Materials Science Forum Vols. 783-786

Abstract: We present a computer inexpensive method to calculate the profile of the high resolution diffraction pattern of a single crystal superalloy. The displacement field is taken as the sum of the individual fields of cuboidal inclusions with uniform eigenstrains. Early simulations are compared to experimental data.

Abstract: Development for superconducting wires of materials such as Nb₃Al and the high-temperature superconductors (HTS such as REBCO, Bi2223, and Bi2212) has been carried out for high-field magnet applications. It is known that these types of wire exhibit very different characteristics and performance for different applications. The development of Nb₃Al wire for high-field accelerator magnet has resulted in remarkable achievements in critical current using a Rapid Heating and Quenching (RHQ) method by High Energy Accelerator Research Organization (KEK) and National Institute for Materials Science (NIMS). As one example of a characteristic of Nb₃Al, the strain sensitivity of the critical current in the RHQ-Nb₃Al wire is better than that of Nb₃Sn wire. A strain study is needed to further the development of a high-filed magnet; therefore, we have carried out experimental studies using the neutron diffractometer at J-PARC Takumi. Researchers have recently achieved the highest critical current density for REBCO wires in a high-field above 15 T. For this reason, REBCO wire has been considered for high-field magnet NMR applications in Riken. But several obstacles remain, including coil degradation, shielding current and thermal runaway. In this paper, R&D on recent advances for applications will be presented.

Abstract: The multi-reflection grazing incidence X-ray diffrection was used to determine residual stress gradient in the mechanically polished Al-Mg alloy and CrN coating. Also, the root mean square values of the third order lattice strain was determined using Wiliamson-Hall method. The results obtained for Al-Mg alloy show that the stress field in the surface layer as well as the microstructure (density of dislocation) depend strongly on the sample preparation. A very high residual compressive stress, which does not change significantly with depth, was measured in the CrN coating. Moreover, a large value of the measured third order strains in the coating was found.

Abstract: Urgent needs concerning energy efficiency and environmental politics require novel approaches to materials design. One recent example is thereby the implementation of light-weight intermetallic titanium aluminides as structural materials for the application in turbine blades of aero-engines as well as in turbocharger turbine wheels for the next generation of automotive engines. Each production process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and / or subsequent heat-treatments. To develop sound and sustainable processing routes, knowledge on solidification processes and phase transformation sequences in advanced TiAl alloys is fundamental. Therefore, in-situ diffraction techniques employing synchrotron radiation and neutrons were used for establishing phase fraction diagrams, investigating advanced heat-treatments as well as for optimizing thermo-mechanical processing. Summarizing all results a consistent picture regarding microstructure formation and its impact on mechanical properties in advanced multi-phase TiAl alloys can be given.

Abstract: Measuring theory of two types of X-ray stress analysis method was compared with each other. One is the conventional method, in which zero-or one-dimensional detector is used for obtaining diffracted beam and stress is determined using the standard sin²ψ method. Another is the new type of X-ray stress analysis method, in which two-dimensional detector is used to obtain whole Debye ring and stress is determined using the cosα method. An experiment was conducted to investigate the validity.

Abstract: Recent status of the technical development of the Bragg-edge neutron transmission imaging and its application to material science is presented. The neutron Bragg imaging has the advantages in measuring large area with reasonable spatial resolution, and it is a non-destructive method capable of looking inside a bulk material. Therefore, various information that are quite different from EBSD, synchrotron microtomography and X-ray/neutron scattering can be obtained by this method. We carried out quantitative imaging to obtain crystalline microstructural information in ultralow-carbon steels that received the high pressure torsion (HPT). The real-space distributions of texture and grain/crystallite size of HPTed steels of four torsion numbers were quantitatively visualized at once. As a result, we could deduce unique distributions of microstructural information depending on each torsion number, and correlated them with real-space distributions of the Vickers hardness. We also successfully developed a versatile strain tomography technique that can obtain tensor values for strain although traditional CT techniques can deal with only scalar values. The new CT algorithm, the tensor CT method, is based on our original algorithm called FBP-EM. The strain tensor tomography using FBP-EM was successfully applied for the experimental measured result obtained with the VAMAS neutron strain analysis international standard sample.

Abstract: Prevention of weld cracking is necessary for ensuring the reliability of high strength steel structures. Tensile residual stress in the weld metal is one of the major factors causing the weld cracking, therefore, it is important to clarify the residual stress distribution in the weld metal. Conventional stress measurement, the stress relief method using strain gauges and the X-ray diffraction technique, can only provide the stress information in the surface region of the steel weld. The neutron diffraction is the only non-destructive method that can measure the residual stress distribution inside the steel weld [1-3]. The neutron stress measurement was applied for the 980MPa class high strength steel weld and it was revealed that high level of tensile residual stress can affect the weld cracking to a significant degree [4-5]. Recently, it was reported that Ni-Cr type steel weld exhibit higher resistance to the weld cracking compared with conventional low alloy type weld. Increase of tensile residual stress is prevented by lower transformation temperature of the Ni-Cr type weld metal and retained austenite phase is dispersed in the martensite microstructure. It is considered that lower level of tensile residual stress and the existence of retained austenite may prevent hydrogen accumulation in the weld metal [6]. However, retained austenite and the residual stress conditions in the Ni-Cr type high strength steel weld is not well understood. In this study, neutron diffraction analysis was conducted on the Ni-Cr type steel weld joint with the tensile strength level of 980MPa in order to investigate the effect of the retained austenite and the residual stress distribution on the weld cracking.

Abstract: Considering the significant role that residual stresses play in determining the lifetime-service of materials, it is mandatory to have a good understanding of and a means of predicting those that develop during welding processes. For this purpose, a User MATerial subroutine (UMAT) is developed to study the effects of various parameters that influence solid state phase transformations and residual stress evolution during welding of SA508 ferritic steel. The temperature dependent elastic and kinematic hardening parameters for each of the individual phases that can potentially develop during cooling from elevated temperatures are measured and are used for calculating stress development during low (75 mm/min) and high (300 mm/min) speed gas-tungsten arc welding (GTAW) on SA508 grade 3. These two speeds are selected to cover a wide range of cooling rates in the heat affected zone so that different phase proportions would be present. The results of the numerical simulations for residual stresses are compared against those measured by neutron diffraction. It is shown here that a low speed weld results in bainite formation whereas a high speed weld results in bainitic as well as subsequent martensitic phase transformations where each welding rate results in different residual stress development.

Abstract: There is a confused and contradictory literature on the role of small concentrationsof niobium on the development of the so-called local brittle zones in steels. These zones consistof a mixture of martensite and austenite and hence, their formation should be predictable usingmodern microstructure calculation methods. Following an assessment of the most relevant literature,a mathematical model is presented which enables three quantities to be calculated, the fractionof martensite, the carbon concentration of the martensite, and its ability to influence toughness.Examples are presented for particular linepipe steels, and then the generic effect of alloying elementsother than niobium, on the development of local zones.

Abstract: The improvement of optimization numerical methods for constitutive equations has been the first aim of this research. A subordinate optimization algorithm, based on Newton method, through “ad hoc” assessment, has been developed. Application to hot torsion data of AISI4145 with two grain sizes has been carried out. It is concluded that the Newton method is an excellent algorithm for the optimization of strain dependent constitutive equations. Two models are presented as alternative to the generalized Garofalo model: the normalized stress exponent model (NSE) and the Generalized Sherby model (SG). The NSE model is the most precise to restitute the experimental stress-strain curves.