Mechanism Of Hydrogen Induced Delayed Crack In EBW Of Ti55 Alloy

Titanium alloys are widely used in the aerospace industry,and they occupy a more important place in the military aerospace region. However,hydrogen induced delayed cracks can be induced in the weld of some titanium alloys in certain welding condition. But the mechanism concerning hydrogen induced delayed cracks is not clear. So in the basis of the theory of fracture mechanics,the stress intensity factor for slow crack growth,Kth and the crack growth rate are studied on the pre-cracked compact tensile specimens(CT) of EBW of Ti55 which have different hydrogen contents in order to control the initiation and growth of the hydrogen-induced cracks effectively.In this paper,firstly,the rule of permeating hydrogen accorded with the exponent function [H]=79.91e-1.89/t by the chromatographic method for the determination of hydrogen content. Then pre-cracked CT specimens of EBW of Ti55 charged with hydrogen were strained in tension at room temperature and a crack-growth monitor was used to detect the earliest stage and extension of slow crack growth so as to confirm the Kth and da/dt. The microstructure of base metal,weld and its fractograph was observed and analyzed by means of the optical microscopy,scanning electron microscopy and X-ray diffraction. Meanwhile,the microstructure and type of hydride precipitation were examined by TEM.The results confirm that the solid solubility of hydrogen in Ti55 is 79.19ppm. At hydrogen contents below 79.19ppm,Kth decreases with increasing hydrogen,whereas da/dt increases as the hydrogen content is increased. At a certain hydrogen content,the variation in crack growth rate with applied stress intensity factor falls into three distinct regions:at the first stage,da/dt increases with increasing KI;at the second stage,da/dt has no connection with KI;at the last stage,da/dt increases rapidly as KI increases. With increasing hydrogen content,the fracture characteristics from dimple to quasi-cleavage. It is suggested that the mechanism of hydrogen induced delayed cracking is the diffusion of hydrogen upa stress gradient to form hydride precipitate at crack tip.