Showing papers by "Hengzhi Fu published in 2014"

Microstructure control and mechanical properties of Ti44Al6Nb1.0Cr2.0V alloy by cold crucible directional solidification

Abstract: The as-cast Ti44Al6Nb1.0Cr2.0V alloy master ingot was prepared by vacuum consumable melting technology. Some bars were cut from this ingot and they were directionally solidified by cold crucible under different pulling velocities. The samples could be well directionally solidified when the power (P) was 45 kW and the pulling velocity (V) was 8.33 μm/s or 11.67 μm/s. The results show that the interlamellar space of the directionally solidified samples decreases from the average 1650 nm of as-cast to less than 565 nm and is more homogeneous. The microcrack in the master ingot can be eliminated completely and the room temperature (RT) tensile property is also improved after cold crucible directional solidification (CCDS). The ultimate tensile strength (UTS) is 602.5 MPa and the elongation is 1.20% as P=45 kW and V=11.67 μm/s, compared with as-cast 499 MPa of UTS and 0.53% of elongation. Trans-granular and trans-lamella fractures are predominant modes. The relationship between CCDS interlamellar space (d) and the pulling velocity can be described as d = 1783.2 V − 0.554 and r 1 2 = 0.972 , where r 1 2 is the corresponding regression coefficient. The CCDS interlamellar space and nanoindentation hardness (HN) in the lamella region can be described as H N = 17.95 d − 0.145 and r 2 2 = 0.986 , and they are changed as H N ′ = 14.03 d ′ − 0.104 and r 3 2 = 0.975 when the cast condition is considered. The nanoindentation hardness of the B2 phase and the block γ phase are about 8.89 GPa and 8.15 GPa, respectively; both of them keep almost the same in different conditions.

In situ fabrication of highly-dense Al2O3/YAG nanoeutectic composite ceramics by a modified laser surface processing

Abstract: Alumina-matrix eutectic in situ composite ceramics present excellent high-temperature mechanical properties, which have been considered as promising next-generation ultra-high temperature structural materials. A modified laser surface processing is developed to in situ fabricate highly-dense Al2O3/YAG bulk nanoeutectic ceramics with large size and homogeneous three-dimensional network of nanoeutectic microstructure by introducing two-side remelting and high-temperature preheating. The crack and porosity are avoided, and the eutectic structure achieves a good continuous growth between two solidified layers. The eutectic phases show sharp interface bonding with a defined orientation relationship. The dislocations and crack deflection at high-density phase interfaces importantly contribute to the enhanced fracture toughness.

Microstructure and mechanical property of directionally solidified NiAl–Cr(Mo)–(Hf, Dy) alloy at different withdrawal rates

Abstract: The microstructure and mechanical property of directionally solidified Ni–30.95Al–32Cr–6Mo–0.05Hf (at%)–0.1Dy (wt%) alloy were investigated by means of OM, SEM, TEM, EPMA, three-point bending tests and high-temperature tensile tests. With the withdrawal rate increasing from 6 to 120 μm/s, the microstructure changes from planar eutectic to cellular or dendritic eutectic, and both the interlamellar spacing and eutectic cell size decrease gradually. The relation between interlamellar spacing and withdrawal rate can be obtained as λ ¯ = 4.55 V − 0.40 . The corresponding room temperature fracture toughness first increases and then decreases, which should be attributed to the microstructural evolution at different withdrawal rates. Moreover, some toughness mechanisms are responsible for the improvement of fracture toughness, such as crack bridging, interface debonding and microcrack linkage. The high-temperature tensile strength is low at the planar growth rate (10 μm/s), and it increases gradually with the increase of withdrawal rate. The strengthening mechanism is also discussed.

Faceted-nonfaceted growth transition and 3-D morphological evolution of primary Al6Mn microcrystals in directionally solidified Al-3 at.% Mn alloy

Abstract: A comprehensive understanding of the growth pattern of intermetallic compounds (IMCs) during solidification is critical to both the crystal-growth theory and its property optimization. In this article, growth pattern and three-dimensional (3D) morphology of primary Al6Mn IMC were investigated in directionally solidified Al-3 at.% Mn alloy at a wide range of growth rates. A transition from faceted ( 100 µm/s) was observed with increasing growth rates. Correspondingly, 3D morphologies of primary Al6Mn change from a solid polyhedron to a hollow structure, and then to a dendrite. This kind of change is associated with the competitive growths of different crystal planes determined by the crystallographic anisotropy and growth kinetics of Al6Mn. A growth model based on atomic cluster attachment is proposed to reveal the growth transition, and a growth-rate ratio between different crystal planes is used to appropriately reveal the formation mechanism of different morphologies at low rates.

Effect of microstructures on the room temperature fracture toughness of NiAl–32Cr–6Mo hypereutectic alloy directionally solidified at different withdrawal rates

Abstract: The effect of microstructures on the room temperature fracture toughness of NiAl–32Cr–6Mo (at%) hypereutectic alloy was investigated. The solidification microstructure changed from planar eutectic to cellular eutectic and dendritic eutectic with increasing withdrawal rate. The fracture toughness of alloy with planar eutectic microstructure solidified at 10 μm s−1 was 23.74 MPa m1/2, and then it dropped to 15.35 MPa m1/2 when the alloy solidified at rate of 15 μm s−1. But higher fracture toughness of 22.92 MPa m1/2 was obtained when the alloy solidified at 25 μm s−1 and had a perfect cellular microstructure. In this case, the production efficiency can be markedly improved. The fracture surfaces of the NiAl–32Cr–6Mo hypereutectic alloy showed quasi-cleavage fracture mode, some cleavage steps and tearing ridges were observed. The alloys with cellular and dendritic eutectic microstructures exhibited transcellular fracture morphologies; the conically shaped ridges or valleys were observed in each eutectic cell. Owing to the perfect cellular microstructure, the bonding strength of the cell boundary was high; the eutectic cells could largely deform consistently and offer higher resistance of the crack propagation.

Formation of stray grains during directional solidification of a superalloy AM3

Abstract: Stray grains were found during the preparation of single crystal superalloy AM3 by seeding technique and the researching on the competitive growth of bi-crystal. Stray grains were mainly observed at the diverging 〈001〉 corner of the mold wall. Therefore, increasing orientation deviation angle would intensify the possibility of the formation of stray grains. This was because the solute was inclined to enrich at the diverging 〈001〉 corner of mold wall, leading to the relatively large undercooling, and accordingly resulted in the formation of stray grains.

Lamellar orientation control of directionally solidified Ti–46Al–0·5W–0·5Si alloy by self-seeding technology

Abstract: Lamellar structures of Ti–46Al–0·5W–0·5Si (at-%) alloy solidifying through the primary α phase were successfully aligned parallel to the growth direction using a self-seeding technology in a Bridgman type directional solidification furnace. At the growth rate of 20 μm s−1 and temperature gradient of 12·1 K mm−1, the α phase grows along the direction. Therefore, the parallel lamellar structures, whose lamellar orientation is direction, can grow continuously until the end of solidification under these solidification parameters. The original lamellar structures of the unmelted region parallel to the growth direction and suitable solidification parameters are necessary for TiAl alloys solidifying through the primary α phase to control the lamellar orientation. The self-seeding technology gets rid of the cutting and fixing of seeds and simplifies the processing of controlling the lamellar orientation of TiAl alloys. It can promote the engineering applications of the lamellar orientation control of TiAl...

Determining the Effects of Growth Velocity on Microstructure and Mechanical Properties of Ti-47Al Alloy using Electromagnetic Confinement and Directional Solidification

Abstract: Directionally solidified Ti-47Al samples without contamination were obtained by the electromagnetic confinement and directional solidification technique. With increasing growth velocity, the solid/liquid interface was changed from cellular to dendritic. The grain boundary turned vague, and the lamellae within the grains became distorted. When the growth velocity increased to 200 μm/s, some γ phases and α phases appeared in local region owing to the enrichment of Al solute. The columnar grain size (λ), interdendritic spacing (λ 1), and interlamellar spacing (λ L) were measured, and the effects of growth velocity on these parameters were discussed. As the growth velocity increased, both the microhardness and the 0.2% offset compression yield stress increased continuously. However, the value of yield stress decreased slightly when the growth velocity increased to 200 μm/s, which resulted from the appearance of γ phases.

Local melting/solidification during peritectic solidification in a steep temperature gradient: analysis of a directionally solidified Al–25at%Ni

Abstract: Melting of primary Al3Ni2 phase and solidification of Al3Ni peritectic phase during directional solidification of an Al–25at%Ni peritectic alloy have been investigated. In a steep temperature gradient of up to 50 K/mm and at a pulling rate of 20 μm/s, an incomplete coverage of peritectic Al3Ni phase on the surface of the primary Al3Ni2 phase has been observed. Below the peritectic temperature in the presence of the incomplete coverage, melting of primary Al3Ni2 on the one side and solidification to the Al3Ni peritectic phase on the other side proceed swiftly via diffusion through the interphase liquid layer. Theoretical calculations based on an incomplete-coverage-related melting/solidification model are in close agreement with the experimental measurements.

Effect of solidification rate on microstructures and orientations of Al‐Cu hypereutectic alloy in thin crucible

Abstract: The microstructures and orientations of Al-40%Cu hypereutectic alloy prepared in a 0.45 mm high-purity alumina crucible at different directional solidification rates (5, 10 and 100 μm/s) were investigated. With solidification rate increasing, the primary phase Al2Cu of the alloy transits from complex dendrite morphologies to regular faceted square shapes. The surfaces of regular square morphology about Al2Cu phase are {110} facets. The orientation text results by XRD showed that at a growth rate of 5 μm/s, the phase displayed texture orientations at (310), (110), and (220). At 100 μm/s, the phase possessed a very strong preferred orientation at (310). The experimental results showed that growth in small-size samples at a higher growth rate can achieve a regular microstructure with preferred orientation. The growth morphologies are deduced from attachment energies descripted the microstructure evolution of the primary phase.

Formation mechanism of banded structure during directional solidification of Sn–Cd peritectic alloy under convection condition

Abstract: Directional solidification experiments of Sn-0.75 wt%Cd and Sn-1.6 wt%Cd peritectic alloys have been conducted under convection condition to investigate the formation mechanism of banded structure. Many types of banded structure were obtained, which cannot be interpreted by the Karma’s model. The reason for this conflict is that there are many banded structure formation mechanisms such as abundant nucleation, regrowth, fast radial cellular growth and radial competitive growth under convection condition, but the Karma’s model only considers the abundant nucleation and ignores other mechanisms. The analyses showed that these formation mechanisms changed along with an increase in alloy composition. Based on these analyses, a simple modified banding window, which considered these different formation mechanisms, has been presented. Compared with the banding window defined by the Karma’s model, this modified banding window contained it and could predict different banded structure formations under convection condition appropriately.

Effect of boron doping on microstructure of directionally solidified Ti-46Al-2Cr-2Nb alloy

Abstract: In this paper, Ti–46Al–2Cr–2Nb and Ti–46Al–2Cr–2Nb–0·2B alloys were directionally solidified under different cooling rates. Based on the comparison, the effects of minor B doping on the primary dendrite arm and lamellar spacing were investigated. TiB was formed during solidification before the formation of the lamellar structure. The primary dendrite arm spacing was slightly reduced by TiB when the α phase was solidified as the primary phase. However, the lamellar spacing significantly increased in the Ti–46Al–2Cr–2Nb–0·2B alloy because TiB reduced the undercooling temperature required for the formation of the γ phase.

Effects of Withdrawal Rate and Temperature Gradient on the Microstructure Evolution in Directionally Solidified NiAl-36Cr-6Mo Hypereutectic Alloy

Abstract: The effects of withdrawal rate and temperature gradient on the microstructure and growth interface morphology in directionally solidified Ni-29Al-36Cr-6Mo(at.%) hypereutectic alloy were investigated. Under the temperature gradient of 250 K/cm, well-aligned eutectic microstructure with lamellar morphology was obtained at the withdrawal rate of 6 μm/s. When the withdrawal rate was 10 μm/s, the microstructure changed to Cr(Mo) dendrites + eutectic lamellae. With the increasing withdrawal rate, the interdendritic eutectic growth interface changed from planar to cellular, the number of primary Cr(Mo) dendrites became greater, and the microstructure was refined. When the temperature gradient increased to 600 K/cm, the coupled eutectic growth zone of NiAl-Cr(Mo) alloy was expanded; a well-aligned eutectic microstructure could be obtained at higher rate of 10 μm/s. Furthermore, the planar/cellular transition rate of the interdendritic eutectic growth interface increased. Even at the same withdrawal rate, the number of primary Cr(Mo) dendrites was less and the microstructure was finer under the temperature gradient of 600 K/cm.

Phase and Microstructure Selection During Directional Solidification of Peritectic Alloy Under Convection Condition

Abstract: A phase and microstructure selection map used for peritectic alloy directionally solidified under convection condition was presented, which is based on the nucleation, constitutional undercooling criterion (NCU criterion), and the highest interface temperature criterion. This selection map shows the relationships between the phase/microstructure, the G/V ratio (G is the temperature gradient, V is the growth velocity), and the alloy composition under different convection intensities and nucleation undercoolings. Comparing with the results from directional solidification experiments of Sn–Cd peritectic alloys, this selection map was generally in agreement with the experimental results.