Structural evolution during the plastic deformation of nanostructured steel

摘要

Nanostructured bainitic steels represent a major advance in technology, because it is now possible to produce very large quantities of three-dimensional objects containing an incredible density of interfaces. This can be done at a reasonable cost so that thousands of tonnes can actually be produced.The nanostructure usually is a mixture of slender plates of bainitic ferrite embedded in a matrix of carbon enriched austenite. This austenite is stable under ambient conditions but can decompose into a mixture of ferrite and carbides at temperatures in the vicinity of 400°C. However previous research has discovered that it is possible to make the alloys heat resistant either by increasing the silicon concentration which makes carbides less stable, or by adding solutes that increase the thermodynamic stability of the austenite.The purpose of the work presented in the thesis was to examine the stability of the steels using a variety of characterisation techniques. It is discovered for example that austenite can actually grow at temperatures as low as 400°C in one of the alloys, and dramatically influence the resulting mechanical properties. This has been studied systematically and modelled mathematically to show the different kinds of behaviour that occur when mechanical properties are measured at temperature.In the alloy variant that does not contain nickel as an austenite stabilising element, it is the precipitation of carbides at relatively high testing temperatures that leads to a significant deterioration of properties. The temperature at which this occurs is much higher than in “conventional” nanostructured steels.In another scenario it has been demonstrated that it is possible to precipitate intermetallic compounds which then influence the progress of transformation. Detailed kinetic measurements and microscopy have shown that the intermetallic compounds influence kinetics either by depleting the parent phase from certain solutes, or by stimulating the intragranular nucleation of bainitic ferrite.The major outcome of this work is that there is a new understanding of the thermal stability of high silicon or high nickel bulk nanostructured bainitic steel. In addition, it is possible now to specify the optimum stability of the austenite in the nanostructure with respect to achieving the best mechanical property combi- nations.