Introduction

Titanium alloys are widely believed to undergo rapid coarsening in the β-phase region [1]. Boron is incorporated into titanium alloys [2, 3] to prevent this phenomenon. The low solubility of boron at room temperature in titanium alloys leads to the precipitation of TiB whisker from the matrix [4].

The Boron addition could restrict the grain coarsening of the anterior β-grain and change the morphology of the α-phase to be equiaxed, which is a consensus acknowledged by most researchers [2, 5, 6]. Adding boron to cast titanium matrix composites would refine the anterior β-grains [7, 8]. In our previously published paper [9] we highlighted an interesting phenomenon that the size of the anterior β-grains is consistent with the size of spherical TA15 titanium metallic powder (raw material) during the solid-state phase transition, although titanium alloys are kept at a relatively high temperature (about 200°C higher than the β-transus) and for a certain time without restricting TiB whiskers distributed at the anterior β-grain boundaries. This phenomenon seems to have some scientific value and deserves to be studied and replicated in experiments to confirm. Using titanium alloy powders with different sizes to control the microstructure of titanium alloys and design the material’s performance would be feasible.

Equiaxed α-microstructure is highly sought-after in titanium alloys as a beneficial microstructure. It should be noted that in published articles, equiaxial α-grain formation is only reported under two conditions. One is in titanium alloys after thermo-mechanical processing in the α + β region [10]. Another is in TiB whisker-reinforced titanium matrix composites cool from the temperature above the β-transus at a slow cooling rate [6, 11].

When a titanium alloy is cooled from the β-phase file, the α-grain boundaries would first nucleate at the anterior β-grain boundaries and provide nucleation sites for the α-laths or α-colonies. This is a Widmanstätten nucleating and growing mechanism accepted by most researchers [6, 12, 13]. There is a detail, however, that there is only one-grain boundary α between the two adjacent anterior β-grains. Furthermore, a particular grain boundary α belongs to or is shared by one of the anterior β-grains, which was not noticed before.

In this paper, the TA15 alloys were prepared by vacuum hot-pressing sintering using spherical TA15 powders of two different sizes to verify the abovementioned phenomena and discuss the assignment of the α-grain boundaries.

Experimental Procedure

Spherical powders of Ti–6.6Al–1.7Mo–2.3V–1.9Zr alloy with two different sizes (100 and 200 μm) were used as raw materials, as shown in Fig. 1. Due to the high cooling rate during the gas atomization in the course of powder preparation, the as-received powders were composed of martensite microstructure (Fig. 1c).

Fig. 1.
figure 1

SEM (a, b) and OM (c) images of as-received spherical TA15 powders produced from coarse (a, c) and fine (b) powders

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The powders were sintered in a hot pressure sintering furnace under vacuum of 6 ∙ 10–2 Pa, pressure of 25 MPa and temperature of 1200°C for 45 min in a graphite mold. After exposure at constant temperature and pressure, the as-sintered billets were cooled at a very slow furnace cooling rate. The powder size was measured by a laser particle analyzer (BT-2003), and the microstructure was examined by an optical microscope (OM, Olympus GX71). The microstructure of the as-sintered specimens were examined by scanning electron microscopy (SEM, Zeiss) equipped with an electron backscatter diffraction (EBSD) system and TSL OIM Analysis 7. The dimensions of the tensile test specimens were 15 mm × 4 mm × 2 mm. The tests were performed on an Istron-5569 universal testing machine at a constant crosshead speed of 0.5 mm/min at ambient temperature.

Results and Discussion

The SEM images of the as-sintered TA15 alloys made from powders of two different sizes are shown in Fig. 2. It is clearly seen in Figs. 2a, b that the size of the anterior β-grains is consistent with the size of the as-received spherical TA15 powders (Fig. 1). In the previous research [2, 11], it was generally assumed that in the TiB whisker-reinforced titanium matrix composites, the boron additions would refine the grain size of anterior β-grains, resulting in the refinement of α-grains. However, the size of anterior β-grains is only related to the size of the metal powders used, as shown in Fig 2. This result means that even if sintered at a temperature higher than the β-transus for a relatively long time, the preformed β-grains of TA15 alloys produced by vacuum hot-pressing sintering using spherical TA15 powders would not coarsen. In addition to our previously published article [9], this result also reveals that during solid phase transition (β → α), TiB precipitates are not the only factor restricting anterior β-grain magnitudes.

Fig. 2.
figure 2

SEM micrographs of TA15 alloys fabricated from coarse (a) and fine (b) powders

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It should be noted that in the published articles the formation of equiaxed α-grains is reported in only two cases, i.e., the recrystallization processes occurring during mechanical working at temperatures within the α + β range and the TiB reinforcement of titanium matrix composites cooled in the furnace at a low rate [6, 10, 11]. It's interesting to observe that the fine powder TA15 alloys have equiaxed α-grains (Fig. 2b). It can be concluded that the formation of equiaxed is connected to the size of previous β-grains and cooling rate. The formation of equiaxed α is first reported without thermomechanical processing or the influence of TiB precipitation. This interesting phenomenon will be further analyzed in future work using a finer powder. The effect of powder size on microstructure and mechanical properties will also be investigated.

ESBD was employed to examine the TA15 alloys fabricated from TA15 powders of two different sizes in order to better understand the relationship between powder size and microstructure. Figs. 3a, b demonstrate the IQ (image quality) and misorientation angle between adjacent α-grains of TA15 alloys. It could be seen that α-morphology is changed from α-colonies (coarse powder) to equiaxed α (fine powder) when powders with different sizes are applied under identical preparation conditions. It is consistent with the result in Fig. 2. The percentage of high-angle grain boundaries (HAGBs) is much larger than that of low-angle grain boundaries (LAGBs). Upon analyzing the data in-depth, it can be seen that there are almost no LAGBs in the α-colonies. It means that in the process of the nucleation and growth of α colonies, the orientation of α-laths in a colony is unanimous. As shown in Fig. 3b, the percentage of equiaxed α is much larger than that of α-colonies. Therefore, the microstructure of TA15 alloys fabricated from the fine powder could be called equiaxed microstructure, which is beneficial for the ductility of titanium alloys [14, 15]. The content of LAGBs is increased to 20% in TA15 alloys fabricated from the fine powder. Sufficient recovery is undertaken during the sintering and slow furnace cooling process. The sub-boundaries of adjacent equiaxed grains in a parent β-grain are not caused by the deformation during the sintering process but are sub-boundaries of adjacent equiaxed α-grains. It could be further inferred that the equiaxed α might be generated by the fracture of α-lath.

Fig. 3.
figure 3

EBSD results for the TA15 alloys fabricated from powders of two different sizes: a, b) IQ and grain boundaries, with lines representing HAGBs and LAGBs; c, d) IPF of the residual β-phase; e, f) misorientation angle of α-Ti; a, c, e) coarse powder; b, d, f) fine powder

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Figures 3c and d show the inverse pole figures (IPF) of the residual β-phase in TA15 alloys fabricated from TA15 powders of different sizes. The β-phases belonging to different anterior β-grains have different orientations, which can be shown in the EBSD results. Figures 3e and f demonstrate the misorientation angles of α-Ti in two TA15 alloy samples. From Fig. 3e, the peaks of the curve are approximately at 10°, 60°, 63°, and 90°, illustrating that the solid phase transition (β → α) process follows the Burges relationship [16]. This phenomenon is not obvious in TA15 alloys fabricated from fine powder due to two reasons. The primary reason is that TA15 alloys fabricated from fine powder have more grain boundaries and a random angle of grain boundaries compared to those TA15 alloys fabricated from coarse powder. Furthermore, TA15 alloys fabricated from fine powder have many sub-boundaries, which is the second reason (Fig. 3b).

In titanium alloys, the Widmanstätten structure is considered to be the most important and common microstructure. Sympathetic nucleation and interface instability are proposed as two formation mechanisms [17]. In this paper, the sympathetic mechanism is more reasonable due to the integrated α-grain boundaries and a clean interface between α-grain boundaries and α-colonies (Fig. 3a). That is to say, α-precipitates would first nucleate and grow along the prior β-grain boundaries. Upon further cooling, parallel intergranular α-plates would nucleate from the newly formed α-grain boundaries and grow into its parent β-grain, forming α-colonies. Most researchers consider this mechanism to be more reasonable and accepted [6, 12, 13]. The attribution of the α-grain boundaries between the two adjacent anterior β-grains has not been discussed before. Analysis by means of synthesis in Figs. 3a, b is adopted for the purpose of an in-depth understanding of the relationship between α-grain boundaries and α-colonies. The angle between the α-grain boundaries (point A in Fig. 3a) and α-colonies (region B in Fig. 3a) is approximately 70°, while the angle of the α-grain boundaries (point A in Fig. 3a) and α-colonies (region C in Fig. 3a) is approximately 60°. According to Burges relationship [16], the α-grain boundaries (point A in Fig. 3a) between anterior β-grains β1 and β2 belong to β2 (β1 and β2 were shown in Fig. 3c). To summarize, the microstructure evolution during vacuum hot-pressing sintering can be explained as follows. In this process, there is a transition from solid phase to powder compact. In the temperature-rise period, when the temperature reaches β-transus, the solid-state phase transition occurs (α → β). The anterior β-grain only grows to the original powder interface at that point, which means that it won't coarsen, leading to its size exceeding that of the original powder. During the subsequent cooling process, when the temperature reduces to β-transus, the solid-state phase transition occurs (β → α). Firstly, the nuclei of grain boundaries α occur at the grain boundary of the anterior β-grain. Then the nuclei of grain boundaries α grow together enclosing the anterior β-grain. The grain boundaries α would provide nucleation sites for the α colonies of its parent β-grain and another adjacent prior β-grain. Finally, α-colonies grow into the prior β-grain forming the Widmanstätten structure. The sintering process is accompanied by a powder compact formation.

The two typical microstructures shown in Fig. 2 should definitely affect the mechanical properties of the TA15 alloys. Figure 4 presents the tensile curves of TA15 samples fabricated from the TA15 powders of two different sizes. As seen, the TA15 alloy fabricated from the fine powder shows an improvement in tensile properties (ultimate tensile strength and ductility) compared to that of the TA15 alloy fabricated from the coarse powder. To be specific, the tensile strength increases from 849 to 898 MPa and the ductility grows from 5.5 to 6.5%. It is apparent that the grain size of the TA15 sample made from coarse powder is coarser than that of the TA15 sample made from fine powder (Figs. 3a, b). Based on the well-known Hall–Petch relation, tensile strength and grain size are inversely proportional [18, 19]. This explains why TA15 alloys made from fine powder have a relatively higher tensile strength. The formation of equiaxed α-grains is beneficial to the plasticity of alloys [14, 15]. Thus, the fine powder TA15 alloys have a higher ductility than the coarse powder TA15 alloys.

Fig. 4.
figure 4

Tensile curves of the TA15 alloys fabricated from powders of two different sizes

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Conclusions

The microstructure of TA15 alloys varies depending on the size of the initial powder, whether it is Widmanstätten pattern for coarse powder or equiaxed for fine powder. The size of anterior β-grains is in line with the size of the as-received spherical TA15 powder. Grain boundaries α could provide nucleation sites for the α-colonies of its parent β-grain and the adjacent anterior β-grains.

Declaration of Interest

There are no conflicts of interest.

Fundings

This project is supported by the National Natural Science Foundation of China [grant number: 52305364]; and the Postgraduate Research & Practice Innovation Program of Jiangsu Province [grant number: KYCX22_3636].