Pinning Effect of In-Situ TiCp and TiBw on the Grain Size and Room Temperature Strength of ( TiC + TiB ) / Ti Composites †

(TiCp + TiBw)/Ti composite was firstly prepared by spark plasma sintering (SPS) and hot extrusion from Ti-B4C system, and then isothermally heat treated at 400–800 °C for 24 h to study the pinning effect of the in situ formed reinforcements. Microstructure and phase composition was investigated by SEM, XRD and TEM, variation of the grain size of Ti matrix was studied by EBSD analysis, and room temperature strength was also measured after different heat treatments. Results show that, comparing with pure Ti materials, both the grain size of Ti matrix and the room temperature strength of the composite almost keep stable after heat treatments, suggesting the pinning effect of in situ formed TiCp and TiBw is effective to suppress the growth of Ti matrix grains during heat treatments.


Introduction
Titanium matrix composites (TMCs) were considered to be the most potential candidates for automotive and spaceflight applications due to their excellent properties, such as high specific strength, high specific modulus, good chemical stability and heat resistance (Feng and Zhou, 2004;Liu and Huang, 2013).Up to now, B 4 C ceramic powder became one of the most popular additives for Ti matrix, where an in-situ reaction took place and resulted in two kinds of ideal ceramic reinforcements, viz.particle-like TiC and whisker-like TiB (hereafter referred as TiC p and TiB w ), during the subsequent high temperature consolidation process (Zhou and Qin, 2011;Ni and Geng, 2008a).
As a structural materials used under a moderate temperature, the stability of the strength is an important factor to evaluate the reliability of Ti materials.Although there are many factors that influence the strength of TMCs, such as the grain size, O and N contents, as well as the amounts and morphologies of the reinforcements (Li and Sun, 2013;Zhang and Chen, 2010;Sun and Li, 2012;Koo and Park, 2012), the strengthening efficiency is accomplished based on the strength of matrix (Meyers and Chawla, 2009), which is always decided by the grain size in the case of pure Ti matrix.
In many researches, small particles, such as precipitations, carbon nanotubes, organic or amorphous particles, have been demonstrated to be effective to stabilize the grain size, and thus maintain the strength of materials (Xue and Zeng, 2012;Vanherpe and Moelans, 2010).For (TiC + TiB)/Ti composites, there are two kinds of reinforcements with different morphologies.However, it is regretful that there are few reports about the pinning effect of the in-situ formed reinforcements on the grain growth and the resulting mechanical properties of the composites.

Experimental details
In this work, (TiC + TiB)/Ti composite was prepared by in-situ reaction from commercial pure Ti (TC450, ~25 μm) and B 4 C powders (~0.5 μm).Ti and 1.61 wt% B 4 C powders were firstly mixed and then consolidated and reacted by spark plasma sintering (SPS-1030S, SPS Syntex) at 1000 °C for 60 min under a vacuum < 6 Pa.Graphite moulds were used and the pressure was 30 MPa.Then, the SPS billet with a diameter of 42.0 mm was hot extruded to a rod with a diameter of 7.0 mm by a hydraulic press machine Shibayama).Before hot extrusion, the billet was pre-heated at 1000 °C for 3 min and the extrusion speed was 3.0 mm/min.Subsequently, the extruded rods were cut and isothermally heat treated at 400, 500, 600, 700 and 800 °C for 24 h in a vacuum < 10 Pa, respectively.Finally, the heat treated rods were machined to tensile test specimens with a diameter of 3.0 mm and gauge length of 15 mm.For the purpose of comparison, pure Ti samples were also prepared and posttreated by the same way as (TiC + TiB)/Ti composite.
Room tensile tests were carried out on a universal testing machine (Autograph AG-X 50 kN; Shimadzu) with a cross-head speed of 0.6 mm/min.The strain was recorded by using a CCD camera associated with the machine.Phase compositions were identified by X-ray diffraction (XRD-6100, shimadzu).Microstructures were investigated by field-emission scanning electronic microscopy (JXA-8530F, JEOL).Grain size of the Ti matrix and pure Ti was measured by electron backscatter diffraction (EBSD) technology with a TSL (TSL Digiview IV, EDAX) instrument.Oxygen and nitrogen contents were investigated by an inserted gas fusion nitrogen-oxygen determination instrument (EMGA-830, Horiba).Both the tensile strength and the O/N contents measurements were repeated three times to get an average value and to check the reliability of the results.

Microstructure and phase composition of as extruded composites
Fig. 1 shows microstructure charactering results of the composites prepared by SPS and hot extrusion from Ti-B 4 C powder system.It can be found from the SEM image (Fig. 1a) that, the as-extruded composite consists of matrix, particle-like phase and whisker-like phase.XRD pattern (Fig. 1b) shows these three phases are α-Ti, TiC and TiB.Furthermore, the SAED patterns show the matrix is α-Ti, whisker-like phase is TiB and particle-like phase is TiC.Such a result also agrees well with other published papers (Ni and Geng, 2008b;Radhakrishna Bahat andSubramanyam, 2002, Jia andLi, 2014;Geng and Ni, 2008).

Microstructure evolutions by isothermal heat treatments
Fig. 2 shows the microstructures of (TiC + TiB)/Ti composite and pure Ti materials after isothermal heat treatments at different temperatures for 24 h.Combining Fig. 1a with Fig. 2 a1-c1, it can be found that the micro- structures of (TiC + TiB)/Ti composite do not change after heat treatments, especially the amount and morphologies TiC p and TiB w .On the other hand, the grain size of pure Ti materials increases obviously after isothermal heat treatments.However, it is regretful that the grain size of Ti matrix in (TiC + TiB)/Ti composite cannot be found from the metallographic microstructures.
In order to clarify the pinning effect of TiC p and TiB w on the growth of Ti matrix grains, EBSD analysis was carried out, and the results are shown in Fig. 3.It is clear that, with the increase of isothermal heat treated temperature, the size of Ti grains in pure Ti materials increases obviously, but that in the (TiC + TiB)/Ti composite does not change significantly.Such a result shows without doubt that, the in-situ formed TiC p and TiB w have important pinning effect on the growth of Ti matrix in the case of high temperature exposure.
It is noticeable that, the size of Ti grains in both of pure Ti and (TiC + TiB)/Ti composite increases first and then decreases subsequently.Such a phenomenon can be attributed to the recrystallization of the extruded materials in the following heat treatments, which can be also confirmed from the EBSD images, especially that of pure Ti materials (Victoria-Hernandez and Yi, 2014).Because of the larger deformation energy stored in the (TiC + TiB)/ Ti composite, the recrystallization temperature for (TiC + TiB)/Ti composite is much lower than that of pure Ti materials.

Variations of room temperature strength
Fig. 4 shows the room temperature typical nominal S-S curves of (TiC + TiB)/Ti composite and pure Ti materials after isothermal heat treatments at different temperatures for 24 h.It can be easily found that, the tensile strength of pure Ti decreases continuously with the increase of isothermal heat treatment temperature from 400 to 700 °C, and then increases when the treating temperature increases to 800 °C (Fig. 4a).However, the strength of (TiC + TiB)/Ti composite after isothermal heat treatments almost keep a constant, except a small decrease after heat treating at 400 °C for 24 h.
From the viewpoint of strengthening mechanism, the strength of (TiC + TiB)/Ti composite at room temperature can be written as (Li and Sun, 2013): where, σ TMCs and σ Ti are the strength of (TiC + TiB)/Ti composite and pure Ti, Δσ sol , Δσ refine , and Δσ reinf are the strength increments caused by O/N solution, grain refinement and in-situ formed reinforcements TiC p and TiB w .
According to the results of microstructure characterizations, isothermal heat treatments do not change the amount and morphologies of in-situ formed reinforcements TiC p and TiB w .On the other hand, the results of EBSD analysis show the variation of grain size of Ti matrix.Therefore, in order to clarify the influence of pinning effect on the room temperature strength of (TiC + TiB)/Ti composite, it is necessary to investigate the change of O and N contents.Fig. 5 shows the O and N contents of (TiC + TiB)/Ti composite after heat treatments at different temperatures for 24 h.For the purpose of comparison, the O and N contents of pure Ti materials have also been measured.It can be found that, isothermal heat treatments do not change the O or N content significantly for both (TiC p + TiB w )/Ti composite and pure Ti materials.Therefore, it can be confirmed that, the room temperature strength are mainly influenced by the variations of the grain size of Ti matrix.Combining the EBSD results (Fig. 3) and nominal S-S curves of both pure Ti and (TiC + TiB)/Ti composite (Fig. 4), it can be found that, the changing trend of tensile strength agrees well with that of the grain size of Ti matrix.Such a phenomenon indicates without doubt that, the pinning effect of in-situ formed TiC p and TiB w is effective to avoid the growth of Ti matrix, and then keeps the stability of the room temperature tensile strength of (TiC + TiB)/Ti composite after isothermal heat treatments.

Conclusions
In this paper, the pinning effect of in-situ formed TiC p and TiB w on the growth of Ti matrix grains and the resulting room temperature strength of (TiC + TiB)/Ti composite were investigated systemically.The main conclusions can be drawn as follows: 1. Particle-like TiC and whisker-like TiB phases can be fabricated successfully by in-situ reaction from the Ti-B 4 C powder system.
2. The amount and morphologies of TiC p and TiB w are stable during isothermal heat treatments, and their pining effect can suppress the growth of Ti matrix grains.
3. Compared with pure Ti materials, the room temperature tensile strength is stable even after isothermal heat treatment at 800 °C for 24 h, suggesting a good pinning effect of in-situ formed TiC p and TiB w .

Fig. 3
Fig. 3 Size of Ti grains in pure Ti and (TiC + TiB)/Ti composites after isothermal heat treatments at different temperatures for 24 h.

Fig. 4
Fig. 4 Typical nominal S-S curves of (a) pure Ti and (b)(TiC + TiB)/Ti composite after isothermal heat treatments at different temperature for 24 h and then tested at room temperature.

Fig. 5
Fig. 5 O and N contents of pure Ti and (TiC + TiB)/Ti composites after isothermal heat treatments at different temperatures for 24 h.