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

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 plates from pre-existing β grain boundaries, and eutectoid decomposition (+ 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 plates, supersaturated in Cu, are formed along with a highly refined lamellar eutectoid product between these plates. In contrast, intermediate (9 K/s) and slow (2 K/s) cooling results in considerably coarser proeutectoid 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 and the lamellar eutectoid product, which seems to promote the cooperative growth of lamellar + Ti₂Cu. Irrespective of the cooling rate, nucleation of the lamellar eutectoid (+ Ti₂Cu) product appears to only occur at the interface between the proeutectoid plates and the β matrix.