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.
In addition to the measurement of the CTE, a Thermal Mechanical Analysis (TMA) type dilatometers allows the measurement of mechanical properties. This can also be used to identify phase changes, since mechanical properties such as the Young’s modulus also change during phase transitions.
The Linseis L78 quenching dilatometer is commonly used to measure the phase transformation (austenite, ferrite, martensite, perlite, bainite, …) temperatures in steels. This type of push rod dilatometer helps simulating manufacturing methods such as salt bath heat treatment, oil quenching, etc.
These transformation temperatures shift depending on the heating and cooling rates. For some phase transitions high heating and cooling rates are required. This speed is created by an induction heater and an induction coil with a gas cooling helix. E.g. a sample can be heated to 1000°C and quenched to room temperature within less than 30 seconds. An axial force can be applied to sample by adding the deformation option to the system and thereby increasing the capability of the unit to simulate stress hardening processes.
Researchers can use the dilatometer to create CCT and TTT diagrams and for developing new metal compositions. These diagrams help identify processing conditions that can be used to achieve a desired multiple phase composition in a metal alloy giving it unique mechanical properties.
The glass transition temperature Tg can be measured for non-crystalline materials like, glasses or polymers using a dilatometer. The Tg marks the point at which the molecular movement increases sharply and the transformation of the material from a solid to a rubbery, viscoelastic state begins.
At the Tg, the CTE increases and can be detected as a slope change of the expansion curve. If the sample is heated further, the expansion curve reaches a maximum at the softening point. After that the viscosity decreases and the sample begins to flow. A TMA can also use the decrease in the Young’s modulus to determine Tg.
Linseis dilatometers are equipped with softening point detection 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.