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Xiaosheng Zhou,Zongqing Ma,Liming Yu,Yuan Huang,Huijun Li,Yongchang Liu 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.1
With Al addition, dual-phase oxide dispersion strengthened (ODS) steels consisting of martensite and ferrite are fabricatedby spark plasma sintering. It is found that Al addition has a negligible effect on martensite lath size, while the amount andsize of ferrite grains are related to the Al content. M23C6(M = Fe, Cr) carbides have been identified within the ferrite grainsor along ferrite boundaries. With increasing Al concentration, more fine Y–Al–O oxide nanoparticles are formed. Uponannealing treatment, homogeneous and refined distribution of ferrite grains is obtained, which may involve the particlestimulatednucleation of recrystallization caused by the large sized M23C6. As Al is increased from 0.05 to 0.1 wt%, thetensile strength of the annealed steel is decreased, as well as its ductility. For the annealed 9Cr-ODS steel containing 0.1 wt%Al, in tensile loading the large sized M23C6along ferrite boundaries would facilitate the cracking along boundaries betweenthe hard annealed ferrite and soft annealed martensite, producing the mixed fracture of dimple and intergranular fracture.
Hot Deformation Behavior and Recrystallization Mechanism in an As-Cast CoNi-Based Superalloy
Yong Guan,Yongchang Liu,Zongqing Ma,Huijun Li,Hongyao Yu 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.6
The hot deformation behavior of as-cast CoNi-based superalloy was investigated by means of hot compression tests. Thecorresponding microstructure evolution after hot deformation was examined by optical microscope, as well as the TEMtechnique. The constitutive equation and processing map as a function of deformation temperature and strain rate were developed. Results show that the efficiency peak of the processing map is 0.38 at the temperature of 1130 °C and the strain rate of0.01 s−1. The discontinuous dynamic recrystallization (DDRX) related to the strain-induced grain boundary migration (SIBM)is the dominant recrystallization mechanism during hot working. Meanwhile, the DRX also occurs by forming sub-grainsaround MC carbides and shear band. In addition, the existence of fine γ′ precipitates are found to retard the recrystallization.
Junpeng Yang,Qi Cai,Yongchang Liu,Chong Li,Zongqing Ma,Huijun Li 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.4
The W-modifi ed Ti 2 AlNb-based alloys synthesized at 1100 °C by spark plasma sintering were solution treated at 1300 °Cfor 4 h and then aged at 800–1000 °C for 1 h. The phase composition, microstructure evolution, and microhardness of theaged alloys are investigated in this study. A signifi cant enhancement of hardness, ~ 750 HV, is obtained in the alloy aged at900 °C, while that of the one without W addition is only ~ 470 HV. The hardness is also higher than that of common β-Ti andTi–6Al–4V alloys. As the ageing temperature increases, the B2/O structure evolves from B2 + O colonies to Widmannstättenstructure, followed by a “disordering to ordering” procedure. This process also involves the variation of the angle betweenadjacent O phase from 90° to 60°. Specifi cally, a herringbone Widmannstätten B2 + O structure is constructed in the alloysaged in the α 2 + B2 + O phase region, which is related to the diff usion of W and the substitution of W for Nb in the latticeof B2 or O.