Microstructure Dual-Refinement and Mechanical Properties Manipulation of TiBw/TA15(Si) Composites by Thermo-mechanical Processing and Heat Treatment

Titanium matrix composites with near-α titanium alloy as the matrix exhibit high specific strength and good high-temperature strength. However, the thermo-mechanical processing is limited by the brittleness of the reinforcement, making it difficult to refine the grain size and control the silicide precipitates. The present work aims to prepare titanium matrix composites with refined grain size and silicide particles. To this end, network-architecture TiBw/TA15(Si) composites were prepared using low-energy ball milling and vacuum hot-pressing. After refining the initial lamellar microstructure of the material through a solid solution treatment, the evolution of the TA15(Si) matrix and TiBw reinforcement during the hot deformation were studied. Ideal thermo-mechanical processing parameters in dual-phase region for TiBw/TA15(Si) composites after solid solution treatment was obtained, and the strain distribution in the compressive sample was revealed. Subsequently, TiBw/TA15(Si) composites with fine grains and dispersed silicide particles were obtained using solid solution treatment and isothermal hot-pressing, achieving a simultaneous improvement in room temperature strength and ductility. The influence mechanism of solid solution treatment on the microstructure and properties of the material was analyzed. Finally, TiBw/TA15(Si) composites with different lamellar content and morphologies were obtained through heat treatment of the deformed material. The effect of heat treatment on the microstructure of the material and the influence of different microstructures on the room temperature and high-temperature tensile mechanical behaviors of the material were discussed. After compositional design through thermodynamic calculation, TiBw/TA15(Si) composites with different Si and Zr contents and a network architecture were prepared by low-energy ball milling and vacuum hot-pressing sintering using spherical TA15 coarse powder and TiB2, ZrB2, and Si fine powders. Silicide particles precipitated on the α/β interfaces in the matrix, and the precipitation amount increased with the increase of Si and Zr element contents. The tensile strength of materials with different compositions ranged from 1012 MPa to 1120 MPa, and the elongation ranged from 1.2% to 6.7%. When the Si content was between 0.3 wt.% and 1.0 wt.% and the Zr content was between 0.3 wt.% and 1.0 wt.%, the materials exhibited good strength and plasticity. By observing the microstructures of different materials subjected to heat treatment at 960°C to 1040°C for 30 min followed by quenching, it was found that silicide particles can be completely dissolved below β transformation temperature when the Si content is below 0.5 wt.%, which is suitable for microstructure control in dual-phase region. Considering the comprehensive mechanical properties and silicide dissolution temperature, a composite with a chemical composition of 3.4 vol.% TiBw/TA15-0.3 wt.%Si was selected for the study of dual-phase region thermo-mechanical processing and microstructure control. After the solid solution treatment in single-phase region, the initial microstructure of TiBw/TA15(Si) was refined, after that, the high-temperature deformation behavior of the material was studied. The hot compression tests were conducted in dual-phase region at temperatures ranging from 870°C to 950°C and strain rates ranging from 1 s-1 to 0.001 s-1. It was found that the compressive stress-strain curves of the composite in this range exhibited dynamic recrystallization features, and the compressive stress decreased with increasing deformation temperature and decreasing strain rate. Except for the fully lath overheated-microstructure observed at 950°C / 1 s-1, the deformation microstructure of the material under the remaining conditions consisted of equiaxed α grains and residual α/β laths. The microstructure became finer with lower deformation temperature and higher strain rate. Higher recrystallization degree was observed below 910°C at lower strain rates, while the opposite was observed above 910°C. Observation of the microstructure produced by different deformation degrees revealed that the formation of equiaxed grains in the material involved three steps: α/β lath distortion, formation of boundaries within α/β laths, and lath decomposition. The activation energy for hot deformation of the TiBw/TA15(Si) composite after single-phase solid solution pretreatment was determined to be 623 kJ/mol. Based on the constructed processing map, the ideal deformation parameters were determined to be in the temperature range of 900°C to 950°C and strain rates of 0.01 s-1 to 0.001 s-1. The orientation distribution of TiBw reinforcements during high-temperature plastic deformation in materials has been studied. The rotational behavior of TiBw reinforcements during deformation was analyzed, revealing that the orientation distribution of the reinforcements is influenced by the direction and magnitude of the principal strain applied to the material. The direction of the principal strain determines the principal directions of covariance matrix for the reinforcement's orientation distribution, while the magnitude of the principal strain determines the parameters in the probability density function. Based on these findings, a method was proposed to calculate the local plastic deformation in materials using the orientation distribution of TiBw reinforcements. Error analysis through computer simulations showed that this method exhibited good accuracy in the range of 20% to 80% compression strain. Using this method, the strain distribution in hot-deformed TiBw/TA15(Si) composites was analyzed, and the accuracy of the calculations was confirmed by comparing them with the deformation of the network structure of TiBw. Based on the results of hot compression tests, TiBw/TA15(Si) composites were subjected to a solid solution treatment followed by isothermal hot pressing at 920 °C and a strain rate of 0.003 s-1 with 75% height reduction. This process yielded a microstructure with fine equiaxed α grains and dispersed fine silicide particles, with an average grain size of 1.6 μm and particle size of 70 μm, respectively, achieving a dual-refinement of both α grains and silicide particles. The composite exhibited a yield strength of 1100 MPa and an elongation of 7.7%. A comparison showed that the solid solution pretreatment led to finer grains and more dispersed silicide particles in the material after isothermal hot pressing, resulting in a significant increase in yield strength. Heat treatment of the hot-pressed composite in the range of 980 °C to 1050 °C revealed that higher temperatures led to higher contents of α/β laths and coarser microstructures, while faster cooling rates resulted in finer lath structures. Water quenching after the heat treatment at 1010 °C for 30 min produced excellent room temperature strength and ductility, with a tensile strength of 1350 MPa and an elongation of 6.7%. Slip trace analysis and transmission electron microscopy analysis revealed that the equiaxed microstructure in the material facilitated multi-system slip, enhancing deformation uniformity. The prismatic slip in lamellar microstructure obtained with air cooling was suppressed by the Burgers orientation relationship, the strain-softening of the lamellae resulted in deformation dominated by single-system slip and strain localization. The transformed β phase obtained through water quenching exhibited limited deformability, which was primarily accommodated by the prismatic slip of primary α laths. High-temperature tensile tests under different conditions showed that decreasing the deformation temperature, increasing the strain rate, refining the grain size, and decreasing the α/β lath content transform the deformation mechanism into grain boundary sliding, which significantly enhances the deformability of TiBw/TA15(Si) composites. The composite with fine equiaxed microstructure could achieve an elongation up to 348% at 800 °C and a strain rate of 0.0003 s-1, which showed a good potential for superplastic forming at low temperatures.
Type of thesis
The University of Waikato
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