Study on the Formation and Precipitation Mechanism of Mn 5 Si 3 Phase in the MBA-2 Brass Alloy
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INTRODUCTION
HIGH-STRENGTH special brasses possess a high wear resistance and good bearing properties. Therefore, they are commonly used to manufacture friction components of machines, particularly the ring synchronizer in the automobile where resistance to wear is the chief requirement.[1–4] The matrix of special brasses can be classified as a + b¢ or b¢ phases. The a phase is a Cu-rich solid solution with an fcc crystal structure, while the b¢ phase has an ordered bcc crystal structure and transitions to disordered b phase at elevated temperatures.[5] Alloying elements such as aluminum, silicon, iron, manganese, and tin are added to strengthen the brasses by entering into solid solution, forming intermetallic compounds, or influencing the volume fraction of b¢ phase in the microstructure. In the case of the special brasses containing additions of Mn and Si, particles of manganese silicide are formed, which is indicated to be Mn5Si3.[1–3,6] These particles, with a hexagonal crystal structure and high hardness, are responsible for the high wear resistance of the special brass. It is widely accepted that the microstructural features (e.g., size, morphology and distribution) of hard particles greatly influence the wear characteristics of the alloys.[7–10] To modify the microstructural features of Mn5Si3 phase in the solidification microstructure of the HANG LI and PENGCHAO ZHANG, Doctors, JINCHUAN JIE, Lecturer, CHUNXU JIA, Master, and TINGJU LI, Professor, are with the School of Material Science and Engineering, Dalian University of Technology, Dalian, 116024 Liaoning, China. Contact e-mail: [email protected] TONGMIN WANG, Professor, Laboratory of Special Processing of Raw Materials, Dalian University of Technology. Contact e-mail: [email protected] Manuscript submitted April 15, 2015. Article published online March 31, 2016 2616—VOLUME 47A, JUNE 2016
special brass, it is beneficial to understand its formation mechanism. There are a few researches concerning this respect. Sun et al.[6] believed that the primary Mn5Si3 in the brass was formed from the melt. But there is little evidence provided to support the thesis. In addition, the Mn5Si3 precipitates were observed in the microstructures of both C67400 brass and LMtsSKA brass quenching from elevated temperatures.[6,11] However, the formation mechanism of Mn5Si3 phase in the special brass is still unclear. It is therefore of great significance to find ways and means, necessary for study on the formation of the primary Mn5Si3 in the brass melt. Many studies[12–14] have shown that real-time observation of microstructure evolution at high temperatures can be achieved by applying synchrotron X-ray radiography with satisfying spatial and temporal resolution. Moreover, the rapid cooling from the melt is commonly used to suppress the formation of precipitates during solidification and preserve the primary phase formed at high temperatures.[15–17] Hence, in this study the application of synchrotron X-ray radiography and rapid cooling is considered to investigate the formation mech
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