The Study of Microplasticity Mechanism in Ti-50 wt.%Nb Alloy with High Hydrogen Content

Abstract
The upper yield point (≈700MPa) appears at the compression test curves (ε=0.024 sec^-1) of b.c.c. Nb-50wt.%Ti due to the increase of hydrogen content from 0 to 0.2 wt.% and more and leads to the non monotonous increase in compressive lower yield stress from 400 to 550 MPa. Taking into account close connection between macro- and microplasticity of metallic materials the low frequency (≈2Hz) amplitude dependent internal friction (ADIF) spectrum (γ=1 .. 60.10^-5) in hydrogenized Nb-50 wt.% Ti and Nb samples are studied. The ADIF investigation of the closed hysteresis loop "loading-unloading" shows the dependence of its width from the hydrogen content which evidences the fact of dislocation unpinning from hydrogen atmospheres in the 1/2 cycle of loading. The study of ADIF spectrum for samples with different hydrogen content before and after torsion deformation (γ≈2%) shows the sharp increase of IF level at γ=1 .. 10.10^-5 after ≈1 hour of natural ageing. At that time the ADIF curves change its shape from Γ-shape to U-shape. The amplitude range of the IF increase depends on the hydrogen content. It is the interaction of hydrogen atoms with dislocations that caused the above mentioned effect which has not been observed in hydrogen free samples. The time estimation for the formation of thermodynamically stable hydrogen atmospheres on dislocations shows that hydrogen atmospheres could not follow the dislocation during compressive tests and that leads to the upper yield point appearance.