Determination of dendrite coherency point characteristics using first derivative curve versus temperature

Abstract

The dendrite coherency point (DCP) temperature refers to the state of a solidifying alloy at which a coherent dendrite network is established during the formation of grains. Several relatively complex methods for detection of the DCP temperature have been developed. There are four main DCP temperature testing approaches: (i) the rheological technique, (ii) thermal analysis of the minimum temperature difference between two cooling curves, (iii) thermal analysis of the second derivative of one cooling curve, and (iv) the thermal diffusivity measurement technique. This paper follows up the proposed thermal analysis of one center cooling curve for the determination of the DCP characteristics such as: temperature, time, instantaneous solidification rate, and fraction solid. The first derivative of the cooling curve is plotted versus the temperature and time and the thermal characteristics of all metallurgical reactions, including the DCP are determined with the same accuracy achieved using the two thermocouple technique developed by Backerud et al. [4, 5]. Statistical analysis of the DCP temperature using the one versus two thermocouple techniques shows R 2 equal to 0.99. This research revealed that utilization of dT/dt versus the temperature curve methodology also allows for analysis of the α-Al dendrite nucleation and growth characteristics and consequent determination of the grain size. On-going work on this new methodology for characterization of other solidification events will be presented in subsequent papers.