Antiphase boundary-like stacking fault in α″-martensite of disordered crystal structure in β-titanium shape memory alloy

The ordering and antiphase boundary (APB)-like fault found in the α″-martensite of β-Ti shape memory alloys are studied. Long-range chemical ordering was not found, but APB-like faults were observed in every martensite plate studied by transmission electron microscopy. These faults have morphology similar to the APBs observed in ordered phases. The superlattice reflections observed in some previous works were a consequence of multiple diffractions. APB-like faults were not observed in the parent phase, leading to the conclusion that the faults were introduced by the martensite transformation. The fault took the form of a wavy tube running perpendicular to the habit plane. The fault was a ‘transformation-induced APB’ with an additional small displacement due to the pre-existing athermal ω phase. The displacement vector was determined to be [–3/50, ?23/50, 1/2]. Geometrical aspects of the formation of APB-like faults are also discussed.     

Incompatibility and preferred morphology in the self-accommodation microstructure of β-titanium shape memory alloy

The frequency distribution of habit plane variant (HPV) clusters and the deviation from twin orientation relationships (ORs) at the junction plane (JP) are investigated by transmission electron microscopy together with theoretical evaluation of the kinematic compatibility (KC) at the JP in a β-titanium shape memory alloy. Even though there are more than 10 types of possible HPV clusters, only three types are formed. V-shaped couplings of HPVs by {111} type I twins (V_I: 49%) and by ?211? type II twins (V_II: 42%) are the predominant types. A triangular morphology due to coupling of {111} type I twins is observed with a frequency of only 9%. These preferred morphologies are well explained by the degree of incompatibility (the rotation necessary for compatible connection of HPVs). The exact twin OR and KC are maintained at the JP in a V_I cluster instead of KC at the habit plane (HP), whereas the JP in a V_II cluster is incompatible and the ?211? type II twin OR shows slight deviation at the JP by about 0.4°. The competition between KC at the JP and KC at the HP (invariant plane) is responsible for the frequency distribution of HPV clusters and the character of the interfaces in the self-accommodation microstructure.     

Ab initio calculation of characteristics of a hcpti–c system in α-titanium

The relative energies of possible phases in a Ti–C system in the hcp-lattice of titanium are calculated. A probability of formation of a more energetically favorable structure is shown than that of NaCl, which is well-known from the phase diagram of Ti–C. The results of calculations of lattice parameters, elasticity moduli, and charge density distribution in the predicted phase are presented. The relationship between the atomic structure peculiarities and the chemical bonding type in this phase is discussed.     

In situ TEM characterisation of dislocation interactions in α-titanium

In situ straining in the transmission electron microscope and diffraction-contrast electron tomography has been applied to investigate dislocation interactions in α-Ti. Dislocation debris, in the form of small loops, was seen to form from sequential cross-slip events. Electron tomography provided direct three-dimensional visualisation of the dislocation structures, allowing accurate identification of slip planes, dislocation line directions and spatial relations between dislocations.     

Dislocation nucleation centers of thinly lamellar α-martensite in iron alloys

Probable dislocation-type nucleation centers of -martensite crystals with {259}-{3 10 15} habits, which are observed in iron alloys, are selected. It is demonstrated that 30° dislocations of the initial phase are most consistent with the wave pattern of the growth of such crystals. A particular dislocation nucleation center corresponds to each martensite crystal with a given morphology.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 79–83, September, 1991.     

Structure of the B2 phase in Ti–25Al–25Mo alloy

The structure of the B2 phase has been investigated in Ti–25Al–25Mo alloy using Rietveld refinement of X-ray and neutron diffraction data in as-cast and solution-treated conditions. Different initial structure models have been used for the refinement. The site occupancy of the various chemical constituents in the B2 phase has been calculated and compared with earlier investigations. The relative merits of neutron diffraction over X-ray diffraction for structural refinement of the B2 phase in Ti–25Al–25Mo alloy have been demonstrated.     

Dislocation centers in α-martensite formation and dual couplings of thinly-laminated martensite crystals

The spectrum of observable dual couplings of crystals of thinly-laminated martensite with habiti {3 15 10} is discussed within the framework of the concept of the dislocation formation of -martensite crystals. A spectrum of dislocation centers of formation arising together with the developing martensite crystal is proposed. Here it is essential to divide the arising dislocation centers of formation into two groups associated with different displacement mechanisms. This difference permits understanding of the difference in the frequency of observation of couplings and the introduction of the concept of a comb structure — a set of secondary parallel crystals of martensite, associated with the same initial crystal.Urals Forestry Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 64–67, June, 1994.     

Directional solidification of Ti–49 at.%Al alloy

Ti–49Al (at.%) alloy was directionally solidified in Bridgman-type directional solidification furnace. The specimens were directionally solidified under an argon atmosphere with the different growth rate (V=5–30 μm/s) at a constant temperature gradient (G=12.1 K/mm), and with the different temperature gradient (G=2.8–12.1 K/mm) at a constant growth rate (V=10 μm/s). The dendritic spacings (λ ₁) were measured from both transverse and longitudinal sections of the specimens. The dependence of λ ₁ on the growth rate (V) and temperature gradient (G) were determined by using linear regression analysis. According to the experimental results, the value of λ ₁ decreases with the increase of values of V and G. The experimental results were compared with the current theoretical and numerical models, and similar previous experimental results.     

HAADF-STEM study of β′-type precipitates in an over-aged Al–Mg–Si–Ag alloy

Coarse, rod-shaped precipitates growing along ?100?Al directions in an Al–1.0?wt% Mg₂Si alloy with 0.5?wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62?m (189) and composition Al₃Mg₃Si₂Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg₉Si₅ phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates     

In situ and tomographic analysis of dislocation/grain boundary interactions in α-titanium

In situ straining in the transmission electron microscope and diffraction-contrast electron tomography have been applied to the investigation of dislocation/grain boundary and dislocation/twin boundary interactions in α-Ti. It was found that, similar to FCC materials, the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation. That is, grain boundary strain energy density minimization determines the selection of the emitted slip system.     

Thermal analysis of precipitation reactions in a Ti–25Nb–3Mo–3Zr–2Sn alloy

A study was undertaken on a Ti–25Nb–3Mo–3Zr–2Sn alloy using differential scanning calorimetry (DSC) in order to improve understanding of the precipitation reactions occurring during aging heat treatments. The investigation showed that isothermal ω phase can be formed in the cast and solution treated alloy at low aging temperatures. An exothermic peak in the temperature range of 300 to 400°C was detected for precipitation of the ω phase, with approximate activation energy of 176 kJ/mol. The ω phase begins to dissolve at temperatures around 400°C and precipitation of the α phase is favoured at higher temperatures between 400°C and 600°C. An exothermic peak with activation energy of 197 kJ/mol was measured for precipitation of the α phase. Deformation resulting in the formation of the stress induced α″ phase altered the DSC heating profile for the solution treated alloy. The exothermic peak associated with precipitation of the ω phase was not detected during heating of the deformed material and increased endothermic heating associated with recovery and recrystallisation was observed.     

Continuous-cooling α-to-γ transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy

The continuous-cooling transformation behaviour of Ti–45.5?at.%?Al–0.05?at.%?B alloy was quantitatively measured using a real-time resistivity–temperature–time measurement apparatus operating under a high vacuum. The addition of a small amount of B does not significantly alter the α–γ-phase equilibria but significantly raises the α–γ lamellar start temperature of Ti–45.5?at.%?Al alloy at most cooling rates. Furthermore, it markedly increases the critical cooling rate for the ordering reaction. The effect of B addition, which greatly stabilizes the lamellar structure up to a fast cooling rate, is to accelerate the lamellar formation kinetics; the lamellar spacing was nevertheless distinctively larger in a B-doped alloy. This is because lamellae in B-doped alloy nucleate heterogeneously on titanium borides at the grain boundary; the borides are effective nucleation sites particularly since local Ti depletion can occur near the interface of the growing titanium borides during cooling. In the absence of B addition, the lamellar structure starts to form only at temperatures below T ₀, suggesting that a large undercooling is required for the nucleation of lamellae even at the grain boundaries. On the other hand, the B addition greatly retards the kinetics of the α-to-α₂ ordering reaction by markedly increasing its critical cooling rate without a large change in the ordering temperature. This is believed to be due to its tendency to segregate strongly to the antiphase boundaries.     

Single-pulse KrF laser ablation and nanopatterning in vacuum of β-titanium alloys used in biomedical applications

We report the KrF laser (248 nm) nanosecond single-pulse irradiation in vacuum of a mirror-polished surface of a model -titanium alloy. A series of single-pulse laser-annealing experiments is performed with increasing fluence adjusted to obtain conditions from submelting (a few mJ/cm²) up to intense laser ablation (30 J/cm²). The structural and morphological changes are followed by atomic force microscopy and scanning electron microscopy, as well as with other analytical techniques, and compared with the starting equilibrium mixture of the untreated sample. The results are discussed and related to the theoretical estimation of fluences corresponding to the -transus, melting, boiling and intense ablation (phase-explosion) thresholds as well as to the estimates of melt depth and lifetime evaluation as a function of fluence. By carrying out laser annealing in vacuum the surface oxidation is avoided while roughening and phase transformations are favoured within the laser-heated depth. The mirror-polished starting surface is composed of nanometric -precipitates embedded in a -matrix, whose surface is systematically 5–10 nm above the -surface. Upon laser annealing with increasing fluence the -nanocrystals transform progressively into -phase, resulting in an increasing relief. At higher fluence the conditions for increasing roughness are studied. Submicrometre roughness is expected to improve cell adhesion to -titanium alloys and hence osteointegration. PACS 81.05.-t; 68.37.Ps; 68.55.Jk     

Effect of impurity atoms on α2/γ lamellar interfacial misfit in Ti–Al alloy: a systematic first principles study

The effect of transition metal solutes on the lattice parameters of γ-TiAl and α₂-Ti₃Al were studied by first principles calculations to find suitable elements for controlling the α₂/γ interfacial misfit in lamellar Ti–Al alloys. Better agreement was found between the calculated and experimental phase and site preferences of impurity atoms than in a previous first principles study. The calculated lattice parameters suggest that elements in groups 6–11 of the 4th period (Cr, Mn, Fe, Co, Ni and Cu) are effective for increasing the misfit, leading to increasing density of misfit dislocation and, in turn, higher yield strength and ductility. This effect is caused by the change in the lattice parameter of the γ-TiAl phase rather than those of α₂-Ti₃Al phase. This prediction agrees qualitatively with experimental data from a previous study although the effects of temperature are not taken into account. Further improvements should be possible by considering those effects. Nevertheless, the results highlight the effects of impurity addition on interfacial misfit at a level which cannot be achieved by classical concepts such as atomic size in a hard sphere model. The results will also be valuable in further more quantitative predictions and in understanding the effects of temperature, including off-stoichiometry, thermal expansion and vibration entropy.     

Strain behavior of the hydrogenated submicrocrystalline Ti–6Al–4V alloy

Comparative studies of the influence of 0.002–0.12 mass % hydrogenation on the structure and phase composition of the submicrocrystalline and coarse-grained Ti–6Al–4 V alloys are performed. The evolution of the strain processes in the hydrogenated alloy is studied for both alloys upon tension at a temperature of 293 K depending on the hydrogen content and alloy state. It is established that the presence of hydrogen in the nanostructured Ti–6Al–4 V alloy in the solid solution leads to a decrease of its yield stress and an increase of its tensile strength and total strain before failure. The possible reasons for the increased duration of the uniform strain stage and the effect of strain hardening of the alloy in the presence of hydrogen in the solid solution are discussed.     

Corrosion behavior of high-level waste container materials Ti and Ti–Pd alloy under long-term gamma irradiation in Beishan groundwater

Titanium and titanium–palladium alloys are important potential materials for nuclear waste container, which will endure both intense γ-irradiation and groundwater erosion. Therefore, it is very important to investigate the corrosion behavior of the container materials. In this research, the cumulative dose effect of TA8-1 type titanium–palladium alloy(TA8-1) and TA2-type pure titanium(TA2) under γ-irradiation was studied based on the geological disposal of nuclear wastes. The irradiation experiments were performed at room temperature using60 Co gamma sources with a 5.0-k Gy·h-1 intensity for 40, 80 or 160 days, respectively. The p H value and conductivity of Beishan groundwater were investigated.The results showed that the p H value changed from alkaline(8.22) to acidic(2.46 for TA8-1 and 2.44 for TA2), while the un-irradiated solution remained alkaline(8.17 for TA8-1 and 8.20 for TA2) after 160 days. With the increase of irradiation dose, the conductivity increases rapidly and then tends to become stable, which indicates that the titanium dioxide corrosion layer formed on the surface of the sample surface effectively prevents further corrosion. Meanwhile, XRD and SEM–EDS analysis results show that the main components of corrosion products are Ti O_2 and Ti O. The titanium on the surface of the sample is oxidized, resulting in slight uneven local corrosion. The results show that TA8-1 and TA2 are suitable to be used as candidate materials for high-level waste(HLW) disposal containers due to their excellent performance under long-term and high-dose irradiation corrosion.