Berlin 2012 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 14: Topical Session Bulk Nanostrucured Materials III - Microstructure and Characterization I
MM 14.6: Talk
Monday, March 26, 2012, 17:00–17:15, H 0107
Laser ultrasound for evaluation of microstructure of HPT nickel — Victor V. Kozhushko1, •Heinz Krenn1, and Reinhard Pippan2 — 1Karl-Franzens-University Graz, Graz, Austria — 2Erich Schmid Institute, Austrian Academy of Sciences, Leoben, Austria
The high pressure torsion (HPT) method is employed for grain size refinement of polycrystalline nickel. The grain structure of a disk specimen of about 30 mm diameter and 2.5 mm thickness is changed from coarse grains at the center to ultrafine grains (150 nm) at the periphery of the HPT-sample. The evaluation of the elastic properties was carried out by means of laser induced ultrasound. A noncontact sensor (5 mm pancake coil) based on an electromagnetic acoustic transducer (EMAT) and a permanent cylindrical magnet with the field of ~0.25 T was attached to one side of the specimen. The coil detects the transient magnetic field due to the inverse magnetostrictive effect from the pressure pulse excited by the laser. The pronounced compression phase has nanosecond duration. The attenuation of the propagating ultrasound pulse is mainly due to the interaction with dislocations [1]. The scattering at grain boundaries produces longitudinal and shear waves which arrive to the sensor later than the primary pulse. Signals measured at different radii of the nickel specimen clearly showed a decrease of the ultrasonic attenuation with the HPT-refinement of the microstructure.
Financial support by the FWF Austrian Science Fund is appreciated (No. S10407-N16).
[1] V.V. Kozhushko et al., Key Eng. Mat. 465 (2011) 374