Processing and properties of materials with or without adiabatic shear susceptibility
1 online resource (151 pages) : PDF
University of North Carolina at Charlotte
In this dissertation, two topic areas related to adiabatic shear banding (ASB) phenomenon were studied. Body centered cubic (BCC) metals and alloys may exhibit ASB under uniaxial high strain rate compression if the grain size is refined into the ultrafine grained (UFG, grain size d > 100 nm but < 1000 nm) or nanocrystalline (NC, d < 100 nm) regime through severe plastic deformation (SPD). Such adiabatic shear localization behavior is an essential property for a kinetic energy (KE) penetrator which exhibits a "self-sharpening" effect. This means that the head of the penetrator remains sharp during the penetration so that most of the KE is primarily imparted to destroy the target. However, UFG/NC BCC refractory metals processed by SPD could not be used as KE penetrators due to the limitation of their physical dimensions. In this work, heterogeneous multi-layer structures of W/Fe/W and W/V/W have been produced using cold rolling and diffusion bonding to achieve a hierarchical structure whereby the limiting dimensions were extended and during testing the products exhibited ASB. The ASBs were observed to propagate through the heterogeneous layers and the bonding interfaces remained intact upon high rate loading. Subscale heterogeneous projectiles have been fabricated for ballistic testing. ASBs identified at the head of the projectile residuals suggest an early onset of shear localization behavior during the ballistic event.In contrast to the former topic, a twinning induced plasticity (TWIP) steel with a composition of Fe-15Mn-2.5Si-2Al-0.6C and face centered cubic (FCC) structure has been found to exhibit strong strain and strain rate hardening upon the mechanical loading, resulting in outstanding ASB resistance. The strain and strain rate hardening mechanisms have been experimentally investigated as a function of strain rate under uniaxial tension and compression. The steel sample is characterized by a constant strain hardening rate accompanied by high strength and high ductility under tension. This extraordinary strong strain rate hardening behavior was discussed in the context of deformation kinetics: high strain rate sensitivity (SRS) and low activation volume compared with coarse-grained (CG) FCC counterparts. It is reported for the first time: a marginal size effect has been revealed in this TWIP steel and it is attributed to an extremely small activation volume. According to the Zener-Hollomon equation, increasing the strain rate has an equivalent effect to that of a decrease in deformation temperature which favors the formation of twins with small thickness and spacing.
ACTIVATION VOLUMEADIABATIC SHEAR BANDINGPLASTIC INSTABILITYSTRAIN RATE SENSITIVITYTUNGSTENTWIP STEEL
Kecskes, LaszloCherukuri, HarishXu, TerryChen, Don
Thesis (Ph.D.)--University of North Carolina at Charlotte, 2018.
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