Internal friction and impact toughness of structural BCC alloys

The paper deals with acoustic properties (Young's modulus E and logarithmic decrement δ) and impact toughness of the BCC alloys: low activation ferritic-martensitic steel EK-181 (produced in the RF) and V-(4-10)Ti-(4-5)Cr alloys (produced in the RF, USA and Japan). The materials have been investigated in ∼77-300 K temperature range where BCC metals usually become brittle at low temperatures. The acoustic measurements were made by composite oscillator technique mainly in the amplitude-independent damping (δi) range at frequencies of longitudinal vibrations of about 100 kHz. The data have been obtained for annealed, plastically bent and bent-then-straightened specimens. The δi(T) dependences for the pre-bent samples show a similar behavior as the impact toughness of the alloys. So, the ductile to brittle transition temperature (DBTT) for the BCC alloys can be determined by an acoustic non-destructive technique. It has been found that the acoustical DBTT depends on microstructure of specimens (internal stresses) and is different for the EK-181 steel of different thermal-mechanical treatments and for the RF, USA and Japan vanadium alloys. A comparison of the δi(T) curves for the BCC EK-181 steel and for the FCC austenitic EK-164 steel is shown and discussed as well. A viscous dragging of edge dislocations is found to control the ductile to brittle transition of the BCC alloys.