THE STUDY OF MICROPLASTICITY MECHANISM IN TI-50WT.PERCENT-NB ALLOY WITH HIGH HYDROGEN CONTENT

The upper yield point (approximate to 700 MPa) appears at the compress ion test curves (epsilon=0.024 sec(-1)) of b.c.c. Nb-50 wt.%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- a nd microplasticity of metallic materials the low frequency (similar to 2 Hz) amplitude dependent internal friction (ADIF) spectrum (gamma=1. .60 . 10(-5)) in hydrogenized Nb-50 wt.% Ti and Nb samples are studied . The ADIF investigation of the closed hysteresis loop ''loading-unloa ding'' shows the dependence of its width from the hydrogen content whi ch evidences the fact of dislocation unpinning from hydrogen atmospher es in the 1/2 cycle of loading. The study of ADIF spectrum for samples with different hydrogen content before and after torsion deformation (gamma approximate to 2%) shows the sharp increase of IF level at gamm a=1..10 . 10(-5) after approximate to 1 hour of natural ageing. At tha t time the ADIF curves change its shape from Gamma-shape to U-shape. T he 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 fr ee samples. The time estimation For the formation of thermodynamically stable hydrogen atmospheres on dislocations shows that hydrogen atmos pheres could not follow the dislocation during compressive tests and t hat leads to the upper yield point appearance.