Effects of solidification rate on segregation and fluid flow tendency in mushy zone of directionally solidified superalloy Inconel 718

Abstract

The microstructure and composition of the interdendritic liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates between 2 and 100 μm s^?1 have been investigated by SEM and EDAX techniques. The interdendritic liquid segregation profiles along the mushy zone are presented. The liquid density difference and Rayleigh number in the interdendritic liquid were calculated and analysed as well. It was found that when the solidification rates increased in the range 10–70 μm s^?1, segregation of Nb decreased, but segregation of Mo was most serious at 20 μm s^?1. The liquid density difference increased the most for rates from 20 to 40 μm s^?1 as temperature decreased. The maximum relative Rayleigh number was highest at 10°C below the liquidus temperature at 20 μm s^?1, which indicated the conditions where fluid flow most easily occurred for Inconel 718. The relative Rayleigh number synthetically considers the factors affecting fluid flow and can give a reasonable prediction for fluid flow tendency.     

静磁场对定向凝固镍基高温合金组织影响的研究进展

综述了国内外静磁场对镍基高温合金组织影响的研究现状,重点分析了施加不同方式、强度的静磁场对定向凝固镍基高温合金枝晶组织、元素偏析、凝固缺陷及高温力学性能的影响规律,并从变截面处杂晶的控制、晶体取向偏离的控制以及对凝固特性的影响机制等方面提出了静磁场在定向凝固镍基高温合金研究中潜在的发展方向。     

A band microstructure in directionally solidified hypo-peritectic Ti-45Al alloy

We report the first experimental observation of discrete bands in directionally solidified hypo-peritectic Ti-45Al alloy. The banded structure consists of two phases and the lamellar orientation within these phases is different. Lamellar boundaries within one phase inclined at an angle of 45° to the growth direction and within the other phase aligned parallel to the growth direction. A number of models were proposed to explain the band formation in TiAl alloys. It was found that the phase precipitation in hypo-peritectic TiAl alloys was influenced by the solidification condition and the growth morphologies between primary and peritectic phases.     

Effect of microstructure on crack propagation in high-temperature fatigue of directionally solidified Ni-based superalloy

In order to clarify the crack propagation properties of an anisotropic material (Ni‐based directionally solidified superalloy), longitudinally loaded specimens (L‐specimens) and transversely loaded specimens (T‐specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T‐specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three transgranular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.     

Influence of a weak static magnetic field on the primary dendrite arm spacing of a directionally solidified Ni-based superalloy

Magnetic field applied in the solidification can influence the primary dendrite arm spacing (PDAS) of alloys. The effect of a weak static magnetic field on the PDAS of Ni-based superalloy DZ417G with different dimension is studied in this paper. The results show that the PDAS for directionally solidified superalloy DZ417G under the weak static magnetic field is significantly decreased. With increasing the intensity of magnetic field from 0 to 0.05 T, the PDAS is decreased from 0.3 mm to 0.23 mm and from 0.61 mm to 0.48 mm for the ingots with 4 mm and 10 mm in the diameter, respectively. When the intensity of magnetic field is larger than 0.05 T, the PDAS is slightly increased. The influence of the interactive competition between the magnetic damping and thermoelectromagnetic convection caused by magnetic field on the PDAS for superalloy DZ417G is discussed.     

Solute redistribution and segregation in solidification processes

In this review paper, research on solute redistribution coefficients for equilibrium, near equilibrium and non-equilibrium solidification processes were first analyzed. Then, different models advanced since the 1950s for solute redistribution during the directional solidification process with a planar interface were summarized. The so-called second-order opposite diffusion compensation method (SODCM) presented by the author was described in detail. Thirdly, solute redistribution in the dendritic solidiication process and the segregation were discussed. It is concluded that the problem can be described resulting Ψ_S –W_L functions. Among the different models, that obtained by the present author in 1994 was explained. Fourthly, the formation o macro–segregation mainly caused by liquid flow in the mushy zone is analyzed. A parameter about the driving force for the liquid flow is described.The parameter is considered to be the controlling parameter for the formation of macro-segregation. The relationship between the parameter and the compositions in steel is also given in the paper as an example.     

Influence of Ti content on microstructure,mechanical properties and castability of directionally solidified superalloy DZ125L

Directionally solidified DZ125L alloys of various Ti content were developed by low segregation technology. The microstructure, mechanical properties and castability of directionally solidified superalloy DZ125L were investigated. With decreasing Ti content in DZ125L alloy, the size, volume fraction and separation temperature of γ′ decreased. While the tensile strength and rupture life of directional solidified cylindrical samples is also obviously decreased. Lowering Ti content in alloy resulted in free hot cracking casting blades. Compared with IN792 alloy, DZ125L alloy needs much lower Ti/Ta ratio to achieve good castability.     

Laser arc hybrid weld microstructure in nickel based IN738 superalloy

Abstract

A study of the microstructural characteristics of laser arc hybrid welded nickel based IN738 superalloy was performed. Laser arc hybrid welding produced a desirable weld profile in the alloy, similar to what is usually obtained during laser beam welding, and no cracking occurred exclusively in the fusion zone. Elemental partitioning pattern in the fusion zone was studied by electron probe microanalysis and calculating the volume fraction of the weld metal that resulted from the consumption of the filler wire. The result showed that Ti, Ta, Nb, Mo, Al and Zr partitioned into the interdendritic regions of the fusion zone. SEM and TEM examination of the fusion zone showed the presence of secondary solidification reaction constituents, which consists of MC type carbides. The study further revealed that non-equilibrium liquation of various second phases that were present in the alloy prior to welding contributed to intergranular liquation in the heat affected zone (HAZ), which consequently resulted in extensive HAZ intergranular cracking during welding. Although laser arc hybrid welding appears promising for improving the weldability of nickel based IN738 superalloy, a suitable weldability improvement procedure is required in order to minimise HAZ intergranular cracking and thereby enhance the applicability of this technology to the joining of the superalloy.     

Effect of boron and zirconium on directional solidification behaviour and segregation of DS IN738 superalloy

The effect of boron and zirconium on the directional solidification behaviour and segregation of IN738 superalloy has been studied. It was found that additions of boron and zirconium enlarge the solidus-liquidus temperature interval and increase the amount of residual melt during solidification. Boron gives the alloy a tendency to form developed dendrites, while zirconium enhances a cellular solidified structure. Both boron and zirconium are shown to be rejected to the residual melt during solidification, and shown to change the segregation of alloying elements by interaction.     

定向凝固镍基合金DZ444声学特性的各向异性

选取定向凝固镍基高温合金DZ444不同方向片状试样,利用电子背散射衍射等技术表征晶体取向和微观组织,利用脉冲回波技术分析纵波声速和声衰减系数。结果表明:两声学特性呈各向异性,随着试样平面法向与凝固方向之间夹角φ由0°到45°再到90°,纵波声速由5533m/s增大到6595m/s后又降至5634m/s,而声衰减系数逐渐增大,变化约0.19dB/mm;对信号频谱分析发现,表面回波与一次底波的主频差值、主频幅值差值及表观积分反射系数均逐渐增大,这主要是由微观组织和晶体取向差异造成的。     

Microsegregation in directionally solidified dendritic-cellular structure of superalloy CMSX-4

A directionally solidified sample of superalloy CMSX-4 was investigated to show the effect of crystal orientation on the segregation distribution. The solute distribution of alloying elements across a dendritic cell was measured. Due to the preferred crystal growth in <100> orientation the segregation profiles in this direction is much flatter than that in <110> orientation.     

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

The creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.     

Low cycle fatigue behavior of a directionally solidified cobalt base superalloy

An investigation has been made of the low cycle fatigue behavior and microstructure evolution of a directionally solidified cobalt-base superalloy at room temperature, 700 and 850°C under the control of different total strain amplitudes. The results show that at room temperature the cyclic hardening of the alloy appears during the first few cycles, and then a long saturation stage begins. At 700°C, the alloy exhibits a pronounced initial hardening, and a secondary hardening after a short saturation. At 850°C, the alloy shows a continuous cyclic hardening until fracture. Examination by TEM indicates that the initial hardening of the alloy at room temperature is caused by the pile-ups of the stacking faults at the stacking fault intersections, while the saturation is related to the formation of the hexagonal close-packed (HCP) zones and twins. The mechanism of initial hardening at 700°C is similar to that at room temperature, while the stress saturation is due to interaction obstacle to stacking-fault becoming weaker, because of thermal activation. The secondary hardening is attributed to the formation of sessile dislocation tangles. The continuous cyclic hardening at 850°C is related to the interaction between the precipitates (M₂₃C₆)and dislocations.     

Effect of pre-exposure on crack initiation life of a directionally solidified Ni-base superalloy

Blades and vanes are just two of several industrial gas turbine (IGT) components often subjected to long periods of elevated temperature before, during, and after high stress operating conditions. In these systems, cyclic loading is induced by repeated start-ups, firings, and shut-down ramps. Combinations of complex thermal and mechanical service conditions in the presence of aggressive reactants facilitate crack initiation via oxide spike formation. In the current study, the effect of pre-exposure on the oxide spiking damage mechanism and crack initiation life is characterized for a representative directionally solidified (DS) Ni-base superalloy, e.g. DS GTD-111. Comparisons of unexposed and pre-exposed samples reveal that 100 h of either creep pre-strain and/or thermal pre-exposure strongly influences the dominant damage mechanism that leads to crack initiation under subsequent fatigue cycling. A mechanistic model for crack initiation is modified to capture the influence of pre-exposure on life.     

Influence of dwell times on the thermomechanical fatigue behavior of a directionally solidified Ni-base superalloy

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T = 100–750 °C and optional dwells of 20 min at 750 °C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {1 1 1}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.     

Effect of notch on fatigue behaviour of a directionally solidified superalloy at high temperature

The effect of notch types and stress concentration factors (K_t) on low cycle fatigue life and cracking of the DZ125 directionally solidified superalloy has been experimentally investigated. Single‐edge notched specimens with V and U type geometries were tested at 850 °C with stress ratio R = 0.1. High temperature in situ optical method was used to observe crack initiation and short crack propagation. Scanning electron microscope observation of fracture was used to analyse the failure mechanism. The results reveal that fatigue resistance decreases with K_t increasing from 1.76 to 4.35. The ratcheting is found to be affected by both K_t and the nominal stress from the displacement–force curve. In situ observations indicate that the cracking does not occur at the notch apex but at the location where the max principal stress or Hill's stress is the highest. According to the scanning electron microscope observations, the failure of the notched specimens strongly depends on the anisotropy microstructures.     

Influence of high-temperature air pre-exposure on mechanical strength of a directionally solidified cobalt-base superalloy

The present work investigated the influence of air pre-exposure at 850°C for 1000 h on the mechanical properties and microstructures of the directionally solidified cobalt-base alloy DZ40M. The results show that the air pre-exposure did not embrittle the alloy, but resulted in a marked strengthening. Microstructural observations and energy dispersive X-ray spectroscopy (EDS) indicates that the air pre-exposure caused a substantial external and internal oxidation, which is assumed to protect the alloy from being oxidized further and make a significant contribution to alloy strengthening, respectively. The immunity of the alloy to air embrittlement is attributed to elimination of transverse grain boundaries by directional solidification.     

低Re含量第二代柱晶高温合金DZ59的蠕变断裂性能

采用液态金属冷却(LMC)工艺和成分优化设计,制备出一种高温性能优异的低Re含量第二代柱晶高温合金DZ59,通过扫描电镜(SEM),透射电镜(TEM)等方法对其组织特征及蠕变断裂进行了表征,研究了合金的蠕变断裂性能.结果表明,DZ59合金的高温蠕变断裂性能超过一代单晶合金,并接近二代单晶合金的水平.发现柱晶高温合金的二次晶界反应(SGRZ)现象,表明SGRZ受温度和应力的控制,由于增加了合金横向晶界的受力界面,在高负荷下可能成为蠕变空洞萌生和扩展的位置.     

The characteristics of serrated flow in superalloy IN738LC

Serrated flow was investigated in superalloy IN738LC, a nickel-base γ′ age-hardened alloy. In this material serrated flow appeared between 350 and 450 °C and strain rate of (8.77 × 10⁻⁵ to 8.77 × 10⁻³) s⁻¹. Activation energy for this process was calculated to be 0.69–0.86 eV which is in good agreement with the values reported for similar alloys. Results show that the diffusion rate of substitutional solute atoms at this temperature range is too low to cause this effect. This suggests that the interaction of solute atoms and moving dislocation is responsible for the observed serrated flow in this alloy.     

The effect of ternary addition of silicon on the structure and microstructure of rapidly solidified equiatomic TiNi alloys

The influence of a small amount of silicon (up to 6 at %) on the structure and microstructure of rapidly solidified equiatomic TiNi alloys is reported. It is shown that a small amount of silicon suppresses the B2 TiNi formation and promotes a competitive nucleation of Ti2Ni phase. Further additions of silicon promote the formation of glass. The behaviour of the ternary replacement of both titanium and nickel by silicon has been studied. It is shown that replacement of titanium increases the tendency for the formation of silicide phases while the latter is absent when silicon replaces the nickel atoms. This revised version was published online in November 2006 with corrections to the Cover Date.