Thermochemical surface hardening of Ti-6Al-4V: On the role of temperature and treatment media

The present work addresses the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 on thermochemical surface treatment in different temperature regimes as well as in different gaseous and plasma-based media. Grades G5 and G23 were subjected to gaseous surface hardening using various gas compositions and temperature regimes. Two different series of gaseous surface treatments were carried out: 1) Carbo-oxidizing of G23 in mill-annealed condition was performed in CO gas at (high) temperatures ranging from 777 to 1027 degrees C. Treatment at 1027 degrees C resulted in the deepest case with the formation of a thin surface layer of titanium sub-oxides supported by a thick TiCxO1-x layer as the dominant phase. Post-nitriding of the carbo-oxidized specimen raised the hardness of the TiCxO1-x phase by the incorporation of nitrogen, yielding values up to similar to 2900 HV and multi-layered structures. 2) Carbo-oxidizing of G5 with a bi-modal microstructure in CO/CO2 gas mixture at (intermediate) temperatures in the range 677-777 degrees C. Treatment at 777 degrees C resulted in a surface hardness of similar to 1900 HV and a similar to 60 mu m-thick diffusion zone. In addition to gaseous surface hardening, also plasma-assisted methods were applied and compared to their gaseous counterparts. Intermediate- and low-temperature plasma treatments were carried out on G23: 3) Plasma (carbo-nitro)oxidizing was performed in a CO2 and N-2 gas mixture at 750 degrees C and (carbo-) oxidizing in CO2 gas at 650 degrees C. 4) Plasma nitriding was conducted in a N-2/H-2 gas mixture at 750 degrees C and 850 degrees C. The widely different types of treatments applied in this work represent the diversity of the gaseous and plasma-based thermochemical methods that can be applied for surface engineering of titanium alloys.

Automated summary

This study investigated the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 under different temperature and gaseous conditions. Various surface treatments were applied, resulting in different hardness values and structure formations. The diversity of treatments in this work represents the potential of gaseous and plasma-based thermochemical methods for surface engineering of titanium alloys.