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Phase transformations in modern titanium alloys
Šmilauerová, Jana ; Janeček, Miloš (advisor) ; Kalvoda, Ladislav (referee) ; Strunz, Pavel (referee)
This study deals with phase transformations in metastable β titanium alloys, focusing on the investigation of evolution of nanoparticles of thermodynamically metastable ω phase. For the purpose of this research, single crystals of two metastable β titanium alloys - LCB (Ti-6.8Mo-4.5Fe-1.5Al) and Ti-15Mo (in wt. %) - were grown in an optical floating zone furnace. It was established by differential scanning calorimetry that the phase transformations occurring in the material did not change significantly as a result of the single crystal growth process. Using single crystal X-ray diffraction, the shape and the size of ω particles were determined in a series of aged samples. The lattice parameters of ω particles and the β matrix, as well as the misfit between the two structures were calculated. The β phase was found to be locally deformed in compression around ω particles. Small-angle X-ray scattering (SAXS) experiments revealed a spatial ordering of ω particles in the β matrix in a disordered cubic array with the basis vectors along 100 β directions. The SAXS data also allowed the evaluation of the mean ω particle sizes and distances and confirmed that the ω particle growth obeys the t1/3 law following from the Lifshitz-Slyozov-Wagner theory. In situ SAXS performed during isothermal ageing at selected...
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Effect of molybdenum content on phase transformations in binary Ti-Mo alloys
Valešová, Veronika ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
Title: Effect of molybdenum content on phase transformations in binary Ti-Mo alloys Author: Veronika Valešová Department: Department of Physics of Materials Supervisor: PhDr. RNDr. Josef Stráský, Ph.D., Department of Physics of Mate- rials Abstract: β titanium alloys are highly appreciated group of structural materials. They provide an extraordinary combination of strength, toughness and fatigue resistance. These alloys are used predominantly in aerospace sector, chemical industry and medical engineering. We have studied three binary titanium alloys: Ti-12Mo, Ti-15Mo and Ti-18Mo (wt %). Apart from low temperature hexagonal close packed α phase and high temperature base centered cubic β phase, a metastable ω phase can be also found in this type of alloys. The ω phase is created in a diffusionless process by shifting the planes in the β phase and affects both mechanical properties and the α phase precipitation. We have examined the phase transformations occurring in the studied alloys using both in-situ and ex-situ methods. Resistometry, dilatometry and differential scanning calorimetry were used as the indirect in-situ methods. For the ex-situ methods, scanning electron microscopy and microhardness measurement were used. Complex characterization of β metastable titanium alloys showed the formation of the...
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Phase transformations in ultra-fine grained titanium alloys
Bartha, Kristina ; Stráský, Josef (advisor) ; Kalvoda, Ladislav (referee) ; Vojtěch, Dalibor (referee)
Title: Phase transformations in ultra-fine grained titanium alloys Author: Kristína Bartha Department: Department of Physics of Materials Supervisor of the doctoral thesis: PhDr. RNDr. Josef Stráský, Ph.D., Department of Physics of Materials Abstract: Ti15Mo alloy in a metastable β solution treated condition was processed by high pressure torsion (HPT) and equal channel angular pressing (ECAP). The microstructure after HPT is severely deformed and ultra-fine grained, while ECAP deformation results in rather coarse-grained structure with shear bands containing high density of lattice defects. Two types of thermal treatments - isothermal annealing and linear heating - were carried out for the solution treated condition and both deformed materials. Wide spectrum of experimental techniques was employed to elucidate the differences in phase transformations, especially in α phase precipitation, occurring in deformed and non-deformed material upon thermal treatment. It was shown that the α phase precipitation is accelerated in the deformed materials due to a high density of lattice defects, which provide a dense net of preferred sites for nucleation and also fast diffusion paths necessary for accelerated growth. The enhanced precipitation of the α phase in deformed materials also affects the stability of the ω...
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Metastable beta titanium alloys for biomedical use
Preisler, Dalibor ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
In this work the effect of die-forging and annealing on the mechanical properties and microstructure of biocompatible metastable alloy Ti-35.3Nb-7.3Zr -5.7Ta-0.7O (wt. %) was studied. Light and scanning electron microscopy, microhardness tests and tensile tests were used to study all prepared conditions. As-cast and as-annealed conditions showed chemical inhomogeneities exhibited as dendritic structure, grains with size of hundreds of micrometers and numerous pores having size of units of micrometers. Precipitation of alpha phase particles was negligible in as-annelaed conditions and only grain boundary alpha paticles formed. Die-forged condition showed grain sizes in the wide range of hundreds of micrometers down to micrometers. Both as-annealed and die-forged conditions showed increased microhardness compared to as-cast condition, but there were no significant differences between different annealing conditions. Yield stress of 870 MPa was found for as-cast condition while as-forged condition exhibited even higher yield stress of 1120 MPa. In both conditions, good room-temperature ductility was demonstrated. Due to its high strength, this alloy is perspective for manufacturing of joint implants.
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Study of Phase Transformations in Ti Alloys
Zháňal, Pavel ; Harcuba, Petr (advisor)
In this work phase transformations in metastable β Ti-15Mo alloy were stud- ied using electrical resistivity measurements. The alloy was subjected to a solu- tion treatment at a temperature higher than β-transus and quenched in water. In this condition, the microstructure of Ti-15Mo alloy consists of a metastable β-matrix and ω phase particles. During in-situ electrical resistivity measurement in a specially designed furnace, significant temperature points which indicate phase transformations in the material were detected. The dependence of elec- trical resistivity on the temperature changes during heating between increasing and decreasing according to the ongoing phase transformation. The changes were observed at temperatures 225, 356 and 560 ◦ C. A further study of these phase transformations using electrical resistivity measurements was performed on var- ious heat treated specimens. In order to control the microstructure evolution in the material, scanning and transmission electron microscopy was used. Mecha- nical properties were studied using Vickers microhardness testing. The obtained results serve to identify the type and sequence of phase transformations which take place in the Ti-15Mo alloy.
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Phase transformations and microstructure changes in TIMET LCB alloy
Šmilauerová, Jana
In the present work, phase transformations in TIMETAL LCB titanium alloy and their influence on mechanical properties were studied. Different initial con- ditions were prepared by solution treating above β-transus immediately followed by heat treatment in α/β temperature regime. These resulted in different grain boundary α thicknesses and contiguities at a fixed α volume fraction. The sub- sequent ageing response of this material was studied by low temperature ageing at 400 ◦ C, 450 ◦ C and 500 ◦ C. Phase transformations were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and resistivity measurements. Mechanical properties were investigated using microhardness measurements and tensile tests. It has been proved that metastable ω phase is formed during annealing at 400 ◦ C and 450 ◦ C. ω particles further transform to very fine precipitates of α phase when exposed to annealing for longer time periods. These fine precipitates significantly con- tribute to increase of microhardness and achieving high value of yield stress. Keywords: Metastable beta Ti alloys, phase transformations, microstructure changes, ω phase
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Microstructure and mechanical properties of ultra-fine grained titanium alloys
Václavová, Kristína ; Stráský, Josef (advisor)
Title: Microstructure and mechanical properties of ultra-fine grained titanium alloys Author: Bc. Kristína Václavová Department / Institute: Department of Physics of Materials Supervisor of the master thesis: PhDr. RNDr. Josef Stráský, Ph.D. Abstract: Metastable β-Ti alloys Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL LCB) were subjected to severe plastic deformation by high pressure torsion. Microhardness of Ti-15Mo and TIMETAL LCB alloys increases with increasing inserted deformation, i.e. with increasing number of HPT rotations and also with increasing distance from the centre of the sample. The highest microhardness after HPT exceeds significantly the microhardness of two-phase α + β heat-treated material. Increasingly deformed microstructure was also demonstrated by scanning electron microscopy and by electron back-scattered diffraction (EBSD). Significant twinning was observed in both studied alloys. Mechanism of multiple twinning contributes notably to the fragmentation of grains and thus to the refinement of the microstructure. Defect structure in Ti-15Mo alloy was studied by positron annihilation spectroscopy. It was proved that dislocations are the only detectable defects in the material by positron annihilation spectroscopy and that dislocation density increases with the number of HPT revolution and...
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Effect of molybdenum content on phase transformations in binary Ti-Mo alloys
Valešová, Veronika ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
Title: Effect of molybdenum content on phase transformations in binary Ti-Mo alloys Author: Veronika Valešová Department: Department of Physics of Materials Supervisor: PhDr. RNDr. Josef Stráský, Ph.D., Department of Physics of Mate- rials Abstract: β titanium alloys are highly appreciated group of structural materials. They provide an extraordinary combination of strength, toughness and fatigue resistance. These alloys are used predominantly in aerospace sector, chemical industry and medical engineering. We have studied three binary titanium alloys: Ti-12Mo, Ti-15Mo and Ti-18Mo (wt %). Apart from low temperature hexagonal close packed α phase and high temperature base centered cubic β phase, a metastable ω phase can be also found in this type of alloys. The ω phase is created in a diffusionless process by shifting the planes in the β phase and affects both mechanical properties and the α phase precipitation. We have examined the phase transformations occurring in the studied alloys using both in-situ and ex-situ methods. Resistometry, dilatometry and differential scanning calorimetry were used as the indirect in-situ methods. For the ex-situ methods, scanning electron microscopy and microhardness measurement were used. Complex characterization of β metastable titanium alloys showed the formation of the...
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Phase transformations in ultra-fine grained titanium alloys
Bartha, Kristina ; Stráský, Josef (advisor) ; Kalvoda, Ladislav (referee) ; Vojtěch, Dalibor (referee)
Title: Phase transformations in ultra-fine grained titanium alloys Author: Kristína Bartha Department: Department of Physics of Materials Supervisor of the doctoral thesis: PhDr. RNDr. Josef Stráský, Ph.D., Department of Physics of Materials Abstract: Ti15Mo alloy in a metastable β solution treated condition was processed by high pressure torsion (HPT) and equal channel angular pressing (ECAP). The microstructure after HPT is severely deformed and ultra-fine grained, while ECAP deformation results in rather coarse-grained structure with shear bands containing high density of lattice defects. Two types of thermal treatments - isothermal annealing and linear heating - were carried out for the solution treated condition and both deformed materials. Wide spectrum of experimental techniques was employed to elucidate the differences in phase transformations, especially in α phase precipitation, occurring in deformed and non-deformed material upon thermal treatment. It was shown that the α phase precipitation is accelerated in the deformed materials due to a high density of lattice defects, which provide a dense net of preferred sites for nucleation and also fast diffusion paths necessary for accelerated growth. The enhanced precipitation of the α phase in deformed materials also affects the stability of the ω...
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Metastable alloy Ti-15Mo prepared by powder metallurgy
Veverková, Anna ; Bartha, Kristina (advisor) ; Chráska, Tomáš (referee)
This diploma thesis focused on manufacturing and characterization of Ti-15Mo metastable beta-Ti alloy prepared by cryogenic milling and spark plasma sintering. Initial powder was prepared by gas atomization and consequently deformed by cryogenic milling (milled powder). Both initial and milled powders were compacted by spark plasma sintering (SPS) at temperatures from 750 řC to 850 řC. Dependence of microstructure and mechanical properties on the parameters of preparation was studied. During cryo-milling, powder particles significantly changed shape from ball-shaped to disc-shaped. Particles were not refined by milling, but severely plastically deformed. SEM observations showed that all prepared samples contain duplex alpha + beta structure. Volume fraction of alpha phase is significantly higher in the sintered milled powder due to increased beta- transus temperature caused by contamination by oxygen and also due to easier alpha phase precipitation caused by refined microstructure. Maximum microhardness of 350 HV was achieved for both types of sintered powders. High microhardness of sintered initial powder can be attributed to formation of omega phase during cooling, while sintered milled powder is strengthened by refined microstructure and small alpha phase precipitates. Cryogenic milling prior to...
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