Allotropic Transformation Temperature Research Articles

Additive manufacturing of TTFNZ (Ti-4.5Ta-4Fe-7.5Nb-6Zr), a novel metastable β-titanium alloy for advanced engineering applications

The alloy Ti‐6Al‐4V (Ti64; wt%) is extensively used for aerospace and medical applications. However, due to potential drawbacks and societal change, intensive research has been conducted to replace this material for medical use, as such various promising alloy systems have been proposed. By implementing additive manufacturing (AM) in the value chain of load-bearing implants, the question of the printability of these promising medical alloy systems arose as none were explicitly developed for AM. In a previous paper, an alloy development methodology for AM (NADFAM [Ackers et al., 2021]) was presented, which allows rapid screening of alloys for laser-based powder bed fusion of metals (PBF-LB/M; ISO/ASTM 52911–1). The method was applied to screen several new titanium alloys and rank the alloys according to target properties for PBF-LB/M of load-bearing implants. Following this screening, a novel metastable β-titanium alloy out of the quinary Ti-Ta-Fe-Nb-Zr system was selected for further investigations, and a refined alloy composition Ti-4.5Ta-4Fe-7.5Nb-6Zr (wt%, denoted as TTFNZ) will be presented and compared to Ti64 in this study. The motivation to investigate a system with a lower allotropic transformation temperature (β/α) than Ti64 derived from the idea that (I) it promises reduced cold cracking tendency in the course of rapid solidification (II) common post-print heat treatments could be performed at lower temperatures or could become redundant using printers with a preheating option and (III) it allows exploring a wide range of microstructures and mechanical properties through thermal processing. The present study reports on: (i) AM feedstock/powder production of TTFNZ, (ii) PBF-LB/M process parameter development, (iii) heat treatment optimisation, related microstructures and their effect on mechanical properties tested via indentation, compression and tensile testing. (iv) Corrosion properties and (v) the PVD coatability of TTFNZ were also studied to discuss its amenability for medical and advanced engineering applications. • First-time atomisation and additive manufacturing of Ti-4.5Ta-4Fe-7.5Nb-6Zr (TTFNZ). • TTFNZ surpasses well-known medical and aerospace alloys in a variety of disciplines. • Investigation of TTFNZ’s microstructure evolution using DSC, EDX/EBSD and XRD. • Compression, tensile and dilatometry results. • Evaluation of TTFNZ’s corrosion properties as well as initial PVD coating trials.

Effects of laser pulse modulation on intermetallic compounds formation for welding of Ti-6Al-4V and AA7075 using AA4047 filler

Laser welding of titanium and aluminum alloys usually generates pores and brittle intermetallic compounds (IMCs), which degrade the weld mechanical properties. In this study, pulse wave (PW) Nd: YAG laser was used to weld Ti-6Al-4V to aluminum alloy 7075 (AA7075) with aluminum alloy 4047 (AA4047) filler using pulse modulation to control the formation of pores and IMCs. The effect of pulse modulation on porosity levels and IMC layer thickness were analyzed. EBSD, XRD and elemental mapping identified that IMC layers of TiAl and Al3(Ti, V) were formed. A numerical model for pulse laser welding Ti/Al alloys was developed and validated with thermocouple measurements. The numerical results revealed that the predicted cooling rates above the allotropic transformation temperature (882 ℃) of Ti for different pulse modulations have significant correlation with the measured IMC layer thickness. The optimized process parameters with pulse modulation were able to control the IMC thickness to within 5 µm and attain a maximum ultimate tensile strength of 158 MPa.

Investigation of Grain Boundary Influence on the Thermodynamic Phase Stability of Nanocrystalline Iron by Using the Molecular Dynamics Simulation Method

The thermodynamics of allotropic transformation in nanocrystalline Fe is studied. For this purpose, the grain boundary structure is initially examined using a novel method based on molecular dynamics (MD) simulation, the results of which are exploited to determine grain boundary thickness and excess volume for different grain sizes. The dilated crystal model and the quasi-harmonic Debye approximation method are subsequently employed to calculate the values of Gibbs free energy for grain boundaries of different allotropic phases. Using the quantitative parameters obtained from the simulations, the relevant Gibbs free energy curves are plotted in terms of grain size and analyzed for different temperatures. This is followed by deriving stability curves for the γ-Fe and α-Fe phases in terms of grain size for different temperatures. Ultimately, the parameters obtained from MD simulation studies and the stability diagrams thus obtained are used to explore the changes in the γ→α allotropic transformation temperature of iron nanocrystal for different grain sizes. It will be that allotropic transformation temperature declines from 900 to 300 K with rising grain size from 4 to 33 nm.

Microstructure Evolution during High-Temperature Deformation of Ti-5Al-5V-2Mo-1Cr-1Fe Alloy under Compression

Hot isothermal compression testing of Ti-5Al-5V-2Mo-1Cr-1Fe alloy exhibiting lamellar (α/β) microstructure was performed using a deformation simulator. The flow curves of the alloy were obtained as a function of temperature around its beta transus (‘Tβ’, the allotropic transformation temperature from HCP to BCC phase) at different strain rates in the range of 10−2-10 s−1. The apparent activation energy ‘Q’ for the alloy under these hot working conditions was found to be ~ 644 and ~ 317 kJ mol−1 in two-phase (α + β) and single-phase β regimes, respectively. Distinct constitutive relationships for the alloy have been established at various temperature regimes. Dynamic material model was employed to generate the processing maps and thereby evaluate the stable domains for hot working of this alloy. Deformation below the ‘Tβ’ led to α lamellae kinking followed by fragmentation and dynamic recrystallization of α phase. While deformation above ‘Tβ’ resulted in serrated β boundary, an indicative of dynamic recovery (DRV) of β phase. The processing maps along with the detailed post-deformed microstructural examination showed that the optimum/safe working domains for processing of the alloy are ‘T’ = 1123-1163 K and ‘ $$\dot{\varepsilon }$$ ’ = 10−2-10 s−1.

The influence of thermal shock t high temperature on Zy-4 alloy

"Zy-4 alloy is a basic material in the construction of the current generation nuclear reactors. Due to the diffusion processes and the chemical reactions that take place, the corrosion in air at high temperatures of the zircaloy-4 alloy determines the appearance of new phases and constituents and the stabilization of the alpha phase at high temperatures, above the allotropic transformation temperature. The paper presents the structural transformations that take place in the alloy following the oxidation under thermal shocks applied in the range 600-1500 ° C, in correlation with the chemical composition, hardness and thermal transfer properties. "

Determination of Polymorphic Transformation Temperature for Alloy VT8

The temperature of polymorphic transformation (Tpt) has been determined for accurate composition of two-phase titanium alloy which has been marked like VT8 through different ways: the method of trial quench and with the help of dilatometric and differential thermal analyzes. The close values of Tpt have been received. Herewith the phase composition has been estimated qualitatively and quantitatively for the hardened samples of alloy VT8 using optical and scanning electron microscopes and the chemical composition with the help of the micro X-ray spectral analysis. The influence of the quenching temperature on the hardness has been investigated. Also, the hardness of the individual structural components and alloy VT8 as a whole unit has been measured. Hardness curves are based depending of the quenching temperature; the results are displayed in tabular and graphical form. The coefficientkhas been found for the calculation of allotropic transformation temperature for this alloy using the known temperature of hardening and quantity of preserved α-phase of hardened structure. The album of the microstructures after quench with different temperatures has been created, this album will allow to reduce the duration of preliminary works for finding specific temperature of polymorphic transformation in production conditions for each new batch of alloy VT8 with another chemical composition (but within the vintage), knowledge of which is necessary for setting the temperature regime of treatment.

Thermal performance determination of binary Fe-Al alloys at elevated temperatures

The thermal expansion and thermal diffusion properties of Fe−Al alloys were measured using dilatometric and laser flash methods. At two allotropic transformation temperatures, the length change of the rapidly solidified pure Fe by injection casting deviates from that of the pure Fe by heating-cooling processing in a dilatometer. The ferro-/paramagnetic transition and second order transition do not influence the thermal expansion behaviors of Fe−Al alloys as temperature increases from 473 to 1373 K. The thermal expansion coefficients of Fe70Al30, Fe60Al40 and Fe55Al45 alloys are apparently larger than those of Fe25Al75 and Fe90Al10 alloys. Once temperature exceeds 877 K, the thermal expansion coefficients of Fe-Al alloys are enhanced with the increase of Al content, except for Fe25Al75 alloy owing to the large packing factor of FeAl3 crystal structure. In the temperature range from 309 to 877 K, the thermal diffusivity as a function of temperature decreases in the sequence of Fe90Al10, Fe55Al45, Fe60Al40, Fe70Al30 and Fe25Al75, and their values are lower than that of pure Fe at the same temperatures. As temperature increases, the thermal diffusivity values of Fe−Al alloys increase, except that of Fe90Al10 alloy decreases in the temperature range from 294 to 979 K and then increases once temperature further increases to 1078 K because of the ferro-/paramagnetic transition. Their thermal expansion coefficient and thermal diffusivity serve as linear or polynomial functions of temperature.

Physical Simulation of Thermo-Mechanical Processing of Ferritic-Bainitic Dual Phase (FBDP) Steel

The present work is dealing with a physical simulation of thermo-mechanical processing of ferritic-bainitic dual phase (FBDP) steel alloy containing 0.1% C, 0.3% Si, 0.9% Mn and 0.7% Cr. The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min. after last hit has been applied and then followed by air cooling. Dilation curves appear that Ac1= 766 °C, while Ac3 was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation a higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure. The flow curve of the steel ensures continuous strain hardening. The strain hardening rate (σf/ε) was directly proportional to temperature difference from pass to pass.

Stability of NiCrAlY coating/titanium alloy system under pure thermal exposure

In this paper, NiCrAlY coating was deposited on TC4 titanium alloy using arc ion plating, and the stability of NiCrAlY coating/TC4 substrate system under pure thermal exposure was analyzed in a wide temperature range. During the thermal exposure, β → γ′ → γ phase transformation takes place in the NiCrAlY coating. The NiCrAlY coating phases totally decompose above the allotropic transformation temperature of TC4. The allotropic transformation of α-Ti to β-Ti of the substrate significantly influences the stability of NiCrAlY coating/TC4 substrate system. NiCrAlY coating elements are mainly consumed by interfacial reactions below the allotropic transformation temperature of TC4. Above the allotropic transformation temperature of TC4, NiCrAlY coating elements are mainly consumed by element dissolution in β-Ti. The NiCrAlY coating totally degrades for the dissolution of coating elements in the substrate to form thick β-Ti stabilized layer.

Investigation of the Effect of Deformation Stored Energy on the Allotropic Transformation Temperature

The aim of our investigation was to study the effect of deformation on the start temperature of the austenite-ferrite transformation. The deformation was carried out during cooling before the beginning of the transformation. The transformation start temperature was determined by dilatometric measurement with a Gleeble 3800 thermomechanical simulator. A novel method was developed to study the deformation induced ferrite transformation (DIFT) effect and successful experiments were carried out on S460MC grade steel specimens to determine the effect of the deformation stored energy on the transformation temperature of the austenite. Evaluating the dilatograms a strong relationship was observed between the temperature of the deformation and the austenite transformation start and finish temperatures. Lower deformation temperature resulted finer microstructure after austenite transformation.

Effect of Carbon Addition on the Microstructure and Properties of Unalloyed Titanium

The effect of carbon additions on the microstructure and properties of unalloyed titanium with 0.2 and 0.5 wt. % C has been studied. Carbon has been found to appear as TiC carbide in these alloys. Its presence translates into increase in allotropic transformation temperature as well as hardness and strength properties of alloys at the expense of reduction in its ductility. It has been demonstrated that these alloys can be subject to hot deformation, and with low carbon content also to cold deformation, and additionally strengthened by solution treatment.

Study on the interface reaction behavior of NiCrAlY coating on titanium alloy

NiCrAlY coating was deposited on TC4 (Ti–6Al–4V) titanium alloy substrate by arc ion plating (AIP). After vacuum heat treatments, the interface reaction behavior of NiCrAlY coating/TC4 substrate was studied. The results show that the 20μm as-deposited coating, which was made up of γ-Ni, α-Cr and β-NiAl phases with 50–200nm grain size, was continuous and tightly bound to the substrate. The allotropic transformation of α-Ti to β-Ti during the heat treatment had a significant influence on the interface reaction behavior. After vacuum heat treatment below the allotropic transformation temperature (ATT) of titanium, i.e. 750°C, Ni3(Al, Ti)/TiNi/Ti2Ni/Ti3Al and α-Ti/β-Ti stabilized layer formed in turn from the coating to substrate at the interface. At 870°C or 900°C (near/above the ATT of titanium), Cr began to take part in the interface reaction and α-TiCr2 phase was detected. With the increase of heat treatment time at 900°C, including the coating phases, all the interface products decomposed and solved in the substrate to form a thick β-Ti stabilized layer for the higher solid solubility of the coating elements in β-Ti. The decomposition of the coating led to its total degradation. The interface reaction processes were studied in terms of thermodynamic and kinetic factors. Based on Fick's first law, the relation between the thickness of interface reaction layer and heat treatment time was discussed.

Transformation-mismatch plasticity in sub-millimeter iron wires

Wires of 99.5% pure iron with 406 and 508 μm diameter were subjected to a uniaxial tensile stress while being thermally cycled about the α–γ allotropic transformation temperature. The strain increments per cycle are proportional to the applied stress in the range 1–22 MPa, indicating that transformation-mismatch plasticity is the dominant deformation mechanism. The strain increments for the wires have the same magnitude as those reported for bulk iron samples, thus, indicating that the internal mismatch strains responsible for this deformation mechanism are undiminished in the wires, despite their high surface-to-volume ratio. Very high average strain rates (up to 3 × 10 −3 s −1) were achieved through resistive heating and convective/radiative cooling of the thinnest wires.

通電開離面に形成する溶融ブリッジの材質特性

Steel and copper are widely used as current collecting material for power supply in electric railways. In this field experimental studies on material wear and friction phenomena are important for improving stability and reliability of current collection in service operation. However few are reported to the effect of contact bridge formation, which is caused by separation of electrodes, in the material wear and friction phenomena, especially for different materials, such as, steel and copper.In this paper, anode and cathode materials, which are made of same metal (either steel or copper), are separated with very low constant speed. to observe the bridge formation and contact surface after the bridge separation in detail.As a result, melting bridge formation of 0.5mm height was observed while steel electrodes separating. In the similar test of the identical condition of the copper electrode, the observation of the bridge formation was difficult. However, the copper bridge that grew at 0.2mm height was observed, where test was conducted with separating speed slowed down to 1/10. The molten bridge was formed at the boundary on the anode side with high brightness, and this effect was seen for both steel and copper. Although this phenomena appears for both cases of steel and copper, the anode made of steel material is damaged more seriously.Also in this paper, special strips of steel and copper, simulating the contact bridge, are studied for contact drop, contact resistance and temperature increase at contact point to melt down by increasing current slowly from zero ampere.As the result, the resistance of the copper which is a diamagnetic material rose in a smooth quadratic curve for the increase in current (temperature), and the steel which is ferromagnetic showed the rapid increase above magnetic transformation and allotropic transformation temperature. The increase of heat quantity by decreasing of specific heat due to magnetic transformation and the consequent resistance increase are the cause of the formation of large melting bridge in the steel And Thomson effect is considered as the cause of damage of anode.

Structural formation of aluminide phases on titanium substrate

An aluminum layer on titanium substrate has been formed by thermal deposition. The structural formation of aluminide phases on the Ti substrate has been studied. The sequence of structural transformations at the Ti/Al interface is limited by the reaction temperature and time. Kinetics of these transformations are defined by a factor of concentration. At the initial stages at the Ti/Al interface the Al 3Ti alloy starts forming as a result of interdiffusion, and gradually the whole aluminum film is spent on the formation of this layer. The Al 3Ti layer decomposes with increasing temperature (>600 °C). At 800 °C the two-phase (Ti 3Al+TiAl) layer is formed on the titanium surface. The TiAl compound is unstable and later on with the increase of the exposure time at 800 °C gradually transforms into the Ti 3Al. The chain of these successive transformations leads to the formation of the continuous homogeneous layer consisting of the Ti 3Al compound on the titanium surface. At temperatures exceeding the allotropic transformation temperature (>900 °C) the Ti 3Al compound starts decomposing, aluminum atoms diffuse to the substrate and are uniformly distributed within the surface volume. All structural changes taking place at the Ti/Al interface are accompanied by considerable changes in microhardness. Due to the Ti 3Al layer being formed on the titanium surface, its microhardness increased practically by 2.5 times.

Transformation superplasticity of zirconium

A tensile strain of 270 pct was achieved for coarse-grained zirconium subjected to transformation superplasticity conditions, where strain increments are accumulated upon repeated thermal cycling around the allotropic transformation temperature under the biasing effect of a uniaxial tensile stress. The strain increment per cycle was found to consist of two equal contributions from transformations on heating and cooling and to increase linearly with the applied stress. The measured strain increments are in good quantitative agreement with predictions based on the average internal stress during the transformation, which was determined independently from experimental transformation times. As the cycling frequency is raised, the average strain rate increases (a maximum value of 1.3·10−4 s−1 was measured), but the strain increment per cycle decreases above a critical cycling frequency, for which the sample gage section undergoes only a partial phase transformation. The resulting reduction in internal mismatch and increase in internal stress are modeled using the experimental observation that β-Zr deforms by a mixture of diffusional and dislocation creep in the stress range of interest.

Effects of heat treatments on the ductility of cobalt electrodeposits

The effects of annealing treatments on the structure and mechanical properties of cobalt electrodeposits have been studied. Annealing temperatures range from 250–800 °C, i.e. below, as well as above, the allotropic α-h c p ⇋ β-f c c transformation temperature (417 °C). The structural characterization included hydrogen content measurement, relative volume fraction of α and β phases determination by X-ray diffraction, and microstructural investigations by optical and electron microscopy. The results showed that an annealing is a very effective means to optimize the ductility of cobalt electrodeposits. The increase of ductility observed after annealing is essentially due to a decrease of the stacking fault density in the deposits. Annealing treatment above the allotropic transformation temperature also produces, in some deposits, an increase of the ductile β-f c c phase content, but this fact does not lead to supplementary improvements of ductility. This is due to the detrimental influence of hydrogen that always exists in these deposits.

Phase equilibria and microstructural control of gamma TiAl based alloys

Abstract Phase equilibria among the β (bcc or B2), α (hcp) and γ (L10) phases in TiAlM ternary systems at elevated temperatures have been studied, where the M is β stabilizing element for pure titanium. The β(B2) + α + γ three-phase coexisting region exists at temperatures above 1473 K, and it moves towards a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the α ↔ β allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios of M to Ti. The change in the phase equilibria results in new reaction pathways peculiar to the ternary systems, thereby opening more possibility for microstructure control of the gamma based alloys. The vertical section drawn based on these studies demonstrates that the γ phase can be in equilibrium with the β(B2) phase at relatively low temperatures in the ternary systems and create a reaction pathway of α → β(B2) + γ. Upon cooling along the same pathway of α → γ as in the binary alloy, the addition of third element affects the transformation rate, and the massive γ structure is found to be formed even under slow cooling rate.

Transformation-mismatch superplasticity in reinforced and unreinforced titanium

Samples of commercial-purity titanium, with and without 10 vol.% TiC particulates, were thermally cycled about the allotropic transformation temperature of titanium. Thermal ratcheting was small for both unstressed materials. Upon application of an external uniaxial tensile stress, unreinforced titanium exhibited large strain increments, resulting from the biasing by the applied stress of the volume mismatch developed between grains during the transformation. Upon repeated cycling, a strain to fracture of 200% was reached, with a strain per cycle proportional to the external stress, in agreement with existing transformation-mismatch superplasticity models. The metal matrix composite displayed transformation-mismatch superplasticity as well, with a strain to fracture of 135% and a strain per cycle significantly higher than for unreinforced titanium. This novel enhancement of superplastic strain is modeled by considering the internal mismatch between the transforming matrix and the non-transforming particulates.

Phase Equilibria in the Rare Earth Metals (R)-Rich Regions of the R-Al Systems (R= La, Ce, Pr, Nd)

The R-rich regions of the R-Al systems (R = La, Ce, Pr, Nd) have been studied by using differential thermal analysis (DTA), metallographic analysis, quantitative electron probe microanalysis (EPMA) and X-ray examinations (XRD). The coordinates of the R-rich eutectics, the lowering of the allotropic transformation temperatures of the rare earth metals by additions of Al and the melting behaviours of the R-rich compounds have been particularly investigated. The terminal aluminium solubilities in the different rare earth metals and the relative stabilities of the catatectic and eutectoid equilibria are discussed.