Papers by Keyword: Phase Transformation

Abstract: Y₂O₃ coatings were prepared on aluminum substrates using plasma spraying with argon as primary gas. Structure and morphology were characterized by using X-ray diffraction, scanning electron microscopy and metallographic analysis. Results show that Y₂O₃ coating’s porosity is 5.6%. Coatings prepared with helium as secondary gas are purely white. However, there are black spots when hydrogen is used as secondary gas. A phase transformation from cubic to monoclinic tends to take place during the spraying operation under argon and hydrogen atmosphere. However this prejudicial transformation can be suppressed by using helium instead of hydrogen.

Abstract: Nacre is one of the most attractive biological materials for its superior mechanical property which is mainly due to the “brick-mortar” microstructure. Nacre samples are heated in N₂ atmosphere at 200°C, 400°C and 600°C, separately. The microstructures of the fresh and heated samples are characterized by SEM and XRD, and the three-point bending strengths of these samples are tested by universal mechanical testing machine. TG-DSC curves are performed on the fresh nacre and the demineralized one. The results show that biopolymer plays important roles in maintaining the microstructure, toughening mechanism and the phase transformation of aragonite.

Abstract: The effect of heat treatment on phase transformation, precipitation behavior and micro-hardness response of Cu40Zn-1.0Ti brass was investigated via powder metallurgy method. The volume fraction of α phase increased with elevated temperature, equaled to that of β phase at 400 °C, and reached to a maximum value of 55.9% at 500 °C. The solid solubility of Ti in Cu40Zn brass matrix decreased with elevated heat treatment temperature, showed high chemical potential for precipitates reaction in Cu40Zn brass. The micro-hardness of the BS40-1.0Ti brass was primarily dependent on the solid solubility of Ti, but also dependent on the phase ratio of α and β phase.

Abstract: As a new type of environmental pollutant reduction technology, semiconductor nano-TiO2 photocatalytic technology has attracted widely attention in recent years. In order to overcome the disadvantage of pure TiO2 photocatalyst in lower utilization of solar energy and heat durability, the sol-gel method followed by co-precipitation method was employed to synthesize ZnO packed N:TiO2 nano-powders, and EDS, XRD, SEM, UV-VIS analysis as well as detection of degradation rate of methyl orange were used to characterize the composition, phase, micro-morphology and the photocatalytic properties of the nano-powders. The results showed that the excitation band of the composite powders extended from the UV region to the visible light zone, indicating a remarkable improvement of the photocatalytic activities of TiO2 composite powders under both visible light and UV irradiation. After calcined at 950°C, the composite powders still remained mixed crystal structures of anatase and rutile phases with the diameter of the composite powder being about 60nm. After coated with ZnO, the heat durability of the N-doped nano-TiO2 composite powders was significantly improved 250°C.

Abstract: The grinding process is one of the most important finishing processes in production industry. During the grinding process the workpiece is subjected to mechanical and thermal loads. They can induce thermal damages in terms of phase transformation due to critical temperature history. A holistic model helps to describe and predict the influence of these loads on the residual stresses in the surface layer. In this paper, a very promising approach using the Finite Element Method (FEM) to simulate the surface grinding process in terms of thermal and mechanical loads during grinding of hardened and tempered steels with vitrified bonded CBN grinding wheels is introduced. The investigations were conducted for deep, pendulum and speed stroke grinding. The change of workpiece material properties was modelled as a function of temperature and phase history. The results lead to the necessary time depending temperature distribution within the surface layer. Hence, the phase transformation can be calculated. The FEM software "Sysweld" was used to analyze the phase transformation kinetics. Hence, the size of the rehardened zone after grinding can be predicted. The evaluation of the FEM model with micrographs of ground workpiece specimens showed a strong correlation for different grinding parameters. Based on the understanding of mechanical and thermal loads as well as phase transformation kinetics in the surface layers the resulting residual stresses can be determined.

Abstract: Cutting processes lead to mechanical and thermal loading of tool and work piece. This loading entails a direct influence of the cutting process on the surface layers of the manufactured work pieces. As a result, residual stresses and modifications of the micro-structure like white layers can occur in surface-near zones of the work piece. This paper presents the development of a FE-simulation model to predict phase transformations due to cutting processes. Therefore a 2D-FE-cutting simulation including a dynamic re-meshing is combined with a simulation routine to describe phase transformations that was primarily developed to simulate laser hardening. This paper illustrates the implemented mechanisms to determine phase transformations considering short time austenization and shows first experimental results revealing the influence of process parameters on the surfaces microstructure.

Abstract: Effects of equal-channel angular extrusion (ECAE) process at 773K and heat treatment on phase transformations and superelasticity of a Ni-rich TiNi shape memory alloy were investigated by differential scanning calorimeter (DSC) measurement and cyclic tensile test. The R phase transformation of Ni-rich TiNi alloy was stimulated after ECAE processes and within a larger temperature range. The martensitic transformation start temperature Ms of Ni-rich TiNi alloy decreased sharply after ECAE processes at 773 K, then rapidly rose back after the specimen aging at 773K for 30min. Reasons for the changes of phase transformation behaviors have been discussed. ECAE processed TiNi samples exhibit better super-elasticity than solution treated one. With the increase passes of ECAE, the superelasticity becomes more stable, and a completely recoverable strain of 6% is obtained for TiNi sample after 8 passes ECAE.

Abstract: In-situ neutron diffraction experiments under tensile loading were carried out to study the micromechanical behaviour of two iron-manganese based steels, a TWIP (twinning induced plasticity) steel with 30 wt% Mn and a TRIP steel (transformation induced plasticity) with 20 wt% Mn. The former was loaded to 31.3% strain and the latter to 20% strain. The 30 wt.% Mn steel had a fully austenitic microstructure which remained stable over the loading range studied, while stress induced austenite to α´- and ε-martensite transformations occur in the 20 wt.% Mn steel which initially contained an α´-martensite in addition to the austenite. The evolution of lattice strains under tensile loading differs between the two steels, reflected their different plastic deformation mechanisms. A stronger grain-orientation dependent behaviour is observed during deformation for the 20 wt.% Mn in contrast to the 30wt.% Mn steel.

Abstract: Low Transformation Temperature (LTT) alloys were developed in order to control the residual stress development by the martensitic phase transformation already during cooling of the weld metal. The positive effect of such LTT alloys on the mitigation of detrimental tensile residual stresses during welding has already been confirmed on the basis of individual laboratory tests. Within the current project it was experimentally investigated whether the phase transformation mechanisms are effective under increased restraint due to multi-pass welding of thicker specimens. The local residual stress depth distribution was analyzed non-destructively for V-type welds processed by arc welding using energy dispersive synchrotron X-ray diffraction (EDXRD). The use of high energy (20 keV to 150 keV) EDXRD allowed for the evaluation of diffraction spectra containing information of all contributing phases. As the investigated LTT alloy contains retained austenite after welding, this phase was also considered for stress analysis. The results show in particular how the constraining effect of increased thickness of the welded plates and additional deposited weld metal influences the level of the residual stresses in near weld surface areas. While the longitudinal residual stresses were reduced in general, in the transition zone from the weld to the heat-affected zone (HAZ) compressive residual stresses were found.

Abstract: Material that shows hydroxyapatite and β-tricalcium phosphate phases is called biphasic calcium phosphate (BCP). In present work, biphasic calcium phosphate was prepared and characterized for future applications on the utilization of bioactivity of HAp and resorbability of β-TCP properties. It was simply synthesized by precipitation method using eggshell as the calcium source (Ca) in the form of calcium nitrate and ammonium phosphate as the phosphate source (P) to obtain biphasic calcium phosphate. The prepared BCP powders and crystal structure were characterized by X-ray diffraction (XRD), Rietveld refinements and Fourier transform infrared (FT–IR) techniques. The results indicate that BCP was observed at the calcining temperature of 800 oC and above. Furthermore, the crystallinity of BCP increases with increasing temperature from 800 - 1200 oC. The phase fraction of β-TCP is enhanced with pH of a solution from 8.6-10.6 and decrease with the temperature range of 800 - 1200 oC. The formation of BCP arises from its non-stoichiometric composition of materials such as variation of synthesis parameters.