Dynamic Real Time And Non-Contact Grain Size Measurement
|Now, for the first time, it is possible to monitor metallic microstructures in real time, in-situ, and at high temperatures while conducting physical simulations on a Gleeble® thermal-mechanical simulator, thanks to a new laser ultrasonic measurement system.|
| The new system, called LUMet (for Laser Ultrasonic Metallurgy), provides unprecedented capabilities by allowing observation of the internal physical state of the specimen during Gleeble tests.
With LUMet, while the specimen is under test, researchers can gather information on:
|The advantages of LUMet over conventional technology include:
Further, by providing better measurements in real-time, LUMet offers the potential to substantially shorten the time required to solve metallurgical problems involving materials or process.
|How It Works
LUMet makes its unique measurements by generating and detecting ultrasound pulses in a sample under test with lasers. To generate the ultrasound pulse in the sample, a high-power, short-pulse laser produces light pulses of a fraction of a Joule of energy and about 10 nanoseconds duration. Each one of these pulses ablates the surface of the sample a tiny amount (on the order of nanometers), causes intense pressure on the surface of the sample, and sends an ultra -sonic pressure pulse through it.
The ultrasound pulse is detected when it reaches the surface of the sample. A laser interferometer measures the tiny amounts of surface displacement caused by the pulse with sub-nanometer resolution by making the laser light from a second laser interfere with itself. This second laser generates
1 kilowatt of instantaneous power for 50 microseconds, which is long enough to do complete ultrasound experiments as the ultrasound pulse bounces back and forth several times within the sample. Since the ultrasonic pulse travels 5–6 millimeters per microsecond, the 50 microsecond detection pulse is a long time. In addition, the detection pulse can be delayed with respect to the generation pulse, thus allowing measurements on samples of any thickness or after any number of reflections, should there be a need to do so. The two lasers are synchronized and fired up to 50 times per second, providing essentially real-time monitoring of microstructure changes for most practical metallurgical engineering problems.
The loss of amplitude and the time between the various ultrasonic echoes gives the ultrasonic attenuation and velocity. The velocity depends on the material under test; it is different for iron or aluminum. The velocity is also a measure of the elastic modulus, which varies with temperature and phase transformation. So researchers can quantify phase transformations with precise laser-ultrasonic velocity measurements.
With LUMet researchers can also measure the texture of the sample. If they know that some crystallographic orientation is normal to the sample surface, then they know that the velocity will be largely deter -mine by that orientation. They know also that changes in the crystallographic orientation will alter that velocity. As a result, they can compute the average orientation distribution from velocity measurements. Grain size is another parameter that can be measured. If you have a continuous medium and an object embedded in it, the object will scatter the ultrasound.
The larger the object is compared to ultra-sonic wavelengths, the more scattering will result. Because each grain is like a scattering object, the larger the grain size, the more loss of amplitude will be observed. As a result, the measurement of attenuation is also a measurement of grain size. Through LUMet researchers can measure the grain size and growth at temperature of the austenite phase in steels. Previously, only time-consuming and sometimes complex metallographic analysis of quenched samples was used to evaluate former austenitic grains, and steelmakers have long wished for such a real-time technique, since the austenitic grain size at high temperature is very important for the properties of the steel at room temperature. Since LUMet can measure elastic moduli, phases, texture, and grain size, it can be used to characterize in-situ, inside the Gleeble, almost any metallurgical process that affects one or several of these parameters.