The Effect of Grain Boundary Carbides on Dynamic Recrystallization During Hot Compression of Ni-Based Superalloy Haynes $$282^{\mathrm{TM}}$$

In alloys where carbides are the main grain boundary phase, the role of carbides during hot working is not known. Here, we address the effect of grain boundary carbides on the dynamic recrystallization during hot compression of Ni-base superalloy Haynes 282. When excluding variations from experimental factors neither stress evolution nor final microstructure indicated that carbides exerted a significant influence on the dynamic recrystallization. The carbide solvus temperature is not a critical limit during thermomechanical processes.

Null boundary phase space: slicings, news & memory

We construct the boundary phase space in D-dimensional Einstein gravity with a generic given co-dimension one null surface $$ \mathcal{N} $$ N as the boundary. The associated boundary symmetry algebra is a semi-direct sum of diffeomorphisms of $$ \mathcal{N} $$ N and Weyl rescalings. It is generated by D towers of surface charges that are generic functions over $$ \mathcal{N} $$ N . These surface charges can be rendered integrable for appropriate slicings of the phase space, provided there is no graviton flux through $$ \mathcal{N} $$ N . In one particular slicing of this type, the charge algebra is the direct sum of the Heisenberg algebra and diffeomorphisms of the transverse space, $$ \mathcal{N} $$ N v for any fixed value of the advanced time v. Finally, we introduce null surface expansion- and spin-memories, and discuss associated memory effects that encode the passage of gravitational waves through $$ \mathcal{N} $$ N , imprinted in a change of the surface charges.

The Effects of Forming CeFe2 on Phase Structure and Magnetic Properties in Ce-Rich Nd-Ce-Fe-B Permanent Magnetic Materials

The decomposition of the Nd-Ce-Fe-B phase to form CeFe2 has been usually believed to have an important positive effect on the magnetic properties of Nd-Ce-Fe-B permanent magnetic materials. In this work, a new decomposition process of the Nd-Ce-Fe-B phase on the formation of the CeFe2 phase was observed to play a negative role in its magnetic properties. It is demonstrated that the Nd-Ce-Fe-B phase decomposes into non-magnetic CeFe2, accompanied by the precipitation of Fe soft-phase. The kinks usually occurring in the demagnetization curves of Ce-rich Nd-Ce-Fe-B magnets have been determined to be related to the Fe soft-phase. Instead of using CeFe2 as a grain-boundary phase, another Ce-Cu boundary phase has been explored to efficiently improve the coercivity of Ce-rich Nd-Ce-Fe-B magnets, provided that the Ce-Cu boundary phase has an appropriate Ce to Cu ratio. The present results contribute to the mechanism comprehension and high-performance design of Nd-Ce-Fe-B permanent magnetic materials.

Phase Formation, Microstructure and Permeability of Fe-Deficient Ni-Cu-Zn Ferrites, (I): Effect of Sintering Temperature

We have studied the densification, phase formation, microstructure, and permeability of stoichiometric and Fe-deficient Ni-Cu-Zn ferrites of composition Ni0.30Cu0.20Zn0.50+zFe2-zO4-(z/2) with 0 £ z £ 0.06 sintered at temperatures from 900 °C to 1150 °C. The shrinkage is shifted from 1000 °C for z = 0 towards lower temperatures and reaches its maximum rate at 900 °C for z = 0.02. Stoichiometric ferrites show regular growth of single-phase ferrite grains if sintered at Ts ≤ 1100 °C. Sintering at 1150 °C leads to the formation of a small amount of Cu2O, triggering exaggerated grain growth. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites coexisting with a minority CuO phase after sintering at 900–1000 °C. At Ts ≥ 1050 °C, CuO transforms into Cu2O, and exaggerated grain growth is observed. The formation of Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability of the ferrites increases with sintering temperature up to a maximum permeability of µ = 230 for z = 0 or µ = 580 for z = 0.02, respectively, at Ts = 1000 °C. At higher sintering temperatures, the permeability decreases, which is due to the formation of a microstructure with intra-crystalline porosity in large grains, and a non-magnetic Cu oxide grain boundary phase.

A micromechanics-based constitutive model for nanocrystalline shape memory alloys incorporating grain size effects

A micromechanics-based model is developed to capture the grain-size dependent superelasticity of nanocrystalline shape memory alloys (SMAs). Grain-size effects are incorporated in the proposed model through definition of dissipative length scale and energetic length scale parameters. In this paper, nanocrystalline SMAs are considered as two-phase composites consisting of the grain-core phase and the grain-boundary phase. Based on the Gibbs free energy including the spatial gradient of the martensite volume fraction, a new transformation function determining the evolution law for transformation strain is derived. Using micromechanical averaging techniques, the grain-size-dependent superelastic behavior of nanocrystalline SMAs can be described. The internal length scales are calibrated using experimental results from published literature. In addition, model validation is performed by comparing the model predictions with the corresponding experimental data on nanostructured NiTi polycrystalline SMA. Finally, effects of the internal length scales on the critical stresses for forward and reverse transformations, the hysteresis loop area (transformation dissipation energy), and the strain hardening are investigated.