Directional Solidification Of Alloys Research Articles

Formation mechanism and mechanical behavior of gradient nanograin structure in directional solidified Ti3Al alloy: Atomic-scale study

The formation mechanism of Ti3Al alloy during a directional solidification process was systemically investigated by means of molecular dynamics (MD) simulations, and its mechanical behavior was explored by comparing with its nanograined (NG), coarse-grained (CG) and gradient nanograined (GNG) counterparts. It is found that the solidified front forms equiaxed crystals first, then they transform into columnar crystals, and the GNG structure is formed finally. Noticeably, the grains will grow preferentially in the direction parallel to the solidification direction. Besides, it is also found that the directional solidified alloy with the GNG structure has higher tensile strength and better ductility than its NG and CG counterparts. The GNG structure not only suppresses strain localization and grain growth in its small grain regions, but also promotes more cross dislocations in the large grain regions, resulting in a better mechanical performance.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Anisotropy in flow behaviour of directionally solidified Ni-based superalloy under compression at an elevated temperature

Directionally solidified (DS) superalloys consist of coarse columnar grains with <001> preferred orientation parallel to the maximum stress axis; but, grain orientations in the transverse direction are not controlled. However, during service, substantial loading could occur in the transverse direction as well, leading to early intergranular failure of components such as DS turbine blades. The present study investigates the effect of crystallographic orientation as well as specimen orientation (transverse and longitudinal with respect to grain growth direction) on the mechanical response of DS Ni-based superalloy in compression. Compression tests were performed at 700 ºC on transverse (with predominantly <101> or <001> oriented grains) and longitudinal (with <001> oriented grain) samples. Though there were negligible changes in yield stress, variations in hardening behaviour and ductility were observed in all the tested samples. Unlike transverse samples, longitudinal samples showed significant strain hardening with no failure even up to 0.3 true strain. Deformation mechanisms that led to these differences are identified using advance characterization techniques. γ' shearing with antiphase boundary formation was observed in the sample containing <101> grains, whereas γ' shearing with the formation of superlattice intrinsic stacking faults occurred in the sample having <001> grains. In addition, substantial lattice rotation and deformation twinning also contribute to strain hardening, especially in longitudinal samples. The effect of orientation is explained in terms of effective stacking fault energy. The study suggests that grains with <001> orientation undergo significant strain hardening and thereby accommodate large strain as compared to <101> oriented grains in DS alloys.

Temperature-dependent mechanical properties and fracture behavior of directionally solidified Ti47Al2Cr2Nb alloy

The influence of temperature on mechanical properties and fracture behavior of directionally solidified (DS) Ti47Al2Cr2Nb alloy was investigated in detail. Tensile tests and single-edge notched bending tests were performed in the temperature range of room temperature (≈25 °C) to 900 °C. The results revealed that both the yield strength and tensile strength of DS alloy specimens were insensitive to temperature until 850 °C. The yield strength and tensile strength at room temperature were found to be 500 MPa and 610 MPa, respectively, which were close to those at 850 °C (i.e., 534 MPa and 575 MPa). The ductility increased with the increase in temperature and the ductile-brittle transition (elongation >5%) occurred in the temperature range of 700–800 °C. Furthermore, the fracture toughness initially increased with the increase in temperature and reached the maximum value of 45 MPa m1/2 at 850 °C, followed by a decrease. It is worth emphasizing that the DS Ti47Al2Cr2Nb alloy exhibited a brittle trans-lamellar fracture at room temperature, which was similar to cleavage fracture. On the other hand, the fracture at high temperatures was influenced by the propagation and coalescence of microcracks at the lamellar interface, similar to micro-void accumulation fracture.

On the occurrence of buoyancy-induced oscillatory growth instability in directional solidification of alloys

Recent solidification experiments identified an oscillatory growth instability during directional solidification of Ni-based superalloy CMSX4 under a given range of cooling rates. From a modeling perspective, the quantitative simulation of dendritic growth under convective conditions remains challenging, due to the multiple length scales involved. Using the dendritic needle network (DNN) model, coupled with an efficient Navier-Stokes solver, we reproduced the buoyancy-induced growth oscillations observed in CMSX4 directional solidification. These previous results have shown that, for a given alloy and temperature gradient, oscillations occur in a narrow range of cooling rates (or pulling velocity, V p ) and that the selected primary dendrite arm spacing (Λ) plays a crucial role in the activation of the flow leading to oscillations. Here, we show that the oscillatory behavior may be generalized to other binary alloys within an appropriate range of (V p ,Λ) by reproducing it for an Al-4at.%Cu alloy. We perform a mapping of oscillatory states as a function of V p and Λ, and identify the regions of occurrence of different behaviors (e.g., sustained or damped oscillations) and their effect on the oscillation characteristics. Our results suggest a minimum of V p for the occurrence of oscillations and confirm the correlation between the oscillation type (namely: damped, sustained, or noisy) with the ratio of average fluid velocity over V p . We describe the different observed growth regimes and highlight similarities and contrasts with our previous results for a CMSX4 alloy.

The roles of solidification cooling rate and (Mn,Cr) alloying elements in the modification of β-AlFeSi and hardness evolvements in near-eutectic Al-Si alloys

High recycling rates worldwide underline the economic and environmental value of aluminum scrap. However, several alloying elements in recycled Al negatively affect the final properties of castings, among which Fe has the most permissive effect on the mechanical properties. The extremely restricted solid solubility of Fe in Al leads to the formation of brittle Fe-containing intermetallic compounds (IMCs), and a possible strategy is to use modifiers of harmful IMCs by alloying. The present work aims to investigate the substitutional characteristic of Fe, Mn, Cr in the α-IMC taking the Al-12 %Si alloy as a reference, i.e., by analyzing the directional solidification (DS) of binary (Al-12 %Si), ternary (Al-12 %Si-1 %Fe) and quaternary (Al-12 %Si-1 %Fe-1 %Mn and Al-12 %Si-1 %Fe-0.6 %Cr) alloys. For all DS alloys castings, the macrostructure is shown to be characterized by columnar grains associated with solidification cooling rates (Ṫ) from 0.4 to 42.3 °C/s. The microstructure of the four alloys examined is shown to be typified by an α-Al dendritic matrix with interdendritic regions formed by α-Al, Si and different IMCs, which have been characterized and associated with the local Ṫ. The evolvement of iron-containing IMCs morphologies according to the addition of alloying elements in Al-Si alloys, is as follows: plate-like for Fe, plate-like and Chinese script for Mn, and fishbone and trefoil/blocky for Cr. Experimental power laws equations are determined relating the primary (λ1), secondary (λ2) and tertiary dendritic arm spacings (λ3) to Ṫ, for any experimentally examined DS alloy casting. The microhardness (HV) of the alloys is correlated with λ3 and Hall-Petch type equations are derived relating HV to both λ3 and Ṫ. The Al-12 %Si-1 %Fe-0.6 %Cr alloy achieved the highest HV among all the alloys examined.

A Novel Yield Criterion for Nickel-Based Superalloys

Abstract With the significant evolution of modern gas turbine engines, selection of high-temperature resistant alloys in the hot section is known to be the fundamental solution to enhance the capabilities of these engines. In general, the high-temperature components are mainly comprised of polycrystalline, directionally solidified, and single crystal superalloys. Single crystal (SX) superalloys were developed in the 1980s to achieve high fatigue resistance and substantial creep rupture strength by eliminating grain boundaries. Directional solidification methods enabled the solidification arrangement of the materials to be comprised of columnar grains which are aligned parallel to the 〈001〉 direction. These casting types have been frequently used with nickel-based superalloys (NBSAs) to develop modern gas turbine blades. In this work, the yield behavior of generic SX and directionally solidified (DS) NBSAs is studied. By observing various SX and DS alloys, it was concluded with need for a novel criterion that can present anisotropic and tensile/compressive asymmetric yield surfaces. This novel criterion is comprised of the criterion proposed by Hill for anisotropic materials and the method developed by Drucker and Prager for alloys that have different tensile and compressive yield strengths. Additional terms to Hill's criterion are introduced to capture the coupling effect of normal stress and shear stress when the applied loads are not in the direction of principal axes of the material coordinate system for single crystal alloys. The parameters for the criterion are obtained from simple uniaxial tension and compression experiments. Results are compared with various well-established yield criterions. Additionally, the novel criterion is utilized to capture the effective stress and strain of multi-axial loading of turbine blades under nonisothermal conditions

Investigation of Al-20Sn-10Cu alloy directional solidification by laboratory X-radiography

Al-based alloys with a soft phase such as Sn are extensively used for bearing components due to their self-lubricating properties. Al-Sn alloys lack the ability to support heavy loads so the alloying with Cu as a third element provides solution strengthening of the aluminium matrix. A crucial issue in the manufacturing of Al-Sn-Cu alloys is the miscibility gap in the phase diagram of the system. Liquid immiscibility is responsible for severe segregation during the solidification process, due to the large density difference between the Al-rich and Sn-rich liquids, which limits their utilization in industry. It is therefore both scientifically and technically important to accurately understand their solidification path. In the present work, the solidification of a ternary Al-20wt.%Sn-10wt.%Cu alloys was investigated in-situ by using X-radiography. Directional solidification experiments were performed on sheet-like samples in the laboratory device SFINX (Solidification Furnace with IN-situ X-radiography), which consists of a Bridgman-type gradient furnace and an X-radiography system. The solidification sequence was determined based on the observation of the recorded images, enlightening the successive steps of the solidification path. These observations were compared to predictions obtained from thermodynamic calculations. Complementary post-mortem microscopic analyses showed that the dendrite primary trunk and secondary arms developed along <110> crystallographic axes instead of the usually expected <100>.

Investigation on melt concentration during thermal stabilization before directional growth in a Mg–Li eutectic alloy in presence of element volatilization

The initial melt concentration C for following directional solidification has been confirmed to be controlled by temperature gradient zone melting (TGZM) during thermal stabilization in previous studies. However, the accurate prediction of C has not been reported in the case of element volatilization before. In the present work, the hypoeutectic Mg-7.3Li alloy is selected to investigate the relationship between the melt concentration C and thermal stabilization process in presence of lithium volatilization. The increase of C has been shown to be interrupted when the thermal stabilization time is long, which is distinct from other alloys where no element volatilization exists. The theoretical analysis has been performed to predict C after thermal stabilization, and both the TGZM effect and Li loss caused by lithium volatilization during thermal stabilization are taken into account. The present prediction shows excellent agreement with the experimental result in Mg-7.3Li alloy, confirming the influence of lithium volatilization. Thus, this work can shed light on the prediction of C during directional solidification of alloys in presence of element volatilization.

The effect of wettability on gas porosity formation during directional solidification of alloys: Insights from lattice Boltzmann-cellular automata simulations

The wettability between solid and porosity plays an important role in the growth of dendrites and pore morphology during solidification of alloys. In this original article we use a two-dimensional coupled lattice Boltzmann-cellular automata-finite-difference (LB-CA-FD) model to simulate the effect of wettability on the dendritic and pore morphologies. Wettability can be adjusted by adjusting the fluid-solid interaction coefficient. The predicted results show that the wettability can strongly influence the formation of the dendritic and bubble structures. The solid-gas coupled growth occurs for the poor wettability, while the bubbles are engulfed by the growing dendrite for the good wettability. It can be found that the cooling rate can strongly affect the area ratio of the preexisting bubbles although it has little effect on the morphologies of the bubbles and dendritic structures. Moreover, morphological evolution of multidendritic growth encountering multiple bubbles during directional solidification is also investigated.

Kink-band formation in the directionally-solidified Mg/LPSO two-phase alloys

ABSTRACT The variation in the mechanical properties with the volume fraction of the long-period stacking ordered (LPSO) phase in directionally solidified (DS) Mg/LPSO two-phase alloys was examined. Unexpectedly, the yield stress of the DS alloys increases non-monotonically with an increase in the volume fraction of the LPSO phase. The LPSO phase is considered an effective strengthening phase in Mg alloys, when the stress is applied parallel to the growth direction. Nevertheless, the highest strength was obtained in alloys with 61–86 vol.% of the LPSO phase, which was considerably higher than that in the LPSO single-phase alloy. It was clarified that this complicated variation in the yield stress was generated from the change in the formation stress of kink bands, which varied with the thickness of the LPSO-phase grains. Furthermore, the coexistence of Mg in the LPSO phase alloy induced the homogeneous formation of kink bands in the alloys, leading to the enhancement of the ‘kink-band strengthening’. The results demonstrated that microstructural control is significantly important in Mg/LPSO two-phase alloys, in which both phases exhibit strong plastic anisotropy, to realize the maximum mechanical properties.

Application of phase field model coupled with convective effects in binary alloy directional solidification and roll casting processes

Based on the Kim-Kim-Suzuki (KKS) phase field model coupled with the thermodynamic parameters, the transformation process from columnar dendrites to equiaxed crystals during directional solidification of aluminium alloy was simulated, and the effects of phase field parameters on the growth morphology and dendrite segregation were discussed. Furthermore, considering the effect of the microcosmic flow field, the convection influence gradient term is introduced into KKS formula near the solid-liquid interface, and the phase field model considering flow field was applied to the inherent convective environment of the actual roll casting process, also the multiple dendrites growth behavior of magnesium alloy under the action of microscopic convection was further explored. When coupling calculation of microscopic velocity field and pressure field, the staggered grid method was used to deal with the complex interface. The combined solution of Marker in Cell (MAC) algorithm and phase field discrete calculation was realized. In order to further describe the influence of convection on the solidification process, the roll casting experiments are used to verify the impact growth of multiple dendrites under convection. The results show that the dendrites undergo solute remelting and the dendrites melt into equiaxed crystals, showing the phenomenon of Columnar to Equiaxed Transition (CET).

Multi-phase field simulation of competitive grain growth for directional solidification

The multi-phase field model of grain competitive growth during directional solidification of alloy is established. Solving multi-phase field models for thin interface layer thickness conditions, the grain boundary evolution and grain elimination during the competitive growth of SCN-0.24-wt% camphor model alloy bi-crystals are investigated. The effects of different crystal orientations and pulling velocities on grain boundary microstructure evolution are quantitatively analyzed. The obtained results are shown below. In the competitive growth of convergent bi-crystals, when favorably oriented dendrites are in the same direction as the heat flow and the pulling speed is too large, the orientation angle of the bi-crystal from small to large size is the normal elimination phenomenon of the favorably oriented dendrite, blocking the unfavorably oriented dendrite, and the grain boundary is along the growth direction of the favorably oriented dendrite. When the pulling speed becomes small, the grain boundary shows the anomalous elimination phenomenon of the unfavorably oriented dendrite, eliminating the favorably oriented dendrite. In the process of competitive growth of divergent bi-crystal, when the growth direction of favorably oriented dendrites is the same as the heat flow direction and the orientation angle of unfavorably oriented grains is small, the frequency of new spindles of favorably oriented grains is significantly higher than that of unfavorably oriented grains, and as the orientation angle of unfavorably oriented dendrites becomes larger, the unfavorably oriented grains are more likely to have stable secondary dendritic arms, which in turn develop new primary dendritic arms to occupy the liquid phase grain boundary space, but the grain boundary direction is still parallel to favorably oriented dendrites. In addition, the tertiary dendritic arms on the developed secondary dendritic arms may also be blocked by the surrounding lateral branches from further developing into nascent main axes, this blocking of the tertiary dendritic arms has a random nature, which can have aninfluence on the generation of nascent primary main axes in the grain boundaries.

Evolution of Unidirectional Solidification Microstructure and Hydrogenated Treatment of Nb-Ti-Co Quasiperitectic Alloys

Titanium alloys have a wide range of applications, and the internal placement of hydrogen into them can modulate the microstructure of the alloys and thus have great potential for further development. However, few studies have been reported on the application of this technique to Nb-Ti-Co ternary alloys, which needs to be urgently investigated. In this paper, four types of alloys (Nb10Ti61Co29, Nb15Ti55Co30, Nb20Ti50Co30, and Nb25Ti50Co25) are selected near the eutectic point of the phase diagram to study their placement of hydrogen by both static and dynamic processes of hydrogen’s placements, focusing on the effects of the temperature, time, and hydrogen-flow rate of such processes on the amount of hydrogen placements. The relationship between the hydrogen replacement parameters and the mechanical properties of the alloys is constructed. The results show that the placed-hydrogen amount of Nb-Ti-Co as-cast alloy grows with the increase of hydrogen-flow rate and soaking (or holding) time, with an upper limit of the placed-hydrogen amount, and the pattern of the directionally- solidified alloys is similar to that of the as-cast alloys; however, at a certain soaking time and hydrogen- flow rate, although the placed hydrogen amount of both alloys rises with the increase of temperature, the placed-hydrogen amount of Nb-Ti-Co directionally-solidified alloys is always larger than that of the as-cast alloys. However, the amount of hydrogen placement in the Nb-Ti-Co directionally-solidified alloys is always larger than that in the as-cast alloys, and the amount of hydrogen placement decreases significantly as the growth rate of the alloys increases. In addition, the microhardness decreases with increasing growth rate in the directionally-solidified specimens, and the amount of hydrogen placement and microhardness increase with growing Nb content.

Spurious grain formation due to Marangoni convection during directional solidification of alloys in µ-g environment of International Space Station

During directional solidification of alloys in the microgravity environment of the International Space Station (ISS), growth of dendritic array is expected to be under purely diffusive transport conditions. The resulting microstructure is expected to make up uniformly arranged primary dendrites on sample cross-sections without any defects, such as macrosegregation and spurious (misoriented) grains. However, spurious grains have been seen in recently directionally solidified Al-7 wt% Si samples under the joint NASA-ESA project (MICAST) and also in SCN-0.24 wt% H2O samples grown under the NASA-PFMI project. Careful examination of both these experiments shows the presence of voids at the melt-crucible interface which are the likely source of the convective flows responsible for the fragmentation of dendrite-side arms, and their rotation which creates the nucleus for the spurious grains. In this paper, we assume that side-arm of a primary dendrite has gotten detached from the primary trunk during remelting and isothermal hold, prior to the onset of directional solidification on the Space Station, in the vicinity of a pore at the melt-crucible interface. We numerically simulate the influence of the Marangoni convection on the trajectory of such a dendrite fragment in the alloy melt and compare with the experimental observations. The experimentally observed rotation behavior of the fragmented side-arm in transparent SCN-0.24 wt% H2O, observed from the PFMI video, shows a good agreement with simulation results. The predicted rotational speeds of broken-off secondary arm in the Al-7 wt% Si (MICAST) samples is significantly higher than those in the SCN-0.24 wt% H2O (PFMI). The strength of the convection is dependent on the Marangoni Number and location of the surface pore relative to the Mushy zone. Marangoni convection is strong enough to force the rotation of broken-off side arms and should be considered an important source of microstructural inhomogeneity during terrestrial solidification processing applications.

Optimizing microstructure and mechanical properties of directionally solidified Ti44Al6Nb1Cr2V alloy by directional heat treatment

To optimize microstructure and improve mechanical properties of directionally solidified (DS) TiAl alloys containing high alloying elements, DS Ti44Al6Nb1Cr2V alloy was conducted directional heat treatment (DHT) at 1730 K. Results indicate that columnar grains grow up straightly after DHT. It decreases from 17.55° to 10.67° for the angle in the axial direction between columnar grains and ingot. Morphology of B2 phases changes obviously, from reticular to banded microstructure. Lamellar clusters grow up greatly and are parallel to the growth direction of columnar grains. These changes are mainly caused by sufficient atoms diffusion during DHT. α grains grow up adequately and residual B2 phases reduce. Compared with that of DS Ti44Al6Nb1Cr2V alloy (505 MPa and 1.40%), ultimate tensile strength and total strain are 609 MPa and 1.86% for DHT alloy, respectively. They are increased by 21% and 33%, respectively. Fracture shows that many tearing edges and some similar dimples morphology are also found in DHT alloy, besides cleavage planes and cleavage steps. TEM image shows that many dislocations are produced in lamellar γ phases of DS alloy. They wind and intercross each other. Dislocation interaction is lower and the stress concentration is less for DHT alloy, due to thin lamellar γ phases. Furthermore, some stacking faults are also produced in lamellar γ phases, which benefits to improving mechanical properties. Therefore, mechanical properties are improved greatly for DS alloy after DHT. • The angle between growth direction of columnar grains and axial direction of ingot decreases from 17.55° to 10.67°. • Lamellar clusters grow up obviously and distribute along the growth direction of columnar grains after DHT. • Morphology of B2 phases changes from reticular to banded structure and its content reduces after DHT. • The ultimate tensile strength and total strain are 609 MPa and 1.86% for DHT alloy, respectively.

Analysis of gravity effects during binary alloy directional solidification by comparison of microgravity and Earth experiments with in situ observation.

Under terrestrial conditions, solidification processes are influenced to a large degree by the gravity effects such as natural convection or buoyancy force, which can dramatically modify the final characteristics of the grown solid. In the last decades, the coupling of in situ observation of growth from the melt, that enables the study of microstructure formation dynamics, and microgravity experimentation, that allows to approach diffusive conditions, has been implemented for both transparent and metallic materials. The results of these investigations enable to test the validity of advanced solidification theories, to validate or develop numerical models and sometimes to reveal unexpected phenomena. The aim of this paper is to present a selection of conclusive experiments obtained with this combined approach in our group to highlight the gravity effects by a comparative study of experiments carried out on earth and in microgravity conditions.

Effect of lamellae orientation on tensile properties of directionally solidified Ti–46Al–0.5W–0.5Si alloy

For full-lamellar TiAl alloys, mechanical properties can be improved in a wide range of temperature by controlling lamellar orientation parallel to growth direction. However, lamellar controlled TiAl alloys with good properties and large scale are difficult to obtain due to sever interface reaction between TiAl melts and mould materials. In this paper, effect of lamellar orientation on tensile properties of directionally solidified (DS) Ti-46Al-0.5W-0.5Si alloy using oxide ceramics crucible was investigated. DS alloys with parallel lamellar structure had better tensile strength (479MPa) and elongation (0.76%) than that of specimen with equiaxed grains (428MPa, 0.22%). Fracture modes of DS alloys were interlamellar, translamellar mode and both, which depended on the lamellae orientation.

Study on improving microstructure and mechanical properties of directionally solidified Ti44Al6Nb1Cr alloy by cyclic DHT

Abstract To improve microstructure of high-Nb TiAl alloys after directional solidification (DS) further, DS Ti44Al6Nb1Cr alloy was conducted cyclic directional heat treatment (DHT) for 1 and 4 times under pulling rate of 4.17 μm/s at 1750 K, respectively. Results indicate that columnar grains grow up obviously after DHT. Compared with DS alloy (2.56 mm), the maximum width of columnar grains is up to about 9.07 mm for DHT alloy after 4 times (DHT-4). The SEM images and EDS results indicate that phase component of DS alloy has not changed after cyclic DHT for different times. It is still composed of α2, γ and B2 phases. As cyclic DHT times increase, thickness of lamellar phases increases gradually. More massive γ phases precipitate from B2 phases and grow up. The content of B2 phases reduces due to atoms diffusing sufficiently during cyclic DHT. Tensile curves show optimal mechanical properties are obtained in DHT alloy for 1 time (DHT-1). The ultimate tensile strength is 636 MPa and total strain is 2.28%. Compared with that of DS alloy (523 MPa and 3.03%), tensile strength is improved greatly. As cyclic times increase, mechanical properties decrease for DHT alloy, due to massive γ phases coarsening and B2 phases distributing discontinuously. This microstructure could not hinder cracks propagation effectively, which may be the main reasons to result in premature fracture for DHT-4 alloy. At 973 K, the better mechanical properties are still achieved in DHT-1 alloy. The ultimate tensile strength is up to 695 MPa and total strain is 6.21%. The mechanical properties improvement is mainly resulted from directional microstructure, especially the directional lamellar clusters and B2 phases. In addition, dislocations, stacking fault bands and deformation twins are observed in lamellar γ phases of DHT-1 alloy. These microstructure changes play an important role in improving compressive mechanical properties of DHT alloy.

Study on improving directional microstructure of Ti44Al6Nb1Cr alloy by continuous regional phase transformation

To improve the directionally solidified (DS) microstructure and mechanical properties of high-Nb containing TiAl alloys, DS Ti44Al6Nb1Cr alloy was directionally heat treated (DHT) with pulling velocity of 4.17 µm/s at 1750 K. Microstructure and grains orientation were characterized by the OM, SEM, TEM and EBSD. Chemical composition and phase component were detected by the EDS and XRD. Results indicate that wide columnar grains are formed along the axial direction after DHT. Microstructure is still fully lamellar and composed of α2, γ and B2 phases. Crystallographic orientation relationship among phases (α2, γ and B2) is not changed. Lamellar clusters in columnar grains change from blocky to columnar, and the reticular B2 phases become banded structure after DHT. During DHT, the β phase transforms into α phase continuously and α grains grow into columnar clusters. Nb and Cr atoms can diffuse more adequately under the influence of directional heat flow and segregate at boundaries of columnar clusters. Therefore, lamellar clusters and B2 phases are distributed almost in parallel along the axial direction of columnar grains at room temperature. Tensile testing shows that the yield strength, ultimate strength and total strain of the DHT alloy are increased to 574 MPa, 636 MPa and 2.28% from 422 MPa, 523 MPa and 3.03% of as DS alloy, respectively. The tensile strength increases about 36% and this improvement is mainly caused by the directionally aligned B2 phases. In addition, a large number of dislocations and stacking fault bands are formed in lamellar γ phases of DHT alloy after testing, which also plays an important role in improving mechanical properties of DS alloy.