It is essential for materials used at high-temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high-temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low-Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni-base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550 8C in the case of low-alloy steels, 750 8C in the case of an austenitic steel (TP347) and up to 1000 8C in the case of the Ni-base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low-Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short-circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr 2 O 3 with some FeCr 2 O 4 at localized sites, while for specimens with larger grain size a non-protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni-base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high-angle grain boundaries.

Polycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 mm s 21 under four-point bending conditions at 650uC in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be 'dynamic embrittlement', i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is particularly slow for special coincident site lattice (CSL) grain boundaries, bending and oxidation experiments were carried out using specimens that underwent successive steps of deformation and annealling, i.e. grain boundary engineering. It has been shown that an increase in the fraction of special CSL grain boundaries yields a higher resistance to both intercrystalline oxidation and hold time cracking by dynamic embrittlement.

No abstract

No abstract