Hydrogen Embrittlement Of TC4 Alloy Treated By Ultrasonic Surface Rolling Based On The Small Punch Test

TC4 titanium alloy is widely used in aerospace and Marine engineering because of the good performance on fatigue resistance and corrosion resistance.whereas,early studies have shown that obvious hydrogen adsorption occurs on TC4 alloy in hydrogen-rich environment,which resulting in hydrogen embrittlement(HE)and reduction of plasticity and toughness.Therefore,it's urgently to develop TC4 HE resistance technology to improve its' service life in hydrogen-rich environment such as deep sea.According to previous research results,ultrasonic rolling technology can improve the surface quality of parts effectively.Some scholars have applied this technology on some metals and observed obvious improvement of mechanical properties,but there has been no report on the effect of ultrasonic rolling treatment to HE resistance of TC4.In order to study the effects of different ultrasonic roll times on the HE resistance of TC4,the surface properties and microstructure of TC4 were disccussed through a series of characterization.Then online hydrogen charging small punch test(SPT)and online tensile test were conducted to obtain the load-displacement curve and stress-strain curve,TC4 tensile properties empirical equations were calculated according to the the fitting of the two related curve data.Then the mechanical properties of different rolling times TC4 before and after hydrogen charging were obtained by the empirical equations.Finally,the influence law and mechanism of the ultrasonic rolls times on the HE behavior of TC4 were analyzed in combination with the changes of microcosmic morphology,mechanical properties and fracture morphology.The main research contents are as follows:(1)Small punch clamp and online electrolytic hydrogen charging device were designed and manufactured,which avoiding the possible hydrogen escape during pre-charging hydrogen process,and guaranteeing the accurate and reasonable of experimental data,providing a reference for the detection of non-destructive HE of the workpiece in service,which owns a broad application prospect.(2)The surface properties of different ultrasonic roll times(0,5,10,15)TC4were studied.The microstructure changes of TC4 before and after hydrogen charging were analyzed by combining metallographic diagram,XRD composition analysis and surface potential difference diagram of atomic force microscope.We found that ultrasonic rolling can increase the residual compressive stress and densify of TC4 surface,which eliminate the formation and extension of microcracks,and make the penetration of hydrogen atoms become difficult.However,excessive rolling timeswill cause exfoliation and damage of surface,forming hydrogen trap.In addition,during hydrogen charging process,we found that hydrogen first reach the upper limit of solubility in priority ? phase and reacts with it,then the hydride TiH and delta TiH2 gonna precipitation in two phase boundary.(3)The empirical evaluation of TC4 yield strength,tensile strength and elongation were obtained through the SPT load-displacement curve and slow strain rate tensile test stress-strain curve.Finally,the mechanical properties of TC4 were calculated by the empirical correlation formula obtained.(4)Based on the fitting data of the tensile properties and fracture morphology analysis,the influence law and mechanism of different ultrasonic rolling times on the HE of TC4 were studied.The results shows that the HE sensitivity and tensile mechanical properties of TC4 first increase then decrease with the increase of rolling times,and TC4 without rolling present a brittle fracture morphology with obvious rifting edges,the TC4 after rolling present mixed fracture morphology with micro-dimples in near-rolling layer and rifting edges in matrix layer.It's obvious that the residual stress enhancement,surface dislocation proliferation and grain refinement induced by ultrasonic rolling can hinder the entry and diffusion of hydrogen,which improve the solubility of hydrogen and raise the generation threshold of hydride.When rolling 15 times,obvious spherical inclusions were observed in the surface layer.It can be attributed to severe plastic deformation under too many ultrasonic rolling times,which cause the formation of hydrogen traps.As a result,hydrogen atoms accumulated in the hydrogen traps and lead to the formation of brittle hydrid,resulting the deduction of TC4 resistance to HE.There are 32 figures,7 tables and 82 references in this paper.