Abstract
The current study reports the detailed analysis of an observation of the local pinning of a slowly moving austenite-ferrite interface by a single nanosized oxidic particle. The observations were made during an in situ cyclic partial phase transformation experiment on a Fe-0.1C-1.0Mn alloy close to the inversion stage at which the interface migrates at a rather low velocity. The low velocity allowed capturing the interface pinning effect over a period of no less than 16 seconds. From our observations, it was possible to follow the progression of the pinning effect from the initial stages all the way through to the release of the interface. The pinning force exerted by the individual particle having a diameter of 140 nm on the austenite-ferrite interface was estimated as 175 nJ m−1, while the maximum pinning length was approximately 750 nm to either side of the particle, leading to an interface line tension of 170 nJ m−1. The observed pinning behavior is compared with the most relevant models in the literature.
Highlights
The interaction of oxide particles with grain boundaries can reduce austenite grain growth at high temperatures (1200 °C) in Fe-0.15C-1.0Mn-1.0N steels deoxidized with Ti and Zr[16] and improve stability of the microstructure in austenitic stainless steel heat treated at 1150 to 1250 °C for 100 to 1000 hours.[17]
During cyclic partial phase transformation (CPPT) experiments, the austenite-to-ferrite and ferrite-to-austenite transformations undergo two distinct transformation stages[32]—the ‘‘normal’’ transformation and the ‘‘inverse’’ transformation which during which the transformation continues even though the T1 temperature has been reached and heating of the specimen has begun
Direct TEM observations of the interaction between an oxide particle and a migrating austenite-ferrite interface displayed the expected features of Zener pinning including the localized inhibition of migration at the particle, a reduction in the overall migration velocity for the interface as a whole and a build-up of the pinning force until the interface was released
Summary
SECOND-PHASE particles can provide a significant local pinning force on moving grain boundaries and moving interfaces and this effect can be, and is being, used to control the final grain size in polycrystalline metals exposed to a thermal treatment.[1,2,3,4] In microalloyed steels, carbide, nitride or carbonitride forming species, such as Nb or Ti, are widely used to assist microstructural control during thermomechanical processing by arresting static recrystallisation of austenite at low temperatures and inhibiting austenite grain coarsening.[5,6,7,8,9] A first theoretical analysis of the interaction of pre-existing (stationary) particles with migrating grain boundaries during recrystallization (i.e., conditions in which the same phase is present on both sides of the interface, but there is a large difference in dislocation density) was formulated by Zener and presented by Smith.[10,11,12] He argued that the inhibiting effect on migration arises from the reduction in interfacial energyManuscript submitted January 21, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS A when the grain boundary contacts the particle Given his explanation, similar considerations are expected to apply to the interaction of particles with interphase boundaries migrating under diffusional transformation conditions. Similar considerations are expected to apply to the interaction of particles with interphase boundaries migrating under diffusional transformation conditions In such a case where different phases exist on either side of the interface the driving force for interface motion generally is much higher than for recrystallization conditions and the effect may be less notable.[13] Previous experimental studies of the d-ferrite-to-austenite and a-ferrite-to-austenite transformations indicate that the transformation interfaces interact with particles in the parent phase[14,15] and transformation kinetics were found to be retarded in a similar fashion to Zener pinning during grain growth.[4,14] The interaction of oxide particles with grain boundaries can reduce austenite grain growth at high temperatures (1200 °C) in Fe-0.15C-1.0Mn-1.0N steels deoxidized with Ti and Zr[16] and improve stability of the microstructure in austenitic stainless steel heat treated at 1150 to 1250 °C for 100 to 1000 hours.[17]
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