Mechanical behavior of precipitation strengthened nanostructured bulk materials produced by inhomogeneous severe plastic deformation

A product’s service life of various power industry components can be extended with application of nanostructured metals and alloys. Implementation of nanomaterials, in particular nanostructured steel, encourages replacement... [ view full abstract ]

A product’s service life of various power industry components can be extended with application of nanostructured metals and alloys. Implementation of nanomaterials, in particular nanostructured steel, encourages replacement of existing products with new nano-products that are more environmentally friendly throughout their lifecycle. For example, the thermal stability of the modified nanostructured steels will ensure the long-term creep resistance of these popular structural materials by avoiding the growth of coarse grains and precipitates at the expense of fine precipitates. Severe plastic deformation (SPD) offers a new processing alternative to bottom-up conventional nanomaterial’s production methods, in spite the currently existing difficulties towards adaptation of the SPD processes to practical use. Interpretation of the mechanical behavior of the nanostructured materials produced by SPD, in which several strengthening mechanisms operate simultaneously, presents a significant challenge. However, the potential benefits are clear. For example, precipitation strengthening can be used to obtain an improved creep resistance, which remains constant even after long-term exposure to high temperature. In this contribution, general findings and some new evidence from different factors affecting mechanical behavior of these materials will be discussed. In particular, we will discuss in more details the role of second phase particles in grain refinement and multilayer nanocrystalline structure formation, predominantly the role of Nb(C,N) in these processes. The structures are produced using top-down approach in the novel metal forming processes. We will demonstrate the new possibilities arising from application of the controlled introduction of inhomogeneous deformation during complex processing, such as recently developed Accumulative Angular Drawing (AAD) and Compression Press Bonding (CPB). These processes are strongly affected by nonlinear strain path changes. It will also be shown that the expected effect of decreasing ductility in nano structured materials, attributed to limited dislocation activity at these length scales, can be effectively minimized by introduction of finely dispersed particles, which effectively increased the work hardening rate according to well-known Considere Criterion for plastic instability. The discussed above phenomena are supported by computer modeling results obtained using the multi-scale numerical approach, and some examples will be presented.