https://www.repository.cam.ac.uk/bitstream/1810/254150/1/V.D.%20Divya%20et%20al%202016%20Materials%20Characterization.pdf

Microstructure of selective laser melted CM247LC nickel-based superalloy and its evolution through heat treatment

The high thermal gradients experienced during manufacture via selective laser melting commonly result in cracking of high γ/γ′ Nickel based superalloys. Such defects cannot be tolerated in applications where component integrity is of paramount importance. To overcome this, many industrial practitioners make use of hot isostatic pressing to ‘heal’ these defects. The possibility of such defects re-opening during the component life necessitates optimisation of SLM processing parameters in order to produce the highest bulk density and integrity in the as-built state.
In this paper, novel fractal scanning strategies based upon mathematical fill curves, namely the Hilbert and Peano-Gosper curve, are explored in which the use of short vector length scans, in the order of 100 μm, is used as a method of reducing residual stresses. The effect on cracking observed in CM247LC superalloy samples was analysed using image processing, comparing the novel fractal scan strategies to more conventional ‘island’ scans. Scanning electron microscopy and energy dispersive X-ray spectroscopy was utilised to determine the cracking mechanisms.
Results show that cracking occurs via two mechanisms, solidification and liquation, with a strong dependence on the laser scan vectors. Through the use of fractal scan strategies, bulk density can be increased by 2 ± 0.7% when compared to the ‘island’ scanning, demonstrating the potential of fractal scan strategies in the manufacture of typically ‘unweldable’ nickel superalloys.

https://nottingham-repository.worktribe.com/preview/846227/1-s2.0-S221486041630358X-main.pdf

Fractal scan strategies for selective laser melting of ‘unweldable’ nickel superalloys

Despite the large industrial use of pack aluminizing processes for the protection of parts (e.g. cooling channels) of gas turbine blades, systematic studies relating the formation mechanisms and the chemical composition of pack aluminized layers to important process parameters, including the nature of the base alloy and the temperature, are scarce. In this study, 4 different alloys (pure Ni, Ni–20Cr, Inconel 738 and directionally solidified CM247LC) were subjected to a pack aluminizing process at three different temperatures (950 °C, 1000 °C, 1040 °C), using a pack mix containing a fluorine-based activator; the results were compared to those obtained with a chemical vapor deposition (CVD) aluminizing process performed at 1040 °C with the same fluorine-based gaseous precursor. The microstructural characterization, performed by SEM + quantitative EDX analysis, XRD and nanoindentation testing, shows that, during the heating stage of the pack aluminizing process, Al is transported to the sample surface at temperatures too low to allow significant simultaneous diffusion of Ni; therefore, a δ-Ni 2 Al 3 outer layer is formed by inward Al diffusion below the alloy surface, and its growth then continues during the isothermal stage as well. As a result, the chosen isothermal treatment temperature does not affect growth mechanisms, although it modifies the overall thickness of the aluminized layer. δ-Ni 2 Al 3 is converted to β-NiAl after a subsequent vacuum heat treatment at 1120 °C. In a CVD process, where gaseous precursor are introduced only after attaining the isothermal treatment stage, Al and Ni diffuse simultaneously from the very beginning of the aluminizing process and β-NiAl is directly developed. Less mobile species (heavy atoms, such as W) in the alloy composition hinder all diffusion phenomena, both during pack aluminizing and during subsequent vacuum heat treatment: after aluminizing, precipitates are developed within the δ-Ni 2 Al 3 outer layer and, after vacuum heat treatment, the resulting β-NiAl layer exhibits a compositional gradient.

Diffusion mechanisms and microstructure development in pack aluminizing of Ni-based alloys

Influence of heat treatment on microstructure and tensile properties of conventionally cast and directionally solidified superalloy CM247LC

In this study, the effects of the addition of Zr on the mechanical properties and fracture mechanism of a fine-grained TM-321 superalloy were investigated. The results indicated that the tensile properties of the TM-321 superalloy were enhanced by the addition of Zr at room and moderate temperatures, and the creep life of the TZ3 alloy was twice as long as that of the original TM-321 superalloy (TZ1) at 760 °C and 724 MPa because of three major factors. First, the grain size of the TM-321 superalloy was refined by adding Zr. Second, the higher content of Zr in the TM-321 super alloy might have prevented the formation of coarse and long script-like grain-boundary (GB) carbides, but facilitated the formation of fine and particle-like carbides and inhibited GB sliding. Moreover, Zr was effective in increasing GB cohesion and decreasing GB diffusion, thus exhibiting a superior GB strengthening effect. Third, the Zr addition increased the strength of the γ/γ′ microstructure and resulted in the inhibition of dislocation motion. The fracture modes of creep and tensile tests demonstrated that crack propagation along the GBs was reduced as the Zr content increased, whereas secondary cracks within the grain interior increased.

Effects of Zr addition on the microstructure and mechanical behavior of a fine-grained nickel-based superalloy at elevated temperatures

The CM 247 LC superalloy was remelted and cast to obtain the desired polycrystalline test bars by controlling casting parameters, followed by the investigation of precipitation morphology and mechanical properties. The experimental results show that by well-controlled casting parameters the CM 247 LC owns excellent castability to form a superalloy with fine grain structure, high strength as well as superior creep resistance.

/pdf/characteristics-and-mechanical-properties-of-polycrystalline-16ivsms5v5.pdf

Characteristics and Mechanical Properties of Polycrystalline CM 247 LC Superalloy Casting

This study investigates the effect of Zr minor additions, with Zr content from 0.015 to 0.15 mass%, on microstructure and mechanical properties of fine-grain (65 mm) CM 247 LC superalloy. Tensile test results indicate that minor addition of Zr to fine-grain CM 247 LC can dramatically improve the yield strength at 300 K as well as both the yield strength and the elongation at 1033 K. Under creep conditions of 1033 K/725 MPa or 1200 K/345 MPa, the Zr additions drastically increased the rupture life, creep rate and elongation. The AES observation reveal that Zr may enrich at grain-boundary and boride/matrix interface and dissolve in matrix, carbide or � 0 phase. Thus, Zr may change the primary MC carbide characteristics and inhibit the script-like MC carbide formation. Moreover, Zr is apparently to increase the effects of the cohesive energy of both the precipitated phase/matrix interface and the grain boundaries, also it is beneficial to stress accommodation and retards the crack initiation and propagation.

Effect of Zirconium on Microstructure and Mechanical Properties of Cast Fine-Grain CM 247 LC Superalloy

This study investigates the effect of the solution-treatment, with solution temperatures from 1494 to 1533 K, on the microstructure and mechanical properties of fine-grain (65 mm) CM 247 LC superalloy. Tensile tests reveal that the yield strength of the alloy after multi-step 1527 K/2 h solution-treatment exceeds that after 1494 K/2 h solution-treatment. Additionally, the alloy after multi-step 1527 K/2 h solutiontreatment had a lower creep rate and greater rupture life than that after 1494 K/2 h solution-treatment. The improved mechanical properties are attributed to the high volume fraction of � 0 particles with the proper size and the refinement of carbides, which inhibit the gliding and climbing of dislocations.

/pdf/effect-of-solution-treatment-on-microstructure-and-2v72hsay2u.pdf

Hsin-Erh Huang

Papers

Characteristics and Mechanical Properties of Polycrystalline CM 247 LC Superalloy Casting

Effect of Zirconium on Microstructure and Mechanical Properties of Cast Fine-Grain CM 247 LC Superalloy

Effect of Solution-Treatment on Microstructure and Mechanical Properties of Cast Fine-Grain CM 247 LC Superalloy