The trapping and diffusion behaviors of hydrogen in crystalline metals and amorphous alloy have been investigated by using the hydrogen thermal analysis technique.
In part one, the concept and existing theories of trapping of hydrogen in metals are introduced. Also the relations between the hydrogen trapping and embrittlement are examined. Theoretical backgrounds of the hydrogen thermal analysis method and the methods measuring the trapping parameters like trap activation energy and trap binding energy are explained.
In part two, the effects of the character of the precipitated TiC/matrix interface on the hydrogen trapping behavior in steel is studied. A model for hydrogen trapping of particle-matrix interface is suggested, in which the large driving force for such hydrogen trapping is associated with the lowering of interfacial free energy. Incoherent titanium carbides have a larger trap activation energy of hydrogen than semi-coherent ones. Although the particles are incoherent, the trap activation energy of hydrogen increases with increasing the precipitated particle size.
In part three, the trapping of hydrogen in thoria dispersed nickel has been investigated. The voids are formed at the particle-matrix interface during cold work. Annealing at high temperature removes this extra trapping sites, allowing an intrinsic trapping effect due to thoria particle matrix interface to be measured. The trap binding energy and trap activation energy of hydrogen at the interfaces of $ThO_2$/lattice are estimated as 33kJ/mol and 48.7kJ/mol, respectively, which indicates that trapping into thoria matrix interface is relatively easy.
In part four, the hydrogen trapping behavior by voids in nickel is studied. Voids in nickel act as strong trapping site for hydrogen. It appears that hydrogen in the void exists as a gas phase and chemisorbed state. However, under the condition investigated in this work, i.e. the temperature range of 623 K to 855 K and 1 atm $H_2$ pressure, most of hydrogens trapped in voids seems to be present as a chemisorbed state.
In part five, the hydrogen thermal analysis study in amorphous $Pd_{80}Si_{20}$ alloy provides phenomenological evidences for a distribution of hydrogen among sites of different energy caused by the disorder in an amorphous metal. As a consequence, at low temperature, the sites of low energy are filled first and then with increasing concentration, hydrogen occupies sites of higher energy. At higher temperatures of low concentrations, a distribution of hydrogen in sites becomes independent of concentration. The solubility of hydrogen is exothermic. The pressure-concentration isotherms show deviations from Sievert's law at temperatures between 303K and 473K for hydrogen pressures between 0.05 and 1 atm. This is attributed to the larger enthalpy of dissolution at higher concentrations, which is related to the distribution of hydrogen atmos in sites of different energies. The concentration dependence of hydrogen diffusivity is correlated with the change of site-occupancy with concentration.
In part six, the effect of structural relaxation on the hydrogen solubility in amorphous alloy $Pd_{80}Si_{20}$ has been investigated. The hydrogen solubility is measured by the hydrogen thermal analysis technique. The calorimetric measurements of relaxation phenomena are carried out using a Differential Scanning Calorimetry (Perkin Elmer DSC-4 unit). In an as-quenched sample, an exothermic effect is observed over a broad temperature range up to the glass transition, which becomes smaller or disappears with increasing the preannealing temperature, due to structural relaxation. The hydrogen solubility in as-quenched $Pd_{80}Si_{20}$ alloy decreases after annealing for structural relaxation, which is attributed to atomic rearrangements due to elimination and redistribution of excess free volume quenched into the glass. As a result of structural relaxation, the width of site energy distribution decreases compared to that of as-quenched state, which corresponds to the disappearance of the larger sites with high activation energy.
결정질 금속과 비정질 금속에 있어서 수소확산 및 수소 trapping 현상을 hydrogen thermal analysis 방법을 이용하여 연구하였다. 결정질 금속의 수소취성 현상은 재료내의 여러 구조적 결함에 의한 수소 trapping 현상과 밀접하게 관련되므로 이러한 수소 확산 및 수소 trapping 현상을 이해하는 것이 수소와 관련된 재료의 재성질 악화를 방지하고 개선시키는데 필수적인 것으로 될 것이다. 특히 비정질 금속에서의 수소 거동은 아직도 확실하게 이해되고 있지 않은 atomic structure에 대한 연구에 많은 발전을 가져다줄 수 있는 것으로 평가된다.
결정질 금속에 있어서 수소에 대한 strong trapping sites로 작용하는 비금속 개재물 및 석출물의 trapping nature는 각 particle/matrix계면의 계면 자유에너지와 관계된다는 trapping model이 제시되었다. thoria dispersed nickel에 대한 연구부문에서는 trapping에 의해 금속의 수소 취성이 억제 될 수 있음을 보여 주었다. BCC구조를 갖는 iron이나 steel에서와 마찬가지로 FCC구조를 갖는 nickel 에서도 void는 dislocation이나 grain boundary에 비해 trapping strength가 더 큰 것으로 나타난다.
비정질 금속(Pd80 Si20)에서의 수소 거동에 대한 연구 부문에서는 결정질 금속에서와는 달리 수소가 위치하게 되는 interstitial sites의 energy distribution이 존재한다는 이론적 model에 대한 실험적인 증거를 보여주었으며 이러한 sites에 대한 수소의 분포는 농도와 온도에 따라 변화한다. 즉, 농도가 작지 않고 온도가 낮은 경우에는 low energy sites에 수소가 우선적으로 채워지고 농도가 증가함에 따라 점차적으로 higher energy sites에 채워지는 Fermi -Dirac Statistics에 의해 수소가 분포되며 농도가 매우 작고 온도가 높은 경우에는 sites에 대한 수소의 분포는 농도에 무관한 maxwell-Boltzman Distribution에 의해 주어진다.
비정질 금속에 있어서 수소확산계수의 농도 의존성, Sievert's law로 부터 deviation을 나타내는 수소용해거동 및 structural relaxation에 따른 amorphous structure의 변화 등을 수소의 site-occupancy behavior에 의해 설명하였다. 수소의 분포가 Fermi-Dirac Statistics에 의해 주어질 때 비정질금속에서의 수소확산은 trap이 존재하는 결정질 금속과 유사하게 나타난다. 이 경우 비정질 금속에서 low energy sites는 결정질 금속에서의 trapping sites와 같이 작용한다.
