Microstructure and Oxidization of Mo Alloys by Spark Plasma Sintering

In the scope of refractory material processed by spark plasma sintering development for structural applications in the core of future combustion chamber, Mo-TiC alloys and Mo-TiC-Cr alloys were sintered at 1700°C with a pressure of 30 MPa. Powders of Mo, TiC and Cr were prepared by planetary ball mill in an argon atmosphere. In this dissertation, the structure, element and phase composition of alloys were investigated. The Mo-TiC alloys showed obvious intergranular brittle fracture. The fracture mode of Mo-TiC-Cr alloys, however, was changed to mixed intergranular and transgranular fracture. The obtained results demonstrated that, compared with the peaks of Mo and Mo2C from Mo-TiC, the peaks of Mo and Mo2C from Mo-TiC-Cr were deflected to the right, due to the addition of chromium forming a solid solution. The results of isothermal oxidation tests at 1200°C in air revealed that the chromium compound on the surface of the oxide layer increases and the pores decreases with the increase of oxidation time for Mo-TiC-Cr alloys. The pores of surface of the Mo-TiC alloys were less and smaller than the Mo-TiC-Cr.

Microstructure and properties of W-Cu/1Cr18Ni9 steel brazed joint with different Ni-based filler metals

W-Cu alloy and 1Cr18Ni9 steel were brazed with NiCrSiBFeTi and NiMnSiCuZr filler metals in a vacuum furnace. The main microstructure of the NiCrSiBFeTi brazing seam region was a Ni-based solid solution of W, Cu, Cr, and Fe dissolved in the Ni phase and intermetallic compounds CrB and TiCr2. The main microstructure of the NiMnSiCuZr brazing seam region was a Ni-based solid solution of Cu, Cr, and Fe dissolved in the Ni phase Fe-based solid solution of Ni, Cu, and Cr dissolved in the Fe phase and intermetallic compounds Mn5Si3, Ni3Si, and Ni7Zr2. The elements Ni, Si, and Ti contained in the NiCrSiBFeTi filler metal and the elements Ni, Mn, and Si contained in the NiMnSiCuZr filler metal were more easily diffused into the interface of W-Cu alloy compared with that of 1Cr18Ni9 steel. The shear strength values of NiCrSiBFeTi joint and NiMnSiCuZr joint were approximately 285 and 249 MPa, respectively, and fracture of the two joints occurred at the interface near the W-Cu alloy side. The fracture morphology of the former was identified as cleavage brittle fracture, and the latter was identified as intergranular brittle fracture.

A Study on Tensile Properties of NiCoCrAl/YSZ Multiscalar Microlaminate

One multiscalar microlaminate (MSML) with 5 thick layers of NiCoCrAl whose thickness were different interspersed with 66 thin layer stacks of NiCoCrAl/YSZ was fabricated by EB-PVD. Uniaxial tensile testing was performed and fracture was examined using SEM. The results show that the microlaminate exhibits brittle-like behavior without macroscopic plastic deformation in room temperature tensile tests and the maximum engineering stress is 212MPa. Examination of fracture surfaces from the samples reveals that ceramic layers fail by intergranular brittle fracture between columns, but metal layers display features of both ductile and brittle fracture. It is also found that the thicknesses of metal layers have a great effect on their failure modes. And interfacial debonding and bridging metal layers are observed. Moreover, the resistance of crack propagation in the microlaminate is discussed.

Investigation of the Hydrogen Embrittlement on the 65Mn Steel from the Nuclear Power Plant with Small Punch Method

Damage of metals due to the influence of hydrogen is quite frequent and leads to dangerous failures. The characteristics of the hydrogen embrittlemnt of the 65Mn steel were evaluated with small punch test. With the increment of the amount of the hydrogen absorbed into the alloy at room temperature, the strength and the toughness of the material reduce. From the small punch experimental results, it is found the total impact energy, the fracture strain and the fracture stress decrease with the increment of the cathodic hydrogen charging time. The fracture surfaces change from the typical ductile fracture with big voids to the typical intergranular brittle fracture mode after hydrogen absorbed in the specimens with higher charging current density.