SPECIFIC FEATURES OF INELASTIC PROCESSES IN MATERIALS WITH NANOSCALE DEFECTS

Теоретически проанализирована высокоскоростная деформация сплавов, содержащих зоны Гинье-Престона, в условиях высокоэнергетических внешних воздействий. Анализ выполнен в рамках теории динамического взаимодействия структурных дефектов. Исследуемый механизм диссипации заключается в необратимом переходе энергии внешних воздействий в энергию дислокационных колебаний. Получено аналитическое выражение динамического предела текучести с учетом всех структурных дефектов, содержащихся в сплаве. Показано, что в условиях высокоэнергетических внешних воздействий наноразмерные дефекты влияют на характер зависимости механических свойств от концентрации атомов второго компонента. Зависимость динамического предела текучести от концентрации атомов второго компонента становится немонотонной и имеет минимум. Выполнены численные оценки концентрации, при которой предел текучести становится минимальным. При таком значении концентрации происходит переход от доминирования торможения дислокации зонами Гинье-Престона к доминированию торможения атомами второго компонента. The high strain rate deformation of alloys containing Guinier-Preston zones under high-energy external influences has been theoretically analyzed. The analysis was carried out within the framework of the theory of dynamic interaction of structural defects. The investigated dissipation mechanism consists in the irreversible transfer of energy of an external impact into the energy of dislocation vibrations. An analytical expression for the dynamic yield stress taking into account all structural defects of the alloy has been obtained. It is shown that, under high-energy external influences, nanoscale defects affect the nature of the dependence of mechanical properties on the concentration of atoms of the second component. The dependence of the dynamic yield stress on the atomic concentration of the second component becomes nonmonotonic and has a minimum. Numerical estimates of the concentration corresponding to the minimum yield stress has been made. At this concentration value, a transition occurs from the dominance of the dislocation drag by the Guinier-Preston zones to the dominance of the drag by the atoms of the second component.

Влияние гигантской магнитострикции на динамический предел текучести в условиях высокоскоростной деформации

The high-strain rate deformation of crystals with giant magnetostriction is theoretically analyzed. It is shown that giant magnetostriction has a significant effect on the dynamic yield stress of crystals.

THE EFFECT OF NANOSCALE DEFECTS ON THE DYNAMIC YIELD STRESS OF ALLOYS

Теоретически проанализирована высокоскоростная деформация состаренных сплавов, содержащих зоны Гинье-Престона. Получено аналитическое выражение вклада зон Гинье-Престона в величину динамического предела текучести. Показано, что динамическое торможение дислокаций наноразмерными дефектами существенно отличается от торможения точечными дефектами. The high strain rate deformation of aged alloys containing Guinier-Preston zones is theoretically analyzed. The analytical expression for the contribution of the Guinier-Preston zones to the value of the dynamic yield stress has been obtained. It is shown that the dynamic drag of dislocations by nanoscale defects differs significantly from the drag by point defects.

L-Box Form Filling of Thixotropic Cementitious Paste and Mortar

Rheological properties of cementitious pastes and mortar affect the casting, placement, and setting properties of fresh concrete. Fundamental rheological knowledge thus helps in predicting concrete flowability and workability. Empirical equations correlate actual rheological parameters based on physical material characteristics to workability tests. Still, these equations generally only take the dynamic yield stress of the material into account. This is not sufficient for thixotropic cementitious pastes or mortars, which possess structural buildup at rest. Workability predictions regarding the flow of concrete are thus more complicated with thixotropic materials. During form-filling in L-shaped formworks, the flow velocity of concrete slows down, wherefore rheological parameters change with time. At initial fast flow, high shear rates without structural buildup can be assumed. Dynamic yield stress and a steady state viscosity thus are proper parameters for empirical equations describing concrete flowability. During low shear rates, partial structural buildup takes place. Viscosity and yield stress increase due to agglomeration and affect the flowability of concrete tremendously. Rheological parameters of various cementitious pastes and mortars varying in their solid volume fraction and flowability were investigated in a vane-in-cup rheometer. The workability of the same mixtures was investigated by measuring the flow length in an L-shaped formwork. The effect of yield stress, viscosity, and thixotropic structural buildup on flow length was investigated. Subsequently correlations and discrepancies between flowability parameters and workability equations were analyzed.