In-situ TEM/STEM observations of intermetallic growth and reverse transformation of ultra-fine bainitic steel

Multi-scale manufacturing covering dynamics in matter and processes observed using in-situ microscopy methods provides an immense capability for better understanding the crucial mechanisms in the nanoscale. Phenomena occurring in engineering materials are mainly revealed in a multidimensional manner involving ex-situ conditions. Although steel is classified as a well-known engineering material, there are still pivotal research challenges related to developing new steel grades with higher in-use and mechanical properties, which are controlled by the evolution of the microstructure. In this respect, dynamic microscopy constitutes an indispensable (although still niche) research method that supports the revealing phenomena that cannot be identified using other research techniques.

The motivation for carrying out this research is strictly dictated by current research directions and challenges in the steel sector. Global directives relating to CO₂ emission reduction in industry support the development of new material solutions focused on achieving high-strength properties, which lead to the diminution of the structure’s weight and fuel consumption. Therefore, steels classified as Advanced High-Strength Steels (AHSS) are the subject of extensive research from both the scientific community’s perspective and the industry. A relevant role in this group of steels is reserved for nanocrystalline bainitic steels, which provide a combination of high-strength parameters with satisfactory ductility. Caballero and Bhadeshia [1], [2] developed the strategy for alloy design of carbide-free bainitic steels formed by nanoscale bainitic ferrite plates and retained austenite films. Current scientific research focuses on e.g. understanding the strengthening mechanisms, kinetics of bainitic transformation, maximization of mechanical properties and optimization of heat treatment processes to improve the prospects of industrialization[3], [4], [5].

Within the scope of this investigation, a new material with an ultra-fine bainitic structure prone to intermetallic strengthening is tested to explore the microstructure evolution during the influence of elevated temperature. Hulme-Smith et al. [6], [7], [8], [9], [10] and Królicka et al. [11], [12], [13] developed the bainitic steels subjected to intermetallic strengthening by nickel aluminide (β phase with B2 structure). This alloy design concept involves a synergistic combination of strengthening mechanisms typical for maraging and advanced bainitic steels. Intermetallic strengthening is an effective solution for enhancing thermal stability under the influence of elevated temperatures, which constitutes a limitation related to the in-use properties of nanocrystalline bainitic steels [14]. Considering ex-situ transmission electron microscopy (TEM) observation conditions, the tested steel was characterized by the presence of the B2 phase (nickel aluminide) after tempering at a temperature of 550 °C (Fig. 1). The chemical composition of tested steel supports the synergistic effect of copper on NiAl formation. Jiao et al. [15], based on atom probe tomography (APT) tests, observed the formation of NiAl and Cu nanoscale precipitation and interfacial segregation in the martensite and austenite regarding maraging steel. In terms of the BCT phase, isolated NiAl nanoparticles and NiAl/Cu co-precipitates were revealed. It should be mentioned, that identification of the NiAl/Cu co-precipitates due to their nanoscale is difficult using the conventional TEM methods. Based on the similarity of nanoscale bainitic ferrite and martensite (both BCT structures [16], [17], [18]), the formation of NiAl/Cu co-precipitates is highly likely in tested steel. Considering this limitation, the current research focused on the B2 observation assuming that Cu also influences the formation of the NiAl-B2 phase.

Considering previous work [13], the highest hardness was noticed during additional tempering at 550 °C/1 h. Moreover, a progressive increase in hardness was observed in comparison to the structure without additional tempering. This indicates that the precipitation processes started already at low tempering temperatures (Fig. 1d). The highest increase in hardness was approximately 30 % compared to the initial microstructure after isothermal heat treatment (435 ± 3 HV1 and 577 ± 6 HV1, respectively). Bearing the mind the complexity of the precipitation processes of intermetallic phases, an attempt was made to observe the evolution of the structure as a function of temperature using the in-situ TEM technique. The aim of the research was not only to observe the growth of intermetallic phases but also to reveal the mechanisms of decomposition of the metastable bainitic structure. It was discovered that during influence of elevated temperatures, both retained austenite and bainitic ferrite are decomposed into a mixture of ferrite and cementite [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Overall, the metastable bainitic structures tend to reach a thermodynamic equilibrium state during tempering processes involving various stages without a unique path, the detailed mechanisms of bainitic structure decomposition was in-depth described in literature review [14]. Moreover, the transformation of retained austenite into martensite occurs during the cooling processes caused by the lower stability of carbon-depleted austenite blocks [25], [29], [30], [31]. Therefore, the direct and indirect decomposition processes may be proposed [29], [31]. The performed in-situ TEM tests support the observation of direct decomposition processes, which are difficult to observe under ex-situ conditions. Within the scope of this work, microstructure evolution were observed during the reverse transformation, covering all stages of bainitic structure decomposition processes.

The novelty aspect of the conducted investigations includes both the methodological approach using advanced in-situ TEM/STEM technique during direct heating and constitutes a response to research gaps related to the stability of the bainitic structures and intermetallic strengthening. Concerning steel, in-situ TEM straining experiments dominate the literature [32], [33], [34], [35], [36]. However, real-time observations of heating and cooling in a microscope column constitute a niche. Xia et al. [37] investigated martensitic steel subjected to tempering performed in the microscope to observe the coarsening of cementite. This experiment involved rapid heating up to 500 °C and observations using TEM mode and EDS mappings. Liu et al. [38] observed the carbide formation at the twinning boundary and recrystallization processes regarding the Fe-1.6 C alloy. Bambach et al. [39] revealed the Cu particles during continuous heating in medium-carbon martensitic steel. Wang [40] investigated high-carbon martensitic steel subjected to in-situ TEM heating (between 100 °C and 700 °C). The carbide precipitation sequence was observed (M₃C carbide was revealed). Kawahara et al. [41] performed an impressive in-situ TEM investigation of the transition carbides growth sequence of the low-carbon ferritic steels. In the current work, we focus on continuous heating with isothermal stops in the range of the whole reverse transformation. Moreover, tested material is characterized by a bainitic matrix and chemical composition prone to intermetallic strengthening.