Directionally Solidified Materials

It is a well known fact that the grain boundaries are the sources of weakness as far as creep behaviour is concerned. Addition of alloying elements considerably stabilise the grain boundaries but they at the same time impair the room temperature ductility by embrittling the grain boundaries. Another way of improving the creep resistance has been conceived by either eliminating the grain boundaries by producing a single crystal or by directionally solidifying the material so that the load could be applied parallel to grain boundaries with no shear component along them. Such columnar crystals would show high resistance to creep because no shear force would exist to make them slide along boundaries. They will not have any tendency to separate because no tensile stress would be acting across them. These structural characteristics are achieved by controlling the direction of solidification in a casting. Some parts of the hot section of the gas turbines have been produced by this process.

Another possibility of improvement of creep strength has arisen by way of precipitating the hard phase in the form of smooth rod shaped particle lined up parallel to direction of solidification in directionally solidified materials. This kind of structure has higher strength than normal directionally solidified structure in the same way as fibres reinforce a matrix material. Such materials, known as directionally solidified eutectics are still in experimental stage.