FEATURES OF STRUCTURE FORMATION IN NI-C, AL-C AND NI-AL-C SYSTEMS UNDER HIGH-TEMPERATURE HEATING

The high-temperature treatment of powder mixtures of Ni-C, Al-C and Ni-Al-C (5% wt C (soot)) has been carried out up to the melting temperature of the metals. It was found that melt particles coagulate with the formation of spherical particles of Ni, Al, and the intermetallic compound NiAl, respectively. Ni particles are characterized by the almost perfect spherical shape and a multilayer graphite coating. NiAl particles have a thin graphene (graphite) coating.

Effect of Ag on the Hot Corrosion of NiAl Intermetallic Compound

The low ambient tensile ductility and inadequate high temperature strength and creep resistance have limited the applications of intermetallic compound NiAl as a structure material at high temperatures. A small addition of Ag could increase the strength of NiAl. In this study, hot corrosion behavior of NiAl and NiAl-Ag containing 1 and 5%Ag (in weight percent), respectively, in molten (Na,K)2SO4 at 1173 K has been examined by thermogravimetry and physical analysis techniques. The experimental results indicated that the corrosion rates of NiAl and NiAl-Ag tended to increase with exposure time, accompanied with the formation of nodules. Fast localized corrosion is the typical feature of the corrosion of both NiAl and NiAl-Ag. The addition of Ag to NiAl accelerated its corrosion, with a higher corrosion rate observed for the 5%Ag addition. The two-phase microstructure of NiAl-Ag alloys accelerated the formation of nodules in the regions close to Ag phases.

Studies of the structure and the atomic diffusion properties of the Σ = 5 [001] twist GB in B2-type intermetallic compound NiAl

The structural properties, the formation energies and the atoms’ diffusion behaviors by vacancy mechanism near the Σ = 5 [001] twist GB of the B2-type intermetallic compound NiAl have been investigated by using the modified analytical embedded-atom method and a molecular dynamics simulation. Both the largest displacement and rippling effect occur at the first layer near the GB. The Ni vacancies at uncoincident sites are most easily formed on the first and second layers of the Ni- and Al-terminations, respectively. Furthermore, the Ni vacancy at an uncoincident site on the second layer of the Al-termination tends to migrate to the coincident Ni site of the first layer of the Ni-termination along a six-step jump path. The Ni vacancies at either the coincident or uncoincident site of the first layer tend to migrate in the first layer and finally return to their original site. Therefore, there is a collective tendency for the Ni vacancies to appear in the GB without local disorder.