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  • Microstructural evolution of Mn-based

    maraging steels and their influences on

    mechanical properties

    By

    Feng Qian

    Thesis submitted for the degree of Doctor of Philosophy

    Department of Materials Science & Engineering

    The University of Sheffield

    March 2015

  • Abstract

    i

    Abstract

    A set of Mn-based maraging TRIP steels was designed by Max-Planck-Institut fr

    Eisenforschung GmbH (MPIE) for light weight and safe automotive applications.

    According to their research, these Mn-based maraging TRIP steels exhibited a

    simultaneous increase in both strength and ductility upon aging. They attributed this

    surprising effect to the combination of precipitation strengthening mechanism and

    TRIP effect of reverted/retained austenite.

    This thesis carried out a further study on this type of steels with minor modification

    of chemical composition (7-12 wt.% Mn, with additional ~ 1 wt.% Al). The

    unknown precipitates were characterized as L21-ordered Ni2TiAl intermetallic phase

    for the first time. This type of precipitates is not only coherent but also coplanar with

    the martensite matrix. Their special orientation relationship together with the small

    lattice misfit (1.24%) led to the precipitates remaining coherent with the martensite

    matrix even after a long-term aging for 10080 min. Analyses on precipitate size

    revealed that the coarsening rate constants follows the diffusion-controlled

    coarsening kinetics form 3~ predicted by LSW theory, but the experimental

    precipitate size distributions (PSDs) is much broader than the theoretical PSD

    function. In addition, a core/shell structure was observed within the precipitates, but

    the exact structure of this structure is still not clear.

    The formation of reverted austenite nanolayers initiated at the onset of aging by a

    diffusionless shear mechanism since the critical Mn concentration for austenite

    reversion at the interface is very low. The accumulated Mn segregation at grain

    boundaries in the following aging led to the austenite nanolayers that grew to lath-

    like reverted austenite, which means the lateral growth of austenite was supported by

    the diffusion of Mn. Due to the low diffusion rate of Mn and the thermodynamic

    resistance to coalescence, the growth rate of lath-like reverted austenite is slow and

    thus the austenite maintained in the range 70-200 nm for a long time. The

    segregation of Ti and Mo on grain boundaries in the initial aging stage resulted in the

    Mn concentration of austenite nanolayers being far from that indicated by the

    equilibrium Fe-Mn phase diagram. The segregation of Ti and Mo gradually vanished

  • Abstract

    ii

    with the enrichment of Mn during the succeeding aging process. The TEM-EDS

    analyses revealed the Mn concentration of lath-like austenite was at the level of ~24

    at.% which is higher than that of retained austenite (8-12 at.%) reported in

    conventional Mn-based TRIP or Q&P steels. Nanoindentation testing revealed that

    the high stability of reverted austenite in Mn-based maraging steels was mainly

    attributed to the high Mn concentration of austenite. The nano-size of reverted

    austenite was also considered to be responsible for the high stability.

    Severe embrittlement occurred in samples aged at lower temperatures or for short

    times. Increasing aging temperatures and duration can significantly improve the

    embrittlement phenomena. An ultimate tensile strength (UTS) of 1120 MPa with

    total elongation (TE) of 18.4% was obtained in the 12% Mn alloy by aging at 500 C

    for 5760 min. It was demonstrated that the dense precipitates contributed to the

    increase in yield strength whereas the work hardening of reverted austenite

    contributed to the enhanced strength and ductility after yielding. The TRIP effect of

    reverted austenite reported by Raabe et al. does not occur to any significant amount

    owing to the high stability of reverted austenite in Mn-based maraging steels.

  • Acknowledgements

    iii

    Acknowledgements

    First of all, I would like to express my sincere gratitude to Professor W. Mark

    Rainforth, for his support and supervision throughout my PhD study. Although he is

    very busy with teaching, research and administration jobs, Professor Mark Rainforth

    is always inspiriting to me. He is really good at encouraging students and has given

    me lots of freedom to realize my idea. Moreover, he is always enthusiastic to share

    the latest news and information in the materials world which largely broaden my

    vision. I also admire his remarkable knowledge of electron microscopy and his

    tutorial on electron microscopy will definitely benefit my future career.

    I greatly acknowledge University of Sheffield, Department for Business Innovation

    & Skills (BIS) and China Scholarship Council (CSC) for giving me the opportunity

    to study in UK and their generous financial support. Another important contributor is

    Tata Steel who provided the materials for this project. I am very grateful to Professor

    Andy Howe for the delight and helpful conservations with him and his efforts on the

    project coordination with industry as well. Dr Eric Palmiere and Dr Martin Jackson

    are also appreciated for their suggestions given in the viva for annual report.

    I must express my sincere appreciation to Dr Peter Kogul, Dr Peng Zeng, Dr Le Ma

    and Dr Cheryl Shaw for the training and technical help at Sorby Centre for Electron

    Microscopy and Microanalysis. They are always here when I meet problems. Thank

    you all. Special thanks go to Dr Joanne Sharp for her helps on precipitation

    characterization using high-end TEM facilities. I am also thankful for Miss Dawn

    Busseys instruction on nanoindentation tests. Many thanks are given to Dr Amit

    Rara, Dr Andy Williams and Mrs Vanessa Dalton who helped me a lot on daily

    research work.

    I am very lucky to meet and work with a number of kind technical staff in the

    Department of Materials Science and Engineering. Thanks a lot to Mr Dean Haylock,

    Mr Kyle Arnold, Dr Lisa Hollands, Mr Michael Bell, Mr Richard Kangley, Mr Ian

    Watts and Mr Stuart Bater for their day-to-day helps on various experimental works

    which made this thesis possible.

  • Acknowledgements

    iv

    I would like to thank my dear friends who helped me a lot on my research work

    during these years. Particular thanks are given to Dr Yonghao Gao for those heavy

    work and IT support. Dr Lin Sun, who helped me a lot with the MatCalc software, is

    also appreciated. The experience of discussing thermodynamics and TEM with Mr

    Xiaoming Yuan and Mr Dongdong Xiao across jet lag will be one of the most

    valuable treasures in my life. I am also grateful for the joy and happiness brought by

    all friends and colleagues in the department which made this journey enjoyable.

    Words are not enough to convey my deepest gratitude to my family for their

    unconditional love and support. My open-minded parents give me their greatest

    patience and understanding through all these years. They give me enough freedom to

    do what I like and to pursue what I want. I am so proud to have them in my life.

  • Table of Contents

    v

    Table of Contents

    Abstract ....................................................................................................................... i

    Acknowledgements ................................................................................................ iii

    Table of Contents ...................................................................................................... v

    List of Figures ............................................................................................................ x

    List of Tables ....................................................................................................... xviii

    Chapter 1 Introduction .............................................................................................. 1

    Chapter 2 Literature review ..................................................................................... 6

    2.1 The development of Mn TRIP steels .................................................................. 6

    2.2 The development of maraging steels .................................................................. 7

    2.2.1 The role of alloying elements in maraging steels ........................................ 9

    2.2.2 Behaviour of intermetallic precipitates in conventional maraging steels .. 10

    2.2.3 Precipitation behaviour in newly-developed maraging steels strengthened

    by other intermetallic phases .............................................................................. 12

    2.2.4 The Mn embrittlement in Fe-Ni-Mn and Fe-Mn alloys ............................. 21

    2.3 Evolution of precipitates................................................................................... 24

    2.3.1 Critical size in precipitation ....................................................................... 24

    2.3.2 Growth and Coarsening of precipitates ..................................................... 25

    2.3.3 Precipitate coarsening theory ..................................................................... 27

    2.3.4 The effect of precipitate size on precipitation strengthening .................