Rapid Solidification Mechanisms Of Multiphase Fe/Ni-based Alloys Under Extraordinary Conditions

Liquid alloys can be highly undercooled and then rapidly solidified under extraordinary conditions such as containerless levitation and free fall.The microstructural characteristics and solidification mechanisms are different from those under conventional conditions.Therefore,the investigations on the rapid solidification of undercooled liquid alloys are beneficial to the enhancement of solidification theories and the development of advanced alloys.In this thesis,the dendritic growth,microstructural evolution and solute distribution of undercooled liquid Ni-Ti,Fe-Cu,Fe-Cu-Si and Fe-Cu-Ge alloys have been systematically investigated by electromagnetic levitation,drop tube and glass fluxing methods.The major results are summarized as follows.1.Dendritic and eutectic solidification of Ni-Ti alloys under containerless conditionsThe microstructural evolutions of undercooled?Ni+Ni₃Ti?and?Ni₃Ti+NiTi?eutectic-type alloys were investigated by electromagnetic levitation and drop tube techniques.The competitive growth of primary?Ni?dendrite and Ni₃Ti compound with?Ni+Ni₃Ti?eutectic causes significant microstructural variations beyond some critical undercoolings.The undercooled eutectic Ni_83.5Ti_16.5 alloy exhibits a microstructural transition of“lamellar eutectic to dendrite”when the undercooling exceeds 40 K.For hypoeutectic Ni₈₅Ti₁₅ alloy,the solidification microstructure is totally composed of segregationless?Ni?phase at the undercoolings larger than 196 K.In the case of hypereutectic Ni₈₁Ti₁₉ alloy,an irregular eutectic structure is formed at a small undercooling of 41 K.In contrast,the undercooled Ni-Ti alloys consisting of Ni₃Ti and NiTi compounds rarely experience the microstructural transition between dendrite and eutectic.The microstructure of eutectic Ni₆₁Ti₃₉ alloy appears as worm-like eutectics at the undercoolings smaller than 314 K.The microstructural morphologies of hypoeutectic Ni₆₆Ti₃₄ and hypereutectic Ni₅₈Ti₄₂ alloys are always characterized by primary dendrites within the experimental undercooling range of0²65 K and 0²06 K,respectively.Theoretical analyses indicate that the dendritic growth of both?Ni?and NiTi phases is mainly controlled by thermal diffusion under substantial undercooling conditions,but the solutal diffusion has a greater effect on NiTi growth.Besides,the growth of Ni₃Ti phase is primarily controlled by solutal diffusion at all undercoolings.Thus the?Ni?phase grows rapidly,resulting in remarkable grain refinement and conspicuous solute trapping.Furthermore,the eutectic coupled zones of regular?Ni+Ni₃Ti?and?Ni₃Ti+NiTi?eutectics are calculated theoretically,the composition ranges of which are 15.6²2.3 at.%Ti and 31.5³9.8at.%Ti,respectively.2.Rapid dendritic growth within liquid Fe-Cu alloy under substantial undercooling conditionsWhen the undercooling exceeds 7 K,the homogenous liquid Fe₅₀Cu₅₀ alloy separates into an Fe-rich zone and a Cu-rich zone.The average compositions of these two zones have a large deviation from the original alloy composition.For the Fe-rich zone,the measured metastable line of separated alloy composition follows the extension of the solidus temperature.The primary?Fe phase nucleates and grows rapidly within the Fe-rich zone.Its dendritic growth rate rises with the enhancement of undercooling,and reaches 15 m/s at the undercooling of 261 K.Thus the solidification microstructure of the Fe-rich zone transforms from dendrites into equiaxed grains with increasing undercooling.In the Cu-rich zone,?Fe phase grows slowly and appears as slender dendrites and spheres.As compared to pure Fe,the growth rate of?Fe phase in Fe₅₀Cu₅₀ alloy is much slower even at large undercoolings.Theoretical analyses demonstrate that the dendritic growth of pure Fe is only controlled by thermal diffusion.For binary Fe₅₀Cu₅₀ alloy,the?Fe phase in the Fe-rich zone experiences a transition from solutal diffusion controlled growth to thermal diffusion controlled growth as the undercooling increases,while the dendritic growth of?Fe phase in the Cu-rich zone is mainly controlled by solutal diffusion.Therefore,the dendritic growth rate of?Fe phase in the Fe-rich zone is slower than that in pure Fe but faster than that in the Cu-rich zone.3.Microstructural evolution and solutal distribution of Fe-Cu-Si alloys under extraordinary conditionsA stable liquid phase separation has been observed in ternary Fe₄₈Cu₄₈Si₄ and Fe_42.5Cu_42.5Si₁₅ alloys,while Fe₃₅Cu₃₅Si₃₀ alloy experiences a metastable liquid phase separation beyond the undercooling of 24 K.The homogeneous liquid alloy separates into an Fe-rich liquid and a Cu-rich liquid during the phase separation.The phase separation morphology of ternary Fe₄₈Cu₄₈Si₄ alloy was investigated under different experimental conditions.Since the Stokes motion driven by the density difference plays a dominant role in the evolution of macrosegregation,the bulk undercooled alloy displays a layered structure in the glass fluxing experiments.In the condition of free fall,surface segregation and Marangoni migration are the driving forces of liquid phase separation and result in the formation of two-layer and three-layer core-shell structures.The liquid phase separation is effectively suppressed under the rapid cooling condition.Therefore,serious macrosegregation has been avoided in the rapidly cooled alloy and the solidification microstructure appears homogenous.The solute distributions of undercooled Fe₄₈Cu₄₈Si₄,Fe_42.5Cu_42.5Si₁₅ and Fe₃₅Cu₃₅Si₃₀alloys were measured,the results of which show that the liquid phase separation induces an enrichment of Si concentration in the Fe-rich zone.In addition,the Si content of?Fe phase is more than that of?Cu?phase,and the Cu content of Fe₅Si₃ phase is larger than that of FeSi phase.4.Phase separation and rapid solidification of highly undercooled Fe-Cu-Ge alloysThe liquid Fe_47.5Cu_47.5Ge₅ and Fe_42.5Cu_42.5Ge₁₅ alloys were highly undercooled by glass fluxing method.The achieved maximum undercoolings are 300 and 268 K,respectively.The undercooled liquid alloys will separate into an Fe-rich zone and a Cu-rich zone,the critical undercooling for Fe_47.5Cu_47.5Ge₅ alloy is 70 K and that for Fe_42.5Cu_42.5Ge₁₅ alloy is 110 K.The average solute concentrations of Fe-rich and Cu-rich zones were measured and the results show that the Ge contents of these two separated zones are close to each other.This indicates that the Fe-rich and Cu-rich liquids have a similar affinity with solute Ge.The undercooled Fe-Cu-Ge immiscible alloys display dual solidification mechanism.In the condition of slight undercooling,the liquid alloy solidifies homogeneously,just like a normal peritectic alloy.For Fe_47.5Cu_47.5Ge₅ alloy,the primary phase is?Fe and the peritectic phase is?Cu?.In the case of Fe_42.5Cu_42.5Ge₁₅ alloy,Fe_0.84Ge_0.16 is the primary phase while Fe₃Ge and?Cu?are the peritectic phases.Liquid phase separation occurs beyond certain undercoolings,thus the peritectic alloy solidifies like a monotectic one.Once the two segregation zones are formed,the solidification process continues in two successive stages.In principle,the nucleation and growth of the primary phase in the Fe-rich zone are the keystone of the solidification process,which is followed by the minor solidification stage of Cu-rich zone.