Electrostatic Levitation Processing and Rapid Solidification Mechanism of Refractory Metallic Materials

Author:Yang Shang Jing

Supervisor:wei bing bo


Degree Year:2018





Refractory metal materials such as tungsten and tungsten alloys have important engineering applications in the fields of national defense and civil industry.However,the extremely high melting temperatures have hindered the deep theoretical study of their liquid physical and chemical properties and rapid solidification mechanism.If supplemented with laster heating,electrostatic levitation technology should be another way to this end.In this paper,the electrostatic levitation process is optimized to design.Combined with molecular dynamics simulation method,the liquid thermalphysical properties and rapid solidification of9 refractory pure metals and 4 binary alloys are studied.The major results are summarized as follows.1.Optimal design and dynamic control of electrostatic levitation processThe electrostatic levitation capability is significantly improved by precisely designing the positioning optical path.Large size metal materials with diameter up to 10 mm were successfully levitated for the first time.Meanwhile,the functions of electrostatic levitation experiment system were expanded.Trigger nucleation and liquid quenching functions were added,and precisely controlling sample temperature was realized.The density,thermal expansion coefficient,surface tension,viscosity,specific heat and emissivity of metals in stable and undercooled states were accurately determined combined with purple background light technology,digital image processing method and droplet oscillation method.High-speed camera and photodiode were used to study the rapid solidification process of undercooled melt in electrostatic levitation.2.Rapid solidification of molten refractory pure metal in deeply undercooled stateRapid solidification of 9 refractory pure metals W,Re,Ta,Mo,Nb,Hf,V,Zr and Ti under electrostatic levitation condition was achieved.With HCP crystal structure,the undercooling level about 787 K(0.24Tm)of pure Re was the largest,which was much higher than that of 0.18Tm for other BCC structure metals.The relationships of density,thermal expansion coefficient,specific heat and emissivity with temperature of the 9 pure metals at undercooled liquid and high temperature solid states were experimentally measured.The relationship between solidification platform time and undercooling after recalescence was obtained.By extrapolation method,the hypercooling limit of pure Nb and pure Zr were determined to be 706 K(0.26 Tm)and 524 K(0.25 Tm).In adiiton,their liquid average specific heat and emissivity were derived.Meanwhile,the surface tension and viscosity of pure Nb and pure Zr were measured by droplet oscillation method.The nucleation mechanisms of pure Nb and pure Zr in electrostatic levitation were studied by combining mathematical statistic method and classical nucleation theory.The statistic results showed that their undercooling degree obeyed the Gaussian distribution.The nonlinear relationship between the critical nucleation size and the undercooling of pure Nb was calculated by molecular dynamics simulation,and the solid-liquid interface energy of0.367 J·m-2 was obtained.In order to study their rapid solidification mechanism,the relationships between dendritic growth velocity of pure metals were determined.Among them,the growth velocity of pure Ti was the largest,95 m·s-1 at undercooling of 329 K.Theoretical analysis shows that the controlling factors of the dendrite growth for refractory pure metals changed from the thermal-diffusion control to the interface kinetic factor with the increase of undercooling.The crystal growth velocity of Ta,Nb,Zr and Ti under the control of a single interface kinetic term was further calculated with the change of undercooling,which showed a trend of increasing first and then decreasing.3.Rapid dendrite growth mechanism in binary Zr-Ti/Si AlloysThe rapid solidification mechanism of Zr90Ti10 and Zr80Ti20 alloys was studied.Their dendritic growth velocity and undercooling showed a power-function relationship.At their maximum undercooling of 366 K and 379 K,the growth velocity reached 71 m·s-1 and 64m·s-1,respectively.The rapid solidification process had an effect on the subsequent solid phase transition.The enhancement of liquid undercooling changed the martensite microstructures.Under electrostatic levitation condition,the undercooling levels of liquid Zr99Si1,Zr97Si3and Zr95Si5 hypoeutectic alloys increased with the rise of Si content,which were 392 K(0.19TL),423 K(0.22TL)and 451 K(0.23TL),respectively.The dendritic growth velocity of(βZr)primary phase increasesd with the enhancement of undercoooling,and the mechanism changed from solute-diffusion controlled to thermal diffusion controlled,the critical undercoolings of which were 125 K,240 K and 350 K.Under rapid solidification condition,the microstructures of Zr-Si alloys were composed of(βZr)dendrite and(αZr+Zr3Si)eutectic,and the microstructures were significantly finer with the increase of undercooling.4.Thermophysical properties and rapid solidification of liquid Nb-Zr alloysThe experimental measurement displayed that the density of binary Nb-Zr alloy system decreased linearly with temperature increasing.Based on this,its potential function was modified.The molecular dynamics calculation showed that the atomic order of liquid Nb-Zr alloy increased with the decrease of temperature and the increase of Nb composition.The rapid solidification of Nb95Zr5,Nb90Zr10 and Nb95Zr15 alloys was realized.The maximum undercoolings were 534 K(0.20TL),498 K(0.19TL)and 483 K(0.18TL),and the corresponding maximum dendrite growth velocities were 38.5,34.0 and 27.1 m·s-1.In addition,the microstructures were remarkably refined with the increase of undercooling.It was found that metastable phase may be formed in the rapid solidification process of Nb90Zr10 alloy from the double recalescence and density evalution,and the metastable phase was predicted to be Rh structure by molecular dynamics simulation.5.Characteristics of rapid dendrite growth in refractory W-Ta alloysThe stable and undercooling densities of liquid W-x%Ta(x=25,50,75)alloys were measured,which decreased linearly with increasing temperature.The densities at liquid line temperatures were 16.12,15.48 and 14.87 g·cm-3,respectively.The undercooling of liquid refractory W-25%Ta,W-50%Ta and W-75%Ta alloys reached752 K(0.21TL),745 K(0.21TL)and 773 K(0.23TL),respectively,and their dendrite growth velocity increased with the enhancement of undercooling.At the maximum undercooling,the growth velocity reached 35.2 m·s-1,33.7 m·s-1 and 33.4 m·s-1 respectively.In addition,the microstructure of W-Ta alloys changed from coarse equiaxed dendrites to well-developed fine dendrites,and the segregation degree decreased.