High resolution NanoSIMS imaging of deuterium distributions in 316 stainless steel specimens after fatigue testing in high pressure deuterium environment

Abstract

It is irrefutable that the presence of hydrogen reduces the mechanical performance of many metals and alloys used for structural components. Several mechanisms of hydrogen-assisted cracking (HAC) of steels have been postulated. The direct evidence of the mechanisms by which hydrogen embrittles these materials has remained elusive. This is by virtue of our difficulty to directly observe the hydrogen distribution at spatial resolutions less than 100 nm and analysis volumes greater than 1 × 109 atoms at microstructural features such as grain boundaries, dislocations, twins, stacking faults and sub-micron inclusions that are all potential hydrogen trapping sites postulated to be responsible for the degradation of mechanical performance. Here, we report on an experimental methodology combining an elaborate fatigue testing protocol in an enriched gaseous deuterium environment with NanoSIMS (secondary ion mass spectrometry) imaging for detection of deuterium at spatial resolutions as low as 100 nm and accompanying TEM analysis. Type 316 stainless steel compact tension specimens were precharged in deuterium followed by fatigue testing at high stress ratio (0.7), low delta K (~11 MPa √m), and a frequency of 1 cycle per minute using a sawtooth waveform with a rise time of 30 s in high pressure (68.9 MPa) gaseous deuterium (99.999% purity) environment at room temperature. High resolution NanoSIMS imaging was then used to measure the deuterium distribution at the tip of and in the wake of secondary and tertiary fatigue cracks as well as at MnS inclusions. The use of deuterium eliminates the difficulties of interpreting hydrogen measurements by SIMS relating to the ubiquitous presence of hydrogen in all high vacuum systems and guarantees that deuterium measured by the NanoSIMS must be attributed to the fatigue testing protocol. This methodology has allowed us to directly observe the distribution of hydrogen in dislocation tangles ahead and in the wake of fatigue crack tips and at the interface of MnS inclusions. The protocol provides an avenue by which the path and speed with which hydrogen proceeds along its embrittling course of action may be directly followed through modifications of the fatigue testing parameters and/or alloy type and allows a means to validate at least qualitatively recently published models of enhanced hydrogen transport by dislocations.