Theory and Methods for Martensite Design: Session 6
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Thursday 2:00 PM
July 13, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Ricardo Komai, QuesTek Innovations LLC

2:00 PM  
The Relationship between Amorphization and Martensitic Transformation in Ti-Ni and Ti-Ni-Fe Alloys: Takeshi Nagase1; Atsushi Sasaki1; Hiroyuki Yasuda1; Tomoyuki Terai1; Takashi Fukuda1; Tomoyuki Kakeshita1; 1Osaka University
     Ti-Ni alloys are well known to show solid state amorphization (SSA) through the defect accumulation via severe plastic deformation, irradiation, and so on. In the present study, the relationship between SSA and martensitic transformation (MT) in Ti-Ni based alloys was investigated by High Voltage Electron Microscopy (HVEM). The total dose required for SSA increased with decreasing of the characteristic MT temperature in Ti50Ni50-xFex alloys [1]. Ti50Ni44Fe6 alloy, which was observed to exhibit anomalies in its physical properties [2], shows the particular SSA behavior [3]. The relationship between SSA and MT was discussed based on the lattice softening.[1] T. Nagase, A. Sasaki, H. Y. Yasuda, H. Mori, T. Terai, T. Kakeshita, Intermetallics, 19 (2011) 1313-1318., [2] M. S. Choi, T. Fukuda, T. Kakeshita, Scripta Materialia, 53 (2005) 869-873., [3] T. Nagase, A. Sasaki, H. Y. Yasuda, T. Terai, T. Fukuda, T. Kakeshita, Acta Materialia, 109 (2016) 201-209.

2:20 PM  
Effect of Hydrogen on Substructure of Martensite in Fe-31Ni Alloy: Akinobu Shibata1; Nahoko Saji1; Hirotaka Tai1; Nobuhiro Tsuji1; 1Kyoto University
    Carbon and nitrogen have been widely used in steels to control microstructures and properties. In contrast, hydrogen, which is also an interstitial element same as carbon and nitrogen, is considered to be the element that degrades properties, for example hydrogen embrittlement. However, there remains a possibility to utilize hydrogen effectively in steels, if we can understand accurately effect of hydrogen on microstructures and properties. On this basis, the present study investigated martensitic transformation behavior of austenite containing hydrogen in an Fe-31Ni (mass %) alloy. In the Fe-31Ni alloy without hydrogen, lenticular type martensite, whose substructure consists of midrib, twinned region, and untwinned region, formed by sub-zero cooling. We found by careful microstructure observations that existence of hydrogen increased fraction of twinned region inside lenticular martensite plate. This result suggests that hydrogen facilitates twinning deformation as a lattice invariant deformation.

2:40 PM  
The Effects of prior-γ Grain Boundary Segregation of Phosphorus, Manganese and Molybdenum on Intergranular Fracture Stress in Low Carbon Martensite Steels: Masahide Yoshimura1; Manabu Hoshino1; Masanori Minagawa1; Masaaki Fujioka1; 1Nippon Steel & Sumitomo Metal Corporation
     Influence of prior-γ grain boundary segregation of alloy elements on intergranular fracture stress is important for the mechanism of temper embrittlement. There are a few efforts based on pure-iron, but no report on low carbon martensite steels. In this study, the effect of segregation of phosphorus, manganese and molybdenum was investigated. The samples melted by changing the amount of phosphorus, manganese and molybdenum based on Fe-0.1%C-3%Mn-90ppmP were employed. The martensite steels with 500μm prior-γ were made by quenching and tempering. The segregation were measured by Auger electron spectroscopy, and the intergranular fracture stress was regarded as the yield strength at ductile brittle transition temperature of Charpy V-notch test. This study revealed that the segregation of phosphorus weakened the fracture stress mostly in order of phosphorus and manganese, and that of molybdenum strengthened the fracture stress quantitatively. Mn-P co-segregation was not observed. The segregation of phosphorus decreased by added molybdenum.

3:00 PM  
Effect of γ-precipitates on Thermal and Acoustic Noises Emitted during Austenite/martensite Transformation in NiFeGaCo Single Crystalline Shape Memory Alloys: Melinda Bolgár1; Lajos Daróczi1; László Tóth1; Elena Panchenko2; Ekaterina Timofeeva2; Yuriy Chumlyakov2; Dezső Beke1; 1University of Debrecen; 2Tomsk State University
    It is shown that the presence of precipitates of γ-phase in NiFeGaCo single crystals influences both the formation of the martensite phase and the thermal and acoustic noises measured during the phase transformation. While in the homogeneous crystals the transformation underwent by single interface motion, in crystals with γ-phase precipitates many martensite needles were formed. These differences are accompanied with differences in the critical exponents of the power law for distributions of the energy and the amplitude of acoustic emission and thermal noises (measured by Differential Scanning Calorimeter). We found a definite correlation between the relative changes of these exponents as well as the integrated energies of acoustic emission during heating and cooling: if the later ratio was negative (negative asymmetry) then the changes in the exponents had opposite sign. In crystals with γ-phase precipitates negative asymmetry while in homogeneous crystals without γ-phase precipitates positive asymmetry was obtained.

3:15 PM  
Analyzation of the Correlation between Martensitic Microstructure and the Cleavage Fracture Behaviour of a PH 15-5 Stainless Steel at Low Temperatures: Dominik Brandl1; Martin Stockinger2; Sarah Ploberger3; Stefan Marsoner1; Gerald Ressel1; 1Materials Center Leoben Forschung GmbH; 2Böhler Schmiedetechnik GmbH & Co KG; 3Böhler Edelstahl GmbH & Co KG
    Structural components in aircrafts have to fulfil strict requirements on mechanical properties, such as fracture toughness even at low temperatures. Generally, these parts are produced by means of a thermomechanical process to control the microstructure. To predict the mechanical properties of the material this work focuses on the correlation between the martensitic microstructure and the fracture behaviour of a PH 15-5 maraging steel. Therefore, electron-back-scatter-diffraction analysis of cross-sections of fracture surfaces are carried out in order to determine the critical microstructural features where the crack is deflected during cleavage fracture. These deflection points are correlated with the packet-, block- or subblock-boundaries of the martensitic structure. The findings of this investigation are used to adapt the thermomechanical treatment in order to optimise the low temperature toughness of the PH15-5 alloy.

3:30 PM Break