Analyzing Method of Hydrogen Trapping Sites in Metal Crystals and the Relationship between Surface Defects and Hydrogen Absorption.

Transcription

1 Analyzing Method of Hydrogen Trapping Sites in Metal Crystals and the Relationship between Surface Defects and Hydrogen Absorption Gen Katano KEK, High Energy Accelerator Research Organization, Japan Introduction about researches of Hydrogen Embritllement Principal and Experimental procedure of Tritium Autoradiography Results of Autoradiography observation High-Strength Steel and Ni3Al Investigation of trapped hydrogen feature Summary

2 Four steps of investigation of the cause of Hydrogen Embrittlement Where are hydrogen paths? Hydrogen can enter into metal along tunnel structure at grain boundary, matrix or other routes. What role does hydrogen play at the cracking? What kinds of hydrogen trap sites exist? What is the hydrogen bonding energy? How to prevent hydrogen embrittlement? Improvement?

3 Formally structure in metal crystals Dislocation Grain boundary Interstitial atoms tunnel structure arrangement of atoms at cross section

4 Cracking mechanism supported by hydrogen 1 3 stress stress Hydrogen gathering 2 4 Slip face Other slip face is active. Cracking direction change. Slip face Makes atom dislocation.easier A slip face is active. Slip face One after another cracking direction change And cracking progress.

5 Approaches to solve the problem of Hydrogen Embrittlement Analysis of Hydrogen dynamics and bonding Energy to trapping sites Hydrogen location analysis Autoradiography Luminography Microprint Method Channelling Cracking Gathering Cracking process crack Observation Computer simulation Hydrogen Embrittlement has two stages

6 Similar process as photography Use tritium instead of Hydrogen Principles of Tritium Autoradiography Doping tritium to sample Make the photo emulsion layer on the specimen Tritium decays _ H He + e + ν β-rays hits photo emulsion 3 3 _ After development silver particles remain Observe silver particle using SEM(scanning electron microscopy) or TEM(transmission electron microscopy) Photo emulsion Exposed emulsion particle β -ray tritium Development Silver particle Collodion protection film

7 Typical photograph of the Tritium Autoradiography SEM Image Observation of Autoradiography on High Strength Steel

8 Experimental procedure of doping tritium into metal structure Electrochemical cathodic charging cathod DC power supply - + Gas bubble e e e e Copper wire sample (HHS or Ni3Al) Pt anode Current density 25A/m 2 Dorping time 2.0 h NaOH aqueous solution (0.10 kmol/m ) 15 Tritium decay rate Bq/m 3 3 _ e e e Oxygen Hydrogen Tritium + 2H 2e H2 + _ H + e H (in Metal) e

9 Observation of the Autoradiography by SEM Other types Grain boundaries SEM Image Tritium atoms entered into the metal along the grain boundaries and at several other types of entry paths.

10 SEM and TEM Electron beam Grain boundaries Electron beam Dislocations Other phases With SEM we can only get hydrogen distribution Information. This method have no information about internal structure. With TEM we can see not only silver particles but also metal structure.

11 Observation of the Autoradiography by TEM on HSSs Cementite Cementite Ferrite Cementite Ferrite Silver grain Ferrite TEM Image (photo) TEM Image (Enhanced) Tritium atoms entered the metal along the boundaries line of Sandwich structure. In shown photo, we can see that hydrogen enters HSS at boundaries and in Cementite structure.

12 Ferrite-ferrite boundaries trap many hydrogen atoms Boundary line Ferrite Silver particle

13 Structure of intermetallic compound Ni3Al Tetrahedral site Ni atoms Al atoms Octahedral site Volume large in comparison to Tetrahedral site L12 type crystal structure has two type interstitial sites.

14 Observation of trapped hydrogen distribution in Ni3Al Trapped tritium is uniformly distribution over the sample (not only at grain boundary). Grain boundary Silver particle

15 Hydrogen in Ni3Al dislocation Dislocation lines Silver particle Dislocation trap tritium!

16 Hydrogen in damaged structure Ni3Al structure was damaged by high energy (4MeV) Ni atoms. Damaged structure was doped by tritium and autoradiography was made. Damaged structure traps hydrogen well!

17 Prevention of Hydrogen embrittlement by baking. Same doping procedure as before but the HSS sample was left to anneal at 100 C for 1h, before making Autoradiography Ferrite-Cementite boundary sample One tritium remains Cementite Ferrite Cementite Cementite Ferrite Ferrite Cementite Ferrite Most of the hydrogen escaped from trap sites!

18 Anneal 100 C for 1h, before making Autoradiography Ferrite-ferrite boundary sample All tritium left the sample Only one silver particle cannot be seen in this area. Ferrite Silver particle

19 Summary of Autoradiography observation at High-Strength Steel We observed hydrogen atoms trapping. grain boundaries ferrite-cementite boundary cementite ferrite-ferrite boundary. Boundary structure traps many hydrogen atoms, but they leave after annealing at 100 C for 1h. Therefore bonding energy is low at these trap sites.

20 High-strength steel Ni3Al Hydrogen distribution Non-uniform uniform Hydrogen trap site grain boundaries boundaries of sandwich structure dislocations damaged structure bcc structure fcc structure

21 Two types trapping sites Low bonding energy trapping site High bonding energy trapping site 100 C C Typical peaks of hydrogen in thermal analysis by N.Suzuki etc.

22 Suggestion to Niobium or metals in general Grain boundaries are important to hydrogen path and trapping sites by bcc metals. Metal crystal have plural hydrogen trapping sites. Low energy bonding sites release hydrogen easily. Therefore these hydrogen have greatly effect to hydrogen gathering. Thank you very much for your kind attention.