Untersuchung des Wasserstoffeinflusses auf die Nukleation und Bewegung von Versetzungen mit einem elektrochemischen nanoindentierenden AFM
Final Report Abstract
During the first phase of this project the in-situ nanoindentation technique was used to investigate the hydrogen effect on nano-mechanical behavior (bulk properties) of different alloys. Our results show that the hydrogen decreases the pop-in load by reducing the activation energy for dislocation nucleation. However, the activation energy for dislocation nucleation is related to different items like the shear modulus μ and dislocation core radius, which may alter (in different orders) by hydrogen charging. Therefore, the main objective of this work was studding quantitatively the effect of hydrogen on each of the mentioned parameters. As a case study we selected different iron aluminum intermetallics; because they attract a huge interest for substitution with the routine stainless steels. The iron aluminides have much higher yield stress for a wide range of temperatures and the cost of production of alloys is much less for such a low-density material. Furthermore, iron aluminides allow for the conservation of less accessible elements such as nickel and molybdenum. However, the reaction of reactive Al atoms with water results in hydrogen atoms which are responsible for the ductility of Fe-Al based intermetallic alloys, especially those with higher concentrations of Al. Therefore, one main objective of this work is a more in-depth analysis of the effect of different ternary alloying elements like (Cr) on the embrittlement of alloys in various environments. The results of nanoindentation in air show the influence of Cr on various mechanical properties like Young’s modulus, Gibbs free energy needs for homogeneous dislocation nucleation (HDN) and hardness (Paper B and C). The increase of the Young’s modulus (in agreement with the results of ex-situ bending tests) and Gibbs free energy after addition of Cr is due to the enhancement of interatomic bonds. Additionally, the bending test of micro pillars was utilized to locally examine hydrogen embrittlement of iron aluminides. Conventional mechanical tests are costly, time consuming, and due to their large scale, not very successful in obtaining mechanistic information. Therefore, we used the technique of bending test of micro pillars, due to its comprehensive range of possibilities, to achieve an essential understanding about the influence of hydrogen on the mechanical properties in various intermetallics. Our experimental results show that hydrogen decreases the Young´s modulus in both samples but this reduction occurs more in the binary alloys. It is due to the weakening effect of hydrogen on the interatomic bonds in agreement with the HEDE model. Additionally the flow stress of the alloys, especially in the binary samples, was seen to decrease with hydrogen charging, which may be due to the effect of hydrogen on the reduction of the line energy of dislocation, which causes reduction of the energy needed for dislocation nucleation and additionally due to the shielding effect of hydrogen in agreement to the HELP theory. However, the advantage of the in-situ bending test is its ability for ranking the sensitivity of different alloys to HE based on the influence of hydrogen on the elastic and/or plastic properties. This information could help us to evaluate the most probable mechanism of HE for each alloy.
Publications
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Direct observation of hydrogen-enhanced plasticity in super duplex stainless steel by means of in situ electrochemical methods, Scripta Materialia 62 (2010), 242-245
Afrooz Barnoush, Mohammad Zamanzade, Horst Vehoff
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Effect of substitutional solid solution on dislocation nucleation in Fe3Al intermetallic alloys, Philosophical Magazine 92 (2012), 3257-3268
Afrooz Barnoush, Mohammad Zamanzade
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Effect of chromium on elastic and plastic deformation of Fe3Al intermetallics, Intermetallics 41(2013), 28-34
Mohammd Zamanzade, Horst Vehoff, Afrooz Barnoush
