Ai Dang Shan

Tensile Properties of MoSi2 at Elevated Temperatures

The tensile properties of two MoSi2 alloys with different grain sizes (1 micrometer and 10 micrometer) were evaluated in vacuum at temperatures ranging from 1400 to 1600K and initial strain rates ranging from 1×10-5/s to 1×10-3/s. For the alloy with 10micron grain size an m vale of 0.35 and an activation energy value of 350 kJ/mol were observed in the lower strain rate range while an m value of 0.12 and an activation energy value of 760 kJ/mol were observed in the higher strain rate range. For the alloy with 1micron grain size, a uniform m value of 0.55 and an activation energy value of 160 kJ/mol were observed. Moreover these two alloys showed remarkable ductility (maximum 33%) in the test temperatures. The deformation mechanism and the remarkable ductility are discussed in the light of the microstructural observations through SEM and TEM.

Effect of Mo Addition on Microstructure and Properties of Nb/Nb5Si3 In-Situ Composite

Nb/Nb5Si3 in-situ composites are very attractive structural materials for these materials perform a good balance in mechanical properties, including a high strength at high temperature (>1373K) and reasonably high fracture toughness at room temperature. Metastable phase Nb3Si plays an important role in the properties of Nb/Nb5Si3 composites by affecting microstructure and volume fracture of ductile phase. In this paper, Nb-10Si-xMo and Nb-18Si-xMo (x=0,5,15) are prepared by arc melting and annealed at 1473K for 100h. Single edge-notched bending (SENB) test was used to study the fracture toughness of Nb-Si-Mo alloys. The stability of metastable phase is analyzed by XRD. The room temperature fracture toughness of Nb-10Si is 10.47MPa(m)1/2 and higher than that of binary Nb-18Si alloys at near-eutectic compositions. The addition of Mo improves the fracture toughness of as cast Nb-Si alloys from 4.1 MPa(m)1/2 to 9.9MPa(m)1/2 at near-eutectic compositions and decreases it from 10.47 MPa(m)1/2 to 8.8MPa(m)1/2 at hypoeutectic compositions.

Characterizing the Shear Deformation during Asymmetric Rolling

Asymmetric rolling has been considered as a possible way to obtain severe plastic deformation (SPD) since it will give an extra shear deformation to the processed materials during rolling. Previous researches have confirmed such a shear deformation. Very recently, the method of inserting-block is used to characterize the shear deformation through direct observation, but when the reduction is more than 70%, the lineation scratched on the side face of internal mark becomes vague and illegible. In order to directly observe the shear deformation of metallic material with large reduction, the internal mark method is employed in this research and asymmetric rolling was performed with pure aluminum and iron at room temperature. In severe plastic deformation, the shear deformation caused by asymmetric rolling was clearly observed and measured through employing internal mark method. Remarkable extra shear deformation during asymmetric rolling was confirmed. Very high equivalent strains were achieved when sheet samples were asymmetrically rolled to high reduction ratio. These strain values fall into the range of SPD.

Principals of Indirect Equal Channel Angular Drawing for Processing of Ultrafine Grained Materials

In this paper indirect ECA drawing method was supposed to be feasible for application. This method will ensure same deformation degree as that in ECAP while keep the size of the billet unchanged so that multiple processing can be done. With this method, materials with both rectangular and round section can be processed and there is no limit to the length of the materials. Die configuration was set up and the equations for relationship among die parameters, billet parameters and its properties, drawing forces and applying pressures are given.

The Control of Microstructures in (Mo1-x, Nbx) Si2 Intermetallics

The intermetallics of (Mo1-xNbx) Si2 consisted of C11b phase and C40 phase when x is equal to 0.05, 0.10 and 0.15, and consisted of only C40 phase when x is equal to 0.20. Systemic heat treatments were conducted on the alloys. The evolution of microstructure was investigated with varying annealing temperature and time. Through the appropriate heat-treated processing, the microstructures of the alloys can be controlled to form full lamellar structure

Microstructure and Properties of Directionally Solidified NbSi2/ Nb5Si3 Composites

NbSi2 is an attractive material for high temperature applications due to its high melting point, low density and good oxidation resistance. The high-temperature strength of NbSi2 is expected to be further improved by incorporation with Nb5Si3, which performs a high creep resistance and strength at high temperature due to its complex crystal structure. In this paper, directionally solidified NbSi2/ Nb5Si3 in-situ composites have been prepared using an optical floating zone method. Scanning Electron Microscopes (SEM) and X-ray diffraction (XRD) have been used to investigate the phase constitution and microstructure. The orientation relationship between Nb5Si3 and NbSi2 is investigated by transmission electron microscopy (TEM). High-temperature properties of alloys are tested by compression at the strain rate of 1×10-4/s at 1673K and 1773K. It was found that high temperature strength and phase constitution of directionally solidified alloys depended on the addition of Mo.

Microstructure and Mechanical Properties of Nano-Grained Dual-Phase Ni-Based Alloy

To provide insight into the mechanical behavior and microstructural evolution of bulk nanograined (NG) Ni-based alloys during annealing, the Ni-based alloy sheets with grain size about 50 nm was produced through severe cold-rolling at room temperature, and then the cold rolled (CRed) Ni-based alloys were annealed at different states. The evolution of the nanostructure of the CRed Ni-based alloy during annealing and corresponding change in mechanical properties was investigated. The results showed that the CRed Ni-based alloy exhibited prominent enhancement in the yield strength (YS), ultimate tensile strength (UTS), which increased respectively from 253 MPa to 1455 MPa, 684 MPa to 1557 MPa. Further increase of the YS and UTS were obtained in the annealed-CRed Ni-based alloy with dual-phase. The YS and UTS of the NG dual-phase Ni-based alloy was respectively 2013 MPa and 2061MPa, which was annealed at 700 °C for 1h. In terms of the microstructural evolution, lower density of defects on the grain boundary were observed and the nanograins can be maintained about 100 nm even when annealed for 30 h at 700 °C, which suggests high thermal stability at this temperature. Both the high thermal stability and strength are due to the formation of the γ′ precipitates and slight grain growth of the NG matrix.

Serrated Flow in an 11Cr Ferritic/Martensitic Steel

Serrated flow behavior of an 11Cr ferritic/martensitic steel was investigated through tensile tests at initial strain rates of 2×10-510-3 s-1 at temperatures ranging from room temperature to 973 K. Serrated flow occurred at three temperature regions of room temperature, 573 K and 773973 K when tensile tests were conducted at a strain rate of 2×10-4 s-1. Serrations are also observed in the steel during tension at temperatures of 573 K and 773973 K at a strain rate of 2×10-5 s-1. With increasing tensile temperature, the yield stress and ultimate tensile stress of the steel were gradually decreased and quickly dropped at temperatures higher than 773 K, while the elongation of the steel was decreased to a minimum at 600 K, and then dramatically increased at temperatures higher than 600 K.

Interface Microstructure of Diffusion Bonded Fe3Al/Al with Ultrafine Grain Layer

The influence of ultrafine grains produced by severe plastic deformation technology on the weldability of Fe3Al and Al dissimilar materials was investigated. An ultrafine grain layer was produced on Fe3Al intermetallic compound by surface mechanical attrition treatment. Then the SMATed Fe3Al was diffusion bonded with 1060Al at 550°C for 90 min in the vacuum of 10-3 Pa. The microstructures of surface ultrafine grain layer and transition zone at the interface of SMATed-Fe3Al/Al joint were observed by scanning electron microscopy. The grain size of surface ultrafine grains was characterized by X-ray diffractometry. The elements distribution at the interface and the phase constituents of transition zone were measured by energy disperse spectroscopy. The results showed that a deformed layer about 10-20 μm wide and surface nanocrystallines about 35nm were produced after 15 min surface mechanical attrition. SMATed Fe3Al was well bonded with Al and 11-30 µm wide transition zone formed. The transition zone consisted of FeAl and FeAl3 phases. The surface nanocrystallines helped the atom diffusion and the formation of diffusion bonded joint with rough surface and lower pressure.

Growth of Crystals and Formation of C40/C11b Lamellar Structure in (Mo0.85Nb0.15)Si2

In this paper, high-quality single crystals of (Mo0.85Nb0.15)Si2, around 8 mm in diameter and 90 mm in length, have been grown by optical heating floating zone method. Effects of chemical composition and growth rate on forming C40 structured single crystal were studied. Aligned C40/C11b lamellar structured can be formed in the as-grown crystals after post annealing at temperatures between 1473 and 1873 K. Chemical composition as well as annealing temperature are found to be two important factors to form C11b lamellae in the C40 matrix. Fully C40/C11b lamellar structure was formed after annealing at 1873 K in the present work. The aligned C40 and C11b lamellae follow a crystallographic orientation relationship of (0001)C40//(110)C11b. Dislocations were observed in some coarse C11b lamellae but never in C40 lamellae of the duplex structure. This is probably due to accumulation of misfit strain during formation of C40/C11b lamellae.