Solidification microstructure formation in HK40 and HH40 alloys

The microstructure formation processes in HK40 and HH40 alloys were investigated through JmatP ro calculations and quenching performed during directional solidification. The phase transition routes of HK40 and HH40 alloys were determined as L → L ＋ γ→ L ＋ γ ＋ M_7C_3 →γ ＋ M_7C_3 →γ ＋ M_7C_3 ＋ M_（23）C_6→γ ＋ M_（23）C_6 and L → L ＋ δ→ L ＋ δ ＋ γ→ L ＋ δ ＋ γ ＋ M_（23）C_6→δ ＋ γ ＋ M_（23）C_6, respectively. The solidification mode was determined to be the austenitic mode（A mode） in HK40 alloy and the ferritic–austenitic solidification mode（FA mode） in HH40 alloy. In HK40 alloy, eutectic carbides directly precipitate in a liquid and coarsen during cooling. The primary γ dendrites grow at the 60° angle to each other. On the other hand, in HH40 alloy, residual δ forms because of the incomplete transformation from δ to γ. Cr_（23）C_6 carbide is produced in solid delta ferrite δ but not directly in liquid HH40 alloy. Because of carbide formation in the solid phase and no rapid growth of the dendrite in a non-preferential direction, HH40 alloy is more resistant to cast defect formation than HK40 alloy.

CRACK GROWTH BEHAVIOR AND FRACTURE FEATURE OF GH2132 UNDER CREEP-FATIGUE CONDITIONS

The superalloy GH2132 is equivalent to A286. The tests were carried out at 550°C under various cyclic frequencies (hold time) and load levels, and the fracture surfaces were examined by using a scanning electron microscope. It was shown that the fracture mode of creep-fatigue and the effect of cyclic loading on crack growth change with the growth of crack and the increase of net-section stress, and both are reversed when the net-section stress is up to the yield stress of material. When σn0.2, cracking is predominantly cyclic-dependent transgranular and cyclic loading accelerates creep crack growth, whereas when σn>σ0.2, the case is reversed.

WIDE-BAND LASER CLADDING: WEAR AND CORROSION RESISTANCE OF A Ni-BASED ALLOY

A novel wide-band laser cladding system, with high rate of cladding, has been developed in the present work. The system mainly consisted of a 5kW CO2 laser, an automatic powder feeder and a wide-band scanning rotative polygon mirror which can produce a linear or rectangular focused laser beam. Using this system, a Ni-Cr-Si-B alloy powder was cladded on the surface of type 321 austenitic stainless steel in order to improve its wear and corrosion resistance. The pitting corrosion, high temperature oxidation and wear tests were conducted in order to evaluate the properties of the laser cladded layer. The results demonstrated that the cladded layer can significantly improve the adhesive wear and pitting corrosion resistance of the substrate. Moreover, the cladded layer exhibited good oxidation resistance, which is almost the same as that of GMR-235D Ni-based superalloy.

EFFECT OF MODIFICATION ON CARBIDE GRANULATION AND ITS MECHANISM

By the modification, network carbide is disconnected at 1100 degree C through uneven dissolution at higher energy places in carbide, where there is twin or lattice distortion, and is granulated at 1130 degree C through element diffusion. The stability of M//7C//3 carbide is decreased because of the modification reducing the segregation of Cr and Mo, thus the temperature, at which uneven dissolution of carbide commenced, is decreased. Also lattice distortion or defect such as twin in carbide is increased by the modification, which prompts widespread disconnections in carbide. In addition, the modification prompts element diffusion to accelerate the kinetics process of carbide granulation. (Edited author abstract) 6 Refs.

Synthesis of nanocrystalline NiCrC alloy feedstock powders for thermal spraying by cryogenic ball milling

Nanocrystalline NiCrC alloy powders with a qualified particle size distribution for thermal spraying were synthesized using the cryogenic ball milling （cryomilling） method. The morphology, microstructure, size distribution, and phase transformation of the powders were characterized by scanning electron microscopy （SEM）, laser scattering for particle size analysis, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. After cryomilling for 20 h, the average grain size of the as-milled powders approached a constant value of 30 nm by XRD measurement. The average particle size slightly increased from 17.5 to 20.3 μm during the 20-h milling. About 90vol% of the powders satisfied the requirement for thermal spraying with the particle dimension of 10-50 μm, and most of the powders exhibited spherical morphology, which were expected to have good fluidity during thermal spraying. The Cr2O3 phase formed during the cryornilling process as revealed in the XRD spectra, which was expected to enhance the thermal stability of the as-milled powders during the followed thermal spraying or other heat treatment.

Safety and efficacy of cobalt chromium alloy based sirolimus-eluting stent with bioabsorbable polymer in porcine model

Background First generation drug-eluting stents （DESs） were based on 316L stainless steel and coated with a permanent polymer. The vessel wall of these DESs was inflammatory and late in-stent thrombosis was reported. Hence, cobalt chromium based DES coated with a bioabsorbable polymer was an alternate choice. Methods Cobalt chromium based DES with bioabsorbable polymer （Simrex stent） as well as control stents （Polymer stent and EXCELTM stent） were implanted into porcine arteries. At a designated time, angiography, quantitative coronary angiography （QCA） analysis, histomorphometry, and electron-microscopical follow-up were performed. Results A total of 98 stents of all the three groups were harvested. At week 24, percent diameter stenosis （%DS）, late loss （LL）, and percent area stenosis （%AS） of Simrex was （12.9±0.4）%, （0.35±0.02） mm, and （24.5±4.2）%, respectively, without significant difference in comparison to commercialized EXCELTM stent. Slight inflammatory reaction was seen around the stent strut of Simrex, just as in the other two groups. Electron-microscopical follow-up suggested that it might take 4-12 weeks for Simrex to complete its re-endothelialization process. Conclusions Cobalt chromium based, bioabsorbable polymer coated sirolimus-eluting stent showed excellent biocompatibility. During 24 weeks observation in porcine model, it was proved that this novel DES system successfully inhibited neointima hyperplasia and decreased in-stent stenosis. It is feasible to launch a clinical evaluation to improve the current prognosis of DES implantation.

Effect of furnace atmosphere on sintering process of chromium-containing steel via powder metallurgy

During a powder metallurgy process such as sintering,the primary role played by the atmosphere in furnace is to prevent an excessive oxidation of powder compacts in case of the formation of oxides as residuals on powder surfaces.In particular,the adjustment of furnace atmosphere is the key to eliminate the phenomenon"decarburization"likely to occur in carbon-containing compacts.A continuous belt furnace was used to stabilize the potentials of carbon and oxygen in zones divided by sintering,delubrication,and cooling.Chromium and manganese,which are sensitive to oxygen,were added to improve mechanical properties in a cost-effective way.Powders of steel containing chromium were sintered in an atmosphere composed of CO,O_(2),and H_(2).The effects of atmosphere,lubricant,and graphite on oxidation(or reduction)and decarburization(or carburization)were investigated.Superior quality was achieved under the control of delubrication atmosphere.It is indicated that in a protective atmosphere,the chemical reactions occurring at various stages took remarkable effect on the quality of sintered compact.The potentials of oxygen and carbon in a continuous belt furnace were monitored and analyzed using an on-line thermal measuring unit consisting of thermocouple,oxygen probe,and carbon monoxide sensor.The avoidance of oxidation and decarburization promises desired micro structure and carbon content and satisfactory properties through the adjustment of technical parameters,e.g.,the composition of gases in delubrication and various sintering zones,the rate of gas inlet,and cooling rate.

A plating method for metal coating of fiber Bragg grating

We present a method for metal coating optical fiber and in-fiber Bragg grating. The technology process which is based on electroless plating and electroplating method is described in detail. The fiber is firstly coated with a thin copper or nickel plate with electroless plating method. Then, a thicker nickel plate is coated on the surface of the conductive layer. Under the optimum conditions, the surfaces of chemical plating and electroplating coatings are all smooth and compact. There is no visible defect found in the cross-section. Using this two-step metallization method, the in-fiber Bragg grating can be well protected and its thermal sensitivity can be enhanced. After the metallization process, the fiber sensor is successfully embedded in the 42CrMo steel by brazing method. Thus a smart metal structure is achieved. The embedding results show that the plating method for metallization protection of in-fiber Bragg grating is effective.