Rapid Dendritic Growth And Metastable Phase Separation Within Liquid Peritectic Fe-Cu Based Alloys

The rapid dendritic growth and metastable phase separation of highly undercooled alloy melts are the important subjects in the field of material science.The systematic study of these two aspects can promote the development of solidification theories and realize the industrial application of metastable alloy materials.In this dissertation,the rapid dendritic growth and metastable phase separation of liquid peritectic Fe-Cu,Fe-Cu-Sn and Fe-Cu-Ge alloys are extensively investigated by electromagnetic levitation,glass fluxing,drop tube and melt spinning techniques.The major results are summarized in the following four respects.1.Dendritic growth mechanism of liquid binary peritectic Fe-Cu alloys under electromagnetic levitationThe specific heats of liquid peritectic Fe-Cu alloys are determined by the adiabatic copper calorimeter.The enthalpy changes of liquid peritectic Fe-Cu alloys show linear relationships over wide temperature ranges and specific heat data of these alloys are almost constant.Meanwhile,molecular dynamics simulation is used to theoretically predict the specific heat values.The excessive specific heat,enthalpy change,entropy change,Gibbs free energy change,thermal diffusivity and thermal conductivity versus temperature are also calculated based on the measured specific heats.The dendritic growth kinetics of peritectic Fe-Cu alloys are investigated by using electromagnetic levitation technique.The single phase structure does not form for highly undercooled Fe-Cu alloys.The phase constitutions of hypoperitectic Fe_93.5Cu_6.5,peritectic Fe_92.8Cu_7.2 and hyperperitectic Fe_88.5Cu_11.5 alloys are all composed of?Fe and（Cu）solid solutions.The maximum undercoolings achieved in the experiments are 194 K（0.11 T_L）,401K（0.23 T_L）and 468 K（0.27 T_L）for three alloys,respectively.With the enhancement of the undercooling,the dendritic growth velocities of primary phases for three alloys increase as power functions,and reach 4,69 and 68 m·s^-11 at the maximum undercoolings.For hypoperitectic Fe_93.5Cu_6.5 alloy,the dendritic growth ofδFe phase is mainly controlled by thermal diffusion.For peritectic Fe_92.8Cu_7.2 and hyperperitectic Fe_88.5Cu_11.5 alloys,the dendritic growth ofδFe and?Fe phases is successively controlled by thermal diffusion,solute diffusion and interface kinetics effect with the increase of the undercooling.The theoretical analyses show that almost segregationless solidification of peritectic Fe-Cu alloys is realized if undercooling is sufficiently large.2.Dendritic growth investigation on ternary peritectic Fe-Cu-Ge alloys under containerless stateDendritic growth characteristics of highly undercooled peritectic Fe_88.2Cu_6.8Ge₅ and Fe_84.1Cu_10.9Ge₅ alloys are investigated by electromagnetic levitation method.XRD and SEM analyses show that these two alloys are composed of?Fe and（Cu）phases at different undercoolings.The dendritic growth velocities of primary phases show power functions with the rise of undercooling,which reach 1.7 and 1.3 m·s^-1 at the maximum undercoolings of 282and 255 K.The microstructures of two alloys show significant grain refinement during rapid solidification.The ternary peritectic Fe_47.5Cu_47.5Ge₅ and Fe₄₅Cu₄₅Ge₁₀0 alloy droplets show metastable liquid phase separation in the rapid solidification process.With the decrease of alloy droplet,the microstructures of these two alloys evolve from three-layer core-shell and two-layer core-shell structure into dendrite morphology.The primary phase are all?Fe phase.The ternary peritectic Fe_42.5Cu_42.5Ge₁₅ alloy does not exhibit metastable liquid phase separation during solidification.The microstructure is characterized by dendrite and the phase constitutions are Fe_0.84Ge_0.16,Fe₃Ge and（Cu）phases.The Fe_0.84Ge_0.16.16 dendrites are primary phase.The experimental results reveal that the increase of Ge content can suppress the metastable liquid phase separation gradually.3.Metastable phase separation and microstructural evolution of highly undercooled ternary peritectic Fe-Cu-Sn alloysDuring glass fluxing experiments,the critical undercoolings of ternary peritectic Fe_62.5Cu_27.5Sn₁₀0 alloy for occurrence liquid phase separation and macrosegregation are 65 and142 K,respectively.The dendritic growth velocity for primary?Fe phase is determined.Once bulk undercooling exceeds 142 K,its dendritic growth velocity increases exponentially with undercooling,which reaches 30.4 m·s^-1 at the maximum undercooling of 360 K（0.21T_L）.The metastable liquid phase separation of Fe_62.5Cu_27.5Sn₁₀0 alloy droplets is also explored under the free fall condition.Experimental results show that the metastable liquid phase separation leads to the formation of two-or three-layer core-shell structures and uniformly dispersed structures.Theoretical calculations reveal that the thermal and solutal Marangoni migrations are the dynamic mechanisms responsible for the development of core-shell structure.During the melt spinning experiments,it is found that the cooling rate displays a remarkable influence on the refinement of alloy ribbons.The solidification pathways of undercooled peritectic Fe_27.5Cu_62.5Sn₁₀ alloy are modulated by rapid solidification techniques.Through regulating the liquid undercooling,three types of microstructures:dendrite,dispersive structure and macrosegregation pattern are formed under the normal gravity condition.Below the first critical undercooling of 15 K,the alloy melt displays the normal peritectic solidification.At moderate undercoolings above 15K,the metastable liquid phase separation takes place and the solidified microstructure appears as homogeneously dispersed structure.If undercooling further overtakes the second threshold of 107 K,macrosegregation occurs and the bulk alloy separates into an Fe-rich zone and a Cu-rich zone.It is found that the liquid phase separation time influences the microstructural evolution.Under the free fall condition,if the droplet diameter decreased below 805μm,the metastable liquid phase separation is induced and the microstructural morphology of Fe_27.5Cu_62.5Sn₁₀ alloy droplet evolves from dendrite into dispersive structure.Furthermore,experimental and simulated results reveal that the temperature gradient has great influence on the size distribution of Fe-rich globules.During melt spinning experiments,the microstructures of ternary Fe_27.5Cu_62.5Sn₁₀ alloy ribbons before and after annealing refine significantly with increasing roller speed.4.Dendritic growth and magnetic property of highly undercooled ternary peritectic Fe-Cu-Sn alloysThe experimental results reveal that ternary equiatomic peritectic Fe_33.3Cu_33.3Sn_33.3 alloy appears as peritectic solidification characteristics during rapid solidification.The solidification microstructures are all composed of primary?Fe dendrites together with Cu₃Sn and Cu₆Sn₅ phases.The results reveal that the growth velocities of primary?Fe phase increase in the form of power law with enhanced undercooling,and its maximum value is 33mm·s^-1 at the undercooling of 251 K.With the enhancement of undercooling,the coarse dendrites evolve into broken and equiaxed dendrites.Under the free fall condition,with the increase of surface cooling rate,the morphology of primary?Fe phase transforms from coarse dendrites into broken dendrites.The alloy samples of ternary peritectic Fe_37.5Cu_37.5Sn₂₅5 alloy are all composed of?Fe,Cu₃Sn and Cu₆Sn₅ phases at different solidification conditions.Under the normal gravity condition,the liquid phase separation can be induced if the undercooling exceeds 177 K.During the undercooling range of 8¹77 K,the maximum dendritic growth velocity of primary?Fe phase is 41.5 mm·s^-1.Under the reduced gravity condition,the Fe_37.5Cu_37.5Sn₂₅alloy alloy droplets transform from dendrites into phase separation patterns with the reduction of alloy droplet.During the melt spinning experiments,the microstructures of these two alloy ribbons refine significantly and show soft magnetic characteristics.The coercivity changes with the surface cooling rate.Furthermore,the results indicate that the grain size of?Fe phase is the main factor to influence the coercivity of alloy ribbons.