Hot-Deformed Microstructure and Texture of Ti-62222 Alloy

Abstract

The Ti-62222 (Ti-6Al-2Sn-2Zr-2Mo-2Cr) alloy has considerable potential for structural applications in the aerospace industry owing to its exceptional fracture resistance and specific strength. This study investigates the influence of local strain parameters and solution treatment and aging (STA) on the microstructure, texture evolution, and microhardness of a hot-forged Ti-62222 alloy. The strain distribution was simulated using the finite element method (FEM). The results showed that in the specimens before heat treatment, the morphology of the primary Ti α phase grains elongated perpendicular to the compression direction as the strain increased. In contrast, the post-heat-treated specimens (PHTSs) exhibited similar aspect ratios, regardless of the strain level, owing to grain spheroidization induced by the STA heat treatment process. Spheroidal primary Ti α phase and acicular Ti α’ phase were observed in the specimens before and after heat treatment. Texture analysis revealed that the specimens subjected to heat treatment had a weaker texture than the before-heat-treatment specimens. The near ($11\overline{2}0$)//FD texture tended to develop along the direction perpendicular to the forging direction. The microhardness analysis results indicated that strain had no significant effect on the microhardness of either the as-forged specimen or the PHTS. After heat treatment, the specimens showed consistent microhardness values regardless of the strain level. The PHTS exhibited increased microhardness, attributed to the aging process during STA.

1. Introduction

Titanium alloys are widely used in the aerospace, energy, military, and marine industries owing to higher stiffness, specific strength, and excellent corrosion resistance compared to other structural materials [1,2,3,4,5,6,7,8,9,10,11,12,13]. However, titanium alloys are prone to develop strong textures during plastic deformation [14,15,16,17,18]. This strong texture can result in significant anisotropy in the mechanical properties and plastic deformation behavior, necessitating further investigation [19,20,21]. Kobryn et al. [22] investigated the microstructure and texture evolution during the solidification of Ti-6Al-4V. Peters et al. [23] investigated the influence of texture on the fatigue properties of Ti-6Al-4V.

Among titanium alloys, the Ti-62222 alloy exhibits significant potential for structural applications in the aerospace industry owing to its superior fracture toughness and higher specific strength relative to Ti-6Al-4V [24], one of the most widely used commercial titanium alloys [25]. However, studies on texture evolution during the deformation of the Ti-62222 alloy remain limited.

Considering that the hot workability of titanium alloys decreases significantly with decreasing temperature [26], the production of components typically involves ingot casting which is followed by various hot deformation and heat treatment processes, broadly referred to as thermomechanical processing [21]. Consequently, extensive research has been conducted on the hot deformation behavior of titanium alloys to enhance their mechanical performance. Yang et al. [27] investigated the influence of temperature and strain rate during hot compression of Ti-6Al-4V on the microstructure and texture. The results indicate that at temperatures below 900 °C and strain rates exceeding 0.1 s⁻¹, the microstructure primarily comprises elongated Ti α grains. Compared to the as-received specimen, the texture was stronger in the specimen deformed at 930 °C and weaker in the specimen deformed below 930 °C. Warchomicka et al. [28] investigated the effect of strain rate on the microstructural evolution during hot deformation of the Ti-6Al-4V alloy. The results show that the dynamic recrystallization of the Ti α grain occurs at strains of 1 within the strain rate range of 0.1–5 s⁻¹. In the Ti α- β region, the Ti α grains maintain their shape during deformation up to a strain of 0.2. Zhang et al. [29] investigated the plastic flow behavior and microstructural evolution during the subtransus hot deformation of the Ti-6Al-4V alloy with three initial microstructures. The results indicate that the Ti α’ martensitic starting microstructure is more beneficial for achieving grain refinement in Ti-6Al-4V alloy. To date, most studies on the hot deformation behavior of titanium alloys have focused on conventional titanium alloys such as the Ti-6Al-4V, whereas research on the hot deformation behavior of the Ti-62222 alloy is limited. Additional research is essential to optimize industrial applications of the Ti-62222 alloy.

This study investigates the influence of local strain parameters and heat treatment on the microstructure, texture evolution, and microhardness of the hot-forged Ti-62222 alloy. The strain distribution within the specimens was evaluated using the finite element method (FEM). The microstructures of the specimens were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The textures of the specimens were analyzed using an orientation distribution function (ODF).

2. Materials and Methods

The double cone Ti-62222 specimens—with a height of 77 mm and a minor and major diameter of 55 and 77 mm—were hot forged at 910 °C. A schematic of the hot-forging process is shown in Figure 1a, and the dimensions of the hot-forged specimen are shown in Figure 1b. After deformation, solution treatment and aging (STA) heat treatment were performed to investigate its effect on the microstructure, texture, and microhardness. The heat treatment consisted of solution treatment at 945 °C for 30 min, followed by air cooling and aging at 540 °C for 8 h, and then followed by further air cooling. On completion of processing, the specimens were cut parallel to the forging direction for cross-sectional analysis.

The FEM-based software DEFORM^TM V11 was used to simulate the deformation tests and analyze the deformation distribution within the sample. The plastic flow behavior of the Ti-6222 alloy for the FEM simulation at elevated temperatures was obtained from a compressive stress–strain curve of the alloy in the temperature range between 750 and 1050 °C. Based on the experimental compressive stress–strain curves, the flow stress of the alloy was assumed to be 325 MPa at 750 °C before decreasing linearly with increasing temperature to 40 MPa at 1050 °C. For the simulation, the Ti double cone was set as an elastic-plastic object while the forging dies were set as rigid objects. Their physical parameters, such as the elastic moduli, referred to the material database in Deform-3D. The Ti double cone was meshed with 272,548 meshes. The temperatures of the dies and the exterior were set to 910 °C. The film and friction coefficients between the double cone and the dies were chosen as 5 and 0.3 N/s·mm·°C, respectively. Figure 2 shows the deformation distribution maps of the deformed specimens. Based on the strain levels, arbitrary locations within the specimen were labeled from A to R. The highest strain was observed at location A and the lowest at location R. The microstructural and textural evolution was investigated at each location.

The microstructures of the specimens were observed using an optical microscope (OM, Axiolab 5, Carl Zeiss, Jena, Germany) and SEM (Mira 3, TESCAN, Kohoutovice, Czech Republic). The specimens for the OM and SEM analyses were etched with Kalling etchant, a solution of copper chloride, hydrochloric acid, and ethanol. EBSD analysis (Aztec HKL, Oxford Inc., Oxford, UK) was conducted to further investigate the phase evolution. From OM images, the aspect ratio of the primary Ti α phase and the area ratio of each phase were analyzed using Image J (version 1.54g), an open-source image processing software.

For the texture evolution analysis, the ODF was calculated with multiple pole figure data using MTEX, a MATLAB-based data post-processing software. The MATLAB R2023a software was used for the data processing. The pole figures of the specimens were obtained using X-ray diffraction (XRD; SmartLab, Rigaku, Tokyo, Japan).

The microhardness of the specimens was measured using the HM200 Vickers microhardness test machine (Mitutoyo, Sakado, Japan). Test parameters during the hardness measurement were as follows: load = 3 N, load time = 7 s, and the interval between hardness indentations in the x and y directions = 0.3 mm. The microhardness distribution across the specimen was visualized as a microhardness map to analyze the effect of strain.

3. Results and Discussion

Phase evolution and phase analysis were conducted using an EBSD phase map and an inverse pole figure (IPF) map to investigate the microstructural changes during the high-temperature deformation process. The phase map analysis of the as-forged specimen presented in Figure 3a shows that the microstructure of the hot-forged Ti-62222 alloy consists of two distinct phases, both phases in a hexagonal close-packed structure but with different lattice parameters. The microstructure is composed of polygonal-shaped relatively coarse grains (Ti α) and a matrix comprising a mixture of two distinct phases (i.e., Ti α and Ti α’). The IPF map analysis establishes that grains with similar orientations are formed within a matrix (i.e., the surrounding matrix of primary Ti α grains) characterized by random orientations (Figure 3b). Based on the phase map and IPF map analysis in this study and on previous research on phase transformation during the hot deformation of Ti-62222 [30,31,32,33], the phase evolution process can be inferred as follows: during high-temperature compressive deformation, the high-temperature stable Ti β phase is formed as primary grains. As the temperature decreases, the Ti α+ β phases develop around the primary Ti β grains, followed by a diffusion-free transformation of the Ti β phase into the Ti α’ phase, resulting in regions containing a mixture of Ti α and Ti α’ phases.

In a previous study [28], dynamic recrystallization of the Ti α grain was observed during the hot deformation process of Ti-6Al-4V at a strain of approximately 1 mm/mm. However, dynamic recrystallization of the primary Ti α grain was not observed during the hot deformation process in this study, even at higher strains of up to 2 mm/mm. From these results, it can be inferred that much higher plastic deformation or higher deformation temperature is required to achieve dynamic recrystallization in Ti-62222 in comparison to Ti-6Al-4V. This is probably because of the higher thermal stability of the primary α grain in Ti-6222 in comparison to that in Ti-6Al-4V.

Cross-sectional OM images by location for the specimens before and after heat treatment are shown in Figure 4 and Figure 5, respectively. The primary Ti α phase grains elongated in a direction perpendicular to the compression axis as the strain increased in the as-forged specimens. The primary Ti α phase grains at location A, which experienced the highest strain, exhibited the most elongated morphology. The grain elongation could be observed at locations B, C, and D as well. However, at locations E and R, where relatively lower plastic strains are expected compared to locations A, B, C, and D, no significant grain elongation could be observed. This phenomenon can be attributed to the high-temperature deformation during the hot forging process, where grain elongation occurs in the direction perpendicular to the forging direction. Conversely, Ti α phase grains exhibit uniform morphology after the heat treatment, independent of the strain levels caused by grain spheroidization resulting from the STA heat treatment process. The aspect ratios of the primary Ti α phase by location for specimens before and after heat treatment are shown in Figure 6a. The aspect ratio of the as-forged specimen exhibited a tendency to increase with increasing strain, owing to grain elongation occurring perpendicular to the forging direction during high-temperature deformation. In contrast, the post-heat-treated specimen exhibited consistent aspect ratios irrespective of the strain level, likely due to grain spheroidization facilitated by the STA heat treatment process. The area ratio indicating the fraction of the primary Ti α phase relative to the total area was analyzed by location for specimens both before and after heat treatment, as shown in Figure 6b. The area fraction was approximately 35% before heat treatment and 21% after heat treatment—considered to result from the partial dissolution of the primary Ti α phase and its transformation into the thermally stable Ti β phase during the STA heat treatment process.

The effects of strain and heat treatment on the microstructures of the specimens were investigated using SEM, as shown in Figure 7. Both before and after heat treatment, the microstructure exhibited spheroidal primary Ti α and acicular Ti α’ phases. The acicular Ti α’ phase showed no significant changes in size or morphology regardless of the strain level, both before and after heat treatment. Additionally, it was observed that the heat treatment does not affect the morphology of the acicular Ti α’ phase. However, the spheroidal primary Ti α phase grains in the as-forged specimens were observed to be relatively more elongated at locations A, B, C, and D, which experienced higher strain levels in the forging process. At location E, the grains were less elongated in comparison to locations A–D; no grain elongation was observed at location R, which experienced minimal strain. These findings indicate that during the high-temperature deformation process, the morphology of the spheroidal primary Ti α phase grains becomes increasingly elongated with higher strain levels, whereas no elongation occurs below a certain strain level. In contrast, the post-heat-treated specimens showed no elongation of the spheroidal primary Ti α phase grains, regardless of the strain level, likely due to grain spheroidization induced by the STA heat treatment process. Furthermore, the primary Ti α phase appears brighter in the heat-treated specimen in comparison to the as-forged specimen, indicating that the microstructural defects such as dislocations and low-angle grain boundaries in these phases were reduced due to the annealing effect during the heat treatment.

The ODF analysis results for the specimens were compared by location, both before and after the heat treatment, as shown in Figure 8. The textures of ($13\overline{4}0$)//FD, ($23\overline{5}1$)//FD, ($14\overline{5}2$)//FD, ($12\overline{3}0$)//FD, and ($12\overline{3}1$)//FD were observed in the as-forged specimens. In contrast, in the post-heat-treated specimens, only the textures ($13\overline{4}0$)//FD, ($12\overline{3}0$)//FD, and ($12\overline{3}1$)//FD were observed. These results reveal similarities in the ($13\overline{4}0$)//FD, ($12\overline{3}0$)//FD, and ($12\overline{3}1$)//FD textures between the specimens before and after heat treatment, whereas the ($23\overline{5}1$)//FD and ($14\overline{5}2$)//FD textures observed in the as-forged specimens disappeared after heat treatment. Moreover, the overall texture strength of the post-heat-treated specimens was weaker than that of the as-forged specimens. These results confirm that the strong texture formed in Ti-62222 after deformation can be weakened by heat treatment. The before and after heat treatment specimens exhibited the weakest texture at location R, where the strain level was the lowest, indicating that the deformation process significantly contributes to texture development in the Ti-62222 alloy. Texture analysis demonstrated a clear tendency for the near ($11\overline{2}0$)//FD texture to develop along the direction perpendicular to the forging direction, confirming that the forging process significantly influenced the orientation of the texture. As the near ($11\overline{2}0$)//FD texture is characteristic of typical Ti β forging textures, this analysis suggests that forging was conducted in a region comprising both Ti α and Ti β phases.

The microhardness was measured as the distance from the center of the as-forged and post-heat-treated specimens. The measured microhardness values are presented as microhardness maps and graphs in Figure 9. In the as-forged specimens and post-heat-treated specimens, consistent values were observed across the entire specimen, regardless of the strain level. These results indicated that the hot deformation of Ti-62222 had no significant effect on the hardness of the specimen. The average microhardness of the as-forged specimen was approximately 330 HV, whereas the post-heat-treated specimen exhibited a higher average microhardness of approximately 431 HV. In a previous study [34], Pinke et al. found that air cooling after solution heat treatment had no significant effect on the hardness of Ti alloys. However, subsequent aging treatment at 550 °C increased the microhardness due to precipitation of the fine Ti α phase from the Ti β phase. As shown in Figure 6b, the area ratio of the primary Ti α phase decreased after heat treatment. This phenomenon is attributed to the partial phase transformation of the primary Ti α phase into the thermally stable Ti β phase during the solution heat treatment in the STA process. Additionally, a portion of the transformed Ti β phase underwent further phase transformation into an acicular Ti α’ phase due to the rapid cooling rate during air cooling after solution treatment. Subsequently, the isothermal ω phase formed within the remaining Ti β phase as the aging process progressed further, leading to an increase in hardness. Therefore, the hardness increase in the post-heat-treated specimens can be attributed primarily to the formation of the acicular Ti α’ phase and isothermal ω phase within the Ti β phase owing to the phase transformation induced by the STA heat treatment.

4. Conclusions

This study investigated the microstructure, texture evolution, and microhardness of hot-forged Ti-62222 alloys. The deformation distribution within the specimens was analyzed by simulating deformation tests using FEM to evaluate the effects of strain levels. STA was performed to examine the impact of the heat treatment. Additionally, phase map and IPF map analyses were conducted to investigate the phase evolution process during the hot deformation of Ti-62222.

• During high-temperature compressive deformation, the thermally stable Ti β phase formed as primary grains. During cooling, Ti α+β phases developed around the primary β grains, followed by a diffusion-free transformation of the Ti β phase into the Ti α’ phase. This phenomenon resulted in regions containing a mixture of Ti α and Ti α’ phases in the hot-forged Ti-62222. The dynamic recrystallization of the Ti α phase is not observed during the hot forging process up to a strain of 2 mm/mm. It is inferred that much higher deformation is required for the dynamic recrystallization of Ti-62222 to occur during the hot deformation process, owing to the higher thermal stability of the α phase of this alloy in comparison to Ti-6Al-4V.
• Both as-forged and post-heat-treated specimens exhibited spheroidal primary Ti α phase and acicular Ti α’ phase. The primary Ti α phase exhibited grain elongation, which became more pronounced with increasing strain levels. In contrast, the acicular Ti α’ phase showed no significant changes in size or morphology regardless of strain level. Spheroidization of the primary α phase occurred after the STA heat treatment.
• Texture analysis established that the strong textures formed during deformation weakened after heat treatment. Particularly, the ($13\overline{4}0$)//FD, ($12\overline{3}0$)//FD, and ($12\overline{3}1$)//FD textures were preserved, whereas the ($23\overline{5}1$)//FD and ($14\overline{5}2$)//FD textures disappeared after the heat treatment. The development of a near ($11\overline{2}0$)//FD texture along the direction perpendicular to the forging direction confirms the considerable influence of hot deformation on texture orientation.
• The microhardness measurement results indicated that strain had no significant effect on the microhardness of either the as-forged or the post-heat-treated specimens. In contrast, the post-heat-treated specimen exhibited higher average microhardness; this can be attributed to the aging process during STA.