Shamil Kh. Mukhtarov

Influence of Severe Thermomechanical Treatment on Formation of Nanocrystalline Structure in Ni 718 and Ni 718Plus Alloys and their Mechanical Properties

The influence of severe thermomechanical treatment via multiple forging on the formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has been investigated. It was observed that a step-wise decrease of processing temperature from 950 down to 575°C allowed the refinement of the initial coarse grain structure to a NC state. Investigations of structural changes in the deformed samples have shown that extending the temperature interval of dynamic recrystallization to low homologous temperatures resulted in the formation of a fine-grained recrystallized structure. The temperature of NC structure formation in ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the presence of the additional γ′-phase, which increased the recrystallization temperature. This decreased the total strain required to produce NC structure, as compared with Alloy 718. It was observed that increasing the total strain and decreasing temperatures step-wise during deformation via multiple forging resulted in a uniform structure across the cross-section of the samples. The room temperature mechanical properties of the investigated alloys with various grain sizes are also compared.

Effect of Grain Size on the Superplastic Behavior of a Nanostructured Nickel-Based Superalloy

The present paper deals with the review of earlier studies and original investigation of microcrystalline (MC) and nanostructured (NS) nickel based INCONEL® alloy 718 processed by severe plastic deformation (SPD) via multiple isothermal forging (MIF). The alloy with a mean grain size of 1 µm - 80 nm has been studied in terms of its thermal stability, superplastic and mechanical properties. It was established that the NS state with the 80 nm grain size can be considered as thermally stable up to the temperature 600°C (0.56Tm). The increase of annealing temperature beyond 600°C causes static recrystallization. Investigations of mechanical properties at room temperature have shown that the decrease of a mean grain size provides the enhancement of strength and reduction of plasticity. Thus, the alloy with a grain size of 80 nm has shown the ultimate strength - 1920 MPa and ductility - 4.8%. Mechanical properties of the NS state of the alloy after annealing are discussed. It has been established that alloy 718 with a grain size of 80 nm displays superplasticity at 600°C and a strain rate of 1.510-4s-1. The values of relative elongation and strain rate sensitivity coefficient m are 350% and 0.37, respectively.

Microstructure and Properties of Nanostructured Alloy 718

Nickel-iron Alloy 718 is widely used for fabricating parts by superplastic deformation. Refinement of grains down to a nanostructure (NS) size improves the alloy’s processing properties. Thermomechanical treatment has been carried out to form a NS state in bulk alloy by multiple isothermal forging (MIF) at gradually decreasing temperatures. Investigation of superplastic properties and processing behavior of Alloy 718 has been performed. The alloy with a grain size of 80 nm displays superplasticity (SP) at a temperature which is lower than for a conventional fine grained alloy by about 350°C. The values of the relative elongation  and the strain rate sensitivity coefficient m are 350% and 0.37, respectively. The experimental data on the influence of grain size on solid-state weldability in the range of SP have been obtained. The application of the effect of low temperature SP yields lower temperatures of superplastic forming (SPF) and pressure welding (PW) as compared with conventional SP of fine-grained material. The experiment of the combined process of SPF and PW by counter-forming of two polished sheets, demonstrates its low temperature processing feasibility using NS specimens. The SPF processing of NS sheets in a cylindrical die has been investigated. It has revealed that macro-deformation is uniform in cross and longitudinal sections. Mechanical properties of Alloy 718 in NS condition and after strengthening heat treatment have been discussed.

Manufacturing of Axisymmetric Components out of Superalloys and Hard-to-Deform Steels by Roll Forming

Heat-resistant alloys are the basic material of gas turbine engine (GTE) design. Fine-grained structure in these alloys can be formed by isothermal forging and then different axisymmetric GTE components as wheels, shafts, rings can be superplastic roll formed. Examples of the superplastic and isothermal deformation use for manufacturing components out of superalloys and steels for critical applications are given. The possibility of roll forming parts as rings with a diameter up to 800 mm and as flange - cone with a diameter up to 600 mm out of superalloys (Inconel 718, EK79, EP741NP), accordingly, on SRZHD-800 and modified PNC-600 mills were showed. The macrostructure investigations of the components after the roll forming showed that the homogeneous structure was formed. The microstructure at the flange portion was fine-grained and at membrane zone was coarse-grained. Cone part was roll formed at isothermal condition from pre-stamped chromium martensitic steel sheet. Manufacturing technology of roll forming was tested by computer and physical simulation. Service properties of components were obtained by subsequent heat treatment. The effectiveness of the technology associated with increased service properties of components and decreases the labor content by automated equipment.

Deformation Nanostructuring and Superplastic Processing of Metallic Materials

Theoretical and practical aspects of fabrication and processing of bulk metallic nanomaterials are presented. The effect of different deformation modes on the structure formation is shown. Development of nanotechnology with respect to fabrication of gas turbine engines (GTE) parts made of nanostructured superalloys is exemplified.

Superplastic Behavior of ATI 718Plus Superalloy

Complex shaped, ultra thin-walled parts can be manufactured using superplastic forming. Hot working temperature for the production of fine-grained billets (d=5-15 μm) out of ATI Allvac 718Plus® superalloy is in the range of 982-1038°C. An ultrafine-grained structure (d=0.3 μm) was produced by multi-axial forging with a gradual decrease of the forging temperature from 950 to 700°C. Superplastic properties of the alloy were carried out in the temperature interval of 700-950°C. It has been revealed that the fine-grained alloy provided superplastic elongations about 300% at 950°C and strain rate of 10-4 s-1. The highest elongation of ultrafine-grained alloy was about 1450% and very low flow stresses were reached at 900°C and strain rate of 3×10-4 s-1. The ultrafine-grained alloy showed superplastic properties also at 700°C (0.62Tm). The microstructure and superplastic properties of the alloys 718 and 718Plus are compared.

Microstructure and Microhardness of a Nanostructured Nickel-Iron Based Alloy

This paper presents an overview and some original results about the mechanical properties and phase analysis of a nanostructured (NS) nickel-iron based alloy INCONEL 718. This structure was obtained by severe plastic deformation (SPD) via high pressure torsion (HPT) and multiple isothermal forging (MIF) of the alloy with an initial coarse-grained (CG) structure. Materials before and after SPD were analyzed by scanning, transmission electron microscopes and atom probe tomography (APT). Experimental data indicate that after HPT at room temperature - phase was partly dissolved and that precipitation of the -phase occurs during post deformation aging. A hardness up to 8 GPa was recorded for the NS alloy after SPD and annealing at 600°C.

Superplastic Roll Forming of Axial Symmetric Articles from Superalloys

A new method of microstructure design in axisymmetric gas turbine engine components made of superalloys is proposed. The method is based on obtaining three types of microstructures by means of superplastic roll forming: coarse-grained one with coherent nano-sized precipitates of the  phase, the second with ultrafine grains of both phases and the third combining coarse deformed matrix grains surrounded by thin layers of fine matrix grains and coarsened precipitates of  phase.