc-melted in Ar atmosphere, then sealed in quartz tubes and subjected to heat treatment at 1273 K for 168 hours.
Data Loading...
