An overview of the machinability of aeroengine alloys

Microstructural and compositional design of Ni-based single crystalline superalloys ― A review

Three industrial gas turbine blades made of conventionally cast (CC) IN-738 and GTD-111 and directionally solidified GTD-111 Ni-base superalloys were examined after long-term exposures in service environments. All three blades exhibit similar, service-induced microstructural changes (MCs) including γ′ coarsening and coalescence, excessive secondary M23C6 precipitation, and primary MC degeneration, regardless of the chemical composition and the grain size. Special attention was paid to the primary MC decomposition. It is shown that the primary MC decomposition occurs by carbon diffusion out of the carbide into the γ + γ′ matrix, resulting in the formation of Cr-rich M23C6 carbides near the initial carbide/matrix interface. A transition zone is shown to develop between the original MC core and its perimeter, demonstrating the gradual outward diffusion of carbon and a slight inward increase in nickel concentration. The hexagonal Ni3(TiTa) η-phase was also found in the MC transition zone and on the MC-γ/γ′ interface. The primary MC decomposition can be expressed by the reaction MC + γ/γ′ → M23C6 + η. Finally, it is shown that the grain-boundary (GB) MC decomposes more rapidly than that in the grain interiors. This is consistent with the more rapid GB diffusion that leads to the acceleration of the MC diffusional decomposition processes.

Mechanism of primary MC carbide decomposition in Ni-base superalloys

Dependence of creep strength on the interfacial dislocations in a fourth generation SC superalloy TMS-138

A new in-house designed series of Ni based superalloys with stepwise increased Re and Ru additions has been investigated, to systematically determine the influence of Re and Ru on γ/γ′-microstructure and high temperature creep properties. Improved creep resistance and thus also a higher alloy temperature capability of up to 87 K/at.% was found for additions of Re. Additions of Ru revealed a lower temperature capability improvement of up to 38 K/at.% for low Re-containing second generation alloys. However, in third and fourth generation alloys with higher Re-contents, no significant influence of Ru on creep rupture strength was observed. The creep properties are discussed with respect to the γ′-volume fraction, γ′-size and γ′-coarsening rate, as well as the γ/γ′-lattice misfit and the γ/γ′ partitioning coefficient of the different Re and Ru containing alloys. The presented data shows, that these microstructure parameters are strongly influenced by additions of Re, but only marginally by additions of Ru. A further influence on creep rupture strength is given by the solid solution hardening of the γ-matrix, which is discussed based on solid solution hardener concentrations either experimentally derived or calculated from ThermoCalc data.

The effect of Re and Ru on γ/γ′ microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys

The crystallization kinetics of the Ni45Ti23Zr15Si5Pd12 alloy has been investigated by DSC and XRD. The process of crystallization is multi-stage proceeding. The isothermal activation energy obtained from Arrhenius equation increases with the process of crystallization. Based on the JMA equation, the average Avrami exponent for the main crystallization process is larger than 3.0, indicating the crystallization to be a three-dimensional nucleation and growth. The large atomic size difference and negative heats of mixing increase the thermal stability of the Ni-based amorphous alloy.

Crystallization kinetics of the Ni-based bulk metallic glass

As a new kind of engineering material,the metallic glass can be used widely due to its excellent properties such as high strength,hardness and elastic energy and low corrosion resistance. The size of metallic glasses is generally small,which is the main limitation for their application.How to prepare a larger size metallic glass becomes one of main targets in metallic glasses research.To prepare larger size metallic glasses,one method is to optimize their chemical compositions,and another is to join small size metallic glasses together by welding.In this paper,the Zr_(55)Al_(10)Ni_5Cu_(30) bulk metallic glass (BMG) has been successfully jointed by friction welding under the conditions of rotational speed from 4×10~3 to 5×10~3 r/min,friction pressure from 80 to 100 MPa,friction time from 0.2 to 0.4 s,upsetting pressure 200 MPa and upsetting time 2 s.The welded joint has been examined using SEM,XRD and TEM,and it is proved that the welded zone still keeps an amorphous structure.The plasticity of this metallic glass has strong temperature sensitivity near the glass transition temperature T_g.Above T_g, the metallic glass possesses a good plasticity which is necessary for friction welding of metallic glasses.

FRICTION WELDING OF Zr_(55)Al_(10)Ni_5Cu_(30) BULK METALLIC GLASS

Apposite phosphorus content can improve the mechanical properties of the finegrained GH4133 nickel-base superalloy. The grain size of the alloy was refined to ASTM8—10 grade by controlling the thermal processing and the heat treatment. The tensile specimens fail by intergranular fracture even when the phosphorus level is as high as 0.140% (mass fraction). The yield and ultimate strengths of the refined alloy are 200 and 150 MPa higher than that of the alloy with a standard microstructure of ASTM 4—5 grain size, respectively. And the above two strengths of the alloy with 0.140% phosphorus are 100 and 50 MPa higher than those of the alloy with a industrial controlled phosphorus level, say 0.005%. The impact specimens fail by intragranular fracture when the phosphorus level is below 0.023% and the fracture toughness of the alloy is higher than 70 MJ/m~2. Some parts of the fracture surface of the impact specimen with 0.140% phosphorus exhibited intergranular fracture characteristics and the fracture toughness is lowered to about 40 MJ/m~2. Phosphorus extends the rupture life of the GH4133 alloy and its optimum addition is around 0.011%. The interaction between phosphorus atmosphere and dislocations is discussed to understand the effect of phosphorus on different type of mechanical properties.

Effect of Phosphorus on Mechanical Properties of Fine-Grained GH4133 Nickel Base Superalloy

In this paper,the microstructures and properties of two kinds ofAl-Li-Mg-Zr alloys have been studied.The Mg acts as an important solution strength-ening element.The concentration of Mg do not change the tendency of aging hardening ofAl-Li-Mg-Zr alloy.The strengths and hardness of the alloy aged at 463K for differenttimes increased as increasing the content of Mg.This is due to the decrease of Li solubilityin the matrix,which is caused by the addition of Mg,and therefore enhancing the coarsen-ing of δ′particles.On the contrary,the plasticity of the alloys decrease with the increase ofMg in the alloy,and the elongation is the smallest at peak aging condition.The tensile cracksurfaces of the alloy aged at different times are shown as a transgranular and intergranularmix-cracking mode.

RELATIONSHIP BETWEEN MICROSTRUCTURES AND PRORERTIES OF Al-Li-Mg-Zr ALLOYS

The influence of carbon and aluminium on deoxidation during vacuum induction melting (VIM) refining Ni-base superalloy using CaO crucible was studied The equilibriums of C-O and Al-O reactions in Ni-base superalloy were calculated from thermodynamics The calculated results show that C-O reaction is far from equilibrium state and Al-O reaction could decrease oxygen content to below 6×10-6 The C-A1, Al-CaO and C-Al2O3 reactions were also analyzed Thermodynamics calculation shows that the above reactions can take place during the VIM refining process These reactions conducted the concentrations of C and Al decreasing to about 50 times than those required for deoxidation

INVESTIGATION ON DEOXIDATION DURING VIM REFINING Ni-BASE SUPERALLOY BY USING CaO CRUCIBLE

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