Shaping of metals by thixoforming relies on the unusual flow behavior of semisolid slurries containing nondendritic solid phase. The microstructure of an alloy stirred during freezing consists of rounded particles of solid, as opposed to the dendrites associated with conventional solidification. In the semisolid state, these slurries are thixotropic, in that their apparent viscosity is dependent on shear rate and time. Here, a technique of rapid compression testing is outlined, carried out under conditions similar to normal industrial thixoforming, to assess slurry flow behavior and to examine the correlation between feedstock production routes, microstructure, and resistance to flow. Samples are heated to the desired temperature in the semisolid state with various soaking times and rammed at constant velocity against a platen backed by a load cell. The load-displacement curves produced from the tests may show an initial peak, believed to originate from a skeletal structure which rapidly breaks down under shear. The load signal during flow decreases with increasing soaking time and with temperature, and the initial peak eventually disappears in all alloys investigated. Quantitative metallography indicates that the lower loads correspond to greater spheroidicity of the solid particles within the slurry. The curves have been analyzed to derive the viscosity as a function of average shear rate and demonstrate that the semisolid slurries exhibit pseudoplastic flow behavior which is dependent on the compression velocity and is far removed from steady-state conditions.
Creep deformation and failure is one of the most critical life limiting factors of structural components used at elevated temperatures, such as in nuclear power plants. Understanding of the mechanisms of creep in nuclear power plant steels, such as Type 316H austenitic stainless steels, is still incomplete. It has been observed that long-term creep curves of initially solution-treated (ST) 316H stainless steels exhibit multiple secondary stages at the operational temperature and stress range. This paper probes the internal mechanisms for this complex phenomenon by correlating and quantifying the evolution of microstructural state (dislocations, precipitation and solid solution elements) and its mechanistic influence on the material's creep properties. This is examined for the first time by a multi-scale self-consistent crystal plasticity framework combined with a simple classical phase transformation model and thermal solute strengthening model. The novel integrated model is capable of describing a broad range of physical processes, including dislocation multiplication (hardening) and climb-controlled recovery, precipitation nucleation, growth and coarsening (Ostwald Ripening) and thermal solute dragging. The mechanisms responsible for the observed multiple secondary stages in the creep curves of initially solution-treated 316H stainless steels are explained through the strengthening and softening effects associated with these processes.
Current energy drivers are pushing research in power generation materials towards improved efficiency and improved environmental impact. In the context of new generation ultra-supercritical (USC) power plant , this is represented by increased efficiency, service temperature reaching 750 o C, pressures in the range of 35 -37.5 MPa and associated carbon capture technology. Ni base alloys are primary candidate materials for long term high temperature applications such as boilers. The transition from their current applications, which have required lower exposure times and milder corrosive environments, requires the investigation of their microstructural evolution as a function of thermo-mechanical treatment and simulated service conditions, coupled with modelling activities that are able to forecast such microstructural changes. The lack of widespread microstructural data in this context for most nickel base alloys makes this type of investigation necessary and novel. Alloy INCONEL 617 is one of the Ni-base candidate materials. The microstructures of four specimens of this material crept at temperatures in the 650 o C-750 o C range for up to 20000 h have been characterised and quantified. Grain structure, precipitate type and location, precipitate volume fraction, size and inter-particle spacing have been determined. The data obtained are used both as input for and validation of a microstructurally-based CDM model for forecasting creep properties.
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