Transformations of the Microstructure and Phase Compositions of Titanium Alloys during Ultrasonic Impact Treatment. Part I. Commercially Pure Titanium

Alexey Panin,Olga Perevalova,Marina Kazachenok,Anton Nikonov,Elena Sklyarova,Andrey Dmitriev

doi:10.3390/met11040562

Abstract

Experimental and theoretical studies helped to reveal patterns of surface roughening and the microstructure refinement in the surface layer of commercial pure titanium during ultrasonic impact treatment. Applying transmission electron microscopy technique, a gradient microstructure in the surface layer of the ultrasonically treated sample, where the grain size is varied from nano- to micrometers was revealed. It was shown that the surface plastic strains of the titanium sample proceeded according to the plastic ploughing mechanism, which was accompanied by dislocation sliding, twinning, and the transformations of the microstructure and phase composition. The molecular dynamics method was applied to demonstrate the mechanism of the phase transformations associated with the formation of stacking faults, as well as the reversible displacement of atoms from their sites in the hcp lattice, causing a change in coordination numbers. The role of the electronic subsystem in the development of the strain-induced phase transformations during ultrasonic impact treatment was discussed.

Highlights

Ultrasonic impact treatment (UIT) is an effective method of surface hardening that significantly improves the functional properties of structural materials, as well as their welded joints [1]
Since the plate movement was relative to the vibrated striker during the UIT processing, pile-ups of the displaced titanium were strained and re-strained again along the striker perimeter at each subsequent impact [8]
Dimensions of the pile-ups and the distances between them were determined by the UIT processing parameters and the commercially pure titanium (CP-Ti) mechanical properties

Summary

Introduction

Ultrasonic impact treatment (UIT) is an effective method of surface hardening that significantly improves the functional properties of structural materials, as well as their welded joints [1]. Surface processing with a high-strength striker vibrating at an ultrasonic frequency results in a notable change in the microstructure of metals and alloys due to the refining of existent grains and the formation of new ones. It has been shown that strains of the surface layer of titanium and its alloys (which are widely used in the aerospace, nuclear and other industries) are developed according to the plastic ploughing mechanism during repeated penetrations of a striker into the treated specimen [6,7,8] In this case, intense sliding and twinning of dislocations take place, causing the formation of sub-grains with low-angle misorientations, as well as their subsequent transformation into nanocrystalline grains with high-angle misorientations

Methods

Results

Conclusion