Effect of ultrasonic assisted-ECAP processing on the microstructure, mechanical properties, and fluoride-induced corrosion performance of pure titanium

The influence of the ultrasonic assisted equal channel angular pressing (UA-ECAP) process on the microstructural characteristics, mechanical properties, and fluoride-induced corrosion behavior of grade 2 commercially pure titanium (CP-Ti) is investigated. The UA-ECAP process, particularly in the first pass, induces the formation of twins and slip bands. Continued slip band activity leads to non-equilibrium structures and ultrafine grains in subsequent passes. Mechanical properties analyses show hardness progression from 147 HV (annealed state) to 233 HV (3-pass UA-ECAPed) and yield strength increase from 465 MPa to 765 MPa with elongation reduction from 24.6 % to 16.6 %. Fluoride-induced corrosion of various samples in 1 wt% HF solution is investigated through electrochemical and immersion tests. After 28 days of immersion, the 3-pass UA-ECAPed sample exhibits a corrosion rate around half that of the annealed one and lower than other UA-ECAPed samples. Microscopic observations of corroded samples reveal the 3-pass UA-ECAPed sample's film is denser and more uniform, as compared with the other samples. Glow discharge optical emission spectroscopy (GDOES) confirms the corrosion films on all samples consist of an outer region rich in oxygen/fluorine and an inner region rich in oxygen; however, the film exists on the 3-pass UA-ECAPed sample exhibits over twice thicker than other samples and with higher contribution of oxygen in it as well. A proposed physical model is provided to explain the 3-pass ECAP sample's enhanced corrosion resistance.