MECHANISMS OF STRAIN LOCALIZATION IN ULTRAFINE-GRAINED TUNGSTEN

Tungsten;Shear Band;Ultrafine Grained;Mechanical Properties;Materials Science & Engineering

Butler, Brady G.Hemker, Kevin J ;

Johns Hopkins University

关键词: Tungsten;    Shear Band;    Ultrafine Grained;    Mechanical Properties;    Materials Science & Engineering;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/60972/BUTLER-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

The material properties of tungsten are described in a long list of extremes. It has an extraordinarily high strength, density, stiffness, thermal conductivity, and melting-point. This remarkable combination of properties allows tungsten to push the envelope of possibilities in the fields of energy, aerospace and defense. Within the defense industry, there is a considerable effort to take advantage of these properties in ballistic applications. The high strength and density of tungsten make it particularly suitable for anti-armor, kinetic energy penetrator munitions. The key performance metrics for a kinetic energy penetrator are the ability to withstand the harsh conditions of launch, and efficiently perforate heavily armored targets.However, these basic requirements are actually at odds with each other.An ideal material shows highly stable plasticity during launch, while plastic instabilities, in the form of shear banding, improve ballistic efficiencies down range (i.e. terminal ballistics). Unfortunately, commercial tungsten tends to perform poorly in both instances.The strong tensile loads experienced during launch generally result in brittle failures, while tungsten actually shows a high degree of plastic stability under the pressures and strain rates of terminal ballistics. By contrast, ultrafine-grained tungsten (100-1000 nm) has shown significant promise for achieving ductility or strain localization based on the novel microstructures enabled by the processing conditions; however, the specific mechanisms of these disparate behaviors remain unclear. The goal of this study was to investigate the conditions for stable plastic flow, brittle fracture, and strain localization in sintered ultrafine-grained tungsten, and tungsten alloys, for the purpose of improving performance in ordnance applications. In particular, the mechanisms of plastic instability and strain localization were investigated to control the initiation of shear bands. Powder metallurgical processing and alloy development were effective in mediating issues related to plasticity. Mechanical testing was performed to understand the initiation and development of shear bands during deformation. Finally, these ultrafine-grained materials were characterized extensively in order to identify the mechanisms of deformation. Over the course of this research, it was discovered that commonly accepted models of strain localization in ultrafine grained metals were unsuccessful in describing the physical processes that lead to shear banding in these materials.In particular, it was determined that strain softening mechanisms, based on texture and temperature, were insufficient for describing the initiation of shear bands in ultrafine grained metals.However, these instabilities can be described based on traditional concepts of dislocation mediated plasticity, as long as the relationships between grain size, dislocation density, and deformation are accounted for properly.These discoveries have significant implications for the viability of tungsten in ballistic applications, and extreme engineering environments in general.