Unexpected Effect of Nb Addition as a Microalloying Element on Mechanical Properties of δ-TRIP Steels

The concept of microalloying was applied to the δ-TRIP （transformation-induced plasticity） steel to inves- tigate the feasibility of increasing the mechanical properties and understanding the effect of microalloying on the morphology and structure of the steel. A hot rolled δ-TRIP steel with three different contents of Nb （0, 0.03, 0.07 mass%） was subjected to the microstructural and mechanical examination. The high Al and Si concentration in these steels guaranteed the presence of the considerable δ-ferrite phase in the microstructure after the casting and the subsequent hot rolling. The obtained results showed that Nb dramatically affects the microstructure the dynamic re- covery and recrystallization behavior, as well as the grain shape and thus the stability of austenite after the thermo- mechanical process of hot rolling. The results also revealed an unexpected effect of Nb on the mechanical properties. The addition of Nb to the δ-TRIP steel led to a significant decrease in the ultimate strength （from 1144 to 917 MPa） and an increase in ductility （from 24% to 28%）. These unconventional results could be explained by the change in the steel microstructure. The work-hardening＇behaviors of all samples exhibit three stages of the work-hardening rate evolution. At the stage 2, the work-hardening rate of the studied steels increased, being attributed to the TRIP effect and the transformation of austenite to martensite.

Fabrication of (Ti,Hf)-rich NiTiHf Alloy Using Graphitic Mold and Crucible

In this research, fabrication of a （Ti,Hf）-rich NiTiHf alloy by using vacuum induction melting （VIM） process and a graphitic crucible was investigated. For this purpose, casts with the nominal composition of Ni49Ti38Hf15 were prepared in graphitic crucible and mold. Optical microscopy （OM）, scanning electron microscopy （SEM）, energy dispersive X-ray spectroscopy （EDS）, X-ray diffraction （XRD）, and differential scanning calorimetry （DSC） tests were employed to characterize the samples. Results demonstrated that microstructure of the first cast was composted of a B2 austenite phase as well as a great amount of two differently formed （Ti,Hf）C carbides. Moreover, no austenite ＊-＊ martensite transformation peak was detected in the DSC curve of this sample, indicating a drastic decline in the transformation temperatures. In the succeeding cast, however, owing to the formation of carbide layers on the inner surfaces of the graphitic crucible and mold during the initial casting process, the amounts of carbides decreased remarkably. This cast exhibited transformation temperatures above 100℃, while XRD pattern denoted the presence of B19t monoclinic martensite phase at room temperature. All in all, results confirmed that VIM process using graphitic mold and crucible can be considered as an appropriate method for the fabrication of （Ti,Hf）-rich NiTiHf high temperature shape memory alloys.