Portevin-Le Châtelier effect in wrought Ni-based superalloys: Experiments and mechanisms

Abstract

The Portevin-Le Châtelier (PLC) effect is a plastic instability in alloys at certain strain rates and deformation temperatures. This plastic instability exhibits serrated yielding in the temporal domain and strain localization in the spatial domain. Wrought Ni-based superalloys often exhibit the PLC effect. To guarantee the safe and stable operation of equipment, it is important to study the PLC effect in wrought Ni-based superalloys. This paper provides a review of various experimental phenomena and micromechanisms related to the PLC effect in wrought Ni-based superalloys, which have been reported in various publications in recent years and include work from our own group. The influences of stacking fault energy and γ′ precipitates on the PLC effect in wrought Ni-based superalloys are also discussed in detail. Additionally, several suggestions for the future study of the PLC effect in wrought Ni-based superalloys are provided.

Introduction

Wrought Ni-based superalloys play a dominant role in aero engines and industrial gas turbines based on several advantages [[1], [2], [3]]. First, the austenitic structure of nickel can withstand a wide range of temperatures and bond with numerous alloying elements, which is beneficial for material strength. Second, such alloys have excellent oxidation resistance. Third, desired mechanical properties can be obtained by optimizing the morphology, distribution, and volume fraction of γ′ precipitates. Generally, wrought Ni-based superalloys have service temperatures based on a high degree of alloying to intensify solution or precipitation strength [[4], [5], [6]]. For example, a Ni‒Co-based superalloy, which contains Co and Ti, can increase the service temperature by nearly 50 °C [7]. However, a high degree of alloying makes plastic deformational behavior (i.e. the Portevin-Le Châtelier (PLC) effect) more complex.

Plastic deformation behavior is the most fundamental and important mechanical behavior of alloys. The stress‒strain curves of uniform deformation are smooth before necking occurs in tensile tests. However, most alloys exhibit plastic instability with yield points [8] or serrated flow [9] phenomena. The yield point phenomenon generates an abrupt drop in the stress‒strain curve at the end of the elastic region. This drop in stress is followed by fluctuations in the curve, which correspond to local deformation bands (Lüders bands) in a tensile sample. The serrated flow phenomenon in the plastic region is also known as the PLC effect, which is common in most Ni-based superalloys [[10], [11], [12]]. The PLC effect often generates serrations in stress‒time curves and step changes in strain‒time curve. Additionally, tensile samples exhibit the propagation of strain localization, which is often referred to as PLC banding. Deformation bands, including Lüders bands and PLC bands, have a negative impact on the surface quality and further processing of alloys. Lüders bands only appear at the beginning of plastic deformation and can be eliminated via predeformation, while PLC bands can appear at any time during plastic deformation and cannot be removed effectively. Therefore, it is necessary to study the effects of deformation conditions on PLC and avoid the deformation region in which the PLC effect occurs.

Portevin and Châtelier first conducted detailed experimental studies on the plastic instability of aluminum alloys and proposed the concept of “serrated yielding” [13,14]. This plastic instability phenomenon was named as the PLC effect by Cottrell to honor their efforts [15]. Dynamic strain aging (DSA) has been widely accepted as the main mechanism of the PLC effect [[16], [17], [18]]. A serrated flow and negative strain rate sensitivity are the predominant characteristics in the DSA regime [19]. The critical strain of a serrated flow, which depends on the strain rates and temperatures, is a crucial factor in the PLC effect. Based on variations in critical strain, DSA can be divided into the normal DSA and inverse DSA regimes [20]. DSA is based on interactions between solute atoms and mobile dislocations. Therefore, all factors (including stacking fault (SF) energy and γ′ precipitates) that affect these interactions have an influence on the PLC effect [21,22].

Additionally, significant attention has been paid to the PLC effect in Ni-based superalloys based on its influence on mechanical properties. Valsan et al. [23] determined that DSA influences the deformation and fracture behaviors of the Nimonic PE 16 superalloy in a temperature range of 450–550 °C at low strain rates. Gopinath et al. [24] identified dislocations pinned by solutes because DSA can offset softening during the low-cycle fatigue testing of the U720Li alloy. Rezende et al. [25] reported that DSA and precipitates are responsible for the high strength of the Inconel 718 superalloy at elevated temperatures. Pu et al. [26] inferred that the intermediate-temperature embrittlement of the Ni-based UNS N10276 superalloy may be associated with a strong PLC effect. Therefore, the PLC effect can be utilized to enhance mechanical strength, but should be avoided during hot deformation processes. Understanding the PLC effect has great significance for designing and applying wrought Ni-based superalloys.

This paper presents a review of PLC effects in wrought Ni-based superalloys. Our main focuses can be divided into the experimental phenomena and micromechanisms of PLC effects in wrought Ni-based superalloys. Additionally, shortcomings and prospects are discussed in the final section.