Effect of carbon and boron additions on segregation behavior of directionally solidified nickel-base superalloys with rhenium

The phase transformation temperature, segregation behavior of elements and as-cast microstructure were investigated in experimental nickel-base superalloys with different levels of carbon and boron. The results show that the liquidus temperature decreases gradually but the carbide solvus temperature increases obviously with increasing carbon addition. Minor boron addition to the alloy decreases the liquidus temperature, carbide solvus temperature and solidus temperature slightly. Apart from rhenium, the segregation coefficients of the elements alter insignificantly with the addition of carbon. The segregation behavior of rhenium, tungsten and tantalum become more severe with boron addition. The volume fraction and size of primary carbides increase with increasing carbon addition. The main morphology of the carbides is script-like in the alloys with carbon addition while the carbide sheets tend to be concentrated and coarse in the boron-containing alloys

Effect of hot extrusion and heat treatment on microstructure of nickel-base superalloy

Effects of hot extrusion (HEX) and heat treatment on prior particle boundary (PPB), MC carbides,γ′precipitates and grain size of nickel-base FGH96 superalloy were studied. The results show that PPB consists of largeγ′, MC carbides enriched with Ti, Nb and a modicum of oxides. Thereafter, it can efficaciously tune γ′ precipitate size from micrometer down to nanometer region and simultaneously results in the annihilation of PPB by HEX process. The activation energy for grain growth of as-HEXed FGH96 superalloy was measured to be 402.6 kJ/mol, indicating that γ′ precipitate serves the critical role in inhibiting grain growth under sub-solvus heat treatment. Moreover, the results reveal that grain growth is primarily restrained by MC carbide in the case of super-solvus temperature.

Coarsening behavior of γ' particles in a nickel-base superalloy

The coarsening behavior of γ particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000,1080 and 1140°C for different times,the relation of average particle size to time obeys the cube law ( a /2)3= kt,where k is 15.49 × 103,77.5 × 103 and 230.04 × 103 nm3/min,respectively. The particle size distributions are better fit to the LSW theoretical distributions when aging at 1000°C within 1440 min....

Effect of Ta addition on solidification microstructure and element segregation of IN617B nickel-base superalloy

IN617B nickel-base superalloy is considered as a good candidate material in 700℃advanced ultrasupercritical coal-fired power plants.The effect of Ta addition on solidification microstructure and element segregation of IN617B alloy was investigated by OM,SEM,TEM,EDS,EPMA and thermodynamic calculation.The results showed that the solidification microstructure exhibited a dendritic segregation pattern with many primary carbides distributed in interdendritic regions,such as network M_(6)C,lath M_(23)C_(6) and granular Ti(C,N).The addition of Ta promoted the precipitation of Ta-rich MC significantly inhibiting the precipitation of M_(6)C and M_(23)C_(6),and reduced the segregation degree of Al,Mo and Ti alloying elements.The addition of Ta decreased the melting temperature of MC carbide,but did not impact the solidification path,that was,L→γmatrix→MC or Ti(C,N)→M_(6)C→M_(23)C_(6),where MC and Ti(C,N)tended to form symbiotic microstructure with M_(6)C.This study will provide theoretical basis and data support for the alloy optimization and casting structure control of IN617B nickel-based superalloy.

Optimality analysis of multiplex A-TIG welding flux for nickel-base superalloy

Orthogonal experiment is employed to study a new kind of multiplex flux for nickel-base superalloy. This activated TIG welding flux is composed of NaF, MgF2 and CaF2, and their proportion is 5：4：1. Compared with conventional TIG welding, the penetration increases 164% by the action of the flux. Tensile test result indicates that the fracture strength of the mixed flux A-TIG weld bead is higher than base metal, and it increases along with the decrement of the welding current. The average extensibility of the weldment is beyond 100%, which means perfect ductility. MetaUographs elucidate that there exist lots of deep and evenly distributed dimples on the fiacture section of weld bead while on that of base metal there only exists a few shallow dimples and massive tearing ridge.

Segregation and diffusion behavior of niobium in a highly alloyed nickel-base superalloy

The segregation during solidification and high temperature diffusion during homogenization of niobium in a highly alloyed nickel-base superalloy were investigated.Niobium is seriously segregated into the interdendritic regions with segregation coefficient as high as 4.30.Various niobium-enriched phases including Laves phase,δ phase,（γ＋γ＇） eutectic,MC and M6C types of carbides precipitated in the interdendritic regions.The soluble temperature of niobium-enriched phases and the degree of dendritic segregation were determined by differential thermal analysis and homogenization treatments at varied temperatures and times.The calculation of elemental diffusion indicates that the diffusion rate of niobium increases remarkably with the homogenization temperature increasing,so that the annealing time can be effectively reduced.A three-step homogenization treatment technology without incipient melting was established to eliminate the segregation of niobium and to obtain a uniform microstructure.

Effect of Heat Treatment on High Temperature Stress Rupture Strength of Brazing Seam for Nickel-base Superalloy

In order to enhance the high-temperature stress rupture strength of brazing seam by heat treatment, it was diffusion treated, then solution heat treated, and finally aging treated. The microstructure of brazing seam especially morphology of gamma ' phase and boride was observed and the strength of brazing seam was measured in this process. The results show that heat treatment can enhance high-temperature stress rupture strength by improving the microstructure of brazing seam. The strength of brazing seam after solution heat treatment decreases in comparison with that only after diffusion treatment while aging treatment after solution heat treatment increases the strength of brazing seam.

Laser Shock Processing of an Austenitic Stainless Steel and a Nickel-base Superalloy

An austenitic stainless steel 1Cr18Ni9Ti and a solid solution-strengthened Ni-base superalloy GH30 were shock processed using a Q-switched pulsed Nd-glass laser. Microstructure, hardness and residual stress of the laser shock processed surface were investigated as functions of laser processing parameters. Results show that high density of dislocations and fine deformation twins are produced in the laser shock processed surface layers in both the austenitic stainless steel and the nickel-base superalloy. Extensive strain-induced martensite was also observed in the laser shock processed zone of the austenitic steel. The hardness of the laser shock processed surface was significantly enhanced and compressive stress as high as 400 MPa was produced in the laser shock processed surface.

Na_2SO_4- and NaCl- Induced Hot Corrosion Behaviors of a Nickel-Base Superalloy with Aluminide Diffusion Coating

Hot-corrosion behaviors of nickel-base superalloy and aluminide diffusion coating have been investigated in conditions of contents of Na2SO4 and NaCI molten salts at 900℃ by means of XRD and SEM. Hot-corrosion scale of the superalloy and aluminide diffusion coating were analyzed and their surface morphologies were observed. The results demonstrate that both coated and uncoated specimens are not susceptible to various contents of NaCI. That may be resulted from the AI2O3 scale formation. Growth stress was characterized by the formation of convoluted scales.

Mechanical properties and microstructure changes after long-term aging at 700℃ for a nickel-base superalloy

Mechanical properties and microstructure changes have been investigated on anew nickel-base superalloy after long-term aging at 700 deg C. It is found that the majorprecipitates of the tested alloy are MC, M_(23)C_6, M_6C and y' in the course of long4erm aging at700 deg C. The carbides maintain good thermal stability with the aging time up to 5008 h. The growthrate of gamma prime precipitates is relatively high in the early aging period and then slows down.The coarsening behavior of gamma prime follows a diffusion-controlled growth procedure. The roomtemperature Rockwell hardness of the alloy aged at 700 deg C increases slightly at the initial stageof aging, but it decreases with the prolonged time. It mainly depends on the size of gamma prime.In comparison with Nimonic alloy 263, the new alloy characterizes with higher tensile andstress-rupture strengths at high temperatures. The new nickel-base superalloy offers a combinationof microstructure stability, strength, ductility and toughness at 700 deg C.

Antiphase Boundary Strengthening in a Single Crystal Nickel-base Superalloy

An investigation has been made into strengthening mechanism in a single crystal nickel-base superalloy DD8 by transmission electron microscopy. The results show that the stress rupture strength of the alloy increases with decreasing misfit, and the antiphase boundaries (APBs) formed in the ordered γ' phase, rather than the misfits, play a dominate role in strengthening of the single crystal Ni-base superalloy DD8.There are three kinds of mechanisms for forming the APBs which were observed in the present materials. One is mis-arrangement of the local ordered atoms in the γ' precipitates due to the local strain; the second arises from the 1/2<110> dislocations cutting into the γ', and the third is the formation of the APBs induced by the 1/2<110> matrix dislocation network. The contribution of the antiphase boundary energy to the strength of the alloy can be expressed by:where τ is the resistance to deformation provided by the APB energy; S is the long-range order degree in γ'; Tc is the transition temperature from order to disorder; f is the volume fraction of γ'; rs is the radius of γ'; b is the Burgers vector; a is the lattice constant; G is the shear modulus, and k is the proportional constant.

Surface recrystallization of a single crystal nickel-base superalloy

The recrystallization behavior of a single crystal nickel-base superalloy was investigated by shot peening and subsequent annealing. Two kinds of recrystallization microstructures, which are intensively dependent on the annealing temperature, are shown in the nickel-base superalloy after shot peening and subsequent annealing. Surface recrystallized grains are obtained when the superalloy is annealed at solution treatment temperature. The nucleation of recrystallization originates from the dendritic core, where rapid dissolution of γ＇ particles occurs. Cellular recrystallization is observed after annealing at lower temperatures. Cellular structures induced by high diffusivity of the moving boundary and more γ＇ particles dissolution led by residual stress are developed from the surface region. Recrystallized kinetics of the shot-peened alloy annealed at 1050°C accords with the Johnson-Mehl-Avrami-Kolmogorov equation. The low Avrami exponent is caused by the inhomogeneous distribution of stored energy, the decreasing of stored energy during recovery, and the strong resistance of boundary migration by γ＇ particles.

Influence of Co, W and Ti on the Stress-rupture Lives of a Single Crystal Nickel-base Superalloy

The influence of Co, W and Ti on stress-rupture lives of a Ni-Cr-AI-Mo-Ta-Co-W-Ti single crystal nickel-base superalloy has been investigated using a L9 （34） orthogonal array design （OAD） by statistical analysis. At a selected composition range, Ti content was the most important factor to the effect of the stress-rupture lives and then followed by Co content. W content had the minimum effect on stress-rupture lives. The optimal alloy should contain 10 wt pct Co, 8 wt pct W and zero Ti. The optimized alloy also had good microstructural stability during thermal exposure at 870℃ for 500 h.

New approach for assessing the weldability of precipitation-strengthened nickel-base superalloys

A new procedure was proposed for evaluating the weldability of nickel-base superalloys. The theory is on the basis of two microstructural patterns. In pattern I, the weld microstructure exhibits severe alloying segregation, many low-melting eutectic structures, and low weldability. The weld requires a weaker etchant and a shorter time for etching. In pattern Ⅱ, the weld microstructure displays less alloying segregation, low quantity of eutectic structures, and high weldability. The weld needs a stronger etchant and a longer time for etching. Five superalloys containing different amounts of Nb and Ti were designed to verify the patterns. After welding operations, the welds were etched by four etchants with different corrosivities. The weldability was determined by TG-DSC measurements. The metallography and weldability results confirmed the theoretic patterns. Finally, the etchant corrosivity and etching time were proposed as new criteria to evaluate the weldability of nickel-base superalloys.

Hot Deformation Characteristics for a Nickel-base Superalloy GH742y

The hot deformation characteristics of as-cast nickel-base superalloy GH742y after hot isostatic pressing （HIP）（hereafter referred to as-cast alloy） have been investigated by hot compression tests in the temperature range of 1050 to 1140℃, strain rate range of 0.01 s^-1 to 10 s^-1 and strain range of 35% to 50% by means of Gleeble-3500 thermal mechanical simulator. The results show that the as-cast alloy exhibits the poor deformability, and shows wedge-shaped cracking beyond the strain of 35%. At strain rates less than 1.0 s^-1, the stress-strain curves exhibit nearly steady-state behavior, while at strain rate of 10 s^-1, a yield drop and serrated yielding occur. The activation energy values developed on the basis of the experimental data are divided into three domains. The first domain appears at lower strain rate （≤1.0 s^-1） and lower temperature （≤1080℃）, with the lowest mean value of activation energy about 261.4 kJ/mol. The second domain appears at the same strain rate as the first domain, but higher temperature （〉1080℃）, with the intermediate mean value of activation energy about 328.8 k J/tool. The third domain appears at higher strain rate （10 s^-1） and temperature range of 1050 to 1140℃, with the largest mean value of activation energy about 605.05 kJ/mol. Three different constitutive equations are established in corresponding to domains. Microstructural observations in the third domain reveal non-uniform dynamic recrystallization （DRX） of homogeneous γ phase, which leads to the poor deformability and the highest Q value. In contrast, microstructures in the first domain show fully DRX of homogeneous γ phase, leading to the better deformability and the lowest Q value. It is noted that the grain size increases with the increment of strain rate or temperature. These results suggest that bulk metal working of this material may be carried out in the first domain where fully DRX of γ homogeneous occurred.

Effect of Magnesium and Cerium Microelement on Microstructures and Mechanical Properties of K465 Nickel-Base Superalloy

0.02%(mass fraction) Mg and 0.02% Ce were added into nickel-base superalloy, K465, in order to study their effect on microstructures and mechanical properties. The results showed that without addition of Mg and Ce, the grain boundaries was flat, the MC carbide precipitate was noncontinuous with particle shape. With 0.02% Mg addition the MC carbide tended to have tiny granular and blocky morphology, while it had a cursive script-like morphology with 0.02% Ce addition. It was also shown that without any addition of Mg and Ce, the room temperature properties, average tensile strength (σb) and yield strength (σ0.2) were 960 and 790 MPa, the creep rupture life under the condition of 1248 K, 230 MPa was 28.1 h. With the addition of 0.02% Mg, the room temperature properties, average tensile strength and yield strength had a certain enhancement, the creep rupture life (1248 K, 230 MPa) was up to 66.7 h. When the Ce addition was 0.02%, average tensile strength and yield strength were up to 1020～1030 MPa and 855～885 MPa, respectively, but the average elongation percentage fell down to 3.0%, only reaching the required value. The most expressive thing was that the creep rupture life (1248 K, 230 MPa) was only 17.25～18.77 h, far below the required value.

Hot Cracking Susceptibility of 800H and 825 Nickel-Base Superalloys during Welding via Spot Varestraint Test

Hot cracking susceptibility of fillers 52 and 82 in 800H and 825 nickel-base superalloys was discussed using the Spot Varestraint test.The fillers of 52 and 82 were added into nickel-base superalloys via a gas tungsten arc welding(GTAW).Experimental results showed that the hot cracking sensitivity of the nickel-base superalloys with filler at high temperature was lower than that without filler.The hot cracking sensitivity had a slight effect when the filler 82 was added.The total length of crack was increased,the liquid-solid(L-S)two-phase range is higher so that the hot cracking susceptibility will be raised.The morphologies of cracks included the intergranular crack in the molten pool,molten pool of solidification cracking,heat-affected zone of intergranular cracks,and transgranular crack in the heat-affected zone.

Nickel-base superalloys for USC power plants

The advanced ultra-supercritical power plants of the future will utilize steam pressures and temperatures that are too high for traditional ferritic steels,thus requiring austenitic materials.Older nickel-base superalloys such as 263 and 617 were initially evaluated under the European THERMIE project beginning in the 1990s.An entirely new age-hardened alloy 740 which possesses exceptional fireside corrosion resistance and creep strength was also developed for boiler tubing capable of serving at 700C.Subsequently,interest in the USA considered other product forms such as steam header piping and steam turbine forgings for service as high as 760C.A more stable and weldable alloy version now called 740H was developed to meet these more demanding conditions.This paper summarizes the current status of work on alloys 740 and 740H.

Effect of heat treatment on microstructure and high temperature tensile properties of cast nickel-base superalloy with high W, Mo and Nb contents

The microstructural features and high temperature tensile properties of M963 superalloy at as cast, as solutioned and as aged conditions were investigated in detail. The results show that the solution treatment at 1?220?℃ for 4?h, AC causes an increase in high temperature yield strength but a drastic drop in high temperature ductility due to the precipitation of both the secondary carbide M 6C along grain boundaries and at the interdendritic regions and very fine γ ′ particles in the dendrite cores. Aging treatment following the solution treatment can improve the high temperature tensile properties of M963 superalloy due to the coarsing of the γ′ precipitate. One stage aging at 850?℃ for 16?h following the solution treatment causes an increase in both strength and ductility of alloy M963, and two stage aging of 1?089?℃ / 2?h, AC plus 850?℃ / 16?h, AC following the solution treatment further increases the ductility of alloy M963 but slightly decreases its strength.

Hot deformation of spray formed nickel-base superalloy using processing maps

The hot compression testing of hot isostatically pressed(HIPed) spray formed(SF) nickel-base superalloy was carried out by thermal mechanical simulator in the temperature range of 1 050-1 140 ℃ at strain rates of 0.01-10 s-1 and engineering strain of 50%. A processing map was developed on the basis of these data by using the principles of dynamic materials modeling. The microstructural evolution of deformed samples was also examined on the basis of optical and electron microscopic observations. The map exhibits two domains: the instability domain at the temperatures of 1 050 ℃ -1 110 ℃ and strain rate of 0.01 s-1, the stability domain at the temperatures of 1 110 ℃- 1 140 ℃ and strain rates of 1 s-1-10 s-1, with a peak efficiency of about 40%. The dynamic recrystallization(DRX) is observed in the stability domain and the deformed specimens are no cracking or instabilities. However, there is no DRX in the instability domain and the alloy exhibits flow instability with cracks due to poor workability. The optimum hot working condition was determined in the stability domain.