The effect of driving force in Gibbs energy on the fraction of martensite

by Erik Andersson

Institution: KTH Royal Institute of Technology
Year: 2013
Keywords: martensite; model; driving force; Gibbs energy; fraction; Ms-temperature; Engineering and Technology; Materials Engineering; Metallurgy and Metallic Materials; Teknik och teknologier; Materialteknik; Metallurgi och metalliska material; Teknologie kandidatexamen; Bachelor of Science
Record ID: 1370888
Full text PDF: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-123597


The background to this bachelor thesis is an on-going project within the VINN Excellence Center Hero-m. The task in this thesis is to perform a literature survey about the martensite transformation and investigate how the resulting fraction depends on cooling below the M<sub>s</sub>-temperature. Instead of calculating the undercooling for each of the known fractions of martensite the driving force will be evaluated. Several efforts have been made through the years to describe the relationships between fraction transformed austenite and temperature. The approaches to the first models were empirical and derived from collections of data regarding the amount of retained austenite at different quenching temperatures. Lately, studies have been made to derive a thermodynamical relationship using how the Gibbs energy is affected by increments in volume transformed austenite. Two equations are derived by calculating the resulting driving force at different known quenching temperatures and the respective percentage transformed martensite found in previous works. The data for the steels used show a characteristic slope when linearised. A trend for the steels which have a high characteristic slope is that they also have a high M<sub>s</sub> temperature, and the steels which have a low characteristic slope tend to have a low M<sub>s</sub>. Previous relationships which describe the martensitic transformation have considered the importance of the M<sub>s</sub> temperature only in it being a starting temperature for the transformation. To further incorporate the M<sub>s</sub> temperature in the equations presented, further research of the martensitic transformation is required. The approach in this thesis of using thermodynamically calculated data is a base for further investigation of the range of the martensite transformation.