Self-lubricating metal matrix composites (SLMMCs) are materials that offer a combination of high wear resistance and low friction coefficients in severe tribological conditions, resulting from their combination of ceramic reinforcements and solid lubricants. In this work, a combination of in-situ precipitation and vacuum impregnation was employed to fabricate an iron matrix composite reinforced with niobium carbide (NbC), resulting in a novel self-lubricating metal matrix composite. This unique approach enables the use of composites as self-lubricating materials by enhancing interfacial adhesion between the matrix and the reinforcements, thereby preventing the detachment of reinforcement particles—a common cause of third-body wear. Moreover, the surface surrounding the pores is composed almost entirely of NbC particles, acting as an ideal reservoir for graphite by preventing the sealing of these pores. The composite was produced in situ from the reaction between graphite and Fe₂Nb intermetallic powders. A theoretical design using CALPHAD simulations, followed by an experimental analysis to optimize the graphite content and sintering temperature to achieve a microstructure consisting of fine micrometric and submicrometric carbides dispersed in a ferritic matrix. The amount of porosity was tailored for vacuum impregnation with graphite in order to induce solid lubrication. Tribological characterization was performed in a ball-on-flat configuration under dry sliding conditions. The material exhibited wear rates of around 1.23 × 10⁻⁶ mm²/N.m against a harder counter body, while also inducing friction coefficients in the 0.08 – 0.1 range, primarily attributed to the effective utilization of porosity as lubricant reservoirs and the reinforcement of the material due to NbC precipitation. This innovative approach enables the creation of high-performance self-lubricating materials and has the potential to be extended to several other combinations of metals, ceramics, reinforcements, and solid lubricants.

This study focuses on developing a new processing route for ferrous matrix composites reinforced with niobium carbide by producing the reinforcement particles in situ using powder metallurgy. The aim is to improve the interfacial adhesion between matrix and reinforcement compared to traditional ex situ methods. Computational thermodynamics and kinetic analysis were used to optimize the raw materials and processing parameters. The raw materials are mixed, uniaxially pressed, and sintered in a tubular furnace. The study finds that liquid phase sintering improves densification but also leads to clustering, niobium-free regions, and abnormal grain growth. The optimal combination of porosity and microhardness is 16.5 ± 0.7% and 952 ± 82 HV0.05, respectively. Although there is room for further adjustments in processing, this study lays the groundwork for creating valuable materials using Brazilian strategic raw materials and technology.

In situ composite manufacturing techniques can improve the bond between the matrix and reinforcements in a composite material. This is achieved by creating the reinforcements within the matrix through the reaction of precursor materials, which form the desired phases. Thermodynamic data and simulation tools can be used to validate the process and design variables, saving time and resources. The aim of this study is to apply a theoretical framework developed by the authors to the development of a Fe-NbC in situ metal matrix composite. The framework involves using Gibbs free energy and dissociation criteria of reinforcements to validate the feasibility of the composite system and to assess composite formation kinetics and microstructural features based on the driving force and diffusion of elements in raw materials. Here, we demonstrate the application of these tools to the development of a Fe-NbC composite, starting from validating the feasibility to selecting between elemental powders, solid solutions, and intermetallics to achieve the desired microstructure.

Successful implementation of third generation advanced high strength steels (3rd gen AHSS) can be accelerated by developing steels that can be heat treated in existing industrial lines. Here, we develop new carbide free bainitic (CFB) steels in which bainite formation is accelerated by a 0.2 volume fraction of prior martensite and thus can be realized in 5 min, making them suitable for manufacturing in modern continuous annealing lines for bare steel strips. The resulting microstructure consists of bainitic ferrite, tempered martensite, and retained austenite. Carbon and silicon had the most pronounced effect on the mechanical properties among the studied alloying elements (manganese, niobium, chromium, and molybdenum) because of their influence on the fraction and stability of retained austenite. Our proposed treatment, which we call bainite accelerated by martensite (BAM), showed higher strength and lower global formability than traditional CFB without prior martensite (also called TRIP-assisted bainitic ferrite, TBF) and quenched and partitioned (Q&P) steels. Five of the designed steels showed tensile strength higher than 1370 MPa, a total elongation higher than 8%, and hole expansion capacity higher than 30%, and thus meet the requirements for the strongest commercial grades of complex phase steels with improved formability. This work broadens the possibilities of using existing industrial lines for manufacturing novel 3rd gen AHSS.

While experiments show that refining the prior austenite grain size can either accelerate or decelerate bainite formation in steels, kinetic models based on the successive nucleation of bainitic ferrite subunits can only predict an acceleration. In this work we develop a physically-based model for bainite kinetics assuming a displacive growth mechanism which is able to reproduce both faster and slower bainite formation kinetics induced by austenite grain refinement. A theoretical analysis of the model and comparison against published experimental data show that slower kinetics for smaller grains is favored as the difference between the activation energy for grain boundary and autocatalytic nucleation of bainite increases, and as the austenite grain refinement results in finer bainite sub-units. We also theoretically analyze the density of initially present potential nucleation sites for bainite and show that the values of density used in other published bainite nucleation models are mostly underestimated. After using physically consistent values for the density of potential nucleation sites, we were able to calculate the apparent lengthening rate of bainite sheaves which were in line with experimentally measured lengthening rates.