Creep Behaviour and Microstructural Characterization of VAT 36 and VAT 32 Superalloys

Abstract

Superalloys are used primarily for the aerospace, automotive, and petrochemical industries. These applications require materials with high creep resistance. In this work, evaluation of creep resistance and microstructural characterization were carried out at two new nickel intermediate content alloys for application in aerospace industry and in high performance valves for automotive applications (alloys VAT 32 and VAT 36). The alloys are based on a high nickel chromium austenitic matrix with dispersion of intermetallic L12 and phases containing different (Nb,Ti)C carbides. Creep tests were performed at constant load, in the temperature range of 675–750 °C and stress range of 500–600 MPa. Microstructural characterization and failure analysis of fractured surfaces of crept samples were carried out with optical and scanning electron microscopy with EDS. Phases were identified by Rietveld refinement. The results showed that the superalloy VAT 32 has higher creep resistance than the VAT 36. The superior creep resistance of the alloy VAT 32 is related to its higher fraction of carbides (Nb,Ti)C and intermetallic L12 provided by the amount of carbon, titanium, and niobium in its chemical composition and subsequent heat treatment. During creep deformation these precipitates produce anchoring effect of grain boundaries, hindering relative slide between grains and therefore inhibiting crack formation. These volume defects act also as obstacles to dislocation slip and climb, decreasing the creep rate. Failure analysis of surface fractures of crept samples showed intergranular failure mechanism at crack origin for both alloys VAT 36 and VAT 32. Intergranular fracture involves nucleation, growth, and subsequent binding of voids. The final fractured portion showed transgranular ductile failure, with dimples of different shapes, generated by the formation and coalescence of microcavities with dissimilar shape and sizes. The occurrence of a given creep mechanism depends on the test conditions. At creep tests of VAT 32 and VAT 36, for lower stresses and higher temperature, possible dislocation climb over carbides and precipitates would prevail. For higher stresses and intermediate temperatures shear mechanisms involving stacking faults presumably occur over a wide range of experimental conditions.

Creep and Mechanical Behavior Study of Inconel 718 Superalloy

“…According to Oliveira et al . , the results of the creep test with nickel-based superalloy 625 suggested that the Considering creep mechanism at 675 ºC of VAT 32 (superalloys with intermediate nickel content different amounts of carbon, iron, titanium, and niobium) by Gobbi et al 10 , the stress exponent value is 13.72 and activation energy is 616 kJ/mol, results were correlated to formation of higher amount of carbides and intermetallic phases caused by higher mass percentage of carbon and different amounts of alloying elements.…”

“…Nickel-based superalloys are known since the 1930s, and used primarily in aerospace applications [1][2][3][4] . These applications require a material with high mechanical strength 5,6 , good resistance to fatigue and creep [7][8][9][10] , good corrosion resistance 11 , and ability to operate continuously at elevated temperatures 11,12 .…”

“…Creep and creep-fatigue considerations are important in predicting the remaining life and safe inspection intervals as part of maintenance programs for components operating in harsh, high temperature environments. Creep deformation (primary, secondary, tertiary creep) and rupture is one of considerations that are involved in determining the risk of fracture in these structural components that experience harsh environments such as high temperatures and stresses are common in land-based steam and gas turbines, in aircraft engines, and in power-plant components 6,8,10,[25][26][27][28][29] .…”

Creep and High-Temperature Deformation of Metals and Alloys

“…The strict correlation between microstructural features is also of utmost importance for the development and application of other high-temperature alloys. Moving to steels and Ni-based alloys characterized by an austenitic matrix, the strengthening role played by carbides, other precipitates, or dispersoids, has been experimentally investigated in the temperature range of 675-750 • C by Gobbi et al [12] in the case of alloys VAT 32 and VAT 36. In the gamma prime-containing Ni-base superalloys, the creep resistance at high temperature is also affected by the presence of solute atoms in solid solution.…”

Superalloy/Al2O3 type composite compacts obtained by spark plasma sintering from mechanically alloyed powders

“…Usually, superalloys can operate at a high fraction of their melting point. Because of this, superalloys are used in jet engine components such as turbine blades and combustion chamber [6]. Oxide Dispersion Strengthening (ODS) superalloys have been researched in the last decades because the ODS superalloys have excellent high-temperature creep-rupture strength and resistance oxidation and corrosion is higher than those of the superalloys [7,8].…”