Abstract
It is well known that semi-solid forming could only obtain coarse-grained microstructure in a few alloy systems with a low melting point, such as aluminum and magnesium alloys. This work presents that semi-solid forming could also produce novel bimodal microstructure composed of nanostructured matrix and micro-sized (CoFe)Ti2 twins in a titanium alloy, Ti62Nb12.2Fe13.6Co6.4Al5.8. The semi-solid sintering induced by eutectic transformation to form a bimodal microstructure in Ti62Nb12.2Fe13.6Co6.4Al5.8 alloy is a fundamentally different approach from other known methods. The fabricated alloy exhibits high yield strength of 1790 MPa and plastic strain of 15.5%. The novel idea provides a new insight into obtaining nano-grain or bimodal microstructure in alloy systems with high melting point by semi-solid forming and into fabricating high-performance metallic alloys in structural applications.
Highlights
Nanostructured materials often exhibit low ductility at room temperature and very limited or lack of work hardening due to limited dislocation capability[1]
Such a semi-solid state characteristic with highly dense random-packed structure may be of significance and could be employed for fabricating new-structure materials, such as with nano-grained or bimodal microstructure, by semi-solid forming from multicomponent alloy systems having a high melting point
Two evident endothermic peaks are observed at temperatures of 1125 °C and 1180 °C respectively for the 70 h-milled Ti62Nb12.2Fe13.6Co6.4Al5.8 nanocomposite powder, confirming that there exists a semi-solid interval between 1080 °C and 1200 °C for the Ti62Nb12.2Fe13.6Co6.4Al5.8 alloy
Summary
Nanostructured materials often exhibit low ductility at room temperature and very limited or lack of work hardening due to limited dislocation capability[1]. A question arises: if a liquid phase, resulted from preferential eutectic reaction of two solid phases in a multi-phase alloy system with a high melting point, has a highly dense random-packed structure, does the composition of this liquid phase tend to form a nanostructured phase/ microstructure in process of solidification17,18?
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