High temperature tensile properties of directionally solidified Ni-43Ti-4Al-2Nb-2Hf alloy

By liquid metal cooling(LMC) process,the Ni-43Ti-4Al-2Nb-2Hf(%,atomic fraction) alloy was directionally solidified(DS).The microstructure and tensile properties at room and elevated temperature were investigated.It was found that the DS process significantly improves the room temperature tensile strength,increasing by 70% compared with the as-cast alloy.After appropriate heat treatment(HT),the average tensile strength reaches above 1900 MPa,nearly twice of the as-cast one.At 800 and 900 ℃,the tensile strengths are about 308 and 169 MPa,respectively.

Temperature evolution and grain defect formation during single crystal solidification of a blade cluster

In order to investigate the asymmetry of thermal conditions during directional solidification, the temperature evolution and correspondingly developed undercooling in a simplified single crystal blade cluster were numerically simulated. Simulation results demonstrate that the temperature distribution at the blade platforms is obviously asymmetrical. On the outside of the blade which directly faces the heating element, the liquidus(TL) isotherms progress relatively smoothly. On the inside of the blades facing the central rod, however, the TLisotherms are in concave shape and the slope goes upwards to the platform extremities. The average undercooling extent ?T and undercooling time ?t at the inside are much higher than those at the outside. It was then predicted that the inside platform extremities have significantly higher probabilities of stray grain formation compared to the outside ones. A corresponding experiment was carried out and the metallographic examination exhibited the same side-and height-dependence of stray grain formation in the blades as predicted. On the inside of the blades, all platforms are occupied by stray grains, while the platforms on the outside are nearly stray grain free. The simulation result agrees very well with the experimental observation.

Microstructural evolution and corrosion behavior of directionally solidified FeCoNiCrAl high entropy alloy

The FeCoNiCrAl alloys have many potential applications in the fields of structural materials, but few attempts were made to characterize the directional solidification of high entropy alloys. In the present research, the microstructure and corrosion behavior of FeCoNiCrAl high entropy alloy have been investigated under directional solidification. The results show that with increasing solidification rate, the interface morphology of the alloy evolves from planar to cellular and dendritic. The electrochemical experiment results demonstrate that the corrosion products of both non-directionally and directionally solidified FeCoNiCrAl alloys appear as rectangular blocks in phases which Cr and Fe are enriched, while Al and Ni are depleted, suggesting that Al and Ni are dissolved into the NaCl solution. Comparison of the potentiodynamic polarization behaviors between the two differently solidified FeCoNiCrAl high entropy alloys in a 3.5%NaCl solution shows that the corrosion resistance of directionally solidified FeCoNiCrAl alloy is superior to that of the non-directionally solidified FeCoNiCrAl alloy.

Effect of cooling rate on MC carbide in directionally solidified nickel-based superalloy under high thermal gradient

A series of directional solidification experiments have been performed to study the effect of cooling rate on the precipitation behavior of MC carbide in nickel-based superalloy under the temperature gradient of 500 K·s-1.Results reveal that the morphology of MC carbide changes from coarse block to fine strip,then to Chinese-script,and their sizes reduce gradually with the increasing of cooling rate from 2.53 K·s-1 to 36.4 K·s-1.At low cooling rates,most of these carbides are found to be located at the grain boundary and interdendritic regions,while the coupled growth of some carbides and γ matrix in the center of γ grains is occurred at high cooling rate.The main elements forming MC carbide are Ta,W,and Hf.

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.

Microstructure and mechanical properties of Ni3Al intermetallics prepared by directional solidification electromagnetic cold crucible technique

The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.

Multi-scale coupling simulation in directional solidification of superalloy based on cellular automaton-finite difference method

Casting microstructure evolution is difficult to describe quantitatively by only a separate simulation of dendrite scale or grain scale, and the numerical simulation of these two scales is difficult to render compatible. A three-dimensional cellular automaton model couplling both dendritic scale and grain scale is developed to simulate the microstructure evolution of the nickel-based single crystal superalloy DD406. Besides, a macro–mesoscopic/microscopic coupling solution algorithm is proposed to improve computational efficiency. The simulation results of dendrite growth and grain growth of the alloy are obtained and compared with the results given in previous reports. The results show that the primary dendritic arm spacing and secondary dendritic arm spacing of the dendritic growth are consistent with the theoretical and experimental results. The mesoscopic grain simulation can be used to obtain results similar to those of microscopic dendrites simulation. It is indicated that the developed model is feasible and effective.

Effects of processing parameters on the surface quality of directionally solidified tita-nium alloy slab with cold crucible

Experiments of continuous and directional solidification of titanium alloy slabs were performed using authors-designed multi-function directional solidification apparatus with rectangular cold crucible. Influences of processing parameters on the surface qualities of the solidified slabs were studied. It is shown that the slab surface qualities can be effectively improved with increasing of the turns in coil and input power, and with decreasing of withdrawal velocity and relatively low position of pedestal to the induction coil. The influences of the processing parameters in the descending order are as follows: the turns in coil, input power, withdrawal velocity and the relative position of pedestal. With optimized parameters, quality slabs free from cracks and ripples were obtained. The solidified structure with good surface quality shows directionally solidified structure.

Numerical simulation of boundary heat flow effects on directional solidification microstructure of a binary alloy

The boundary heat flow has important significance for the microstructures of directional solidified binary alloy.Interface evolution of the directional solidified microstructure with different boundary heat flow was discussed.In this study, only one interface was allowed to have heat flow, and Neumann boundary conditions were imposed at the other three interfaces.From the calculated results, it was found that different boundary heat flows will result in different microstructures.When the boundary heat flow equals to 20 W·cm-2, the growth of longitudinal side branches is accelerated and the growth of transverse side branches is restrained, and meanwhile, there is dendritic remelting in the calculation domain.When the boundary heat flow equals to 40 W·cm-2, the growths of the transverse and longitudinal side branches compete with each other, and when the boundary heat flow equals to 100-200 W·cm-2, the growth of transverse side branches dominates absolutely.The temperature field of dendritic growth was analyzed and the relation between boundary heat flow and temperature field was also investigated.