Heng Zhi Fu

Microstructure and Mechanical Properties of an Advanced Niobium Based Ultrahigh Temperature Alloy

The microstructure and mechanical properties including room temperature fracture toughness Kq, tensile strengthσb and elongationδ at 1250°C of the Nb based alloy directionally solidified in an electron beam floating zone melting (EBFZM) furnace have been evaluated. The microstructure is primarily composed of Nb solid solution (Nbss), α-(Nb)5Si3 and (Nb)3Si phases. After directional solidification with the moving rate of electron beam gun R being respectively 2.4, 4.8 and 7.2 mm/min, the primary Nbss dendrites, Nbss + (Nb)5Si3/(Nb)3Si eutectic colonies (lamellar or rod-like) and divorced Nb silicide plates align along the longitudinal axes of the specimens. When R = 2.4 mm/min, the best directional microstructure is obtained. Directional solidification has significantly improved theσb at 1250°C and Kq. The maximumσb occurs for the specimens with R = 2.4 mm/min and is about 85.0 MPa, meanwhile, the Kq is about 19.4 MPam1/2.

Unidirectional Solidification of a Nbss/Nb5Si3 In-Situ Composite

The directionally solidified specimens of Nb-13.52 Si-22.60 Ti–6.88 Hf–2.54 Cr–2.24 Al alloy were prepared in an electron beam floating zone melting furnace at the withdrawing rate of 0.1, 0.3, 0.6, 1.0, 2.4 and 6.0 mm/min. All the primary Nb solid solution (Nbss) columns, Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies and divorced (Nb)3Si/(Nb)5Si3 plates or chains align well along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the microstructure is gradually refined, and the amount of Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies increases. Both the room temperature ultimate tensile strength σb and fracture toughness KQ are improved for the directionally solidified specimens. The tensile fracture occurs in a cleavage way.

Continuous Casting and Directional Solidification of Titanium Alloys with Cold Crucible

Titanium alloy billets were continuously cast and directionally solidified under vacuum and compelling cooling conditions by using round or rectangular cross-section cold crucible. It is found that hot crack is caused by friction between the skull and the inner wall of crucible, a coating of CaO and CaF2 flux can eliminate the hot cracks. The experimental results show that the withdrawal velocity, where the changes was setting as 5, 3, 1, 0.5mm/min respectively, is the significant factor that affects the macrostructure and solidification front shape from concave to flat and then to convex. Directional columnar grains and a single crystal can be obtained when the velocity is controlled to either 1 or 0.5mm/min under a round cross-section crucible, and columnar grains can be also obtained at the speed of 2mm/min under a rectangular cross-section crucible. Finally, the temperature fields during the process are calculated and the trend of solidification front is in good agreement with the experiment. Specimens composed of columnar grains or single crystal exhibit excellent tensile properties. Introduction Titanium alloys are promising engineering materials, which are applied to aerospace and shipbuilding industries owning to their high specific strength, hot resistance and corrosion resistance. However, there are some difficulties in controlling the chemical elements and temperature in melting titanium alloys. Recent directional solidification of titanium alloys experiments were performed in optical floating furnace [1-3] or in resistance furnaces using Al2O3[5], Y2O3[4] or CaO [5] crucible. But titanium will react with all commercial ceramic mould materials, which in turn affect the microstructure and mechanical properties. Water-cooling cold crucible provides an effective method for melting titanium alloys because of its high speed in melting, non-contamination and continuous casting [6,7]. Cold crucible continuous casting and directional solidification includes induction melting, soft contact, continuous casting and directional solidification. Not only aluminum, steel and titanium alloys [8], but also photovoltaic multi-crystalline and single silicon are continuously cast by cold crucible [9]. But directional solidification of titanium alloys with cold crucible has not been reported elsewhere. This paper investigates the methodology of continuous casting and directional solidification with cold crucible when it was applied to Ti-6Al-4V alloy. Experimental method The experimental apparatus is schematically shown in Fig.1. This apparatus comprises: -a furnace chamber made of steel, in which a vacuum can be established by an exhaust pump -a water-cooling segment copper crucible with 8 pieces of vertical slits and 30mm in the internal diameter and 130mm in height. -a four-turn water cooling coil surrounding the crucible and supplying 50 kHz frequency alter current generated by a transistor generator, of which the maximum output power is 100kW -a withdrawal system and a feeder system, both of them can be controlled respectively. The procedure is as follows. A Ti-6Al-4V alloy billet is fixed on feeder and a primer is placed in the crucible. A vacuum of 1Pa is established, then Argon gas is introduced to 200 Pa. Power is gradually Materials Science Forum Online: 2005-01-15 ISSN: 1662-9752, Vols. 475-479, pp 2575-2578 doi:10.4028/www.scientific.net/MSF.475-479.2575

Progress of Directional Solidification in Processing of Advanced Materials

Most of materials have long been considered to be mechanical and/or physical anisotropy. Permitting materials to grow along specific orientation by means of directional solidification technique can optimize their structural or functional properties. The present paper attempts to introduce the research work in the field of processing of some advanced materials by innovative directional solidification techniques performed at State Key Laboratory of Solidification Processing and with author’s intended research work. The paper deals with the specific topics on directional solidification of following advanced materials: column and single crystal superalloys under high thermal gradient, Ni-Cu alloys under deep supercooling of the melt, intermetallic compounds with selected preferential crystal orientation, superalloys with container less electromagnetic confinement, high Tc superconducting oxides, high temperature structural ceramics, continuous cast single crystal copper and copper-based composites. The relevant solidification phenomena, such as morphological evolution, phase selection, peritectic reaction and aligned orientation relationship of crystal growth for multi-phases in the processing of directional solidification, are discussed briefly. The trends of developments of directional solidification technique are also prospected.

Interface Morphologies and Microstructure of a Single Crystal Superalloy under High Thermal Gradient Directional Solidification

The interface morphologies single crystal superalloys CMSX-2 has been studied over a range of cooling rate with large variations in withdrawal speed in directional solidification. The superfine cellular structure was obtained under both high thermal gradient up to 1000K/cm and fast withdrawal rate up to 1mm/sec. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of λ’ phase, reduced microsegregation of alloying elements and smaller size of γ-γ’ eutectics.

Finite Element Analysis of Fretting Wear for Nuclear Inconel 690 Alloy

Finite element method to analyze the fretting wear characteristics based on Hertz theory, Coulomb friction law and a modified Archard wear equation, has been applied to a cylinder-on-flat configuration for typical steam generator tube material Inconel 690 alloy and typical anti-vibration bar material Inconel 600 alloy (Cr plating) in the nuclear power plant, under gross slip and partial slip conditions. The evolutions of contact profile, surface contact variables with increase in wear cycles are predicted. The slip regime is predicted to have significant effects on the fretting wear behavior. Under the gross slip regime, it is found that the peak contact pressure occurs at the center of the contact scar, and the actual relative slip is slightly smaller than the applied value. The contact width increases, and the peak pressure decreases gradually with increase in wear cycles. Whereas under the partial slip regime, the peak contact pressure occurs at the stick-slip boundary, the actual relative slip is much smaller than the applied value, and no relative slip occurs in the stick zone. The contact width increases gradually, and the peak pressure increases rapidly with increase in wear cycles.

Thermoelectric Properties of Directionally Solidified Bi2Te3 Alloys under High Thermal Gradient

Thermoelectric Bi2Te3 bulk alloys were directionally solidified successfully at the pulling rate ranging from 1 μm/s to 50 μm/s under a high temperature gradient of 200 K/cm. Preferred crystal orientations of (0 1 5), (1 0 10) and (1 1 0) faces appeared at the pulling rate of 50 μm/s. In the Bi2Te3 alloys directionally solidified at 5 μm/s, the maximum Seebeck coefficient of -253 μV/K was obtained and the maximum electrical resistivity of 2.26 mΩ•cm was measured at 300 K. Besides, the optimum Power Factor (PF) value reached 3.83×10-3 W/K2m at 1 μm/s and the measured results show that the thermoelectric Bi2Te3 bulk alloys grown at low growth rates supply the large PF value at ambient temperate, while at high temperature, the alloy grown at 50 μm/s has a better PF value.

Preparation Development of High-Quality Solar-Grade Multi-Crystalline Silicon by Directional Solidification

Silicon solar cell is well known as one of the cleanest and most potential renewable resources. As the major photovoltaic (PV) material in PV industry, multi-crystalline silicon (mc-Si) grown by directional solidification has recently attracted increasing attention because of its low production cost, low pollution and high throughput. Deeper understanding of the physic and optic properties, and preparation methods of the materials will lead to improved device design. This paper briefly presents basic directional solidification theory of multi-crystalline silicon, and reviews recent development of solar-grade multi-crystalline silicon. The directional solidification preparation techniques of high-quality solar-grade multi-crystalline silicon are detailed introduced and summarized. Furthermore, the existing problems and further development direction of directionally solidified multi-crystalline silicon for solar cell are discussed.

Fabrication and Characterization of Al2O3/GdAlO3 Eutectic Ceramic In Situ Composite by Laser Zone Remelting

In situ composite of Al2O3/GdAlO3(GAP) ceramic eutectic prepared by directional solidification is an interesting candidate for the manufacture of turbine blades because of its excellent mechanical properties. In the present paper, directionally solidified Al2O3/GAP eutectic in situ composite ceramics are manufactured by the laser zone remelting technique to investigate the rapid solidification process. The laser power and scanning rate necessary to carry out the ceramic melt growth is determined. The characteristic microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The as-solidified Al2O3/GAP eutectic presents an elongated colony structure consisting of only -Al2O3 and GAP phases with an oriented growth array. The eutectic spacing is strongly dependent on the laser scanning rate, rapidly decreasing to the sub-micron range for the samples grown at the highest rate. Besides, the formation condition and evolution of the particular microstructure of the composite during rapid solidification are discussed.

Recent Activities on Directional Solidification at the State Key Laboratory of Solidification Processing

Directional solidification technique permits materials to grow along specific orientation, in order to obtain mechanical and/or physical anisotropy. The present research attempts to introduce the research work in the field of processing of some advanced materials by innovative directional solidification techniques performed at State Key Laboratory of Solidification Processing and with author’s intended research work. The paper deals with the specific topics on state of the art of directional solidification: single crystal superalloy and Nd-Fe-B alloys under high thermal gradient, Cu-Ni alloys under deep supercooling of the melt. The relevant solidification phenomena, such as morphological evolution, crystal growth for multi-phases in the processing of directional solidification, are discussed briefly. The trends of developments of directional solidification techniques are also prospected.