Increase In Grain Boundary Energy Research Articles

Constitutive description, processing maps, and un-explored deformation mechanisms of pure Mg and Mg-6Al(wt%) alloy under hot compression

This study investigates the hot deformation behavior and final microstructure features with and without aluminum (Al) incorporation in magnesium (Mg). To this end, pure Mg and Mg-6Al(wt%) alloy were subjected to high-temperature compression and systematically analyzed the flow stress, activation energies, Zener Hollomon parameters, processing maps, and microstructure evolution, including geometrically necessary dislocations under different g vectors. These all were in an excellent relationship with each other. The microstructure analysis greatly validated the processing map parameters. Amazingly, we have attained an unexplored grain boundary sliding phenomenon under high-strain rate compression in pure Mg. This phenomenon is only reported in high-temperature tensile specimens, especially for those who attained superplasticity. Unfortunately, the mechanism attained at a strain rate (10 s−1) and temperature (573 K), which is referred to as an unstable region. Above the temperatures of 573 K, the Mg lost thermal stability, and an abrupt increase in grain size happened (3–40 µm). The addition of Al restricted the abnormal grain growth at elevated temperatures and retained grain size within < 10 µm. The higher geometrically necessary dislocation density and increase in grain boundary energy are the potential reasons for the thermal stability of Mg-Al alloy. Therefore, incorporating the Al not only enhances the formability of Mg but also provides many different stable zones, thermal stability and increases the power dissipation efficiency.

Effect of Grain Boundary Energy in Recrystallization Simulation by Phase Field Method

In this paper, the effect of grain boundary energy in AZ31 Mg alloy with multi-order parameters phenomenological phase field model has been discussed during the progress of recrystallization. The average grain size of the recrystallization grain at a certain temperature and a certain restored energy but various grain boundary energies have been studied, and the simulated results show that the larger the grain boundary energy is, the larger the average grain size will be, and the speed of grain growth will increase with the increase of grain boundary energy. Additionally, temperature will also increase the grain growth rate.

Effects of Vacancies on Deformation Behavior in Nanocrystalline Nickel

The effects of vacancies on deformation of nanocrystalline Ni have been investigated by experiments and molecular dynamics (MD) simulations. In the experiments, nanocrystalline Ni specimens containing different numbers of vacancies were produced by electrodeposition and annealing, and their mechanical properties were investigated by tensile tests. As a result, the yield stress and fracture stress for the specimen containing more vacancies were lower than those for the one containing fewer vacancies. The MD simulations showed that the grain boundary energy is increased by the presence of vacancies in the grain boundary, however, that an increase in grain boundary energy with straining is reduced by the presence of vacancies in the grain boundary. The results of the experiments and simulations suggested that there is a correlation between the grain boundary energy characteristics and the mechanical properties of the nanocrystalline Ni.

A Molecular Dynamics Study of the Energy and Structure of the Symmetric Tilt Boundary of Iron

The temperature dependences of the energy and structure of the symmetric tilt boundary of bcc and fcc iron were investigated by molecular dynamics simulation. A large energy cusp was observed at the bcc(112)‹110›Σ3 and fcc(111)‹110›Σ3 grain boundary plane, which is a twin boundary, whereas it was not observed at the bcc(111)‹110›Σ3 plane in spite of it having the lowest Σ-value. The grain boundary energy increased at the temperature close to the melting point except for the grain boundary planes that have a large energy cusp. On the other hand, it was newly founded that the grain boundary energies at the bcc(112)‹110›Σ3 and fcc(111)‹110›Σ3 did not increased despite such a high temperature. The increase in grain boundary energy was due to the premelting, which is a localized disorder of the atoms near the grain boundary energy. It was confirmed that the grain boundary energy is affected by the matching at the interface rather than the periodicity described by the Σ-value.

SUS316Lステンレス鋼における超強ひずみ加工による(α+γ)ナノ2相組織の形成

Mechanical milling (MM) process is applied to SUS316L austenitic stainless steel powder and a nano-duplex structure formation behavior is investigated. A nano grain structure with BCC phase forms in surface region of the powder milled for 720 ks. Those nano grains have the grain size of less than 20 nm. The (α+γ) nano-duplex structure, whose grain size is approximately 100 nm, forms in the inner region of the milling powder. The α grain in the surface region is an aggregated structure and consists of equiaxed grains, while those α grains in the inner region disperse homogeneously and indicate more spherical and smooth shape compared with the γ grains around them. TEM/EDS examination revealed that there occurs two kinds of BCC transformation. Those α nano grains in the surface region are considered to form by increase of grain boundary energy. On the other hand, the spherical α nano grains in the inner region form by increase of the grain boundary energy as well as the chemical free energy. Such an increase of chemical free energy is assumed to be caused by the existence of a huge number of vacancies which are introduced by the MM treatment. Although the austenite phase in the SUS316L steel is meta-stable at the room temperature, it hardly transforms to martensite phase even by a heavy cold rolling. Thus, these two kinds of α transformation are extraordinary phenomena and are considered quite different from these conventional strain induced martensitic transformation. A (γ+σ) nano-duplex structure material obtained by sintering (α+γ) nano-duplex powder has a good mechanical property.

Atomistic simulation of grain boundary sliding in pure and magnesium doped aluminum bicrystals

Molecular dynamics and statics simulations are used to study grain boundary sliding and energy in bicrystals with symmetric tilt grain boundaries of Al and Mg doped Al. There is an increase in grain boundary energy in Al bicrystals with the presence of Mg depending on the position of Mg atom. Simulations of sliding show a clear dependence of magnitude of sliding on grain boundary energy.

Synthesis of intermetallics by mechanical alloying

Mechanical alloying (MA), a solid-state powder processing method, is a “far from equilibrium” synthesis technique which allows the development of novel crystal structures and microstructures, leading to enhanced physical and mechanical properties. The application of MA to the synthesis of intermetallics in the TiAl(Nb), AlFe, NbAl, TiMg, AlZr(Fe) and AlMg systems is presented. The ability to synthesize a variety of alloy phases, including supersaturated solid solutions, nanocrystalline structures, amorphous phases and intermetallic compounds themselves, is discussed. No extension of solubility using MA was observed in the intermetallics studied, unlike the situation using rapid solidification (RS). Nanostructured grains were observed in all compositions, their rate of decrease in size and minimum size being related to the following partially interrelated parameters: stability of the intermetallic, grain boundary energy, melting point and the balance between defect creation/recovery. Long-time milling generally resulted in amorphous phase formation largely because of the increase in grain boundary energy per mole with reduced grain size; good agreement with the Miedema model for amorphization was obtained in the AlFe system. Generally, annealing was required to form the intermetallic after MA; however, intermetallics with a large negative enthalpy of formation were detected in the mechanically alloyed condition. Low-temperature compaction allowed the retention of the fine microstructure in the nanometer range, giving an interesting capability to enhance ductility in the normally brittle intermetallics.

Grain boundaries during superplastic deformation

The structure and properties of grain boundaries are considered during superplastic deformation on the basis of recent concepts of the interaction between lattice dislocations and grain boundaries. It is shown that the boundaries contain both, lattice and grain boundary dislocations. The presence of these extraneous defects leads to an increase of grain boundary energy; there is an increase of the vacancy concentration and enhancement of the diffusion coefficient in the boundaries with non-equilibrium structure. The results of numerical estimation of dislocation density and of the change in diffusion parameters agree well with the established experimental data.