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Superalloys have been developed to meet the need for materials that will serve at high operating temperatures without losing their properties. The three major classes of superalloys are nickel-, iron-, and cobalt-based alloys. Cobalt-based superalloys are used between 650 and 1100 °C temperatures due to their wear, creep, oxidation, heat, and corrosion resistance. Haynes 188, an essential family member, is mainly used in gas turbine engines such as combustion chambers, air transfer ducts, fixed blades, exhaust nozzles, and flame holder components. Strengthening is achieved by solid solution or carbide and intermetallic formed in their structures. None of the cobalt-based superalloys are fully solid solution alloys. Almost all contain carbide or intermetallic compounds. This can lead to dynamic strain aging at different service temperatures in the alloy under load, resulting in a reduction in ductility. Considering the temperatures in service conditions, the dynamic strain ageing behavior of Haynes 188 alloy at 25, 250, 400, 550, 700, 850, and 1000 °C was thoroughly examined for the first time in this study. The unsteady yielding with different type’s serration verified that the dynamic strain ageing is formed in Haynes 188 alloy. Therefore, ductility of alloy decreased in the temperature range of 250-700 °C, particularly at 550 °C.

Low-cycle fatigue tests were conducted on superalloy Inconel 625 in the solution-treated condition in air, at different temperatures under fully reversed axial strain-controlled mode at strain rate of 5 × 10 –3  s −1 . Fatigue life was reduced with increase in strain amplitude and temperature. Initial cyclic hardening was observed till peak stress level, followed by softening till fracture. The sudden drop in stress amplitude at 500 °C can be associated to the dynamic strain ageing, also inferred from the serrations present in the cyclic stress–strain hysteresis loops at 500 °C and 700 °C. The fatigue life variation with plastic strain amplitude complies well with the Coffin–Mansion relationship. Deformation substructure showed evidence of dislocation bands, loops, bowing and persistent slip bands at 700 °C. Fractographs of fractured samples depict striations, typical of fatigue fracture. Increase in striation width was noticed with rise in temperature.

The objective of this investigation is to study the cyclic deformation behavior of SA333 Gr-6 C-Mn steel at 300°C. Low cycle fatigue tests were carried out at total strain amplitude between ±0.35 and ±1.25% at a constant strain rate of 1 × 10−3 s−1. Ratcheting tests were conducted at a various combination of mean stress and stress amplitude at a constant stress rate of 115 MPa s−1. The material SA333 Gr-6 steel exhibits cyclic hardening throughout its fatigue life. The material shows non-Masing behavior and deviation (𝛿𝜎𝑜) from Masing behavior increase with an increase in strain amplitude. Ratcheting strain accumulation increases, whereas ratcheting life decreases with an increase in mean stress or stress amplitude. With an increase in mean stress and stress amplitude, ratcheting rate also increases. The material shows hardening characteristic due to dynamic strain aging (DSA) phenomena.

Purpose The purpose of this paper is to study the effect of mean stress and stress amplitude on the asymmetric cyclic deformation behavior of SA333 Gr-6 C-Mn steel. Such type of loading may arise during the service period because of the load fluctuations, thermal gradients and sudden loading like seismic events. Tests were also carried out at different temperatures to understand the effect of it on sensitiveness of the materials deformation behavior. Design/methodology/approach Cylindrical specimen of 8-mm gauge diameter and 15-mm gauge length was fabricated from the pipe section along its axis. Stress controlled ratcheting tests were carried out by using triangular waveform for cyclic loading. The strain accumulations were measured using 12.5-mm gauge length extensometer. Ratcheting tests were carried out at fixed stress amplitude of 400 MPa and mean stress varying from 0 to 75 MPa, whereas at the fixed mean stress of 100 MPa and stress amplitude varies from 300 to 400 MPa at 300°C. To study the effect of temperature on ratcheting behavior, tests were carried out at a load of 100 MPa mean stress and 350 MPa stress amplitude, with a varying temperature between room temperature and 350°C. The stress rate of 115 MPas-1 was kept constant for all the tests. Findings Increase in mean stress and stress amplitude, ratcheting strain and plastic strain amplitude increases, whereas ratcheting life decreases. With an increase in temperature, ratcheting life increases and strain accumulation decreases up to 300°C, whereas on further increase in temperature, strain accumulation increases with reduction in ratcheting life. Minimum ratcheting rate was observed at 250°C and 300°C. The dynamic strain aging (DSA) phenomena lead to the hardening of the material. The investigated steel shows DSA temperature regime lies between 250°C and 300°C. The failure modes at 250°C and 300°C temperature was transgranular, whereas at 350°C complete ductile. Research limitations/implications The stress rate and loading condition may vary to study the ratcheting behavior. Practical implications From this study, the critical cyclic load may be determined. The DSA temperature regime of this material is determined at this stress rate. This could help to evaluate the cyclic deformation behavior of the material with temperature changes. Originality/value In this investigation, the DSA temperature regime has been determined where maximum ratcheting life, minimum strain accumulation and ratcheting rate were observed. The critical load where the minimum life of the material occurred at elevated temperature is 100 MPa mean stress and 400 MPa stress amplitude.

In the current study, dynamic strain ageing (DSA) phenomena in 316L austenitic stainless steel was investigated under as-received and as-welded conditions. A tensile test was carried out on as-received and as-welded samples for the temperatures of 25–800 °C at a strain rate of 1 × 10−3 s−1. Microstructure and fracture surfaces were investigated by optic and scanning electron microscopes (SEM). 316L austenitic stainless steel showed different DSA behavior under as-received and as-welded conditions, which are discussed in terms of microstructure and mechanical properties.

This work reports results from quasi-static nanoindentation measurements of iron, in the un-strained state and subjected to 15% tensile pre-straining at room temperature, 125 °C and 300 °C, in order to extract room temperature hardness and elastic modulus as a function of indentation depth. The material is found to exhibit increased disposition for pile-up formation due to the pre-straining, affecting the evaluation of the mechanical properties of the material. Nanoindentation data obtained with and without pre-straining are compared with bulk tensile properties derived from the tensile pre-straining tests at various temperatures. A significant mismatch between the hardness of the material and the tensile test results is observed and attributed to increased pile-up behaviour of the material after pre-straining, as evidenced by atomic force microscopy. The observations can be quantitatively reconciled by an elastic modulus correction applied to the nanoindentation data, and the remaining discrepancies explained by taking into account that strain hardening behaviour and nano-hardness results are closely affected by dynamic strain ageing caused by carbon interstitial impurities, which is clearly manifested at the intermediate temperature of 125 °C.