Effects of cooling rate on the microstructure and solute partitioning in near eutectoid Ti–Cu alloys

Abstract

The effect of cooling rate on eutectoid decomposition in a near eutectoid (Ti-5.5 at.% Cu) alloy has been investigated in a systematic manner by coupling scanning electron microscopy, transmission electron microscopy and atom probe tomography studies. Thus, the competition between nucleation and growth of proeutectoid α plates from pre-existing β grain boundaries, and eutectoid decomposition (α + Ti₂Cu) via a pearlitic mechanism has been studied as a function of cooling rate, using a Jominy-end quenched sample that was cooled from the high-temperature single β phase. When the alloy was subjected to very fast cooling (160 K/s), proeutectoid α plates, supersaturated in Cu, are formed along with a highly refined lamellar eutectoid product between these α plates. In contrast, intermediate (9 K/s) and slow (2 K/s) cooling results in considerably coarser proeutectoid α plates as well as lamellar eutectoid products. With the decrease in the cooling rate, there was a substantial increase in the volume fraction of the lamellar eutectoid product and the composition of all decomposition products approached their equilibrium values. Also, the slowest cooled sample (2 K/s) exhibited substantially rougher and irregular interfaces between the proeutectoid α and the lamellar eutectoid product, which seems to promote the cooperative growth of lamellar α + Ti₂Cu. Irrespective of the cooling rate, nucleation of the lamellar eutectoid (α + Ti₂Cu) product appears to only occur at the interface between the proeutectoid α plates and the β matrix.

Keywords:

• Ti alloys
• eutectoid decomposition
• pearlitic transformation
• proeutectoid α

Acknowledgements

The authors gratefully acknowledge the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) and CNPq (National Council for Scientific and Technological Development) for their partial financial support of this work. The authors also gratefully acknowledge the use of experimental facilities at the Center for Advanced Research and Technology (CART) at the University of North Texas.