In Situ Observation of Kinetic Processes of Lath Bainite Nucleation and Growth by Laser Scanning Confocal Microscope in
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UCTION
TO satisfy the increasing demand for high strength and excellent toughness of steel in the automotive and structural industries, bainite microstructure has illustrated its vital importance in recent years.[1–3] In the past decades, the bainite morphology could only be detected after phase transformation by conventional metallographic investigations. In recent years, the continuous nucleation and growth process of bainite has been studied by transmission electron microscope (TEM) and other instruments. Bhadeshia et al. measured the lengthening rate of the subunit of bainite sheaf as 75 lm/s by a photoemission electron microscope.[4] Kang et al. provided direct evidence for bainite growth through in situ observation of the growth process of bainite by TEM with a hot stage under isothermal conditions.[5] Li et al. conducted
GAOJUN MAO, RUI CAO, and JIANHONG CHEN are with the Department of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China and also with the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China. Contact e-mail: [email protected] XILI GUO and YONG JIANG are with Atlantic China Welding Consumables, Inc., Zigong 643000, China. Manuscript submitted January 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
phase transformation behavior in a Ti-50.8 at. pct Ni alloy by in situ TEM observation.[6] In recent years, a powerful method for bainite transformation analysis, i.e., in situ observation by a high-temperature laser scanning confocal microscope (LSCM) with an infrared imaging furnace, has been used to analyze the morphological evolution during the cooling process.[7,8] LSCM makes it possible to observe the continuous processes of bainite transformation. This in situ observation of bainite nucleation and growth is based on the relief phenomenon of the new phase produced during phase transformation due to volume change resulted from lattice change.[7] Hu et al. carried out dynamic observation of bainite transformation in a Fe-C-Mn-Si super bainite steel by LSCM. They concluded that bainite growth had the features of interlocking and impingement of bainitic platelets.[7] Zhang et al. investigated the effect of cooling rate on phase transformation in the low-carbon B-treated steel through LSCM. It showed that the typical transformation products in the low-carbon B-treated steels are the microstructures including intragranular bainitic ferrite, nucleated entirely in the austenite grain.[8] They confirmed the findings that had previously been revealed by Ishikawa and Takahashi.[9] Xu et al. observed the nucleation and morphological evolution in a super bainite steel by LSCM. They found that bainite growth was characterized by the impingement of bainite sheaves, which resulted in an interlocked bainite microstructure.[10]
Additionally, analytical investigation of bainite nucleation and growth has been conducted by Zhang et al. They investigated the morphological and crystallographic evo
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