Non-equilibrium Phase Transformation Diagrams in Engineering Alloys B.T. Alexandrov and J.C. Lippold Welding and Joining Metallurgy Group The Ohio State University Summary The limited capacity of the conventional methods for studying of phase transitions (DTA, DSC, and dilatometry), has restricted the available phase transition data for engineering alloys to: 1) equilibrium phase diagrams and close-to-equilibrium conditions, and 2) TTT and CCT diagrams of transformations associated with significant volume changes. Novel technique and device for Single Sensor Differential Thermal Analysis (SS DTA) have recently been developed that allow studying the non-equilibrium phase transitions during actual and simulated processing (welding) of engineering alloys. These have enabled the development of new type of non-equilibrium continuous heating and cooling transformation (CHCT) diagrams that are applicable to the thermal and thermo-mechanical processing conditions for metallic alloys. A typical CHCT diagram represents the whole range of phase transitions in a specific alloy under non-equilibrium conditions (Fig. 1). These transitions may include melting and solidification, liquation and eutectic reactions, liquid- and solid-state dissolution and precipitation, solid state transformations, recrystallization, etc. Such CHCT diagrams allow determination and subsequent control of the phase transitions that are directly related to the service properties and fabricability (weldability) issues of specific alloys. This paper demonstrates the development process of CHCT diagrams in a ?’– strengthened Ni-base super alloy and advanced high strength steel (Fig. 2). Series of thermal histories have been acquired in the range cooling rates between 50 0C/s and 1000 0C/s, which cover the conditions of low heat input welding and quenching. These were processed by the SS DTA technique to determine the thermal effects of phase transformations. The obtained results show that the phase transitions in the investigated alloys occur with significant degree of supercooling that is dependent on the cooling rate.