| 其他摘要 | In order to testify the roles of B, Zr and P in the Fe-Co-Ni based low thermal expansion superalloy system, and investigate the way to enhance the mechanical performance of the alloys, both the effects of B and Zr on the microstructure and properties of an newly developed Al-Cr-modified Thermo-Span alloy and the role of P in IN783 alloy were investigated in this dissertation.
By investigating the B-Zr-doped modified Thermo-Span casting alloys, it was found that B was hardly dissolved in Laves phase, and increasing B reduced the formation of Laves phase and lowered the terminal solidification temperature to around 1127℃ till the eutectic reaction happened forming the lamellar B-bearing phase. By soaking the new Thermo-Span casting alloy at 1100℃×4h+ 1200℃×25h, the residual interdendritic precipitates were markedly reduced and the element distribution was also homogeneous.
Effects of B on microstructure and various properties including coefficient of thermal expansion (CTE), HV hardness, and both smooth and notch stress rupture properties of Zr-containing Al-Cr-modified Thermo-Span alloy were studied. The results showed that increasing B content constrained the formation of Laves phase, while promoted the formation of boride. Boron hardly dissolved in the matrix and increasing of B hardly affected HV hardness of the new alloy. Increasing B not only prolonged the rupture life, but also eliminated the notch sensitivity in the new alloy. The improvements in properties were supposed to be related with the segregation of B at grain boundaries and interfaces. Enhanced segregation of B was believed to improve grain boundary cohesion, tie up vacancies and reduce grain boundary diffusion, which constrained the nucleation and propagation of intergranular microcracks.
During exposure at 650℃, the growth rate of γ’ was slower in new Thermo-Span alloy than conventional Thermo-Span alloy, and γ’was much finer in new Thermo-Span alloy aged at 650℃ for 1000h. A new Nb-enriched phase was formed in the defect-enriched zone adjacent to grain boundaries and large blocky particles. Being exposured at 650℃ for 1000h, the tensile strength at room temperature was lowered, while the strength at 650℃ was varied slightly in new Thermo-Span alloy. The coarsening of γ’eased the creep deformation of the matrix, so the creep rupture life of the tested alloys was decreased, while the elongation was prolonged. As the matrix was easier to creep, the grain boundary had to be more bearable to high strain and stress concentration in order to coordinate the deformation. Consequently, the strengthening effect of B on grain boundary was more critical, which leaded to the longer rupture life for high B-doped aged alloy. The formation of the Nb-enriched new phase softened the grain boundary, which eased the relaxation of stress concentration. As a result, the notch sensitivity was improved in the aged alloys.
The as-cast microstructure and solidification behavior of IN783 alloy were studied. The solidification sequence was determined as following: L→ L+ γ→ L+ γ+ β→ γ+ β+ Laves. Aluminum and niobium promoted the precipitation of Al-enriched β phase and Nb-enriched Laves phase respectively. Two types of β phase were observed: the large blocky one was formed at 1349 ℃ during the solidification process due to the interdendritic segregation of Al, whereas, these small granular particles around Laves phase or at grain boundaries were formed by Al diffusion in the solidified alloy during cooling process. The primary β phase expelled Nb in the residual liquid, which promoted the formation of Nb enriched Laves phase at a lower temperature. Therefore, the large blocky β phase was usually enveloped by Laves precipitates or eutectic (γ/Laves). In most interdendritic zone, Nb segregation was predominant, and only eutectic (γ/Laves) formed finally at about 1170℃~1180℃. In casting IN783 alloy, P segregated with Nb in the interdendritic zone. The solidification of high P-doped alloy was terminated by the formation of P-bearing lamellar γ/Laves eutectic and the incipient melting temperature was not lowered down. During soaking at 1160℃, the lamellar eutectic dissolved and the P-enriched phase grown up gradually. Through homogenizing at 1160℃×12h+ 1180℃×24h+ 1200℃×6h, the large block interdendritic precipitates were almost dissolved and the element distribution was also homogeneous.
By investigating the microstructure and properties of IN783 alloy, it was found that γ′ precipitates did form during β-aging at 845 ℃, they undergo coarsening with the soaking duration prolongs. During γ′ aging process at lower temperatures, fine γ′ was also formed. As a result, a bimodal distribution of γ′ was formed in the matrix after standard age treatment. The formation of β precipitates restricted the formation of coarse γ′ precipitates by consuming Al atoms in the adjacent matrix. Prolonging the β-aging duration markedly enhanced the precipitation of both intergranular and intragranular β precipitates, enlarging the coarse γ′ free zone around β, and the precipitation of coarse γ′ phase in the matrix was also affected. It was found that the creep rupture life was insensitive to the precipitation of intergranular β phase, while the ductility could be enhanced when β phase was coarsened.
For IN783 alloy treated by standard heat treatment, increasing P content promoted the formation of intergranular β phase and phosphide. The precipitation of intergranular β phase was found to be beneficial for preventing the oxygen from penetration along grain boundaries, while the phosphide was easy to be oxidized. However, phosphorus segregation deteriorated the interfacial cohesion of intergranular β/γ matrix and induces many intergranular cracks. Consequently, the stress-rupture life was reduced in high P-doped alloys. |
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