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Integrated Experimental and Computational Study of Precipitation in Martensitic Steels

Time: Thu 2019-12-12 10.00

Location: F3, Lindstedtsvägen 26, Stockholm (English)

Subject area: Materials Science and Engineering

Doctoral student: Tao Zhou , Metallografi, KTH Royal Institute of Technology, Unit of Structures

Opponent: Dr. David San Martin,

Supervisor: Dr. Peter Hedström, Metallografi, KTH Royal Institute of Technology

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Precipitation is a phase transformation process in metallic materials that significantlyaffects properties. The precipitation process that includes nucleation, growth andcoarsening of small particles can be tuned by alloying, deformation, thermal treatment.This opens opportunities for optimizing the properties of metallic materials bytailoring precipitation. An example of high-performance metallic materials withcontribution from precipitation is tempered martensitic steels. By means of highlydispersed nanoscale precipitates within the hierarchic martensitic microstructure,these steels achieve an excellent combination of ultra-high strength and hightoughness. With the objective of accelerating the development of these high-performance steels, an integrated computational materials engineering (ICME)approach, combining advanced characterization, physically based/semi-empiricalmodelling, theory and databases, is used in this thesis to develop computationallinkages from heat treatment to precipitation to strength.Two multicomponent steels, a Cu precipitation-hardened maraging stainless steel anda carbide-strengthened low alloy Cr – Mo – V martensitic steel, are studied in this thesisusing quantitative characterization and modelling. The results suggest that theprecipitation simulations using Langer-Schwartz-Kampmann-Wagner (LSKW)modelling have good agreements with the experiments and show promise for futurepredictive modelling to be used for materials design. The semi-empirical models forindividual strengthening mechanisms and an integration of the strengtheningmechanisms used in this work may also represent the trends in the yield strength offresh and tempered martensite, but it is difficult to predict the early yielding of freshmartensite and the correlation of hardness and strength. This indicates the need tofurther develop the models. Overall, this thesis shows that the ICME approach can beused to study and predict precipitation and precipitation-strengthening inmulticomponent steels. The applied approach differs from traditional trial-and-errortesting and has the potential to save time, money and resources in steel development.