Molybdeno-Aluminizing of Powder Metallurgy and Wrought Ti and Ti-6Al-4V alloys by Pack Cementation process

• Published in: Materials characterization 2016-08, Vol.118, p.494-504
• Main Authors: Tsipas, Sophia A., Gordo, Elena
• Format: Article
• Language: English
• Subjects: Alloys; ALUMINIUM ADDITIONS; ALUMINIUM NITRIDES; AMMONIUM CHLORIDES; CEMENTING; COATINGS; COMPARATIVE EVALUATIONS; LAYERS; MATERIALS SCIENCE; molybdenizing; Molybdenum; MOLYBDENUM ADDITIONS; OXIDATION; Oxidation resistance; Pack cementation; POWDER METALLURGY; POWDERS; SCANNING ELECTRON MICROSCOPY; Surface treatment; SURFACE TREATMENTS; SURFACES; TEMPERATURE RANGE 0400-1000 K; thermodynamic calculations; THERMODYNAMICS; Ti-6Al-4V; TITANIUM; TITANIUM BASE ALLOYS; TITANIUM NITRIDES; X-RAY DIFFRACTION
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2016.06.028

Wear and high temperature oxidation resistance of some titanium-based alloys needs to be enhanced, and this can be effectively accomplished by surface treatment. Molybdenizing is a surface treatment where molybdenum is introduced into the surface of titanium alloys causing the formation of wear-resistant surface layers containing molybdenum, while aluminizing of titanium-based alloys has been reported to improve their high temperature oxidation properties. Whereas pack cementation and other surface modification methods have been used for molybdenizing or aluminizing of wrought and/or cast pure titanium and titanium alloys, such surface treatments have not been reported on titanium alloys produced by powder metallurgy (PM). Also a critical understanding of the process parameters for simultaneous one step molybdeno-aluminizing of titanium alloys by pack cementation and the predominant mechanism for this process have not been reported. The current research work describes the surface modification of titanium and Ti-6Al-4V prepared by PM by molybdeno-aluminizing and analyzes thermodynamic aspects of the deposition process. Similar coatings are also deposited to wrought Ti-6Al-4V and compared. Characterization of the coatings was carried out using scanning electron microscopy and x-ray diffraction. For both titanium and Ti-6Al-4V, the use of a powder pack containing ammonium chloride as activator leads to the deposition of molybdenum and aluminium into the surface but also introduces nitrogen causing the formation of a thin titanium nitride layer. In addition, various titanium aluminides and mixed titanium aluminium nitrides are formed. The appropriate conditions for molybdeno-aluminizing as well as the phases expected to be formed were successfully determined by thermodynamic equilibrium calculations. •Simultaneous co-deposition of Mo-Al onto powder metallurgy and wrought Ti alloy•Thermodynamic calculations were used to optimize deposition conditions•External TiN and internal a Mo-rich layer on all alloy substrates•Titanium aluminides and Ti-Al mixed nitrides are formed on Ti-6Al-4V•The presence of Al and V alloying elements modifies the diffusion of Mo