Papers by Author: Gu Song Li

Abstract: The effects of rapid solidification on the microstructure and mechanical properties of two kinds of NiAl based eutectics were investigated. Rapid solidification resulted in the following effects, which encompass the refinement of the cell size, lamellar spacing and precipitates, deviation from the equilibrium composition, solubility extension, and a transition from a Heusler phase to an Hf-rich solid solution phase. Except for the deviation from the equilibrium composition, these microstructural characteristics are all beneficial to the improvement of mechanical properties. As a consequence, the room temperature compression yield strength and compression ductility were improved significantly and high temperature strength improved slightly. The high temperature compressive flow behaviour can be described by the temperature-compensated power-law equations.

Abstract: The microstructural evolution during superplastic deformation of the extruded stoichiometric NiAl polycrystals were systemically investigated in various conditions of temperature, strain rate and strain by means of optical microscopy (OM) and transmission electron microscopy (TEM). Consequently, The deformation microstructures corresponding to the large tensile elongation consisted of subgrains, low angle grains as well as high angle grains, which indicated that continuous dynamic recrystallization (CDRX) process was operating during superplastic deformation.

Abstract: The microstructure and tensile mechanical properties of directionally Solidified NiAl-15Cr alloy at various temperatures have been investigated in this paper. The results reveal that the microstructure consists of dendritic β-NiAl phase, interdendritic γ/γ’, γ’ phase transient layer and α-Cr precipitation in β-NiAl phase. With the increase of temperature obvious Brittle-Ductile-Transition (BDT) behavior is observed and the BDT temperature (BDTT) is sensitive to initial strain rate. When the initial strain rate increases by two-order magnitude, the BDTT has an approximate 150K increase. In the temperature range of 1123-1373K, the alloy exhibits good tensile plasticity and poor strength comparing with the NiAl/Cr(Mo,Hf) alloy. Superplastic-like deformation behavior with large elongation (exceed 200%) is achieved at 1323K. The balance between strain hardening (by dislocation glide) and strain softening (dynamic recover and recrystallization) is responsible for the large tensile elongation of this alloy.

Abstract: The tensile creep behavior of NiAl-9Mo eutectic alloy has been investigated over a stress range of 50 to 100MPa at the temperatures ranging from 850 to 950°C. All of the creep curves exhibit the very long steady-state stage. The creep parameters and TEM observations indicates the kinetics of the steady-state creep deformation is governed by dislocation climb in the NiAl matrix phase. The crack origination and development at the colony boundary results in the onset of tertiary creep stage and final fracture of the alloy.