Microstructure Evolution in Directionally Solidified Fe-Ni Alloys in Diffusive Regime
- PDF / 971,330 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 26 Downloads / 217 Views
lloys have been widely used as important engineering and functional materials, and the solidification of peritectic alloys has been a hot topic in solidification field in recent years. To reveal the solidification characteristics of peritectic alloys, especially for banding structures, researchers have carried out many studies in low-melting-point Sn-Cd[1,2] and Pb-Bi[3–6]systems in diffusive regime. Some other microstructures and solidification characteristics, such as directionally solidified two-phase growth structure,[7] peritectic coupled growth,[8] transition from island banding to coupled growth,[9] and influences of peritectic reaction on microstructure evolution,[10] were studied in a high-melting-point Fe-Ni peritectic system. Based on the purely diffusive growth model of banded structure,[11] the effects of pulling rate on microstructure evolution in diffusive regime have been studied in highmelting-point Fe-Ni alloys.[12] However, up to now, few comprehensive scientific studies on directionally solidified Fe-Ni peritectic alloys in diffusive regime have been reported. In order to develop diffusion-controlled conditions, fluid flow during directional solidification should be Z.R. FENG, L.S. WANG, J.F. ZHANG, and Y.J. DU, Ph.D.s, J. SHEN, and H.Z. FU, Professors, and W. WANG, Master, are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P.R. China. Contact e-mail: [email protected] Manuscript submitted: September 25, 2011. Article published online September 29, 2012 640—VOLUME 44A, FEBRUARY 2013
suppressed or eliminated. In vertical directional solidification systems, if the solute is heavier than the solvent, thermosolutal convection can be restrained due to the positive temperature gradient and the negative concentration gradient in the liquid. Thus, for the upward directional solidification of thin Fe-Ni samples, melt convection can be suppressed because the rejected solute element (Ni) is heavier than the solvent. However, fluid motion could occur owing to the presence of lateral gradients in temperature and concentration in the liquid under directional solidification.[13–16] In this circumstance, the strength of melt convection can be characterized by a Rayleigh number (Ra), which is given by RaT ¼
gd 4 bT GH maL
½1
where g is the gravity vector, d is the sample diameter, bT is the volumetric thermal expansion coefficient, v is the kinematic viscosity, GH is the lateral temperature gradient, and aL is the thermal diffusivity of the liquid. From Eq. [1], it can be seen that the magnitude of the Rayleigh number is proportional to d4. Therefore, the convection effects can be suppressed efficiently by reducing the sample diameter. Numerical modeling was developed to correlate melt convection with sample diameter, and then directional solidification experiments under diffusive growth conditions were carried out according to the simulation results to study the phase selection and microstructure evolution in diffusive regime. METALLURGICAL AND MATERIALS TRANSACTIONS A
I
Data Loading...
