Key Engineering Materials Vols. 297-300

Abstract: The fatigue strength of TiAl intermetallic alloy coated with TiAlN film was studied in vacuum at 1073K using a SEM-servo testing machine. In addition, three kinds of TiAlN films were given by physical vapor deposition (1, 3, and 10μ m). The fatigue strength of 3μ m was highest. Also, the fatigue strength of 1μ m was lowest. From this result, existence of optimum film thickness was suggested because the difference of fatigue strength arose in each film thickness. The justification for existence of optimum film thickness is competition of 45-degree crack and 90-degree crack. The 45-degree crack is phenomenon seen in the thin film (1μ m), and is caused by plastic deformation of TiAl substrate. The 45-degree crack is the factor of the fatigue strength fall by the side of thin film. In contrast, the 90-degree crack is phenomenon in the thick film (10μ m), and is caused as result of reaction against load to film. The 90-degree crack is the factor of the fatigue strength fall by the side of thick film. In conclusion, the optimum film thickness can perform meso fracture control, and improves fatigue strength.

Abstract: This paper describes the evaluation of the creep behavior with two types of matching filler metals for 2.25Cr-1.6W(T23)/Mod. 9Cr-1Mo(T91) dissimilar weld joint. Through the welding procedure qualification tests prior to the creep tests, optimum PWHT holding times at 745±5oC were determined as 30minutes for T23 matching filler metal and 60minutes for T91 matching filler metal. It was also confirmed that carbon migration across the weld interfaces and the softened area at HAZ occurred during PWHT. Creep rupture tests were carried out at 575-650oC and 70-220MPa for two dissimilar metal welds manufactured by an optimum PWHT condition. From the comparison of creep rupture strength, it was shown that the creep strength of the welded joint including T23 matching filler metal was similar to that of T23 base material and somewhat higher than that of the welded joint including T91 matching filler metal. From metallurgical study on the crept specimen, the creep damage of T23/T91 dissimilar weld joints could occur at T23 side IC HAZ and near the T23/T91 weld interface simultaneously. However, the final failure locations seemed to depend on the filler metal and the cause of this phenomenon might be regarded as the additional effect of carbon migration across the weld interfaces. It is thought that the dissimilar weld including T91 filler metal which failures at T23 side CG HAZ subject to both metallurgical notch and carbon migration become more susceptible to creep damage than that including T23 filler metal in which the carbon depletion occurs at the T23 filler metal with higher creep strength than T23 HAZ. T23 filler metal can therefore be proposed for matching filler metal of T23/T91 dissimilar weld joints

Abstract: A new modified Nb-containing A-286 γ' strengthened austenitic Fe-base superalloy (14Cr- 28Ni-1.5Mo-1W-2Ti-Nb-Al) designated as GH871 in China characterizes with high level of tensile, stress rupture and low cycle fatigue properties at 650°C. However, the stress rupture ductility is low and the crack propagation rates at 650°C creep or creep/fatigue interaction conditions are high. For ductility and crack propagation behavior improvement vacuum melted GH871 can still keep its high strength level, also raise stress rupture ductility and simultaneously to decrease crack propagation rates at 650°C. Our results support GH871 to be used as a disk material in high temperature industry.

Abstract: To assess the failure risk of high temperature structures, it is essential to obtain the creep fracture toughness of materials. In the present paper, Creep crack growth (CCG) tests for 2¼Cr1Mo steel have been carried out by utilizing the side grooved compact tension (CT) specimen and direct current (DC) electrical potential technique. The material parameter of creep fracture toughness, c mat K , were obtained from analysis of 11 CCG tests at temperatures of 500 and 565oC. It has been found that for 2¼Cr1Mo steel c mat K decreases with increasing time and increases with elevating temperature. For this reason, the value of c mat K at lower temperature or longer service time can be used to extract a conservative result in the high temperature defect assessment where the accurate value of c mat K is unavailable. It is also validated from this test that the proposed formula by Ainsworth et al can be adopted in practice.

Abstract: Both the change in DBTT and the degradation of creep properties of the aged CrMoV cast steel which had been actually used as a main steam valve were evaluated from the conventional SP test and the SP creep (SP-C) test using by SP test specimens. The CVN DBTTs of the virgin and aged steels were estimated to be 280 and 369 K by the conventional SP test, respectively. This increase in DBTT after the long-term service exposure was considered to be caused by carbide-induced embrittlement due to coarse carbides. The SP-C test results revealed that the creep rupture strength of the aged steel was inferior to that of the virgin steel. The ratio, F / s , between the load of the SP-C test (F) and the stress of the uniaxial test ( s) was determined to be 2.4 for the same value of the Larson-Miller parameter in the creep rupture data. This linear relationship enables us to convert the results of the SP-C test into those corresponding to a conventional uniaxial creep test. Consequently, the SP testing method is expected to be appropriate tool for the determination of material deterioration and, in turn, the assessment of remaining-life of high-temperature components.

Abstract: The microstructure of directionally solidified (DS) NiAl-28Cr-5Mo-1Hf eutectic alloy was investigated by SEM and TEM. The eutectic alloy is composed of NiAl matrix , Cr (Mo) phase and a small amount of Heusler phase. Tensile testing of this alloy was performed from room temperature to 1373K. The brittle-to-ductile transition temperature (BDTT) is dependent on strain rate, with two orders of increase on strain rate resulting in a100K increase in transition temperature. The observation of fracture surface showed that at temperature above BDTT, the fractograph changes from NiAl cleavage and debonding along NiAl/Cr(Mo) interface boundary to ductile in nature.

Abstract: In this paper, the mechanical behavior of Ni-base single crystals joint created by TLP bonding is presented. Experimental study has been performed on the TLP single crystal under loading conditions of static uniaxial tension, creep and Three-points-bending (T-P-B) at temperature of 850 °C. Specimens made of the base material, Ni-base single crystal specimens without bonding, were also tested under the same loading conditions. The mechanical behaviors of TLP single crystal is compared to the behaviors of base material. It was found that the static strength of the TLP specimens with the boundary normal to the tensile direction was 63% of the strength of the base material. The creep strength and the fracture ductile strength was more than 57% and 55% of that of the single crystal base material, respectively. The macro and micro graphs of fracture surfaces of specimens indicated that the fracture modes of the samples were brittle fracture.

Abstract: Voronoi cell finite element method (VCFEM) is introduced in this paper to describe the elastic-plastic-creep behavior of particle reinforced composites. The interfacial damage is simulated by partly debonding between Matrix and inclusion. A validation of the nonlinear behavior of the cell element has been carry out by comparing VCFEM results with those calculated by the general finite element package MARC and ABAQUS, and good agreements are found. A microstructure with five inclusions is taken as an example to describe the cyclic stress-strain behavior under different particulate orientation condition, and it shows the influence of the topological microstructure of inclusions. Thermomechanical fatigue properties are also investigated and the loops of stress-strain show the great differences of fatigue behavior between the in-phase case and out-of-case.