Cold Crucible Directional Solidification Research Articles

Enhanced fracture toughness stability of (B,Y)-contained TiAl-based alloy prepared by cold crucible directional solidification

Cold crucible directional solidification (CCDS) can be used to prepare the well-DS TiAl-based alloys with good mechanical properties. However, when the DS columnar grains are coarsened, the stability of fracture toughness will not be guaranteed due to the internal random-direction (α2+γ) lamellas. In this study, in order to improve the fracture toughness stability of CCDS TiAl-based alloys, (B,Y) refinement element was added, and the model alloys Ti–44Al–6Nb–1Cr–2V and Ti–44Al–6Nb–1Cr–2V-(0.1B,0.15Y) were used. The two alloys were fabricated by CCDS under the same parameters, and their macro/microstructure and fracture toughness were studied. The results showed that the B/Y compounds hindered the growth of dendrites, narrowed the DS window, and significantly refined the columnar grains. The fracture toughness of well-DS Ti–44Al–6Nb–1Cr–2V-(0.1B,0.15Y) alloy with the refined columnar grains was still at a higher level, and the key was that the stability of its fracture toughness was significantly improved comparing with that of the well-DS Ti–44Al–6Nb–1Cr–2V alloy with the coarsened columnar grains. It was found that the well-DS Ti–44Al–6Nb–1Cr–2V-(0.1B,0.15Y) alloy could provide more longitudinal grain boundary barriers for the radial crack propagation, create more crack deflection when the crack entered the next columnar grain (lamella colony) and avoid the crack long-distance propagation along the bad-orientation (α2+γ) lamellas.

Microstructure evolution and enhanced fracture toughness of Ti-44Al-8Nb-0.1B alloy fabricated by cold crucible directional solidification under different temperature gradients

Microstructure evolution and enhanced fracture toughness of Ti-44Al-8Nb-0.1B alloy fabricated by cold crucible directional solidification under different temperature gradients

Numerical analysis for solid-liquid interface shape at various temperature gradient in electromagnetic cold crucible directional solidification

The straight solid-liquid (S-L) interface is necessary for obtaining a directional solidification ingot with industrial size and well directional solidification in electromagnetic cold crucible directional solidification (CCDS). Overheating and lateral heat dissipation are the significant parameters for the straight S-L interface in CCDS. In this paper, Nb-16Si-22Ti as an example was used for analyzing the influence of temperature gradient and boundary conditions on the shape of S-L interface by Comsol Multiphysics software. A 3-D parametric geometry as original proportional model was established and the accuracy of the model was verified by measuring magnetic flux density norm with small coil method. The simulation results exhibit that the shape of S-L interface formed by 50kHz-2400A is closer to the plane and beneficial to gain directional grain. The heat supplement meets the requirement of heat loss with 50kHz, and there is little difference for temperature gradient in both solid and liquid interface. The axial and radial maximum temperature gradient of 50 kHz-2400 A is 61.46 K/mm and 63.84 K/mm, respectively.

Effect of V on the microstructure and brittle-to-ductile transition of directionally solidified high-Nb TiAl alloy

The influence of V addition on the microstructure and high-temperature tensile performance of Ti46Al7Nb0.4W0.6Cr alloy, fabricated by using the cold crucible directional solidification method, is comprehensively investigated. The results show that V addition can refine the columnar grain width and promote the brittle-to-ductile transition (BDT) of the TiAl alloy. After the addition of 2 at.% V, the ultimate tensile strength and elongation increased at 973–1173 K. The fracture mode change from interlamellar to translamellar; this is attributed to the decrease in the angle between the lamellar alignment and growth direction. The BDT behaviour of V-containing TiAl alloy at 1073 K results from the transition from dislocation-dominated to twin-dominated deformation as indicated by transmission electron microscopic investigations.

Role of growth rate on microstructure evolution, element distribution and nanohardness of phases in directionally solidified multiphase high-Nb TiAl alloy

A multiphase TiAl alloy with nominal a composition of Ti–43Al–5Nb-3.5Cr–1Zr (in at. %) was fabricated using cold crucible directional solidification equipment at multiple growth rates. Effect of growth rate on the microstructure evolution, element distribution and mechanical property of phases in TiAl alloy were experimentally and statistically studied, especially the mechanism that affects the phase nanohardness. Multiphase microstructures consisting of α 2 /γ lamellar colonies and blocky γ phases as well as strip-like B2 phases were investigated through SEM and EDS identifications. With the increase of growth rates, the volume fractions of B2 and blocky γ phase increased accordingly from 7.3% to 13.3% and 4.6% to 12.6%, respectively. Furthermore, refinement of colonies can also be observed, which is attributed to phase interface migration driven by the change in growth rate. The content of Cr and Zr in the B2 phase is positively correlated with the growth rate, because the increase of growth rate will increase the effective distribution coefficient and make the solidification process deviate from the equilibrium state. Nanoindentation test results showed that nanohardness of B2 and blocky γ phases changed along with the growth rates owing to the enrichment or barrenness of alloying elements. The increase of Zr in blocky γ phase will increase its nanohardness, while the increase of Cr in B2 phase will lead to a decrease in its nanohardness, which is ascribed to lattice distortion caused by atomic substitution.

The microstructure and high cycle fatigue performance of as-cast and directionally solidified Ti46Al7Nb alloy under the three-point bending loading

This study systematically investigated the microstructure and high cycle fatigue deformation behavior of a high-Nb TiAl alloy with a nominal composition of Ti46Al7Nb alloy prepared by cold crucible directional solidification (CCDS) method through TEM, SEM, etc. Macro/microstructure and phase component of as-cast and directionally solidified alloys were characterized. The three-point bending fatigue performance at room temperature was tested, and the corresponding fracture and deformation mechanisms were also revealed. Results show that the columnar grains of the directionally solidified TiAl alloy are well-arranged in the direction of the heat flow. Compared with the as-cast alloy, the segregation of the directionally solidified microstructure is significantly improved. After 107 cycles, the three-point bending fatigue limit of the directionally solidified Ti46Al7Nb alloy is about 60 MPa higher than that of the as-cast state. Fracture morphology shows that cleavage fracture is the main fracture mode of the as-cast alloys. Quasi-cleavage fracture composed mainly of trans-lamellar fracture, supplemented by small amount of plastic fracture, is the major fracture mode of directionally solidified Ti46Al7Nb alloy. The fatigue deformation of this alloy mainly occurs in the γ lamellae. Dislocation multiplication and stress-induced generation of subgrain are the main mechanisms of high cycle fatigue deformation of Ti46Al7Nb alloy. Screw dislocations were also found in the γ subgrains generated after cyclic deformation. This research provides a better understanding of the high cycle fatigue deformation of TiAl alloys.

Microstructure formation and columnar to equiaxed transition during cold crucible directional solidification of a high-Nb TiAl alloy

In order to study the factors of columnar to equiaxed transition (CET) of high-Nb TiAl alloys, Ti46Al7Nb0.4W0.6Cr0.1B alloy has been fabricated by cold crucible directional solidification (CCDS) technique under different pulling rate from 3.3 μm/s to 16.7 μm/s. The marco/micro-structure and phase composition near solid–liquid interface have been characterized. Results show that the CET of the high-Nb TiAl alloy occurs with the increase of the pulling rate at the constant temperature gradient. The microstructure of the columnar grain is composed of α2/γ lamellar matrix and a coupling structure of striped-like B2+γ phases. The lamellar colonies in a columnar grain possess the same orientation, while the arrangement direction between the striped-like B2 phase and growth direction is 0° or 45°. A solidification map for CCDS is established which predicts columnar or equiaxed morphology according to the growth rate (R) and temperature gradient (G). The dendrite morphology at the solid–liquid interface after quenching and the CET is controlled by the actual temperature gradient at the tip of the dendrite. Meanwhile, the increase of growth rate and the satisfaction of heterogeneous nucleation conditions are the main factors for CET. The decrease of actual temperature gradient caused by quenching or the increase of liquidus gradient caused by increasing growth rate can increase the maximum supercooling degree ΔTC. When it reached the supercooling degree ΔTN required to form a new nucleus, equiaxed grains will be produced. In addition, the boride in this alloy can act as a heterogeneous nucleation core to promote CET.

High-temperature deformation resistance and creep resistance of a TiAl-based alloy fabricated by cold crucible directional solidification technology

In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys, cold crucible directional solidification (CCDS) technology was employed. A β-type TiAl-based alloy with the nominal composition of Ti44Al6Nb1Cr2V was prepared using the optimized CCDS parameters of 45 kW input power and 0.5 mm·min−1 solidification rate. Thermo-compression testing was utilized to evaluate the high-temperature deformation resistance and creep resistance of the CCDS Ti44Al6Nb1Cr2V alloy. Results show that the CCDS Ti44Al6Nb1Cr2V alloy billets contain aligned columnar grains and a high percentage of small-angle lamellae. Thermo-compression testing results in the radial direction of the CCDS alloy show a much higher peak stress than other reported results in similar conditions. The much higher hardening exponent and deformation activation energy are obtained, corresponding to the excellent high-temperature deformation resistance and creep resistance, which are because of the hard-oriented grains, weaker stress-strain coordination capability of lamella structure and relatively more hysteretic dynamic recrystallization. Thermo-compression testing results in the longitudinal direction of the CCDS Ti44Al6Nb1Cr2V alloy show the much higher peak stress than that in the radial direction, indicating the better high-temperature deformation resistance and creep resistance attributed to the hard-oriented lamellae in this condition.

Microstructure formation and elevated temperature mechanical properties of directionally solidified Ti44Al6Nb1Cr alloy

To improve room and elevated temperature mechanical properties of high-Nb TiAl alloys, Ti44Al6Nb1Cr alloy has been prepared by cold crucible directional solidification (CCDS) technique under loaded power of 40 kW and pulling rate of 8.3 μm/s. Macro/micro-structure and phase composition have been characterized. Tensile properties have been tested at room and elevated temperature and the related mechanisms have been revealed. Results show that equiaxed grains change to columnar grains growing up along the axial direction of ingot after CCDS. Phase component is still composed of α2, γ and B2 phases. Microstructure consists of lamellar clusters and network structure. The lamellar clusters in a grain possess the same orientation. The content and size of B2 phases decrease and some casting defects with small size decrease as well due to sufficient diffusion of Nb and Cr atoms under action of electromagnetic force during CCDS. Tensile results indicate that comprehensive mechanical properties of Ti44Al6Nb1Cr alloy are improved significantly after CCDS. At room temperature, ultimate tensile strength and total strain are 523 MPa and 3.03%, respectively, which are improved by 18.1% and 64.7% compared with that of as-cast Ti44Al6Nb1Cr alloy (443 MPa and 1.84%). Moreover, fracture morphology also indicates that there are many deep ravines between cleavage steps and some similar dimples microstructure on cleavage steps. They are beneficial to improving comprehensive mechanical properties. At 973 K, ultimate tensile strength of Ti44Al6Nb1Cr alloy does not decrease at all, still reaching 530 MPa after CCDS. When service temperature rises to 1173 K, the strain is less than 4.28% showing excellent mechanical stability under tensile stress less than 436 MPa. The excellent stability for mechanical properties is due to thin lamellar phases, directional microstructure and large plastic deformation region.

Microstructures and mechanical properties of directionally solidified C-containing γ-TiAl alloys via electromagnetic cold crucible

TiAl alloys with nominal compositions of Ti–47Al–2Nb–2Cr-(0.2, 0.5)C were prepared via electromagnetic cold crucible directional solidification. Effects of alloying element carbon on the solidification behavior, microstructure and mechanical properties of the TiAl alloy were investigated. Experimental results revealed that alloying with carbon reduced the B2 phase content and refined the lamellar structure. Also, additions of 0.5 at.% carbon changed the primary phase from single β to the mixture of β and α, and promoted the formation of Ti2AlC particles. The yield strength of the alloy enhanced as the carbon content increased, while the ultimate strength of the alloy containing 0.2 at.% and 0.5 at.% C decreased consequently. Dislocation glide governed the plastic deformation of the two directionally solidified C-containing TiAl alloys. The strain rate sensitivity value and activation volume of the both experimental alloys decreased with the increase of strain. The hindrance of Ti2AlC particles to dislocations movements caused stress concentration and premature cracking.

The microstructure and mechanical property of TiAl alloy containing β-stabilizer

In this study, TiAl alloy with the nominal constitution of Ti45Al2Cr2Nb and Ti45Al4Cr2.5Nb were prepared by the cold crucible directional solidification method, the microstructure and mechanical property were investigated by means of XRD, SEM as well as tensile test. The different microstructure of the two TiAl alloy was analyzed, the main reason of the difference is their different segregation pattern, in the Ti45Al2Cr2Nb alloy the s-segregation is the dominating pattern while in the Ti45Al4Cr2.5Nb β-segregation is the dominating pattern, which cause the discrepancy distribution of B2 phase and independent γ phase in the two alloy. In the Ti45Al2Cr2Nb the B2 phase distribute in the lamellar colony and in the Ti45Al4Cr2.5Nb the B2 phase distribute in the boundary of the lamellar colony. Besides, the different microstructure also influence the mechanical property, the brittle B2 phase in the lamellar colony break the continuity of the lamellar and the boundary of B2 could act as the source of the crack during deformation. In the tensile test the ultimate tensile strength of the Ti45Al2Cr2Nb and Ti45Al4Cr2.5Nb alloy are 347 MPa and 539 MPa respectively. Interestingly, the Young modulus that is the intrinsic property of TiAl alloy and not affected a lot by the microstructure also gets higher, which means that the Cr, Nb element strengthen TiAl alloy not only by optimizing the microstructure but also by strengthening the matrix.

Optimization of electromagnetic energy in cold crucible used for directional solidification of TiAl alloy

Directionally solidified TiAl alloys are one of the most potential candidate for turbine blade in advanced aircraft engines. Electromagnetic (EM) cold crucible directional solidification (CCDS) is a novel technique for preparing large-size TiAl ingot without chemical contamination. In order to improve EM utilization, first the utilization of EM energy in CCDS was evaluated. Then based on the numerical calculation, the absorption power in TiAl alloy that induced by EM energy and the uniformity of EM field were studied, which contributes to the configuration design. Results indicated that the energy utilization in CCDS is improved by optimizing crucible configuration, the start-up power per square mm significantly decreases from 1250 to 500 W/mm2. Finally, a square cold crucible with the section 36 mm × 36 mm was fabricated via a configuration optimization and employed to directionally solidifying TiAl alloy. Both the surface quality and the microstructure were controlled in the processing window, suggesting that the optimization utilization of EM energy in CCDS has been achieved. The results can provide semiquantitative and experimental guidelines on the crucible design and the microstructure control of TiAl alloys from the perspective of EM energy.

Microstructure, Mechanical Properties, and Crack Propagation Behavior in High-Nb TiAl Alloys by Directional Solidification

Titanium aluminide (Ti-47Al-6Nb-0.1C) alloys were prepared using a cold crucible directional solidification technique with an input power range of 35 to 55 kW under a withdrawing velocity of 0.4 mm/min. The macro/microstructure was characterized, and the mechanical properties were evaluated. The results show that the directional solidification (DS) ingots exhibit β-solidification characteristics at an input power range of 35 to 55 kW, and a well-developed DS microstructure was acquired at an input power range of 40 and 45 kW. With the increasing input power, the lamellar spacing decreases, resulting in the increasing tendency in the average room-temperature yield strength. The steady-state creep rate strongly depends on the lamellar spacing, and the creep life depends on the DS microstructure. The well-developed DS alloy can significantly improve the creep properties but has little influence on the room-temperature tensile properties. Moreover, after testing the fracture toughness, the crack propagation showed interlamellar cracks, translamellar cracks, and intercolony boundary cracks that primarily propagated along the colony boundary after creep testing.

Improvement of the creep lifetimes and microstructural stability of β-solidifying γ-TiAl by cold crucible directional solidification

The cast and cold crucible directional solidification (CCDS) Ti-44Al-6Nb-1Cr alloys were crept at 800 °C under 170–230 MPa. The creep lifetimes of CCDS specimens significantly increase comparing with the cast specimens. A novel nano-scale γ/β lamellar structure is found at microsegregation zone in CCDS alloy, which exhibits the higher microstructural stability. The ω phases precipitated in β matrix with accumulating Nb and rejecting Cr. The small precipitated phase exhibits a higher specific surface area, which promotes the cavity and crack nucleation/growth at the interface of precipitated phase. The granular and lathy precipitated phases easily fall off during creep, which contributes to the crack propagation. The CCDS technique can improve creep lifetimes by optimizing the microstructure, promoting the formation of the β/γ lamellae, eliminating the horizontal colony boundaries and reducing the proportion of colony boundaries.

Numerical analysis for electromagnetic field influence on heat transfer behaviors in cold crucible used for directional solidification

Cold crucible directional solidification (CCDS) is the technique that the materials in the segmented water-cooled crucible are induction heated and heat transfer controlled by electromagnetic field for directional solidifying crystals without contamination. In order to control crystal growth by the CCDS efficiently, mathematical analysis of heat transfer during the CCDS process is presented by means of a combined electromagnetic, fluid and thermal model. A 3-D model is established and verified to investigate the characteristics of heat transfer in the melt, which considers the coupled effects of the shaped melt and the electromagnetically driven flow on the temperature field and the shape of solid–liquid (S/L) interface. The shaped melt by electromagnetic pressure along with the induction heat in the skin layer reduce and compensate the radial heat loss, which could decrease the radial temperature gradient in the melt with increasing input powers. A vigorous three-dimensional melt flow consisting of meridional flow and azimuthal flow is induced in the melt, which improves the uniformity of the temperature field and decreases the deflection of S/L interface in the center of melt. The solidification front changes from ‘W’ shape to planar when the power reaches 39 kW. Based on the above results and analysis, TiAl alloys are successfully directionally solidified by the CCDS method with planar S/L interface, and the growth direction of columnar crystals are nearly paralleled.

Influence of Mn addition on the microstructure and mechanical properties of a directionally solidified γ-TiAl alloy

TiAl alloys with nominal compositions of Ti-47Al-2Nb-2Cr-(0, 1, 3, 5)Mn were prepared via electromagnetic cold crucible directional solidification (ECCDS), and the influence of Mn addition on the microstructure and mechanical properties of the alloys was investigated. The experimental results showed that the Mn atoms were dissolved within the TiAl alloy, and no Mn compounds were formed. Based on the X-ray diffraction (XRD) analysis results, the peak angle of the Mn-containing sample shifted to a higher value, indicating lattice shrinkage. The Mn addition also increased the volume fraction of the B2 phase. The yield strength of the TiAl alloy monotonically increased as the content of the substitutional-solute Mn atoms increased. The ultimate strengths of the TiAl alloys containing 1 at.%, 3 at.%, and 5 at.% Mn consequently decreased because micro-cracks were preferentially formed in the B2 phase during room-temperature deformation. The introduction of substitutional-solute Mn atoms could result in a reduction of the stacking-fault energy of the γ phase, and facilitate the operation of twinning-controlled plastic-deformation during the intermediate stage of the plastic deformation process. The interaction between the deformation twins and movable dislocations led to a continuous increase in the work-hardening rate.

Creep Behavior of High‐Nb TiAl Alloy at 800–900 °C by Directional Solidification

Cold crucible directional solidification Ti44Al6Nb1.0Cr alloy is crept at 800–900 °C. Experimental results show that creep lifetime significantly decreases with the increasing creep temperature. When creeping at 900 °C under 130 MPa, the TQ twinning is activated in lamellar structures. The TQ twinning shows a strong dependency on temperature during creep under low creep‐stress and it can overcome α2 lamellae and transfer into adjacent γ lamellae. The hardening by mechanical twinning and the softening by α2 lamellar dissolution take place at different zones in lamellar structures and the strain incompatibility between hardening zone and softening zone promotes the microcracks to form in lamellar structures. The deformation characteristic of hard and soft lamellae is studied. Moreover, recrystallization γ phase formed in lamellar structures near colony boundary during creep at 900 °C accelerates the creep failure.

Effects of lamellar spacing on microstructural stability and creep properties in β-solidifying γ-TiAl alloy by directional solidification

Ti44Al6Nb1Cr (at%) alloys with different lamellar spacing were prepared by cold crucible directional solidification. Creep tests were conducted at 750°C under 260 and 300MPa, and the microstructure before and after creep testing were observed and analyzed. The results show that the prepared TiAl alloys have similar macro/microstructure except for lamellar spacing, which are different from the heat-treated TiAl alloys with obviously changed macro/microstructure. The refinement of lamellar spacing can improve creep properties, especially the steady-state of fine lamellar alloy lasted for more than 600h with creep rate at 7.3 × 10−9S−1. The improvement of creep properties by refined lamellar spacing are revealed as following two reasons. (1) Fine lamellar spacing improves the stability of γ lamellae and increases the resistance for dislocation slip in γ lamellae. (2) It disperses stress concentration and delays the formation of globular structure at colony boundary. Moreover, the alternating β and γ laths in α-segregation zone can improve microstructural stability during creep. In β-solidifying γ-TiAl alloy, the local stress concentration on β-segregation at colony boundary promotes colony boundary sliding and the formation of void with globular structure, which further accelerates the creep failure.

Effects of hydrogen on microstructure and hot deformability of directionally solidified Ti–44Al–6Nb–1Cr alloy

Effect of hydrogen on microstructure and the hot flow behavior of the directionally solidified Ti–44Al–6Nb–1Cr (at.%) alloy was studied. Ti–44Al–6Nb–1Cr alloys were prepared by vacuum induction melting (as-cast alloy) and cold crucible directional solidification (CCDS). Effects of hydrogen on the hot deformation behavior were tested on Gleeble-1500D at 1373 K and 1473 K under strain rate of 0.01 s−1. Results show that the volume fraction of B2 phase is increased after CCDS due to the solute distribution. The residual B2 phase in the hydrogenated alloy is more than that in unhydrogenated alloy because hydrogen is beta stabilizing element and stabilizes the B2 phase. The peak stress of the CCDS alloy is higher than that of as-cast alloy at the same deformation condition due to the directional solidified microstructure. Hydrogen can decrease peak stress of the as-cast alloy and CCDS alloy significantly, which is decreased by about 46% and 43% respectively. This is attributed to the hydrogen-promoted dynamic recrystallization and hydrogen-increased the volume fraction of B2 phase.

Microstructure and mechanical properties of Ni3Al intermetallics prepared by directional solidification electromagnetic cold crucible technique

The present work focused on the Ni3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25Al alloy. Ni3Al intermetallics were prepared at different withdrawal rates by directional solidification (DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni3Al and NiAl phases. The primary dendritic spacing (λ) decreases with the increasing of withdrawal rate (V), and the volume fraction of NiAl phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.