Gradient Microstructure Induced by Surface Mechanical Attrition Treatment in Grade 2 Titanium Studied Using Positron Annihilation Spectroscopy and Complementary Methods.

Konrad Skowron,Léa Le Joncour,Mirosław Wróbel,Michał Mosiałek,Ewa Dryzek

doi:10.3390/ma14216347

Abstract

Microstructural changes in grade 2 titanium generated by surface mechanical attrition treatment (SMAT) were studied using positron annihilation lifetime spectroscopy and complementary methods. A significant increase in the mean positron lifetime indicated many lattice defects introduced by SMAT. Two positron lifetime components were resolved in the positron lifetime spectra measured. The longer lifetime revealed the presence of vacancy clusters containing about 3 or 4 vacancies, while the shorter one was attributed to the annihilation of positrons trapped at dislocations. The changes of the positron lifetime indicated a decreasing dislocation density and the presence of a deeper layer with a higher concentration of vacancy clusters at the distance from the treated surface for which the microhardness approached the value for the strain-free matrix. Electrochemical impedance spectroscopy showed the positive effect of SMAT on the corrosion resistance of the titanium studied in a saline environment also after removal of the original oxide layer that was formed during the SMAT.

Highlights

There is a growing interest in nanocrystalline (NC) materials [1], or gradient materials with greatly refined structures of a subsurface layer
It is equal to 1.65 μm and 3.07 μm for surface mechanical attrition treatment (SMAT)-1 and -2 specimens, respectively, while the roughness of the reference sample without treatment was close to 1 μm
The increase of the surface roughness is in agreement with the results reported by Zhu et al who studied the influence of process parameters of ultrasonic shot peening on surface roughness and hardness of cp titanium [43,44]

Summary

Introduction

There is a growing interest in nanocrystalline (NC) materials [1], or gradient materials with greatly refined structures of a subsurface layer. Methods based on severe plastic deformation (SPD) have become popular presently due to their simplicity and ease of applicability for plentiful classes of materials [2]. The application of these methods results in the creation of ultrafine-grained structure, as well as a great number of crystal lattice defects which introduction greatly changes the properties of the material. NC materials can be obtained only by methods that are based on non-homogeneous deformation with large strain gradients [3]

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Conclusion