A SANS and APT study of precipitate evolution and strengthening in a maraging steel

In this work a combination of the characterisation techniques small angle neutron scattering (SANS) and atom probe tomography (APT) are used to study the precipitation in a maraging steel. Three similar maraging steel alloys were aged at different temperatures and ageing times, and then characterised using SANS, APT and microhardness. The alloys consist of two types of precipitates, namely Laves phase and β-NiAl, the precipitates have different composition and hence precipitate ageing, which makes it complicated to model. The SANS experimental set-up was relatively simple and allowed the precipitate size and fraction of a large number of samples to be measured in a single experiment. The APT results were used for constraining the SANS modelling, particularly the composition, shape and distribution of phases. The characterisation led to the following description of precipitation: NiAl phase reaches coarsening at early stages of ageing and shifts its strength mechanisms from shearing to Orowan looping, which cause the characteristic peak strength; the Laves phase is in growth throughout and its strength contribution increases with ageing time. These observations were shown to be consistent with precipitate evolution and strengthening models, and the work of others. Although, there are some issues with the combination of SANS and APT approach, which are discussed, the methodology provides a valuable tool to understand complex precipitation behaviours

Theory for growth of spherical precipitates with capillarity effects

Analytical solutions are presented for the growth of spherical precipitates incorporating the effect of interface curvature on local equilibrium at the interface, and the behaviour of the solution is examined

Growth of needle and plate shaped particles:Theory for small supersaturations, maximum velocity hypothesis

A solution to the diffusion controlled growth of needle and plate shaped particles is presented as their shape approaches respectively a paraboloid of revolution or a parabolic cylinder, under small supersaturation values, when capillarity and interface kinetic effects are present. The solutions show that as supersaturation decreases, the growth rate and needle tip radius approach a common value regardless of interfacial kinetics effects as capillarity is the main factor that retards particle growth. Simple asymptotic expressions are thus obtained to predict the growth rate and tip radius at low supersaturations, assuming a maximum velocity hypothesis. These represent the circumstances during solid state precipitation reactions which lead to secondary hardening in steels

Elucidating white-etching matter through high-strain rate tensile testing

A form of damage in bearing steels subjected to rolling contact fatigue is the formation of localised regions of white material just below the contact surface. These ‘white-etching regions’ are strikingly visible signs of damage during metallographic examination. One mechanism proposed to explain their formation is adiabatic shear localisation. Experiments are reported here using high-strain rate (250 s−1) tensile testing to show that this is not the case.</p

Role of fracture toughness in impact-abrasion wear

Two new low alloyed steels were developed with different fracture toughness values but at similar level of hardness with same composition and microstructural phase. The steels were subjected to impact-abrasion wear test. This work examines specifically the additional role of toughness during impact-abrasion wear, using a newly developed high toughness steel. Microstructural characterisation of the damaged samples revealed that better toughness helps resist both impact and abrasion damage.publishedVersionPeer reviewe

Role of fracture toughness in impact-abrasion wear

Two new low alloyed steels were developed with different fracture toughness values but at similar level of hardness with same composition and microstructural phase. The steels were subjected to impact-abrasion wear test. This work examines specifically the additional role of toughness during impact-abrasion wear, using a newly developed high toughness steel. Microstructural characterisation of the damaged samples revealed that better toughness helps resist both impact and abrasion damage.publishedVersionPeer reviewe