Relationship of microstructure transformation and hardening behavior of type 17-4 PH stainless steel

The relationship between the microstructure transformation of type 17-4 PH stainless steel and the aging hardening behavior was investigated. The results showed that, when 17-4 PH stainless steel aging at 595℃, the bulk hardness of samples attains its peak value （42.5 HRC） for about 20 min, and then decreases at all time. TEM revealed the microstructure corresponding with peak hardness is that the fine spheroid-shape copper with the fcc crystal structure and the fiber-shape secondary carbide M23C6 precipitated from the lath martensite matrix. Both precipitations of copper and M23C6 are the reasons for strengthening of the alloy at this temperature. With the extension of holding time at this temperature, the copper and secondary carbide grow and lose the coherent relationship with the matrix, so the bulk hardness of samples decreases.

Strengthen Vanadium Alloy by Aging and Its Thermal Stability

V-4Cr-4Ti is the leading candidate vanadium alloy for fusion applications as structural material of first wall and blanket. Due to the interaction between Ti and interstitial solutes of C, N, and O, precipitation occurs at elevated temperature. The behavior has been studied in the past few years by short time annealing and results showed that it may greatly affect its mechanical properties Ti-CON type precipitates, appearing at- 700℃ in the solid-solution annealed alloy in high number density and small size, strengthen the alloy significantly and reduce its ductility. As the ductility reduction is in an acceptable level, the strengthening might be utilized for a light and strong vanadium alloy structure. Before a conclusion, uncertainty of its thermal stability should be studied during the high temperature serves. Besides, seldom has been studied for the effect of long time aging on precipitation behavior and tensile properties of the alloy.

Optimization of N18 Zirconium Alloy for Fuel Cladding of Water Reactors

In order to optimize the microstructure and composition of N18 zirconium alloy （Zr-1Sn-0.35Nb-0.35Fe-0.1Cr, in mass fraction, %）, which was developed in China in 1990s, the effect of microstructure and composition variation on the corrosion resistance of the N18 alloy has been investigated. The autoclave corrosion tests were carried out in super heated steam at 400 ~C/10.3 MPa, in deionized water or lithiated water with 0.01 mol/L LiOH at 360 ~C/18.6 MPa. The exposure time lasted for 300-550 days according to the test temperature. The results show that the microstructure with a fine and uniform distribution of second phase particles （SPPs）, and the decrease of Sn content from 1% （in mass fraction, the same as follows） to 0.8% are of benefit to improving the corrosion resistance; It is detrimental to the corrosion resistance if no Cr addition. The addition of Nb content with upper limit （0.35%） is beneficial to improving the corrosion resistance. The addition of Cu less than 0.1% shows no remarkable influence upon the corrosion resistance for N18 alloy. Comparing the corrosion resistance of the optimized N18 with other commercial zirconium alloys, such as Zircaloy-4, ZIRLO, E635 and Ell0, the former shows superior corrosion resistance in all autoclave testing conditions mentioned above. Although the data of the corrosion resistance as fuel cladding for high burn-up has not been obtained yet, it is believed that the optimized N18 alloy is promising for the candidate of fuel cladding materials as high burn-up fuel assemblies. Based on the theory that the microstructural evolution of oxide layer during corrosion process will affect the corrosion resistance of zirconium alloys, the improvement of corrosion resistance of the N18 alloy by obtaining the microstructure with nano-size and uniform distribution of SPPs, and by decreasing the content of Sn and maintaining the content of Cr is discussed.

Improvement of tribological behavior of a Ti-Al-V alloy by nitrogen ion implantation

The tribological properties especially wear and hardness of a Ti-Al-V alloy with nitrogen implantation (energy 60 keV) were investigated. The implantation was carried out at fluences range from 1×1016 to 4×1017 ions/cm2. Glancing angle X-ray diffraction (GAXRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed to obtain surface characterization of the implanted sample. The unimplanted and implanted samples were also annealed at 600 ℃ in order to understand the influence of annealing on the tribological properties of Ti-Al-V. The hardness shows significant improvement at the higher fluence. After annealing at 600 ℃, the friction coefficient exhibits a relative decrease for the nitrogen-implanted samples. In addition, the wear rates of the implanted samples exhibits a great decrease after annealing at 600 ℃. Nature of the surface and reason for the variation and improvement in wear resistance were discussed in detail.

Microstructure Evolution of AZ31 Mg Alloy during Change Channel Angular Extrusion

Microstructure evolution of AZ31 Mg alloy during change-channel angular extrusion （CCAE） was investigated. The grains of AZ31 Mg alloy were refined significantly from 500 mm to 15 mm after CCAE deformed at 523 K. Dislocations were induced at the initial stage of extrusion and they rearranged themselves to form dislocation boundaries and sub-grain boundaries during deformation. When the specimen through the horizontal change channel with the strain increased, the sub-boundaries evolved to high angle grain boundaries （HAGB）. The process of grain refinement can be described as continuous dynamic recovery and recrystallization （CDRR）.

Effects of Hydrogen Ion Implantation on TiC-C Coating of Stainless Steel

Titanium carbide coatings are widely used as various wear-resistant material. The hydrogen erosion resistance of TiC-C films and the effect of hydrogen participation on TiC-C films were studied. Seventy-five percent TiC-C films are prepared on stainless steel surface by using ion mixing, where TiC-C films are deposited by rf magnetron sputtering followed by argon ion bombardment. The samples are then submitted to hydrogen ion implantation at 1.2 × 10^-3 Pa. Characterization for the 75% TiC-C films was done with SIMS, XRD, AES, and XPS. Secondary ion mass spectroscopy （SIMS） was used to analyze hydrogen concentration variation with depth, X-Ray diffraction （XRD） was used to identify the phases, and Auger electron spectra （AES） as well as X-ray photoelectron spectra （XPS） were used to check the effects of hydrogen on shifts of chemical bonding states of C and Ti in the TiC-C films. It is found that TiC-C films on stainless steel surface can prevent hydrogen from entering stainless steel.

Mechanical behavior of C/SiC T-section under pulling load

For the C/SiC T-section structures, fabrication defects such as pores and local delaminations can be easily formed in the intersection zone which significantly affect the load bearing capacity. In this work, the mechanical behavior of C/SiC T-section under pulling load was investigated, and especially the delamination behavior was studied by introducing the cohesive zone model into the finite element modeling. It was found that for C/SiC T-section under pulling load, the maximum critical delamination load was about1075 N in the present work, and the interface delamination was the main failure mode. It was verified that the effective interfacial strength influenced the critical delamination load, and the strain energy release affected the delamination behavior of the T-section specimen. The failure mechanisms of C/SiC T-section under pulling load depend on the interface bonding states. When the interface is well bonded,the failure mechanisms mainly include matrix stripping, matrix fracture and fiber breakage. Otherwise,only the matrix stripping can be found at the interface of the C/SiC T-section specimen.