Font Size: a A A

Rapid Solidification Of Undercooled Peritectic Alloys

Posted on:2001-03-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y LuFull Text:PDF
GTID:1101360002951603Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
The containerless rapid solidification of undercooled peritectic alloys is of importance m research of materials science in space. The present dissertation carried out systematical investigations on the containerless rapid solidification of undercooled peritectic alioys of Fe-Cu, Fe-Ge, Ag-Sb and Cu-Ti systems by means of glass fluxing method and drop tube processing. Bulk samples of Fe-Cu peritectic alloy have been undercooled by a maximum of 277K(O. 155 TL). Their microstructures are characterized by supersaturated a-Fe matrix containing small Cu particles. The size and amount of the Cu particles decrease as undercooling increases. When undercooling is below 125K, 8-Fe is certainly the nucleating phase and subject to the usual peritectic transformation. At still larger undercooling, heterogeneous nucleation enables the direct nucleation of y-Fe and the peritectic transformation can be correspondingly avoided. Significant solute trapping has taken place during the rapid solidification of highly undercooled Fe-Cu alloy, resulting in an extension of the solubility of Cu in a-Fe phase. During rapid solidification the recalescence temperature decreases, whereas recalescence rate increases with the enhancement of undercooling. Theoretical calculations indicate that there is a transition from solute diffusion controlled dendrite growth to thermal diffusion controlled dendrite growth, if 8-Fe nucleates and grows in the undercooled alloy melt. The dendrite growth of y-Fe behaves like a pure metal and experiences a transition from thermal diffusion controlled growth into interface kinetics controlled growth. Fe5oCu5o hypoperitectic alloy experiences metastable phase separation during rapid solidification under drop tube and glass fluxing conditions. In the case of glass fluxing experiment, the maximum obtained undercooling is 81K, and the sample is macroscopically separated into one upper Fe-rich part and one lower Cu-rich part. In these two segregated parts the secondary phase separation appears as Cu-rich blocks precipitated in the Fe-rich part and Fe-rich spheres scattered in the Cu-rich part. When solidified in drop tube, the obtained particles show a microstructure evolution from the full dendritic microstructure to partially separated, then entirely separated, and finally mixed microstructure. Macroscopic phase separation appears as the central Fe-rich part and surrounding Cu-rich part, and secondary phase separation is characterized by 111 2) Cu-rich spheres distributed in the Fe-rich part and Fe-rich spheres scattered in the Cu-rich part. By using glass fluxing method, Fe7oCu3o, Cu7oFe3o, Cug0Fe10 and Cu93Fe7 alloys were undercooled by up to 267K(O.156TL), 278K(O.166TL), 194K(O.124TL) and 143K(O.096TL) respectively. It is found that all of the four alloys showing metastable phase separation during the solidification, the former two show macroscopic phase separation as one upper Fe-rich part and one lower Cu-rich part. During drop tube solidification, no macroscopic phase separation was found, the peritectic phase grew directly from the alloy melt in Cug3Fe7 alloy. The growth of the primary y-Fe dendrite and the kinetics of coalescence of the separated Fe-rich droplets of above alloys are discussed. The growth of primary y-Fe dendrite is mainly solute diffusion controlled but de...
Keywords/Search Tags:liquid metals, high undercooling, containerless processing, rapid solidification, peritectic transformation.
PDF Full Text Request
Related items