MSE Colloquium: Charles Feng, Improvement of grain boundary tolerance by minor additions of Hf and B in second generation single crystal superalloys

Professor, Un of Science and Technology Beijing

Abstract

The formation of low angle grain boundary (LAB) defects can dramatically degrade the creep properties of single crystal (SX) superalloys, resulting in lower casting yield and higher cost. Nowdays, to improve grain boundary (GB) tolerance in SX superalloys is still an important and challenging task for physical metallurgists. Minor alloying additions (C, B and Hf) have been observed as a means of improving the damage resistance of grain boundaries in a range of commercial single crystal superalloys. However, the creep strengthening mechanism by their additions is still unclear yet. 

In this study, a double-seed solidification technique with two levels of misorientation (5° and 20°) was used to produce a series of second generation SX superalloys with different Hf and B additions. It is the first report that the alloy with the combined additions of Hf and B demonstrated the tolerance to GB as high as 20°misoriention under all the creep conditions of 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. However, the effect of independent additions of Hf or B was not as pronounced as or even much worse than that of their joint additions. In order to understand the influence of Hf and B additions on the creep mechanism in these SX superalloys with GB defects, a detailed characterization of the misorientation related microstructures at the GBs has been carried out. The elemental distribution at the GBs has been analyzed by using Auger electron spectroscopy (AES), time of flight secondary ion mass spectrometry (TOF SIMS) and atom probe tomography (APT) techniques. The bicrystals integrated well with each other, although an almost continuous 5 nm thin layer was identified at the LAB by high angle annular dark field (HAADF) images. Cellular recrystallization microstructures along the high angle grain boundary (HAB) were found in the Hf and B free alloy, but they were significantly suppressed by the minor additions of Hf or B. There was no obvious segregation of Hf, but the segregation of B to HAB was clearly identified by all the analyses using TOF SIMS, AES and APT. Relationships among Hf and B additions, GB microstructural evolution characteristics, segregation behaviors of alloying elements and creep properties as well as their effects on creep behaviors and mechanism will be discussed here as well. This study will be beneficial for understanding the role of Hf and B additions to improve the GB tolerance, and optimizing Hf and B additions in nickel-based single crystal superalloys.

Bio

Dr. Qiang (Charles) Feng is the Professor of the State Key Laboratory for Advanced Metals and Materials at the University of Science and Technology, Beijing (USTB). He graduated with the B.S./M.S. degree from USTB in 1991/1994, and received his Ph.D. degree from Polytechnic University at Brookly, NY (currently the Engineering School of NYU) in 2000. He had been working as the Postdoctoral Research Fellow and Senior Research Fellow in the Department of Materials Science and Engineering at the University of Michigan from 2000 to 2005. Professor Feng has authored or co-authored over 100 papers in referred journals and conference proceedings, and his research interests focus on alloy development, microstructure, mechanical property (creep and fatigue) and life prediction of high temperature structural materials, including superalloys, intermetallic alloys and heat resistant steels. Materials systems of interests are mainly applied in aircraft engine and gas turbine industries as well as power generation industries and automotive industries.

Prof. Feng currently serves for Chinese Materials Research Society (C-MRS) as the Deputy Secretary-general, and has been served for International Union of Materials Research Society (IUMRS) as the Officer and Secretary from 2009-2012. He is also the former Deputy Director of National Center of Materials Service Safety (NCMS), USTB. Prof. Feng is currently the member of High Temperature Alloys Committee and Mechanical Behavior of Materials Committee for the Minerals, Metals and Materials Society (TMS). He is also the international committee member of Eurosuperalloys 2018 conference, which is held every four years. He also serves on the editorial board of the Progress in Natural Science: Materials International. He has been honored by Chinese Ministry of Education as the New Century Excellent Talents in University, and he received numerous awards including Young Leader Professional Development Award from TMS as well as Research Mentor Award from the College of Engineering at the University of Michigan.