HIGH TEMPERATURE TENSILE FRACTURE BEHAVIOR OF [221]-ORIENTED DD100 SINGLE CRYSTAL SUPERALLOY

An investigation has been made on the tensile fracture behavior of oriented single crystal DD100 superalloy at 850. The appearance of the fracture surface of DD100 reflects the glide plane decohension fracture mechanism, which is different from the fracture of oriented DD100. The tensile strength and elongation are lower than the oriented DD100. Oxygen in air has little effect on the strength of DD100 but reduces the tensile elongation obviously. Oxygen does not affect the fracture mode of DD100 but strongly accelerates the process of fracture.

B-C Bond in Diamond Single Crystal Synthesized with h-BN Additive at High Pressure and High Temperature

The synthesis of diamond single crystal in the Fe64Ni36-C system with h-BN additive is investigated at pressure 6.5 GPa and temperature range of 1300-1400℃. The color of the obtained diamond crystals translates from yellow to dark green with increasing the h-BN addition. Fourier-transform infrared （FTIR） results indicate that sp2 hybridization B-N-B and B-N structures generate when the additive content reaches a certain value in the system. The two peaks are located at 745 and 1425cm-1, respectively. Fhrthermore, the FTIR characteristic peak resulting from nitrogen pairs is noticed and it tends to vanish when the h-BN addition reaches 1.1 wt%. Furthermore, Raman peak of the synthesized diamond shifts down to a lower wavenumber with increasing the h-BN ~ddition content in the synthesis system.

Inclusions in large diamond single crystals at different temperatures of synthesis

The inclusions in large diamond single crystals have effects on its ultimate performance, which restricts its industrial applications to a great extent. Therefore, it is necessary to study the inclusions systematically. In this paper, large diamond single crystals with different content values of inclusions are synthesized along the(100) surface by the temperature gradient method(TGM) under 5.6 GPa at different temperatures. With the synthetic temperature changing from 1200?C to 1270?C,the shapes of diamonds change from plate to low tower, to high tower, even to steeple. From the microscopic photographs of the diamond samples, it can be observed that with the shapes of the samples changing at different temperatures, the content values of inclusions in diamonds become zero, a little, much and most, correspondingly. Consequently, with the temperature growing from low to high, the content values of inclusions in crystals increase. The origin of inclusions is explained by the difference in growth rate between diamond crystal and its surface. The content values of inclusions in diamond samples are quantitatively calculated by testing the densities of diamond samples. And the composition and inclusion content are analyzed by energy dispersive spectroscopy(EDS) and x-ray diffraction(XRD). From contrasting scanning electron microscopy(SEM) photographs, it can be found that the more the inclusions in diamond, the more imperfect the diamond surface is.

Effect of temperature on photoresponse properties of solar-blind Schottky barrier diode photodetector based on single crystal Ga_2O_3

A solar-blind photodetector is fabricated on single crystal Ga_2O_3 based on vertical structure Schottky barrier diode. A Cu Schottky contact electrode is prepared in a honeycomb porous structure to increase the ultraviolet(UV) transmittance.The quantum efficiency is about 400% at 42 V. The Ga_2O_3 photodetector shows a sharp cutoff wavelength at 259 nm with high solar-blind/visible(= 3213) and solar-blind/UV(= 834) rejection ratio. Time-resolved photoresponse of the photodetector is investigated at 253-nm illumination from room temperature(RT) to 85.8℃. The photodetector maintains a high reversibility and response speed, even at high temperatures.

Isothermal oxidation behavior of single crystal superalloy DD6

The isothermal oxidation behavior of the second generation single crystal superalloy DD6 was studied at 1050 ℃ and 1100 ℃ in ambient atmosphere.Morphology of oxides was examined by SEM and their composition was analyzed by XRD and EDS.The experimental results show that DD6 alloy obeys subparabolic rate law during oxidation of 100 h at 1050 ℃ and 1100 ℃.The oxide scale exposed at 1050 ℃ is made up of an outer NiO layer with a small amount of Al2O3 and an inner Al2O3 layer.The oxide scale exposed at 1100 ℃ is made up of an outer Al2O3 layer with a small amount of NiO,an intermediate layer,mainly composed of Cr2O3 and TaO2,and an inner Al2O3 layer.The γ＇-free layer was formed under the oxide scale at two temperatures.

Effects of directional solidification parameters and crystal selector on microstructure of single crystal of Ni-base superalloys

The single crystal of nickel-base super alloy is widely used for making turbine blades.The microstructure of the alloy,especially the deviation of preferred orientation of single crystal,possesses the most important effects on the mechanical properties of the blades.In this study,the single crystal ingot and blade of DZ417G alloy are prepared by means of the spiral crystal selector as well as the directional solidification method,and the effect of the parameters(i.e.,the shape of samples,the withdrawal rate)and the structure of the spiral crystal selector on the formation of single crystal and the crystal orientation are investigated.This method can prepare not only the single crystal ingot with simple shape but also the single crystal blades with the complex shape,the simple with rod-shape can form the single crystal easily with a relatively fast withdrawal rate,but the blade with complex shape requires much slower withdrawal rate to form single crystal.The length of the crystal selector almost has no effect on the crystal orientation.However,the angle of selector plays an obvious role on the orientation;the selector with a smaller angle can effectively reduce the deviation of preferred orientation;the appropriate angle of selector to obtain optimal orientation is found to be around30°and the deviation of preferred orientation is about30°for this selector.

SINGLE CRYSTAL GROWTH OF Cu BASED SHAPE MEMORYALLOYANDITSTHERMODYNAMICANALYSIS

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Effect of grain size on high-temperature oxidation behavior of Cu-80Ni alloy

The thermogravimetric analysis of binary Cu 80Ni alloys prepared respectively by conventional casting(CA) and mechanical alloying(MA) techniques and presenting widely different grain sizes was performed at 800 ℃ in air in order to study the effect of grain size change on the oxidation behavior of a solid solution alloy. The results show that the kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law and usually are not composed of a single stage. Mixed scales were produced on the CACu 80Ni alloy surface, which consists of a mixture of copper and nickel oxides. However, oxide scale for MACu 80Ni alloy is mainly composed of a thick compact and continuous inner layer of nickel oxide. The reduction in the alloy grain size speeds up the diffusion of the more reactive component nickel from the alloy to alloy/oxide scale interface and completes the transition from a mixed scale to continuous scale of nickel oxide.

Congruent melting of tungsten phosphide at 5 GPa and 3200℃ for growing its large single crystals

As one of important members of refractory materials,tungsten phosphide(WP)holds great potential for fundamental study and industrial applications in many fields of science and technology,due to its excellent properties such as superconductivity and as-predicted topological band structure.However,synthesis of high-quality WP crystals is still a challenge by using tradition synthetic methods,because the synthesis temperature for growing its large crystals is very stringently required to be as high as 3000℃,which is far beyond the temperature capability of most laboratory-based devices for crystal growth.In addition,high temperature often induces the decomposition of metal phosphides,leading to off-stoichiometric samples based on which the materials'intrinsic properties cannot be explored.In this work,we report a high-pressure synthesis of single-crystal WP through a direct crystallization from cooling the congruent W-P melts at 5 GPa and^3200℃.In combination of x-ray diffraction,electron microscope,and thermal analysis,the crystal structure,morphology,and stability of recovered sample are well investigated.The final product is phase-pure and nearly stoichiometric WP in a single-crystal form with a large grain size,in excess of one millimeter,thus making it feasible to implement most experimental measurements,especially,for the case where a large crystal is required.Success in synthesis of high-quality WP crystals at high pressure can offer great opportunities for determining their intrinsic properties and also making more efforts to study the family of transition-metal phosphides.

Pt-Al₂OM₃Composite Material Designed for Cyclic Production of Optical Glasses under High Temperature Conditions

The article considers one of the possible approaches to the solution of an urgent issue of metal consumption reduction, increase of operating life and maximum operating temperature as well as reduction of irrecoverable losses of platinum products and alloys when operating under high temperature conditions, particularly for glassblowing and single crystal growing crucibles. A two-layered composite material based on platinum-group metals and corundum plasma ceramics is thoroughly investigated. A successful experience of crucibles exploitation, designed for production of high temperature optical glasses from the composite and results of the research on composite material specimens are described.

Influence of yttrium addition on high temperature oxidation resistance of single crystal superalloy

Two experimental single crystal superalloys Y-free alloy and Y-containing alloy were cast in the directionally solidified furnace, while other alloying element contents were basically kept unchanged. The isothermal oxidation behavior of two single crystal superalloy was studied at 1100 ℃ in ambient atmosphere. Morphology of oxides was examined by scanning electron microscopy （SEM） and their compostion was analyzed by X-ray diffraction （XRD） and energy dispersive spectroscopy （EDS）. The effect of yt- trium on the high temperature oxidation resistance of the single crystal superalloy was investigated. The results showed that the oxi- dation resistance of Ni-base single crystal superalloy was substantially improved by adding a little amount of rare earth yttrium. Yt- trium could promote the oxide of A1 formation and decreased the proportion of NiO. Yttrium increased the coherence between the oxide scale and the alloy substrate to decrease the spallation of oxide scale. Yttrium reduced the oxide grain size and the thickness of oxide layer.

Increasing high-temperature fatigue resistance of polysynthetic twinned TiAl single crystal by plastic strain delocalization

Polysynthetic twinned(PST)TiAl single crystal possesses great potentials for high-temperature applications due to its excellent combination of strength,ductility and creep resistance.However,a critical property for high-temperature application of such material involving high-temperature fatigue properties remains unknown.Here,the high-temperature high-cycle fatigue performance of PST TiAl single crystal has been studied.The result shows that PST TiAl single crystal can withstand more than 107 cyclic loadings at 975℃ under a stress amplitude of 270 MPa,which is significantly higher than traditional TiAl alloys.Experimental observations and atomistic simulations indicate that the improvement of fatigue resistance is attributed to the plastic strain delocalization in uniform lamellar structure,and the plastic deformation is well-distributed and sufficient in each lamella.Even in theα2 lamella with difficult slippage,a large number of stacking fault structures can be observed.The{c+a}dislocations inα2 tend to dissociate into a Frank partial with b=1/6<2^(-)20^(-)3>,forming a ribbon of I1 fault which ensures the continuity of deformation.

Aging process design based on the morphological evolution ofγprecipitates in a 4th generation nickel-based single crystal superalloy

In order to meet the design requirements of the aging treatment process of a 4th generation nickel-based single crystal superalloy(Ni-SX)developed independently,the effects of aging temperatures and aging times on the precipitation and morphological evolution ofγprecipitates are studied.The morphological evolution behavior ofγprecipitates during the aging process is summarized subsequently and the coarsening behavior ofγprecipitates is discussed by comparing with the Lifshitz-Slyozov-Wagner model(LSW)and the trans-interface diffusion-controlled model(TIDC).It is demonstrated that primary aging temperature and secondary aging time dominate the size and squareness ofγprecipitates respectively,a narrow primary aging temperature range and a suitable secondary aging time are allowed to obtain the optimized morphology ofγprecipitates.The optimal aging process of the Ni-SX investigated in the present work is obtained for 1100-1120°C/4 h and 870°C/16 h,confirmed by the corresponding creep tests.The coarsening growth ofγprecipitates in short-term aging also conforms to the LSW model well.Besides,the aging process design rules of various Ni-SXs of different generations are also summarized.

PST TiAl single crystals for high temperature applications

Recently,the research team led by Prof.Chen Guang(陈光)at the Engineering Research Center of Materials Behavior and Design,Ministry of Education,Nanjing University of Science and Technology,successfully manufactured a new kind of high-temperature polysynthetic twinned(PST)TiAl single

High-temperature High-pressure Hydrothermal Synthesis of Dy3＋-doped YNbO4 Single Crystal and Its Luminescence Properties

Single crystal Dy3＋ doped YNbO4phosphors were prepared via a high-temperature high-pressure hydrothermal procedure. Under excitation at 270 nm, the Dy3＋-doped YNbO4 phosphor shows bright white emission, which is composed of two strong bands at 492 and 576 nm corresponding to the characteristic 4F9/2→6H15/2 and aF9/2→6H13/2 transitions of Dy3＋, respectively. The dominant band was observed at 352 nm, which corresponds to the 6H15/2→6p7/2 transition of Dy3＋. Nearly white light was achieved at 2ex 270, 310 and 388 nm and the CIE（International Commission on Illumination） values were （0.3135, 0.3421）, （0.3088, 0.3380） and （0.3146, 0.3296）, respectively.

Microstructure and properties of novel quinary multi-principal element alloys with refractory elements

Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.

Growth of Sr_2CuO_(3+δ) superconductor single crystals at high pressure

We have successfully synthesized Sr 2 CuO3+δ single crystals under high pressure and high temperature for the first time. The structure analysis show that this material crystallizes into tetragonal structure isostructural La 2 CuO4 with single CuO 2 plane. The magnetic susceptibility as well as resistance measurements indicates that the bulk superconductivity with the critical transition temperature 37 K is achieved in the crystal.

Air oxidation of Cu-50Ni and Cu-70Ni alloys at 800℃

The air oxidation of Cu Ni alloys with 50% and 70% nickel (mole fraction) at 800?℃ was studied. The kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law. Typical double layered scales are produced, which consist of a CuO outer layer and an inner layer containing a mixture of Cu 2O and NiO with many pores. Cu 50Ni presents a small degree of internal oxidation of nickel, which is observed in many binary double phase systems, but is quite rare in single phase systems.

Compressive Deformation Induced Nanocrystallization of a Supercooled Zr-Based Bulk Metallic Glass

The nanocrystallization behaviour of a bulk Zr-based metallic glass subjected to compressive stress is investigated in the supercooled liquid region. Compared with annealing treatments without compressive stress, compressive deformation promotes the development of nucleation and suppresses the coarsening of nanocrystallites at high temperatures.

Regulation mechanism of catalyst structure on diamond crystal morphology under HPHT process

To elucidate the regulation mechanism of catalyst geometry structure to diamond growth,we establish three catalyst modes with different structures.The simulation results show that with the decrease of the protruding height of the catalyst,the low-temperature region gradually moves toward the center of the catalyst,which causes the distribution characteristics of the temperature and convection field in the catalyst to change.The temperature difference in vertical direction of the catalyst decreases gradually and increases in the horizontal direction,while the catalyst convection velocity has the same variation regularity in the corresponding directions.The variation of temperature difference and convection velocity lead the crystal growth rate in different crystal orientations to change,which directly affects the crystal morphology of the synthetic diamond.The simulation results are consistent with the experimental results,which shows the correctness of the theoretical rational analysis.This work is expected to be able to facilitate the understanding of catalyst structure regulation mechanism on diamond morphology and the providing of an important theoretical basis for the controllable growth of special crystal shape diamond under HPHT process.