Multi-strengthening-mechanisms in a novel titanium alloy with (TiHf)5Si3 particle-reinforcement

Titanium and its alloys are widely used as an important class of engineering materials in aerospace, nuclear, chemical plants and biomaterials fields due to their high strength-to-weight ratio and good corrosion resistance [1, 2]. It is well known that stable Ti5Si3 phase with a complex P63 hexagonal structure possessing high melting point normally can be formed in high silicon-containing Ti alloys and therefore can act as reinforcement even at high temperature [3-5]. However, high volume fraction of Ti5Si3 phase may depress the plasticity of the Ti alloys at room temperature due to its brittleness. Thus, increasing needs of the Ti alloys with good performances invoke widespread investigations on the relationship between microstructure and properties of newly designed silicon-containing Ti alloys. Recently, Ti-Si-X alloys with β-stabilizing elements have been proposed and exhibited improved plasticity [6, 7]. Our previous work also proved that the comprehensive mechanical properties of silicon-containing Ti alloys could be improved by adding multiple alloying elements including Nb, Fe, Hf, Ta and W [8]. However, the relationship between microstructure and properties and the underlying strengthening mechanisms of these Ti alloys have not been investigated sufficiently.


Introduction
Titanium and its alloys are widely used as an important class of engineering materials in aerospace, nuclear, chemical plants and biomaterials fields due to their high strength-to-weight ratio and good corrosion resistance [1,2]. It is well known that stable Ti 5 Si 3 phase with a complex P63 hexagonal structure possessing high melting point normally can be formed in high silicon-containing Ti alloys and therefore can act as reinforcement even at high temperature [3][4][5]. However, high volume fraction of Ti 5 Si 3 phase may depress the plasticity of the Ti alloys at room temperature due to its brittleness. Thus, increasing needs of the Ti alloys with good performances invoke widespread investigations on the relationship between microstructure and properties of newly designed silicon-containing Ti alloys. Recently, Ti-Si-X alloys with β-stabilizing elements have been proposed and exhibited improved plasticity [6,7]. Our previous work also proved that the comprehensive mechanical properties of silicon-containing Ti alloys could be improved by adding multiple alloying elements including Nb, Fe, Hf, Ta and W [8]. However, the relationship between microstructure and properties and the underlying strengthening mechanisms of these Ti alloys have not been investigated sufficiently.
In this study, a novel Ti-2Si-2Nb-2Fe-1Hf-1Ta-1W alloy noted as Ti alloy in the following has been fabricated as researching object to study the relation between microstructure and mechanical properties. Multi-strengthening-mechanisms of this novel Ti alloy were discussed based on electron microscopy and correlated statistical analysis.

Experimental
The Ti alloy was fabricated by arc-melting followed by hot forging and hot rolling at 980 ˚C and 820 ˚C, respectively. Details of the fabrication processing, dimension and composition for this Ti alloy were given in our previous work [8]. Solution treatments were carried out at 800 ˚C and 900 ˚C, which are about 50 ˚C below and above β transus temperature of 847 ˚C, respectively, for 40 min followed by air cooling. Then, aging treatments were carried out at 500 ˚C for 6 h followed by air cooling for both solution conditions. Samples were kept for each condition including as-rolled Ti alloy before heat treatment, i.e. five types of samples denoted as as-rolled, 800 ˚C, 800 ˚C+500 ˚C, 900 ˚C and 900 ˚C+500 ˚C.

Microstructure
The overall microstructure of as-rolled sample is shown in Figure 1(a), showing fine equiaxed grains with uniform distribution of particle reinforcements. Figure 1  The microstructures of the Ti alloys after different heat treatments are shown in Figure 2, showing that the average size of the particles does not change much with a slight growth to 1 µm. Figure 2(a) shows that after solution treatment at 800 ˚C, the Ti matrix consists of equiaxed α grains and β+α colonies with thin α lath. After aging treatment at 500 ˚C, the Ti matrix retains equiaxed α grains and β+α colonies with growth of α lath as shown in Figure 2(b). Figure 2(c) shows that after solution treatment at 900 ˚C, the Ti matrix consists of β+α colonies. After aging treatment at 500 ˚C, the Ti matrix retains β+α colonies with growth of α lath as shown in Figure   2(d). The different microstructures between the Ti alloys treated at 800 ˚C and 900 ˚C can be attributed to α/β transformation at 847 ˚C.
The grain size distributions of the Ti alloys under different conditions are plotted in Figure 3. It should be noted that the grain size for equiaxed α/β grains and β+α colonies were measured as grain diameter and colony size, respectively. Statistical results were collected from overall microstructures for each condition by applying intercept method for grain diameter and area calculation for frequency [8,9]. It shows that after solution treatments at high temperatures of 800 ˚C and 900 ˚C, the peak diameter of α/β grains increases from 1 µm to 2.5 µm and 3 µm, respectively. In addition, the peak diameter of β+α colonies increases from 2.5 µm to about 4 µm after corresponding aging treatments.  Figure 4(a) shows clean particle with a diameter of about 1 µm indexed as (TiHf) 5 Si 3 as shown in inset. It should be noted that (TiHf) 5 Si 3 has same crystal structure as Ti 5 Si 3 with partial Hf atoms occupation instead of Ti atoms in unit, and the detail of (TiHf) 5 Si 3 formation has been reported in our previous work [8]. Figure 4(b) shows high density dislocations generated near (TiHf) 5

Discussion
With uniform distribution of particle reinforcements, the strengthening of the Ti alloy can be attributed to i) particle strengthening, ii) solid-solution strengthening, iii) grain boundary strengthening and iv) dislocation strengthening.

Particle strengthening
The novel Ti alloy is reinforced by (TiHf) 5 Si 3 particles and the average diameter of these particles can be kept as about 1 µm even after high temperature heat treatment for a long period as shown in Figure 1 and 2. Moreover, the existence of (TiHf) 5 Si 3 particles located at grain boundaries and triple junctions can resist the growth of matrix Ti grains. Particle strengthening performs in two ways.
On the one hand, the (TiHf) 5 Si 3 particle itself has higher module and sustains higher stress theoretically at equivalent strain than Ti 5 MATEC Web of Conferences 321, 11078 (2020) https://doi.org/10.1051/matecconf/202032111078 The 14 th World Conference on Titanium matrix. On the other hand, the (TiHf) 5 Si 3 particles can hinder the dislocation motion and thereafter case dislocations accumulations close to (TiHf) 5 Si 3 particles.

Solid-solution strengthening
Solid-solution strengthening can be caused by alloyed elements with different atom size compared to metal matrix due to unit misfit and relative variation of strain fields. Local strain fields can be created and interact with dislocations for example impede their motion, leading to increasing strength. Si added in this Ti alloy partially forms Ti(Si) solid solution with very low Si concentration [10] and forms silicide at the same time. Nb, Fe, Ta and W act as solid-solution elements in the Ti matrix. Hf also may act as solid-solution element in the Ti matrix and is likely to solute in the Ti 5 Si 3 phase resulting in the (TiHf) 5 Si 3 compound phase [8].

Grain boundary strengthening
According to the Hall-Petch relationship, the yield strength of Ti increases with decreasing grain size. The average grain diameter d of the Ti alloy under different conditions can be calculated by expectation according to statistical results from Figure 3. The stressgrain size relationship at a strain of 2% with a dash drawn line by idealized modeling is given as shown in Figure 6 and it turns out that the strength of the Ti follows the Hall-Petch relationship well. It should be noted that, the given strain is 2% since the grain size of the Ti alloy is generally small and therefore notable response of dislocation slip demands higher strength instead of yield strength at a strain of 0.2%. In addition, a misfit strain may be created due to mismatch deformation behavior between α and β phase during tension thereafter be relaxed by formation of geometrically necessary dislocations (GND) near phase boundaries [11]. The phase boundaries in this way can also strengthen the Ti alloys to some extent.

Dislocation strengthening
The difference in thermal expansion between (TiHf) 5 Si 3 particle and Ti matrix may give rise to a thermal stress at interface between them during air cooling after heat treatments. This may cause elastic/plastic deformation near the interface to some extent leading to dislocations generated near the (TiHf) 5 Si 3 particles (Figure 4(a)). The dislocation density ρ caused by residual thermal stress can be calculated according to ref [12] as: where B is a geometric constant, b is the Burger's vector of the Ti matrix, Δα is the difference of thermal expansion coefficients between (TiHf) 5 Si 3 particle and Ti matrix, ΔT is temperature variation, and f, R, and D are the volume fraction, aspect ratio and diameter of the particles, respectively. Moreover, the different elastic/plastic properties between (TiHf) 5 Si 3 particle and Ti matrix may cause mismatch in strained Ti alloy leading to the GND formation near the interface. These dislocations resulted from thermo mismatch and mechnical mismatch between the (TiHf) 5 Si 3 particle and the Ti matrix may creat extra strengthening effect on the Ti alloy.

Conclusions
A novel Ti-2Si-2Nb-2Fe-1Hf-1Ta-1W alloy with (TiHf) 5 Si 3 particle-reinforcement has been fabricated, exhibiting good mechanical performance. The (TiHf) 5 Si 3 particles can be kept as about 1 µm even after high temperature heat treatment for a long period. With uniform distribution of the (TiHf) 5 Si 3 particle-reinforcement at grain boundaries and triple junctions, multiple strengthening mechanisms have been found including i) particle strengthening, ii) solid-solution strengthening, iii) grain boundary strengthening and iv) dislocation strengthening.