Phase transitions with dilatometer
A dilatometer is well suited to identify phase transformation in materials since a phase change is also accompanied by a change in the Coefficients of Thermal Expansion (CTE) of the material. The temperature where the CTE changes is called the phase transformation temperature.
A material phase transition involves a materials ability to change its crystal lattice structure to suit requirements of thermodynamics, Gibbs Free energy, or in response to a mechanical stress. Phase transfomations, in many cases, involve mobility of lattice vacancies, anion and cations through thermally activated Arrhenius diffusion processes. Understanding the kinetics of transformation of these phases can be useful for creating unique material properties by changing the phase portions in a material.
The Linseis RITA quenching dilatometer is commonly used to measure the martensite, ferrite, or austenite phase transformations in steels. This type of push rod dilatometer helps duplicate conditions needed to study methods of quench hardening steel. These transformations occur extremely quickly requiring the dilatometer to heat and cool the samples ballistically. This speed is create by an induction heater followed with quench cooling.
As an example, a sample can be heated to 1000°C and quenched cool to room temperature all in less than 30 seconds from start to finish. An axial force can be added to the system by adding the deformation option to increase the capabiltiy of the unit to measure stress hardening. Investigators use the dilatometer to create CCT and TTT diagrams for new metal compositions. These diagrams help identify processing conditions that can be used to achieve a desired mutiple phase composition in a metal alloy giving it unique mechanical properties.
The glass transition temperature Tg is commonly measured for non-crystalline, glass, polymer materials using a dilatometer. A material Tg temperature marks the point where moluecular motion starts to occur while changing the material from a rigid solid to a rubbery / visco-elactic solid.
The CTE increases and can be detected as a slope change of the expansion curve at the Tg. Above the Tg the material becomes visco-elastic and then reaches the softening point at the maximum peak of the expansion curve. As the material is heated above the softening point the material starts to flow as the viscosity becomes lower.
Thermal Mechanical Analysis (TMA) type dilatometers that apply a cyclic force on the sample can identify the changes in the mechanical properties.Through this transition the modules decreases as the material changes from a ridge solid to visco-elastic solid. Linseis dilatometers are equipped with softening point detection software that will automatically stop the furnace heating once the softening point is reached. This helps avoid melting materials in the instrument and destroying the measuring system.