Advanced Materials Research Vols. 284-286

Abstract: The specimens of low angle boundaries were machined from the second generation single crystal superalloy DD6 blades. The microstructures of low angle boundaries (LAB) were investigated from three scales of dendrite, γ′ phase and atom with optical microscopy (OM), scanning electron microscope (SEM), transition electron microscope (TEM) and high resolution transmission electrion microscopy (HREM). The results showed that on the dendrite scale LAB is interdendrite district formed by three dimensional curved face between the adjacent dendrites. On the γ′ phase scale LAB is composed by a thin layer γ phase and its bilateral imperfect cube γ′ phase. On the atom scale LAB is made up of dislocations within several atom thickness.

Abstract: The Mg-3% Al melt was treated by carbon inoculation and Fe addition. The influence of Fe on the formation of nucleation was investigated. That Fe inhibited the grain refinement of the carbon-inoculated Mg-3%Al alloy or not was closely associated with the operating sequence of carbon inoculation and Fe addition. Under the condition of Fe addition firstly and then carbon inoculation, the Al₄C₃ particles and the particles with duplex phase structure of Al₄C₃ coated on Al-C-Fe or Al-Fe could be observed. Both kinds of particles should act as nucleating substrates for a-Mg grains, resulting in no obvious effect of Fe on refining efficiency of carbon inoculation. Under the contrary condition, the potency of Al₄C₃ nucleating substrates was poisoned by transforming Al₄C₃ into Al-C-Fe-rich intermetallic particles, resulting in grain-coarsening.

Abstract: This paper studied the intergranular corrosion behavior of A7N01-T4 Al-alloy and its welding joint through corrosion rate test by chemical analysis and morphology observation by metalloscope, LCSM and element analysis by EDS. The results showed that the corrosion rate of welding joint was higher than that of the base, and HAZ was the worst corrosion part of welding joint. Zn₂Mg strengthening phase of HAZ segregated continuously along grain boundary under the affected of heat-input of welding, and the low potential of this zinc-rich phase made it to be anode in corrosion cell and dissolved preferentially, an active corrosion route along grain boundary was formed, which made further corrosion develop into inner alloy.

Abstract: The influences of hot forging and ageing treatment on the microstructure and mechanical properties of Mg−8Gd−2Y−1Nd−0.3Zn−0.6Zr (wt.%) alloy have been investigated. The results showed that the grains were significantly refined after hot forging. And the secondary phases in this alloy i.e. Mg₅(Gd_1-x-yNd_xY_y) and Mg₂₄(Y_1-x-yGd_xNd_y)₅ phases were fragmented to small particles due to the large strain during hot forging. Tensile tests revealed that mechanical properties were improved due to grain size refinement. Moreover, the as-forged alloy exhibited remarkable age-hardening response and mechanical properties were further improved by ageing treatment. The ultimate tensile strength, yield strength and elongation of the peak-aged (T5) alloy are 286 MPa, 245 MPa and 5.6 % at room temperature, and 211 MPa, 103 MPa and 19.4 % at 300°C, respectively.

Abstract: A series of heat treatments were preformed to study the evolution of microstructure in an austenitic nickel-base Alloy 690 using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the pre-existed chromium carbides of cold-rolled Alloy 690 were dissolved into the austenitic matrix after high temperature mill annealing (MA) at 1353K for 5min. Mill-annealed and thermally treated Alloy 690 had intergranular M₂₃C₆-type carbides. The morphology of intergranular M₂₃C₆ varied from continuous, semi-continuous to discrete with the temperature of thermal-treatment (TT) increasing from 873K to 1073K. The holding time at temperature of 988K had less influence on the intergranular carbide appearance than that of temperature. The minimum chromium concentration in the vicinity of intergranular carbides was 22.3 wt% in the sample thermally treated at 715°C for 2h. With the holding time of TT increasing from 2h to 27h, Cr concentration increased gradually and the chromium depletion became wider.

Abstract: The study on the phase transformation law of High-Carbon Matrix steel 4Cr-3Mo-3V-2W-Ni-Nb, which is processed in different heat treatment, is conducted by means of metallographic observation, hardness determination and phase analysis conducted with the help of X-ray diffraction. The results indicate what is as follow: through quenching at 970°C, the main precipitations of alloy carbide are V₄WC₅, Cr₇C₃ and Fe₃W₃C at 200~300°C, V₄WC₅ and Cr₇C₃ at 400°C, V₄WC₅ and Cr₇C₃ at 500°C, V₄WC₅, Cr₇C₃ and Mn₂₃C₆ at 600°C. The transformation of residual austenite is thorough completed above 400 °C and the cause of secondary hardening is the precipitation of V₄WC₅ and V₄C_2.67.

Abstract: The microstructure and wearing resistance of 5CrMnMo hot working die steel with four different amounts of additions of Ce was studied and compared with that without Ce by analyzing the microstructure, worn-out appearance, average wearing rate and friction coefficient. The results showed that when the addition of Ce was proper, the microstructure was significantly refined, the average wearing rate and friction coefficient were reduced markedly and the grinding crack image was also relatively thin and shallow. Therefore, the wear resistance was improved effectively. When the addition of Ce is 0.20 %( mass percent), 5CrMnMo steel had the least surface average wearing quantities and the best wearing resistance. As compared with the 5CrMnMo steel without Ce, the average wearing rate decreasing margin of that with 0.20% Ce was high to 36.19%.

Abstract: Effects of thermo-mechanical treatment on the mechanical properties and microstructure of 2197 alloy were studied through analyses of the mechanical properties by tensile tests and TEM observation of thermo-mechanical heat treated 2197 alloy plates of various states. Results show that the dominating precipitated phase of peak-aged 2197 alloy during thermo-mechanical heat treatment is T1 phase of which the size is 50~150nm. The precipitation and growth of T1 phase are accelerated due to the existed nucleation sites for heterogeneous nucleation of T1 phase offered by thermo-mechanical treatment, therefore, the time for 2197 alloy to reach the peak-aged state is shorten. The strength of 2197 alloy for peak-aged state is increased through thermo-mechanical treatment because the strengthening effect of T1 phase with higher aspect ratio is bigger than that of δ′ and θ′ phases.

Abstract: The initial discontinuity state (IDS) concept was developed just several years ago in an attempt to describe the as-manufactured or as-produced state of material. As a geometric and material characteristic, IDS is the major parameter of holistic life assessment methodology which establishes the initial analysis condition. In this paper, a number of IDS values were determined for several types of smooth specimens. A crack growth analysis software was used to develop the IDS. The results obtained indicate that the calculated IDS values scatter widely. A statistical analysis shows that the distribution of the calculated IDS values fit well with Weibull distribution.

Abstract: On the basis of analysis of thermodynamics and kinetics on the oxidation behavior of MoSi₂ at 673 – 873 K, an oxidation kinetics model is established. The kinetics model gives a well explanation that the very severe oxidation rate of MoSi₂ at 773 K is because of the maximum oxidation reaction constant near this temperature.