Z. Q. Hu

A Coherent Polycrystal Model for the Inverse Hall-Petch Relation in Nanocrystalline Materials

A coherent polycrystal model for the abnormal grain size dependence is presented. It treats the nanocrystalline material as a coherent precipitate strengthened two-phase alloy in which all the grain boundaries merge into a whole continuous matrix acid each of the grains embeds in the matrix coherently. According to this model, the transition from the normal to the inverse Hall-Fetch relation corresponds to thr role-exchange of the grain bulk and the grain boundary in the deforming mechanism. Theoretical formula based on this model is compared with the available experimental data. Thr results indicate that the model gives an excellent description of the so-called inverse Hall-Petch relation in the nanocrystalline materials

ELECTRICAL-RESISTIVITY OF NANOCRYSTALLINE FE-CU-SI-B ALLOYS OBTAINED BY CRYSTALLIZATION OF THE AMORPHOUS ALLOY

Abstract Nanocrystalline Fe-Cu-Si-B alloys, with grain sizes of 30–90 nm, were prepared by crystallization of the parent amorphous alloy. Electrical resistivity of 30 nm-grained Fe-Cu-Si-B samples measured at room temperature was found to be higher than that of the amorphous material. With increasing grain size, the resistivity declines rapidly, which is in good agreement with the theoretical analysis based on the electron scattering of interfaces.

Formation and lattice distortion of nanocrystalline selenium

Abstract A nanocrystalline selenium (nc-Se) was prepared by completely crystallizing the as-quenched amorphous selenium. The structure of the nc-Se was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The nc-Se with trigonal structure consists of lamellae which are composed of whiskers with a nanometer-sized thickness. It was found that the lattice parameter a increases, but c decreases simultaneously with a reduction of grain size. When the grain size is smaller than 12 nm, the crystal lattice is found to be expanded; whereas for the grains larger than 12 nm, the crystal lattice is contracted.

Effects of heat treatment on microstructures and mechanical properties of a directionally solidified cobalt-base superalloy

DZ40M alloy is a newly developed directionally solidified cobalt-base superalloy. The present work investigated microstructures, room-temperature tensile and stress-rupture properties at 980 degrees C/83 MPa of the alloy in as-cast, solutionized as well as aged states. The microstructure of the DZ40M alloy can be modified by heat treatment. Solution treatment at 1280 degrees C for 4 h dissolved the primary carbides essentially and the alloy became a single-phase supersaturated solid solution. Incorporation of aging treatment at 950 degrees C for 12 h produced a profusion of secondary M26C6 precipitation throughout the matrix. The room-temperature mechanical properties of the alloy are mainly dependent on the microstructures of the matrix. During the stress-rupture tests, the microstructural evolution occurred in the alloy, the primary carbides dissolved sluggishly and the secondary M23C6 precipitated heavily. The precipitation hardening is the most important strengthening mechanism at high temperature for the alloy in ail three states. The stress-rupture properties were dominated by both the matrix and the boundaries of grains and interdendrites. The aged alloy has a superior stress-rupture property, which is attributed to its good microstructural combination of the matrix and the boundaries of grains and interdendrites

Effects of phosphorus on the δ-Ni3Nb phase precipitation and the stress rupture properties in alloy 718

The effects of phosphorus on the phase transformation and stress rupture properties of alloy 718 were investigated. The nucleation of δ-phase, which does not contain phosphorus, was suppressed by the enrichment of phosphorus at grain boundaries. A low level of phosphorus resulted in the formation of faults-containing film-like δ-phase along the grain boundaries, while a higher level of phosphorus favored the long lath-like δ-phase precipitation. Phosphorus greatly prolonged the stress rupture life of the alloy in the range of 0.0008–0.013 wt.%, while it reduced the stress rupture life in the range of 0.013–0.049 wt.%. The effect of phosphorus on the stress rupture properties was closely related to its interaction with oxygen. Phosphorus atoms, in the range of 0.0008–0.013 wt.%, enhanced the resistance to oxygen intrusion along the grain boundaries, protected the grain boundaries from decohesion by oxygen atoms and oxidation, and subsequently prolonged the rupture life of the alloy. The protection effect of P is clearly demonstrated by the phenomenon that the crack initiation site was shifted from the surface to the center in the stress-ruptured samples with increasing addition of P. Over 0.013 wt.%, the protection effect of phosphorus is excessive and phosphorus began to display its inherent effect of damaging the grain boundary strength; the stress rupture life of the alloy was reduced accordingly. Maximum stress rupture life was thus obtained at ≈0.013 wt.% P.

Transformation from the amorphous to the nanocrystalline state in pure selenium

A bulk nanocrystalline selenium (nc-Se) was formed by completely crystallizing a melt-quenched amorphous selenium (a-Se). The transformation process of the a-Se to the nc-Se and its microstructure were characterized by means of differential scanning calorimetry (DSC), x-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Structural analyses indicated that the nanocrystalline selenium samples with average gl ain sizes ranging from 6 to 45 nm were of the trigonal structure, and the morphology of nc-Se was in the form of lamellae, which are made up of fine fibers. A crystallization mechanism of chain folding is suggested to interpret the structural features of the nc-Se and the transformation process from amorphous to nanocrystalline trigonal selenium.

Properties of nanocrystalline Fe-Cu-Si-B alloys generated by crystallization of the amorphous alloy

Abstract Nanocrystalline Fe-Cu-Si-B alloys with grain sizes of 30–90 nm were prepared by crystallization of the amorphous alloy. Properties including microhardness and electrical resistivity were examined in this study. Microhardness measurements showed that a normal Hall-Petch relation between the microhardness and the average grain size was obtained in nanocrystalline Fe-Cu-Si-B alloys. It was found that the electrical resistivity of a 30 nm-grained Fe-Cu-Si-B sample measured at room temperature was higher than that of the amorphous sample. With increasing grain size, the residual resistivity drops rapidly, which is in good agreement with the theoretical analysis based on the electron scattering of interfaces.

Investigation of the lattice structure of nanophases in FeCuSiB alloys

In this paper we present the first investigation on the lattice structure of nanophases, α-Fe(Si) and Fe2B, in nanostructured FeCuSiB alloys obtained via crystallization of the parent amorphous alloy. It is found that the lattice constant of α-Fe(Si) phase is increased, whereas the a-axis is elongated and the c-axis is shortened simultaneously for Fe2B phase with the decrease of grain size. The above results are attributed to a super-saturation of vacancies in the nanophases examined as a result of grain size refinement. The Mossbauer parameters are presented to support the above interpretation.

Mechanically induced structural relaxation in an amorphous metallic Fe80B20 alloy

A melt-spun metallic Fe80B20 glass was subjected to high-energy ball milling. Microstructural changes of the glassy sample during milling were characterized by means of x-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, and Curie temperature measurements. It was found that the metallic glass may relax towards a low energetic configuration by mechanical milling, leading to a reduction of the heat release associated with crystallization of the amorphous phase and an increase of the average hyperfine field as well as of the Curie temperature. These results can be attributed to the occurrence of a strong short-range order in the amorphous state. Our experimental observations suggest that mechanical milling may induce structural relaxation in the amorphous Fe80B20 alloy

Formation kinetics of nanocrystalline FeBSi alloy by crystallization of the metallic glass

In order to clarify the formation mechanism of extremely fine-grained microstructures by crystallization of the FeBSi metallic glass, the nucleation and crystal-growth rates have been determined. The results indicate that the parabolic growth rate increases with annealing temperature, showing a maximum nucleation rate at an intermediate temperature range. Obviously it is possible to obtain nanocrystalline structures by controlling crystallization of the metallic glass on annealing at temperatures near the maximum nucleation rate.