Interdendritic Areas Research Articles

Effect of melt superheat on microstructure of Al4Fe2Mn1.5 Monel alloy

The effect of melt superheat on microstructure of Al4Fe2Mn1.5 Monel alloy made by vacuum melting method was studied. The results show that the alloy consists of dendritic γ matrix and γ′ phase, wherein γ′ phase has two morphologies at different melt superheat. One is divorced eutectic γ′ which distributes in the interdendritic area, the other distributes dispersedly in single particle on the dendritic arm and exists in the petalform shape in the transition area between dendritic arm and interdendritic area. With the increase of superheat, the dendrite becomes finer, the primary dendritic arm is melted off and the secondary dendritic arm spacing decreases. The size of γ′ phase distributed on the dendritic arm becomes smaller and the divorced eutectic γ′ phase increases.

Phase transformation of η and σ phases in an experimental nickel-based superalloy

Both nodular σ phase and platelet η phase were found in the as-cast microstructure of an experimental nickel-based superalloy. The nodular σ phase decomposed into γ phase leaving serrated transformation interfaces between σ and γ during the heat treatment process. The formation mechanism of nodular σ phase and platelet η phase was investigated by analyzing the orientation relationship of σ/γ and η/γ. It was found that σ phase nucleated and grew from the liquid during solidification in the interdendritic area. The η phase had regular platelet morphology and definite orientation relationship with γ phase, which indicated that it was formed by means of solid-state precipitation.

Microstructure and compressive properties of AlCrFeCoNi high entropy alloy

An AlCrFeCoNi high entropy alloy was prepared by vacuum arc melting. Only diffraction peak corresponding to a BCC crystal structure is observed for this AlCrFeCoNi high entropy alloy. The microstructure of this AlCrFeCoNi alloy is polygonal grains with intragranular dendritic segregation. Dendritic segregation area is found to be Al, Ni rich and Cr, Fe deplete, while interdendritic segregation area is Cr, Fe rich and Al, Ni deplete. The distribution of Co is essentially identical. The fine microstructure of dendritic segregation area and of interdendritic segregation area is found to be nanoscale spherical precipitates morphology and basket-weave morphology, respectively. Results of EDS attached on high resolution scanning electron microscope (SEM) revealed that these morphological characteristics are also resulted from elements segregation. This AlCrFeCoNi high entropy alloy exhibits excellent compressive properties. The yield stress, compressive strength and plastic strain of the alloy reaches 1250.96, 2004.23 MPa, and 32.7%, respectively. The fracture mechanism of this AlCrFeCoNi high entropy alloy is observed as cleavage fracture and slip separation.

Effection of Alloying Elements on Microstructures of MX 246 and MX 246A Ni3Al-Based Alloys

The effects of alloying elements on microstructure of two compositions of Ni3Al-based alloys, MX 246 and MX 246A have been studied in this article. The results show that “starburst” γ - eutectic phases are large and their amount is about 18% in MX246. Majority NiZr and a little of ZrC are formed in the boundaries of γ - eutectic phases. Microstructure at 1200°C shows that NiZr phases have been melted completely, which cause the decrease of melting point of phases to about 1125°C. With increasing alloying additions such as Mo and Hf. γ - eutectic phases are dispersive, uniform, small, and their amount is decreased to about 4%. Carbides and borides are formed in MX246A. The volume fractions of the phases in MX246A and the chemical compositions of the phases have been quantitatively analyzed. The results show that carbides and borides are about 1.12% in MX246A and the main carbides are TiC. HfC and the borides are (Ni0.086Cr0.08/Wa0.094Mo0.376Ti0.334 Hf0.024) 3B2. TEM observation results show that with increasing alloying element additions such as Mo and Hf, a lot of γ - interface misfit dislocations are produced in the interface of γ and phase and they are interacted with the nanometer particles. phases are all stable in MX246A at 1200°C. The stability of phase is increased with the addition of Mo and Hf. In addition, there are many white spot phases which are mainly NiMo phases, and a little (NiMo)6C and (MoTi)6C gathering in some interdendrite areas.

Surface treatment of DZ4 directionally solidified nickel-based superalloy by high current pulsed electron beam

Surface treatment of DZ4 directionally solidified nickel-based superalloy was carried out by using high current pulsed electron beam (HCPEB) technique. Microstructure changes in modified surface layer were characterized together with their corrosion property. It is found that the treated surface becomes rougher with increasing the number of HCPEB pulse due to the formation of craters occurring preferentially in the interdendritic areas. The thickness of remelted layer increased slightly when more HCPEB pulses applied and reached about 3 μm after 10 HCPEB pulses. The corrosion resistance of modified surfaces exhibited an effective improvement when measured in 0.5 mol/L sulfuric acid solution. The factors that influence the corrosion resistance of DZ4 alloy were discussed by considering the surface purification effect featured by HCPEB surface treatment.

Effect of Heat Treatment on Microstructure and Mechanical Properties for a Ni<sub>3</sub>Al Base Single Crystal Alloy DDIC6

The effect of heat treatment on microstructures and stress rupture property of a Ni3Al base single crystal alloy DDIC6 was studied in the present investigate. The single crystal specimens were produced by screw selection crystal method. The heat treatment for the alloy was 1300°C/10h+1120°C/4h+870°C/32h and 1300°C/10h+870°C/32h.The microstructures were examined by SEM, TEM and X-ray EDS techniques. The stress rupture tests were carried out in air by constant load creep machines under 1100°C/130MPa with the specimens size of φ5×25 mm. The experimental results showed that the as-cast large size γ′ phases entirely dissolved after 1300°C/10h, and secondary fine γ′ phases precipitated by following aging at 1120°C and 870°C for certain periods of time. The stress rupture life under 1100°C/130MPa increased from 20~30hrs for as-cast condition to 60~100hrs for heat treatment condition. The improvement of the creep resistance of the alloy may attribute to the decrement of the elements segregation at dendrite and interdendritic areas, and the proper size and distribution of γ′ phases.

Oxidation behavior of a single-crystal Ni-base superalloy in air at 900, 1000 and 1100 °C

The isothermal oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 900 to 1100 °C and analyzed by OM, TGA, XRD, EDX, SEM and EPMA. The alloy obeyed subparabolic rate law during oxidation at 900 and 1000 °C, whereas the alloy showed parabolic rate law at 1100 °C exposure. The variations in the chemical composition due to segregations, which resulted from the solidification process, led to the formation of different kinds of oxide scale on the dendritic and interdendritic area during oxidation at 900 and 1000 °C, while the alloy exhibited relatively uniform oxidation at 1100 °C. The growth mechanism of the scale was discussed.

Microsegregation in high Nb containing TiAl alloy ingots beyond laboratory scale

Microsegregation in big ingots of Ti–45Al–(8–9)Nb–(W, B, Y) alloy had been studied. The composition and microstructural morphology of the large ingot exhibited significant microinhomogeneity. Three types of microsegregation were observed in as-cast microstructure of the large ingot. First is the solidification segregation (S-segregation) at interdendritic area, in which the composition is characterized by higher Al, B (boride), and Y (oxide) contents and lower Nb and W contents. Second is the β-segregation at the boundary and triple junctions among α grain due to the phase transformation of β → α. The composition at the segregation area is characterized by higher Nb and W additions that lead to the formation of β particles and γ phase. Third is the α-segregation that forms local lamellar structure composed of β, γ and α plates due to phase transformation of α → α 2 + β + γ. The microsegregation for the PAM ingot is lower than that for SM ingot in terms of the volume fraction of β phase. The reason is that the PAM melting can offer better control of pouring temperature and rather fast cooling rate by water-cooled copper crucible.

Oxidation Behavior of the Single-Crystal Ni-base Superalloy DD32 in Air at 900, 1000, and 1100°C

The isothermal-oxidation behavior of the single-crystal Ni-base superalloy DD32 was studied over the temperature range from 900–1100 °C and analyzed by OM, TGA, XRD, EDX, SED, and EPMA. The alloy DD32 obeyed a subparabolic rate law during oxidation at 900 and 1000°C, while the alloy showed a rather high oxidation rate at 1100°C. The severe composition segregation, which resulted from the solidification process, led to the formation of different scale on the dendritic and interdendritic regions. Internal nitride (AlN) was observed in the subsurface zone of dendritic areas, but absent in interdendritic areas. The oxidation mechanism of the alloy DD32 is discussed by comparing with other alloys.

Synchrotron tomography of porosity in single-crystal nickel-base superalloys

Porosity in the single-crystal nickel-base superalloys CMSX-6, SRR99, CMSX-4 and CMSX-10 was investigated by synchrotron X-ray tomography, supplying information about the three dimensional distribution of pores and a statistical database of pore sizes and shapes. Tomography investigations were completed by scanning electron microscopy. In undeformed superalloys the porosity was found to increase with the content of refractory elements. In CMSX-4 creep tested at 1100 °C a deformation induced growth of porosity was observed. Shape and size of the pores was analysed quantitatively as a function of applied load and creep time. The growth was found to be based on the enlargement of pre-existing pores in the interdendritic areas and on the generation of new small pores. Growth processes at creep temperature result in polyhedral pores. Formation and shape transformation of the creep pores is discussed.

Effect of rhenium addition on isothermal oxidation behavior of single-crystal Ni-based superalloy

The isothermal oxidation behavior of a nickel-based superalloy, which we designate as DD32, containing 4 wt.% rhenium and another alloy (DD32M) without rhenium were investigated at 900, 1000 and 1100 °C. Oxidized alloys were characterized by SEM, EDX, EPMA and XRD. The two alloys revealed similar oxidation rates at 900 °C, while the DD32 alloy showed a slightly higher mass gain than that of the DD32M alloy during 1000 and 1100 °C exposure. Surface and cross-sectional examination indicated that DD32 alloy showed nonuniform oxidation between dendritic and interdendritic areas at all temperatures. Similar different oxidation behavior between dendritic and interdendritic regions was also observed in the DD32M alloy at 900 °C. However, DD32M showed relatively uniform oxidation at 1000 and 1100 °C exposure. Therefore, Re addition clearly induced the nonuniform oxidation of the alloy.

Oxidation Behavior of the Directionally Solidified Ni-base Superalloy DS951 in Air

The isothermal oxidation behavior at 1000, 1050 and 1100°C and the cyclic oxidation behavior at 1000°C of the directionally solidified Ni-base superalloy DS951 were investigated. The oxidized samples were characterized by SEM, EDAX, XRD. The alloy DS951 obeyed a two-stage parabolic rate law during isothermal oxidation at 1000–1100°C. In cyclic conditions, the alloy showed no weight loss even after 1100 cycles. Cross-sectional examination revealed the development of faceted and needle-shaped AlN precipitates in the alloy subsurface region during cyclic oxidation, while no internal corrosion products were found in isothermal tests. Furthermore, a selective oxidation of MC carbides (M was predominately niobium with some chromium and tungsten) was observed along alloy grain boundaries and in interdendritic areas.

Zn/55Al coating microstructure and corrosion mechanism

In the present work Zn/55Al coatings from different suppliers were studied. The results revealed the presence of at least two different phases in the interdendritic and in the dendritic areas that compose the coating. Analyses done in the cross section of test panels exposed for five years in six different atmospheres showed that the corrosion process begins in the aluminium-rich phase present in the interdendritic area.

μ Phase in a Nickel Base Directionally Solidified Alloy

Scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD) were employed to study the μ phase in a directionally solidified experimental nickel base alloy. It was observed that the μ phase distribute at interdendritic area and precipitate mainly from the γ′ phase in this experimental alloy. Orientation relationship between the γ′ and the μ phase was found as (111)γ′||(0001)μ and 〈\\bar110〉γ′||〈11\\bar20〉μ. Detrimental effect of the μ phase on oxidation resistance of the alloys was also found.

Effect of dendrite arm spacing on cleavage fracture toughness of Al–5Si–1Cu alloy

The influence of dendrite arm spacing on mechanical properties, especially on the cleavage fracture toughness of Al–5Si–1Cu alloy, was investigated. Increasing the solidification rate results in a reduction of the secondary dendrite arm spacing λ2. Four different microstructures have been studied. The interdendritic area boundaries are a strong barrier for dislocation slip in the stressed alloy; they cause a change in the density and direction of the slip bands. The values of yield stress R0,2, tensile strength Rm and plane strain fracture toughness KIc for different varieties of Al–5Si–1Cu alloy microstructure have been determined. Relationships between mechanical properties and dendrite arm spacing were described in terms of the Hall Petch equation; a decrease in secondary dendrite arm spacing causes an increase in strength and to a lesser degree, in fracture toughness of Al–5Si–1Cu alloy.

Development of directionally solidified Ni-Al-Mo-B-Y alloy IC6A

A directionally solidified Ni3Al based alloy designated IC6A with the chemical composition Ni - (7.5 - 8.5)Al - (13.0 - 15.0)Mo - (0.02 - 0.1)B - (0.001 - 0.20)Y (wt-%) has been recently developed as a high-temperature structural material for advanced jet-engine vanes operating in the temperature range 1050 - 1100 ° C. Microstructural analysis results show that the regions consisted of Ni3Y, Mo1.24Ni0.76 and γ phases and surrounded by large size γ ′ phases precipitate in the interdendritic areas of alloy IC6A when the content of yttrium is near to the upper limit. Ni3Y and Mo1.24Ni0.76 have no obvious change during high temperature aging. A needle like or rod like phase named Y - NiMo precipitates after aging at 950 ° C for about 200 h. The high temperature cyclic oxidation resistance of alloy IC6A is fairly good owing to the presence of yttrium. The tensile properties, high temperature stress rupture properties and creep properties, high cycle fatigue properties and impact toughness of alloy IC6A were investigated to meet the needs of aeroengine turbine vanes.

Hot ductility of an Fe‐10%Ni alloy during penetration of copper

Today's available theories for hot crack formation are based on the fact that hot cracks are formed in the presence of liquid films in the interdendritic areas or at the grain boundaries, which are exposed to tensile stress. Copper is well known to cause hot shortness in steels. In order to study how the liquid embrittles the material, high‐temperature tensile tests were performed at two strain rates during the penetration of liquid copper into Fe‐10%Ni. The penetration distance was measured in samples that were exposed to strain without fracturing. The ductility (area reduction), strain and ultimate tensile stress were determined. Microprobe analysis was performed on the fractured samples. The transition temperature of ductility was found at 1400–1450°C without copper penetration whereas it occurred at 1025–1078°C during the penetration of copper, i.e. copper starts to embrittle at a temperature below its melting point. The microprobe measurements showed that the diffusion rate of copper into Fe‐10%Ni was enhanced when the lattice was strained. The results are discussed in terms of a new theory concerning vacancy formation and condensation as the dominating mechanism for hot crack formation during solidification.

Effect of hafnium on the castability of directionally solidified nickel-base superalloys

Abstract The effect of Hf additions on the solidification characteristics and castability of the Ni-base superalloy IN792 has been studied. A quenching technique was employed to freeze-in the microstructure during solidification. The crack length in tube-shaped demonstration castings was evaluated. Hf additions were found to reduce the hot-tearing susceptibility significantly. Hf also caused a more gradual decrease of the liquid fraction with temperature in the last stages of solidification, which was observed to improve castability in other alloys previously and which can be correlated with lower strains and strain rates in the vulnerable period where hot tears form. It was also found that the γ/γ′ rosette structure, often referred to as γ/γ′ eutectic in the literature, actually forms by a discontinuous precipitation mechanism from the solid. The amount of γ/γ′ observed in the interdendritic area, therefore, does not reflect the amount of remaining liquid in the last stages of solidification.

Microstructure and Mechanical Properties of Recycled Ni3Al Base Alloy IC6

The effect of recycle proportion on the composition, microstructure and mechanical properties of recycled Ni3Al base alloy IC6 was examined by scanning electron microscopy with X-ray energy dispersive spectrometer (SEM/EDS) and transmission electron microscopy (TEM). The results indicate that the addition of recycled scraps has no obvious effect on the chemical composition, except the content of C and N increases with the addition proportion of recycled scraps. The microstructure of IC6 recycled alloys has no obvious difference compared with the fresh alloy, except some M6C carbides precipitate in the interdendritic area. In contrast with fresh alloy, tensile properties at R. T. of the IC6 recycled alloys have almost no change, the high temperature stress rupture properties decrease slightly, and the thermal fatigue properties deteriorate obviously. The recycle proportion of IC6 alloy should not exceed 50% in production of gas turbine vanes.

Study of precipitates due to the addition of silicon in yttrium modified Ni–Al–Mo–B alloy IC6

The effect of adding 0.10–0.30 wt% Si on the microstructure of the yttrium modified alloy IC6 was examined by scanning electron microscopy (SEM). The chemical compositions of phases precipitated due to the presence of silicon were analyzed by the energy dispersive spectrum (EDS) technique of electron probe micro-analyser (EPMA). The crystal structures of precipitates were studied by transmission electron microscopy (TEM). The results show that two blocky-shaped phases, Mo 1.24Ni 0.76 and Mo 6(Ni 0.75Si 0.25) 7 are formed in the interdendritic area in the alloy with the addition of 0.10–0.20 wt% Si and 0.12 wt% Y, and that a needle-like phase named Y–NiMo precipitates in the interdendritic area in the alloy with addition of 0.30 wt% silicon and 0.12 wt% yttrium besides the formation of the blocky-shaped phases mentioned above.