Vener Valitov

Evaluation of a Ni-20Cr Alloy Processed by Multi-Axis Forging

This paper discusses the development of a novel processing route to produce ultra finegrain bulk alloy forgings; the microstructural response of these forgings to thermal exposure; and the comparison of mechanical properties to those from conventionally processed material. A Ni- 20Cr [wt%] alloy was processed by near-isothermal multi-axis forging to a grain size of approximately 1 μm. A heat-treatment study over the range 900 to 1200°C was conducted to determine the resultant grain size as a function of time and temperature. Tensile properties were measured at room temperature, 500°C, and 930°C. High-cycle fatigue properties were measured at room temperature. The room-temperature tensile strength was approximately 2.5 times greater than that of conventionally processed Ni-20Cr. Fatigue data showed that the room-temperature highcycle fatigue run-out stress was greater than 100% of the yield stress.

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.

Formation of a nanocrystalline structure upon severe thermomechanical treatment and its effect on the superplastic properties of a nickel alloy with nonisomorphic second-phase precipitates

Severe thermomechanical processing of the Inconel 718 alloy has been shown to cause the formation of a nanocrystalline (NC) structure with an average grain size of 0.08 μm. The processing includes discontinuous thermomechanical treatment by the scheme of multiaxial isothermal forging at low strain rates and stepwise decrease of the treatment temperature to 0.6 Tm. The thermal stability of the NC structure has been studied upon heating and superplastic deformation. It is established that the structure refinement to the NC state decreases the temperature threshold of the low-temperature superplasticity to 0.57 Tm (550°C).

Short-range ordering and the abnormal mechanical properties of a Ni-20% Cr alloy

Effect of temperature on mechanical properties of a Ni-20% Cr alloy with an initial grain size of 100 µm in the temperature interval 150–1000°C (0.25–0.8 Tm) has been studied. It is shown that in the temperature range 400–600°C the alloy demonstrates a positive temperature dependence of both the flow stress and the coefficient of strain hardening and exhibits Portevin-le Chatelier effect (PLC). It has been established on the basis of a comparison of data of calorimetric studies and temperature dependence of mechanical properties that the unusual mechanical behavior of Ni-20% Cr alloy is a result of the occurrence of short-range order. The nature of the effect of the short-range order on the mechanical properties is discussed.

Study of the Properties and the Choice of Alloys for Bladed Disks (Blisks) and a Method for Their Joining

The choice of materials for the bladed disks (blisks) that are intended for next-generation aviation gas turbine engines is grounded. As blade materials, single crystals of light heterophase γ′ + γ VKNA-type alloys based on the γ′(Ni3Al) intermetallic compound with an ordered structure are proposed. The choice of novel deformable EP975-type nickel superalloys, which are intended for operation at 800–850°C, as the disk material is grounded. It is shown that the most effective method for forming one-piece joints of an Ni3Al-based alloy and a high-alloy EP975-type nickel superalloy is the new process of solid-phase pressure welding under conditions of high-temperature superplasticity. Solid-phase joints are formed for heterophase Ni3Al-based alloy single crystals and deformable EK61 and EP975 nickel alloys. The gradient structures in the zone of the solid-phase joints that form under the conditions of low- and high-temperature superplasticity at homologous temperatures of ∼0.6Tm and 0.9Tm are studied. The character and direction of the diffusion processes at the joint of an intermetallic alloy single crystal and a deformable polycrystalline alloy are determined.

Deformation mechanisms in Cr20Ni80 alloy at elevated temperatures

AbstractThe mechanisms of plastic deformation of Cr20Ni80 nichrome with an initial grain size of 80 μm were studied in the temperature range 600–950°C and the strain-rate range 1.5 × 10−6−5 × 10−2s−1. Nichrome is shown to exhibit anomalously high values of stress exponent n and a high deformation activation energy Q. These unusual properties were found to be caused by “threshold” stresses below which deformation does not occur. An analysis of the deformation behavior with allowance for threshold stresses reveals the regions of hot, warm, and cold deformation in nichrome. At normalized strain rates

New Technologies Development and Equipment for Local Shape-Forming of the Complicated Parts Made of Heat-Resistant Alloys under Superplastic Deformation Conditions

\dot \varepsilon

Modeling the structure formation during hot deforming the billets of the parts of gas-turbine engines made of heat-resistant nickel alloy

kT/D1Gb < 10−8, the true values of n and Q are ∼4 and 285 ± 30 kJ/mol, respectively. In the normalized-strain range 10−8−10−4n ∼ 6 and the deformation activation energy decreases to 175 ± 30 kJ/mol. This change in the deformation-behavior characteristics is explained by the transition from high-temperature dislocation climb, which is controlled by lattice self-diffusion, to low-temperature dislocation climb, which is controlled by pipe diffusion, as the temperature decreases. At