Materials Science Forum Vols. 539-543

Abstract: Results of experimental research into evolution of the structure and microhardness of the hard magnetic Fe-30Cr-8Co-0,7Ti-0,5V-0,7Si alloy during complex two-level loading (compression + torsion) in isothermal conditions at various temperatures in single-phase region are reported. It was revealed that the deformation leads to a strong refinement of initial coarse-grained structure in the whole volume of the sample, however the generated structure is non-uniform through the body of the sample. In an active zone of deformation, near to mobile head, there is a microcrystalline layer with a grain size of about 5 microns which thickness poorly depends on the formation. With removal from the active zone of deformation the grain size increases, and microhardness decreases.

Abstract: Squeeze-cast 20vol%SiCw/2024 composite was processed by rotary-die equal-channel angular pressing (RD-ECAP) and tensile-tested at elevated temperatures. RD-ECAP does not require billet removal and reinsertion between ECAP passes and can produce large cumulative deformation of type A in the billet at a controlled extrusion temperature. Tensile specimens were machined from the RD-ECAP processed billets along the longitudinal direction. 16 RD-ECAP passes at 623 K produced an average matrix grain size of about 0.8 μm in the composite. The ECAP processed 20vol%SiCw/2024 composite exhibited 330% elongation to failure when tested at 783 K and the high strain rate of 1.17×10-1 s-1. The values of strain rate sensitivity exponent of the SiCw/2024 composite determined over the strain rates between 10-2 and 1.0 s-1, was 0.39, 0.47 and 0.53 at 763, 773 and 783 K, respectively. The grain sizes of the aluminum alloy matrix in the composite after superplastic deformation increased from 0.8 μm to be about 1.5 μm. There was no cavity found within the uniformly deformed area. The apparent activation energy for the superplastic deformation of the 20vol%SiCw/2024 composite was determined to be 284 kJ·mol-1.

Abstract: T122 is a competitive heat resistant steel used for the construction of ultra super critical power stations. The authors experimentally investigated the effect of heat treatment and variation of influential chemical elements, such as vanadium, copper and nitrogen, on the microstructure and properties of the steel, in which the optimal heat treatment schedule during heating was specified and the proper chemical element scope was also decided to achieve the expected service performance. The application and potential of the steel in China today and tomorrow were reasonably briefed. On the other hand, the authors also proposed some intrinsic challenge of the steel, which may undermine the basis of the industrial application of the steel, comparing to other candidates, i.e. T92 steel. An assessment on T122 steel was carefully made from the viewpoint of industrial manufacturing and application.

Abstract: The research activities on ferritic / martensitic 9-12% Cr steels at the Institute of Materials Science, Welding and Forming (IWS) are represented by a network of interacting projects focusing on mechanical properties of base and weld metal, microstructural characterisation of creep and damage kinetics, weldability, microstructure analysis in the course of creep, modelling of precipitation and coarsening kinetics, simulation of complex heat treatments and the deformation behaviour under creep loading. The individual projects are briefly described and the conceptual approach towards a quantitative description of the creep behaviour of 9-12% Cr steels is outlined.

Abstract: The notched low cycle fatigue (LCF) behavior of a P/M (Powder Metallurgy) gas turbine disk superalloy (IN100) was investigated to determine the role of inclusions, such as oxides, that are intrinsic in the process of making powder superalloys. Tests were carried out at temperatures ranging from 426°C to 621°C at several applied stresses. The majority of LCF failures initiated from inclusions (oxides) with minority initiation sites being grain facet in the microstructure. The locations of initiation sites were surface or subsurface, and reduced LCF life was generally associated with surface initiation at the notch root. However, surface initiation was infrequent and observed only at high stresses (i.e., in the presence of large plasticity at the notch root). The stress gradient at the notch root coupled with inclusion size determined the critical conditions for fatigue initiation. In the present paper, these failures and the associated LCF life are discussed in terms of inclusion size and its proximity to the notch root.

Abstract: The present work deals with the influence of niobium in solid solution on the dynamic recrystallization of pure nickel. High-purity nickel and two model nickel-niobium alloys were deformed to large strains via torsion at temperatures between 800 and 1000°C. Niobium additions considerably increased the flow stress, while they lowered the strain-rate sensitivity and increased the apparent activation energy. EBSD of the steady-state microstructures revealed strong grain refinement. Substructure development was favored, whereas thermal twinning was reduced by niobium. More generally, discontinuous recrystallization kinetics were considerably decreased.

Abstract: The precipitation behavior of MX carbonitride during a normalizing heat treatment with and without ausageing was investigated in a modified 9Cr-1Mo steel. The normalizing heat treatment was performed at 1150 oC for 1800 s. Ausageing was conducted at 765 and 500 oC for 1800 to 86400 s during the cooling from the heat treatment. The matrix of the steel was austenite single phase during normalizing and ausageing, except for that ausaged at 765 oC for 86400 s. The initial austenite grain size and hardness were not influenced by ausageing, except for the sample ausaged at 765 oC for 86400 s. Although Nb-rich MX (NbX) and cementite were observed, V-rich MX (VX) was not observed under any of the conditions investigated. The amount of NbX in the steel ausaged at 500 oC was at least twice as large as that under the other conditions, and the amount in the steel ausaged at 760 oC was slightly larger than that in the steel that did not undergo ausageing. The precipitation of NbX took place during ausageing in the austenite matrix. On the other hand, it is well known that VX precipitates during tempering. An equilibrium mole fraction of VX in the austenite matrix calculated by Thermo-Calc. was larger than that of NbX at the ausageing temperatures. It is proposed that VX is an equilibrium phase at the ausageing temperature; however, VX nucleation takes much longer in the austenite matrix. It is postulated that the precipitation of VX is more strongly influenced by the interfacial energy rather than supersaturation. It is concluded that the precipitation of MX carbonitride, especially NbX, can be controlled by ausageing during cooling after a normalizing heat treatment.

Abstract: The effect of boron on microstructure and creep deformation behavior has been investigated for a tempered martensitic 9Cr-3WVNb steel with emphasis on the role of boron free from boron nitrides. Creep tests were carried out at 650oC for up to about 3 x 104 h, using specimens of 10 mm in gauge diameter and 50 mm in gauge length. The addition of boron in combination with no nitrogen addition effectively reduces the coarsening rate of M23C6 carbides by an enrichment of boron in M23C6 particles in the vicinity of prior austenite grain boundaries during creep at 650oC. This stabilizes martensitic microstructure during creep and retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. Excess addition of boron and nitrogen causes the formation of boron nitrides during normalizing at 1050-1150oC, which reduces dissolved boron and nitrogen. The dissolved boron enriches in M23C6 carbides, while the dissolved nitrogen causes the precipitation of fine MX carbonitrides. The variation of creep rates in transient region and of the onset time of acceleration creep with various combinations of boron and nitrogen contents can be explained by the dissolved boron and nitrogen concentrations after normalizing into account.