Efeito do tempo de moagem de alta energia e da carga de compactação na microestrutura e nas propriedades mecânicas de ligas TI-SI-B produzidas por metalurgia do pó

Abstract

Titanium alloys are widely used in advanced technological applications due to their high specific mechanical strength, low density, and excellent corrosion resistance, making them particularly attractive for the aerospace, biomedical, chemical, and energy sectors. The controlled addition of alloying elements allows tailoring of their properties and significantly expands their range of applications. In this context, silicon (Si) and boron (B) stand out for their pronounced influence on the microstructure and mechanical behavior of titanium, promoting increased hardness, wear resistance, thermal stability, and grain refinement. The combined addition of these elements results in Ti–Si–B alloys with more homogeneous microstructures and superior performance under severe service conditions. The objective of this doctoral research was to investigate the influence of high-energy milling time and compaction load on the microstructure, densification, porosity, and mechanical properties of Ti-2Si-1B and Ti-6Si-3B alloys produced by powder metallurgy. The alloys were synthesized from elemental titanium, silicon, and boron powders processed by high-energy milling in a planetary mill at a rotational speed of 200 rpm, using a ball-to-powder mass ratio of 1:20 and milling times of 4 and 8 hours to promote homogeneous mixing. After milling, the powders were compacted under different loads and sintered at 1250 °C for 4 hours, a condition established based on differential thermal and thermogravimetric analyses, which indicated the occurrence of the main microstructural transformations within this temperature range. Material characterization was performed using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, particle size analysis, density and porosity measurements by the Archimedes method, Vickers microhardness testing, nanoindentation for elastic modulus determination, and wettability tests. The results showed that after 4 hours of high-energy milling, both alloys exhibited a predominantly unimodal particle size distribution, with a higher concentration around 50 μm, indicating that mechanical mixing was the dominant mechanism during this processing stage. For the Ti-6Si-3B alloy, the higher content of secondary elements led to a narrower particle size distribution, demonstrating increased milling efficiency. Increasing the milling time to 8 hours resulted in a tendency toward particle agglomeration and broader size distributions, attributed to the high fraction of ductile titanium combined with the high energy input, which favored cold welding. X-ray diffraction analyses revealed the predominance of the α-Ti phase after milling and the formation of intermetallic phases such as Ti₆Si₂B and TiB after sintering, with TiB being more pronounced in the Ti-6Si-3B alloy. The sintered densities ranged from approximately 3.37 to 3.94 g/cm³, with apparent porosity between 11% and 24%. Mechanical testing indicated microhardness values between 400 and 720 HV and elastic moduli consistent with values reported in the literature. Overall, the results demonstrate that low-alloy Ti–Si–B systems produced by high-energy milling and powder metallurgy exhibit homogeneous microstructures and mechanical properties suitable for applications in severe environments, highlighting the importance of careful control of processing parameters.