Dongli Sun

Coating of Y2O3 additive on Al2O3 powder and its effect on the wetting behaviour in the system Al2O3p/Al

Abstract Al 2 O 3 powder was coated with Y 2 O 3 percursor generated by in situ precipitation of Y(NO 3 ) 3 ·6H 2 O in aqueous solution. After calcining at high temperature, the precursor was completely transformed into a crystalline Y 2 O 3 , and Y 2 O 3 -coated Al 2 O 3 powder was obtained. Transmission electron microscope (TEM), high resolution electron microscope (HREM) observation and elemental analysis clearly showed that the Y 2 O 3 was successfully coated on the surface of the Al 2 O 3 particles. The coated powders were also characterized by energy dispersive spectrum (EDAX) and X-ray diffractometry (XRD). The wetting behaviour of molten aluminum on the coated Al 2 O 3 powders was investigated by the sessile drop method. Due to the introduction of Y 2 O 3 , the wettability of the coated-Al 2 O 3 powders by liquid aluminum was dramatically improved in comparison with that of the as-received powders.

Micro-yield property of sub-micron Al2O3 particle reinforced 2024 aluminum matrix composite

Abstract The microstructure and micro-yield strength of sub-micron Al2O3 particle reinforced 2024Al composites and the effect of the thermal-cold cycling treatment on the microstructure and properties were studied. The results show that the dislocations are rare in the microstructure of the sub-micron Al2O3p/2024Al composite in the squeeze casting condition. Aging and thermal-cold cycling treatment does not change this phenomenon. The Al2O3 particles are fine, so the thermal misfit between particles and the matrix is very small during the temperature change, resulting in decreased dislocations. The tiny and uniformly dispersed S′ precipitates and sub-micron particles can effectively pin dislocations, therefore, the micro-yield strength of the composite increases. Depending on the condition of the thermal-cold cycling treatment after aging, both the size and distribution of the S′ precipitates in the composite change, and they have great effect on the micro-yield strength of the composite.

Rare-earth oxide coating for sub-micro particulates reinforced aluminum matrix composites

Abstract The wettability of sub-micro alumina particulates by aluminum in squeeze-casting alumina reinforced 6061Al composites is poor. The fine particles are prone to cluster in aluminum liquid. The properties of composite are far lower than the theoretical values. To disperse the particles uniformly and to improve the material properties, a liquid-phase coating method was used to deposit Y 2 O 3 ceramic on the surface of α-Al 2 O 3 . TEM observations of coating morphology and microstructure of the composite indicate that Y 2 O 3 was successfully coated on Al 2 O 3 particle surfaces, and the coated powders distributed uniformly in the aluminum liquid. The wettability of the powder by the matrix is improved because of the interfacial reaction of Y 2 O 3 with Al. The mechanical properties of the coated-Al 2 O 3p /6061Al composites are also improved. In the composite (as cast) with 30% volume fraction particles, the ultimate tensile strength increased by 29.8%, the yield strength by 38.4% and the elongation by 10.3%. They reached 457.7 MPa, 215.7 MPa and 1.18%, respectively. The fracture surface presents ductile fracture features.

Fabrication of in situ Ti2AlN/TiAl composites by reaction hot pressing and their properties

Ti2AlN/TiAl composites with different volume fractions of reinforcement were successfully fabricated by hot-pressing sintering method (reaction hot pressing) using Ti, Al and TiN powders as starting materials. The synthesis process includes four stages: first, the reactions between Al and Ti powers and between Al and TiN powders respectively occur and result in TiAl3 phase; secondly, Al powders in the sample are exhausted; the remaining Ti cores react with TiAl3 layer to form Ti-Al intermetallics; moreover, a few Ti2AlN particles precipitate from the TiAl3 phase; thirdly, Ti-Al intermetallics react with the remaining Ti cores to form Ti3Al and TiAl phases. TiAl phase and original TiN powers are in direct contact each other; finally, the residual TiN powers react with TiAl phase and result in a plenty of Ti2AlN phase. Compared with TiAl matrix, the hardness, elastic modulus and high-temperature compressive strength of Ti2AlN/TiAl composite are improved obviously and they are all enhanced with increasing the volume fraction of Ti2AlN phase.

Characterization of multi-scale porous structure of fly ash/phosphate geopolymer hollow sphere structures: from submillimeter to nano-scale.

In the present work, the porous structure of fly ash/phosphate geopolymer hollow sphere structures (FPGHSS), prepared by pre-bonding and curing technology, has been characterized by multi-resolution methods from sub-millimeter to nano-scale. Micro-CT and confocal microscopy could provide the macroscopic distribution of porous structure on sub-millimeter scale, and hollow fly ashes with sphere shape and several sub-millimeter open cells with irregular shape were identified. SEM is more suitable to illustrate the distribution of micro-sized open and closed cells, and it was found that the open cells of FPGHSS were mainly formed in the interstitial porosity between fly ashes. Mercury porosimeter measurement showed that the micro-sized open cell of FPGHSS demonstrated a normal/bimodal distribution, and the peaks of pore size distribution were mainly around 100 and 10 μm. TEM observation revealed that the phosphate geopolymer was mainly composed of the porous area with nano-pores and dense areas, which were amorphous Al-O-P phase and α-Al2O3 respectively. The pore size of nano-pores demonstrated a quasi-normal distribution from about 10 to 100 nm. Therefore, detailed information of the porous structure of FPGHSS could be revealed using multiple methods.

In situ observation of G. P. zones in an Al-Zn-Mg alloy under irradiation of electron beam

Harbin Inst Technol, Sch Mat Sci & Engn, Harbin 150001, Peoples R China. Chinese Acad Sci, Inst Met Res, Atom Imaging Solids Lab, Shenyang 110015, Peoples R China.;Sun, DL (reprint author), Harbin Inst Technol, Sch Mat Sci & Engn, Harbin 150001, Peoples R China

Reaction Synthesis Mechanism of TiAl Alloy Fabricated and by Hot Pressed Sintering with Mixed Powders of TiH2 Aluminum

Ti-47Al(at.%) alloys were fabricated through hot pressed sintering using mixed powders of TiH2 and Al and mixed powders of Ti and Al, respectively, as starting materials. The microstructure of alloys were analysed by differencial thermal analysis (DTA),scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the mechanism of the sintering process were studied. Through the experiment it is found that TiH2 powders have been decomposed to Ti and H2 completely before they react with Al powders during the hot press synthesis. The reaction of TiH2 and Al powders is faster than that of Ti and Al powders during the process of hot press synthesis because Ti formed from the decomposition of TiH2 powder has higher activity.

Study on Dimensional Stability of 1Cr18Ni9Ti Stainless Steel and its Mechanism

The influence of stabilizing treatments on the dimensional stability, micro structure and stress state of 1Cr18Ni9Ti austenitic stainless steel was studied by testing the real time dimensional variation of specimens through thermal-cooling cycling on-line measuring method, and then the dimension-stabilizing mechanisms were discussed. The results reveal that the dimensional stability is improved remarkably in the specimens after annealing at 500°C and then thermal-cooling cycling from -60°C to 100°C for four times. The magnitude of accumulative residual relative length change (ARRLC) at cycling end is -0.52×10−5, and the mean residual relative length change (MRRLC) is 0.13×10−5. The improvement of dimensional stability is resulted from the steady dislocation configuration and low residual stress.