Materials Science Forum Vols. 738-739

Abstract: The microstructure of metastable alloys varies with the thermomechanical history of the material. During a thermomechanical loading, different phases can be present in the material simultaneously. They can be at the origin of macroscopic stress and strain. Consequently, it is important to determine the proportion of each phase to understand the transformation kinetic. However, the techniques usually used to carry out these measurements such as magnetic properties measurements, neutron or X-ray diffraction, are heavy and require a lot of resources. This study, thanks to experimental tests based on electric resistance measurements, permits to determine the volume fraction of the different states present in the material. The study has been realized on a 301 stainless steel in order to take into account the effects of elasticity, transformation and plasticity. Then the volume fraction of the different phases of the material has been determined during transformation with the plasticity presence.

Abstract: In the present contribution several advanced electron microscopy techniques are employed in order to describe chemical and structural features of the nano- and microstructure of a Ni_45.5Ti_45.5Nb₉ alloy. A line-up of Nb-rich nano-precipitates is found in the Ni-Ti-rich austenite of as-cast material. Concentration changes of the matrix after annealing are correlated with changes in the transformation temperatures. The formation of rows and plates of larger Nb-rich precipitates and particles is described. The interaction of a twinned martensite plate with a Nb-rich nano-precipitate is discussed and the substitution of Nb atoms on the Ti-sublattice in the matrix is confirmed.

Abstract: The present paper is dedicated to the first observation of the martensitic transformation (MT) and associated shape memory effect (SME) in HfIr equiatomic intermetallic compound. Differential scanning calorimetry allowed determining MT temperatures, which are above 700 K. It was also shown that MT taking place at such high temperatures is associated with SME measured in 3 point bending. MT temperatures and crystal structure changes in the homogeneity range of HfIr compound were also studied. SME behavior changes in the homogeneity range of HfIr compound are discussed

Abstract: Composite material on the basis of a TRIP (transformation induced plasticity) steel with zirconia particles as reinforcement was produced by powder metallurgical technology and conventional sinter process. The goal of such type of material is to obtain exceptional mechanical properties like high deformation energy absorption due to the combination of martensitic phase transformations both in steel and ceramic. The steel matrix was made of the commercial steel AISI 304, which shows a deformation-induced martensitic phase transformation from the austenitic phase (fcc) into the α’-martensite (bcc). The zirconia particles were partially stabilized with MgO and show a stress-assisted martensitic phase transformation from the tetragonal to the monocline phase. Flat specimens were tensile deformed in-situ in a scanning electron microscope in order to follow the damage behaviour of the material. Some zirconia particles were characterized before and after tensile testing both by backscattered electron contrast as well as by electron backscatter diffraction (EBSD) in combination with energy dispersive X-ray spectroscopy (EDS).

Abstract: This paper reports on the tension-compression asymmetry of [001]-oriented Co49Ni21Ga30 single crystals at elevated temperatures. Maximum strains of -4.8 % and 8.6 % in compression and tension, respectively, were found. A linear Clausius-Clapeyron relationship was observed for both stress-states where the smaller slope in tension resulted in a significant increase of the phase transformation temperatures with stress, which reached 180 °C under a constant stress level of 150 MPa. In addition, the material demonstrated a large pseudoelastic temperature range of about 300 °C under both stress state conditions. The results in this study unequivocally indicate the potential of these alloys for applications where elevated temperatures and stress levels prevail.

Abstract: The technique and preliminary results of in situ X-ray diffraction analysis of the martensitic transformation in the newly developed Ti-Nb-Zr SMA for biomedical application are presented. To perform the in situ analysis, an original tensile stage, powered by a Ti-Ni SMA actuator and fit within the “TTK450” thermal chamber of a “PANalytical X’Pert Pro” diffractometer is designed, manufactured and validated. The tensile stage working principle and analysis methodology are described in detail. Preliminary results obtained during in-situ X-ray analysis of the phase transformations in Ti-Nb-Zr SMA are also presented.

Abstract: Typical modes of the microstress distribution were found by X-ray study of Ti-Ni rolled single crystals using the X-ray method of Generalized Pole Figures. The method consists in successive measurements of the peak position 2θ for the same X-ray line of B2 phase by all possible positions of the sample in the course of diffractometric study of its texture. In the case of one-component rolling texture maxima in pole figure break into halves with opposite signs of elastic deformation, whereas by two-component rolling textures the one-sign elastic strain dominates within different maxima.

Abstract: Precipitation decomposition in β metastable phase in TI 5553 alloy were characterized by electrical resistivity and high energy X-ray diffraction for two thermal paths, isothermal transformation at 325°C after direct quenching, or cooling to room temperature and further ageing at 325°C. Both treatments led to different transformation sequences with formation of ω and/or α''. Mean cell parameters of β and α'' were characterized all along the transformation. Results suggest a transformation with poor solute partitioning and building of large stresses in each phase.

Abstract: Abstract. The structure and chemical order of a Heusler alloy of non-stoichiometric composition Ni-Mn-Ga were studied using constant-wavelength (1.538 Å) neutron diffraction at 363K and the diffraction pattern was refined using the FullProf software. At this temperature the structure is austenite (cubic) with Fm space group and lattice constant of a = 5.83913(4) [Å]. The chemical order is of critical importance in these alloys, as Mn becomes antiferromagnetic when the atoms are closer than the radius of the 3d shell. In the studied alloy the refinement of the site occupancy showed that the 4b (Ga site) contained as much as 22% Mn; that significantly alters the distances between the Mn atoms in the crystal and, as a result, also the exchange energy between some of the Mn atoms. Based on the refinement, the composition was determined to be Ni1.91Mn1.29Ga0.8

Abstract: Martensitic α-ε transformation in iron under high pressure was investigated. Using of advanced spectroscopic techniques allow to reveal the influence of nanocrystalline structure on critical points of the above transformation. Comparison both structure evolution and critical points is presented by using the terminology and basic positions of martensitic transformations. The results of experimental observations for coarse-grained and nanocrystalline samples are discussed.