SOLIDIFICATION MECHANISM OF Al/TiC INTRAGRANULAR COMPOSITES FABRICATED BY CONTACT REACTION METHOD

The behaviour of the particles in solidification inter face front was studied, and the equation of the critical interface velocity which related to the supercooling and the volume fraction of particles in the melt was obtained. By the mean time, the two solidification mechanisms of aluminium matrix intragranular composites was put forward.

SOLIDIFICATION MECHANISM OF UNDERCOOLEDSINGLE PHASE ALLOYS

The solidification behavior of bulk undercooled Ni50Cu50 alloy was systematically investigated. Double recalescences were discovered for the first time in a single phase Ni-Cd alloy, in which there is no solid phase transition at elevated temperature. When the alloy melt was undercoolcd below the solidus temperature, the effect of nonequilibrium solidification could make the solid with the norminal composition C0 of the alloy nucleate and grow till tempemture approached To (the temperuture at which the free enengies of solid and liquid with C0 are equal). Following it, the nucleation and growth of the solid with more high melting point component were required, so the secondary recalescence took place. The concentration distribution in crystals was also analysed.

Microstructure characteristics of Ni-43Ti-4Al-2Nb-2Hf alloy prepared by conventional casting and directional solidification

To further investigate the microstructure characteristic and solidification mechanism, so as to provide knowledge for the microstructure control of a NiTi-AI based high-temperature structural material, the microstructure of Ni-43Ti-4AI-2Nb-2Hf （at.%） alloy ingots prepared by conventional casting （arc-melting） and directional solidification （DS） at various drawing velocities （2 mm.min-＇, 18 mm.min-1, 30 mm-min-＇ and 60 mmmin~, respectively） was investigated by means of electron probe microanalyses. Experimental results reveal that the microstructures are composed of NiTi matrix phase,/3-Nb phase and Ti2Ni phase for samples obtained by both conventional casting and DS. Conventional casting has an equiaxial structure, while DS has a slender and acicular cellular structure which grows along the [001] orientation preferentially. Small amounts of white/3-Nb phase and black Ti2Ni phase co-exist at the grain boundaries or intercellular regions. With an increase in drawing velocity, the NiTi matrix phase is inclined to grow along （100） and （200） crystallographic planes, and the cellular arm spacing reduce gradually, but the directionality of the solidified structure weakens significantly. The homogeneous dispersion of,8-Nb phase and the decrease of Ti2Ni phase in DS samples are beneficial to improving the mechanical properties. Solidification mechanism analysis indicates that the dark grey NiTi matrix phase initially precipitates from the liquid phase, and then the divorced eutectic reaction takes place, which produces the light gray matrix phase and/^-Nb phase. Finally, the peritectic reaction happens, which generates the black Ti2Ni phase.

Microstructure and Mechanical Property of 2024 Aluminium Alloy Prepared by Rapid Solidification and Mechanical Milling

Rapidly solidified 2024 aluminium alloy powders were mechanically milled, then consolidated to bulk form. The microstructural changes of the powders in mechanical milling (MM) and consolidation process were characterized by X-ray diffraction analyses and transmission electron microscopy observations. The results showed that mechanical milling reduced the grain size to nanometer, dissolved the Al2Cu intermetallic compound into the aluminium matrix and produced an aluminium supersaturated solid solution. During consolidation process. the grain size increased to submicrometer, and the Al2Cu and Al2(Cu, Mg, Si, Fe, Mn) compounds precipitated owing to heating. Increasing consolidation temperature and time results in obvious grain growth and coarsening of second phase particles. The tensile yield strength of the consolidated alloy with submicrometer size grains increases with decreasing grain size, and it follows the famous HallPetch relation

Microstructure and mechanical properties of Ni3Al intermetallics prepared by directional solidification electromagnetic cold crucible technique

The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.

Code mechanical solidification and verification in MEMS security devices

The virtual machine of code mechanism （VMCM） as a new concept for code mechanical solidification and verification is proposed and can be applied in MEMS （micro-electromechanical systems） security device for high consequence systems. Based on a study of the running condition of physical code mechanism, VMCM＇s configuration, ternary encoding method, running action and logic are derived. The cases of multi-level code mechanism are designed and verified with the VMCM method, showing that the presented method is effective.

Preparation of Graphite Gangue Wall Body Materials By Solidifying Technique

The feasibility of producing graphite gangue wall body materials was discussed by the experiments of four kinds of hardeners solidifying graphite gangue.The result demonstrates that the physical performances of graphite gangue solidified brick samples with 8% hardener D reach the senior grade or the special grade in JC422-91.The study of solidification mechanism indicates that the hardeners produced pastes while being hydrated.In addition,they can activate the mineral compositions in clay to join in the reaction to produce pastes and intensive framework.

Microstructure and mechanical properties of the micrograined hypoeutectic Zn–Mg alloy

A biodegradable Zn alloy, Zn-1.6Mg, with the potential medical applications as a promising coating material for steel components was studied in this work. The alloy was prepared by three different procedures： gravity casting, hot extrusion, and a combination of rapid solidification and hot extrusion. The samples prepared were characterized by light microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. Vickers hardness, tensile, and compressive tests were performed to determine the samples＇ mechanical properties. Structural examination reveals that the average grain sizes of samples prepared by gravity casting, hot extrusion, and rapid solidification followed by hot extrusion are 35.0, 9.7, and 2.1 μm, respectively. The micrograined sample with the finest grain size exhibits the highest hardness（Hv = 122 MPa）, compressive yield strength（382 MPa）, tensile yield strength（332 MPa）, ultimate tensile strength（370 MPa）, and elongation（9%）. This sample also demonstrates the lowest work hardening in tension and temporary softening in compression among the prepared samples. The mechanical behavior of the samples is discussed in relation to the structural characteristics, Hall-Petch relationship, and deformation mechanisms in fine-grained hexagonal-close-packed metals.

A study of microstructure and properties of cast Fe-10Cr-1.5B alloy

In the present study, the microstructure and mechanical properties of cast Fe-10Cr-1.5B（FCB） alloy after different heat treatments were studied. The results showed that the as-cast microstructure of FCB alloy consists of α-Fe, M（M=Cr, Fe, Mn）2（B, C） and M（M=Cr, Fe, Mn）7（C, B）3 type borocarbides, and small amounts of pearlite and austenite. After oil quenching treatment, metal matrix transformed into the martensite from the mixture of martensite, pearlite and austenite. There are many M（M=Cr,Fe,Mn）23（C,B）6 type borocarbide precipitates in the metal matrix, and eutectic borocarbide appears with an apparent disconnection and isolated phenomenon. When the quenching temperature reaches 1,050℃, the hardness of FCB alloy is the highest, but the change of quenching temperature has no obvious effect on impact toughness of FCB alloy. After tempering, the eutectic microstructure of FCB alloy appears with a ＂two links＂ trend. With the increase of tempering temperature, the hardness of FCB alloy decreases gradually and impact toughness increases gradually. Cast FCB alloy oil-quenched from 1,050℃ and tempered from 200℃ has excellent combined properties; its hardness and impact toughness are 61.5 HRC and 8.8 J·cm^-2 respectively.

Mechanism of MSWI Fly Ash Solidified by Microbe Cement

Microbe cement,a new type of gelling material,has attracted wide attention due to the increasing awareness of environmental protection.In this paper,the microbe cement in solidifying municipal solid waste incineration(MSWI)fly ash is investigated and the effect of the microbial induction method in solidifying MSWI fly ash is compared with the traditional chemical reaction strategy by characterizing the resulted calcite and the solidification productions with electronic universal testing machine,X-ray diffractometer(XRD),Fourier transform infrared spectrometer(FTIR),scanning electron microscope(SEM)and atomic absorption spectrometer.The results show that the MSWI fly ash solidified by microbe cement has the highest compressive strength while that of the chemical CaCO3 products is the lowest.The XRD results show that a new hydration gelling substance(Ca2SiO4·0.30H2O)is generated in the MSWI fly ash products.The FTIR results show that the frequency of Si-O bonds and C-O bonds in the products solidified by microbe cement has shifted,while there is no change occurred in the chemical CaCO3 products.The SEM results show that the microstructure of the products solidified by microbe cement is denser than that of chemical CaCO3 products.The test results of heavy metals show that the microbe cement could reduce the leaching concentration of heavy metals in MSWI fly ash.Ultimately,the leaching concentration of Pb meets the standard requirements,while that of Cd is still slightly higher than the standard requirement.

Effect of Oil Shale on Na^+ Solidification of Red Mud-Fly Ash Cementitious Material

Red mud-fly ash based cementitious material mixed with different contents of oil shale calcined at 700 ℃ is investigated in this paper. The effect of active Si and A1 content on the solidification of Na＋ during the hydration process is determined by using X-ray diffraction （XRD）, 27A1 and 29Si magic-angle-spinning nuclear magnetic resonance （MAS-NMR）, infrared （IR）, scanning electronic microscopy （SEM） and X-ray photoelectron spectroscopy （XPS）. It is shown that the content of oil shale has a remarkable effect on the solidified content of Na＋. The hydration process generates a highly reactive intermediate gel phase formed by co-polymerisation of individual alumina and silicate species. This kind of gel is primarily considered as 3D framework of Si04 and A104 tetrahedra interlinked by the shared oxygen atoms randomly. The negative charges and four-coordinated A1 inside the network are mainly charge-balanced by Na＋. The solidifying mechanism of Na＋ is greatly attributed to the forming of this kind of gel.

Influence of high pressure during solidification on the microstructure and strength of Mg-Zn-Y alloys

The effect of high pressure during solidification on the microstructure and mechanical property of Mg-6Zn-1Y and Mg-6Zn-3Y was investigated using optical microscopy, scanning electronic microscopy, X-ray diffraction（XRD） and Vickers-hardness testing. Under atmospheric-pressure solidification, Mg-6Zn-1Y consisted of α-Mg, Mg7Zn3 and Mg_3YZn_6; whilst Mg-6Zn-3Y consisted of α-Mg, Mg_3Y_2Zn_3 and Mg_3YZn_6. Under 6 GPa high-pressure solidification, both alloy consisted of α-Mg, MgZ n and Mg12 YZn. The shape of the main second phase changed from a lamellar structure formed for atmospheric-pressure solidification to small particles formed for solidification at 6 GPa pressure. The dendrite microstructure was refined and was more regular, and the length of the primary dendrite arm increased under 6 GPa high-pressure solidification, which was attributed to increasing thermal undercooling, compositional undercooling and kinetics undercooling. After solidification at 6 GPa pressure, the solid solubility of Y in the second phase and the Vickers-hardness increased from 15 wt.% and 69 MPa for Mg-6Zn-1Y to 49 wt.% and 97 MPa; and from 19 wt.% and 71 MPa for Mg-6Zn-3Y alloy to 20 wt.% and 92 MPa, respectively.

Microstructure, Microsegregation, and Mechanical Properties of Directional Solidified Mg–3.0Nd–1.5Gd Alloy

The microstructure, microsegregation, and mechanical properties of directional solidified Mg–3.0Nd–1.5Gd ternary alloys were experimentally studied. Experimental results showed that the solidification microstructure was composed of dendrite primary a（Mg） phase and interdendritic a（Mg） · Mg12（Nd, Gd） eutectic and Mg5 Gd phase. The primary dendrite arm spacing k1 and secondary dendrite arm spacing k2 were found to be depended on the cooling rate R in the form k1= 8.0415 9 10-6R-0.279 and k2= 6.8883 9 10-6R-0.205, respectively, under the constant temperature gradient of40 K/mm and in the region of cooling rates from 0.4 to 4 K/s. The concentration profiles of Nd and Gd elements calculated by Scheil model were found to be deviated from the ones measured by EPMA to varying degrees, due to ignorance of the back diffusion of the solutes Nd and Gd within a（Mg） matrix. And microsegregation of Gd depended more on the growth rate, compared with Nd microsegregation. The directionally solidified experimental alloy exhibited higher strength than the non-directionally solidified alloy, and the tensile strength of the directionally solidified experimental alloy was improved,while the corresponding elongation decreased with the increase of growth rate.