Papers by Author: Mai Cang Zhang

Abstract: The coarsening kinetics of main strengthening phase γ′ and the growth behavior of grain boundary carbides have been investigated on NiCr20TiAl alloy aged at 550~750°C for 200~10,000h. The precipitates of NiCr20TiAl alloy at standard heat treatment condition are γ′, M₇C₃, M₂₃C₆ and MC. The coarsening of γ′ precipitates proceeds by Ostwald ripening controlled by volume diffusion in the alloy. Grain boundary carbides M23C6 and M7C3 increase with ageing times and temperatures. The morphologies of precipitates after long-time ageing almost remain the same as that at standard heat treatment condition except 750°C.

Abstract: High temperature structure stability of 2 important Nb-containing Ni-base superalloys Inconel 718 (Ni-19Cr-18Fe-3Mo-5Nb-1Ti-0.5Al) and Inconel 740 (Ni-25Cr-20Co-0.5Mo-2Nb -1.8Ti-0.8Al) have been studied on the relationship of microstructure to mechanical properties via SEM, TEM, SAD, XRD and quantitative determination of precipitated phases by micro-chemical analysis. The longest exposure times are 50,000hrs at 650°C for Alloy 718 and 4,000hrs at 704°C, 725°C and 760°C for Alloy 740. The structure instabilities for these 2 Alloys are: 1) strengthening phases γ″/γ′ coarsening; 2) meta-stable strengthening phases change to stable phase, such as γ″→ δ-Ni3Nb for Alloy 718 and γ′→ η-Ni3Ti for Alloy 740; 3) embrittling phases formation, such as α-Cr and σ-phase formation in Alloy 718 and high Si-containing G-phase formation in Alloy 740. On the base of understanding phase changes at high temperature exposures the structure stability of Inconel 718 and Inconel 740 has been improved both by minor adjustment of alloys chemistry. The modifications of Alloy 718 and Alloy 740 are to be developed in the near future.

Abstract: Inconel 718(Ni-19Cr-18Fe-3Mo-5Nb-1Ti-0.5Al) nickel-base superalloy strengthened mainly by Ni3Nb type γ″ and partially by Ni3Al type γ′ precipitation is today’s most widely used superalloy in the world. China has paid special attention on Inconel 718 research and development. Systematic long-term research project has been conducted in close cooperation among our university, research institutes and factories. The goal of this long-term project is in 2 steps. First step is to improve Alloy 718 to get high quality and the second step is for improving the alloy temperature capability from 650oC to 680-700oC. The basic idea for alloy improvement is still to keep the chemical composition in the range of specification by small adjustment or control of minor elements, such as S, P, N, Si and Mg. The main achievements are segregation control by adjustment of S, P, Si, control of N for cleanliness, micro-alloying of Mg for grain boundary strengthening and control of low S and high P for stress rupture life improvement. Modification of Alloy 718 is based on structure stability study and its improvement in adjustment of main second phase strengthening alloy elements Nb, Ti and Al in total amount and the ratios among them. The goal is to achieve more stable second phase strengthening by control of alloy elements. Experimental results show that the future of modified 718 alloys is very attractive to raise the temperature capability improvement from 650oC to 680-700oC.

Abstract: The composition, shape, size and distribution of non-metallic inclusions in a kind of high strength shaft steel enriched CoNi have been investigated. In situ tensile tests in special designed SEM have been conducted to trace the whole process of crack initiation and propagation till to fracture at inclusion. The experimental results show that the non-metallic inclusions in the high strength shaft steel are primarily AlN. Their average size is about 3.6µm. Sometimes, a string of inclusions distributes along the rolling direction. Non-metallic inclusion can induce crack to be initiated by inclusion debonding. When the inclusion size is larger than the critical size, the crack can propagate as the main crack that induces the specimen to fracture.