Contributions of microstructural features to the integrated hardness of TA15 titanium alloy

doi:10.1016/j.msea.2014.12.087

Materials Science and Engineering: A

Volume 628, 25 March 2015, Pages 358-365

https://doi.org/10.1016/j.msea.2014.12.087 Get rights and content

Abstract

Analysis of the contributions of microstructural features to integrated property of TA15 titanium alloy is a crucial issue for microstructure design and obtaining desired material properties. To this end, various microstructural features, such as equiaxed primary α phase (α_p) and its boundary (α_GB), transformed β matrix (β_t) consisting of α plate and residual β phase (β_base) as well as the interface between them (β_GB) were obtained by different isothermal compression and heat treatment conditions. Nanoindentation and Vickers hardness tests were then performed on these features to investigate their contributions to the integrated property. It is found that the hardness of α_p varies slightly with the compression and heat treatment temperatures. The hardness of β_t decreases while the hardness of integrated microstructure increases with increasing compression temperature. Moreover, the hardness values of β_t and integrated microstructure are larger when the heat treatment temperatures are higher. By considering the hardness and volume fraction of each microstructural feature, it is also found that the contributions of microstructural features to the integrated hardness increase in the order of α_GB, α_p, β_GB, β_base. The contributions of α_p and α_GB decrease while those of β_base and β_GB increase with increasing compression temperature. In addition, the contributions of β_base and β_GB to the integrated hardness can be further adjusted by heat treatment under different temperatures.

Introduction

Titanium alloys are widely used in the aviation and aerospace industries due to their exceptional properties [1], [2]. The properties of titanium alloys depend on their microstructural features, such as equiaxed primary α, α colony and α plate within transformed β matrix. In general, high volume fraction of equiaxed α phase increases the ductility but decreases the yield strength of titanium alloys [3]. In contrast, a high volume fraction of lamellar microstructure increases the tensile strength of titanium alloys [4]. In addition, small equiaxed α and α colony as well as thin lamellar α increase the ductility and tensile strength of titanium alloys [4], [5], [6], [7].

Because of the strong influence of microstructural features on the mechanical properties of titanium alloy, a large number of attempts were made to establish relation between them. Zhang [8] investigated the effect of β grain size, α colony size, and lamellar α thickness on the mechanical properties of TA15 titanium alloy, and established a quantitative relationship between them. Sun [9] and Searles [10] quantitatively characterized the microstructural features, such as volume fraction of α phase, thickness of α phase and developed some relational models between these parameters and mechanical properties of titanium alloy. However, the microstructure–property relations created in these studies are based on the morphological parameters of microstructural features without considering the properties of the microstructural features.

In reality, the properties of the microstructual features also have a significantly influence on the integrated property of titanium alloy. Lütjering [5] pointed that element partitioning effect decreases the basic strength of the transformed β matrix in bi-modal structures, which results in lower fatigue and creep strength of the integrated microstructure. Besides, precipitates and dislocations strengthen the α phase and thus lead to an increase in strength of titanium alloys [11], [12]. Therefore, the morphological parameters together with the properties of microstructual features should be concurrently examined to establish the relation between microstructual features and integrated property of titanium alloy, and thus to control the microstructure for obtaining desired properties.

There have been some researches on this issue [12], [13], [14], [15], [16]. Picu [12] deduced the yield strength of α and β phases in Ti–6Al–4V from the yield strength of single phase α titanium alloy and Ti–6Al–4V. The integrated yield strength of the alloy was then predicted using the yield strength and volume fraction of α and β phases. Semiatin [13] and Kim [14] predicted the integrated properties of Ti–6Al–4V by using the properties and volume fractions of its constituent phases (α and β phases). The properties of α and β phases were determined by the properties of single phase α titanium alloys or β titanium alloys which have the same composition as the corresponding phase in the alloy. However, the properties of single phase α titanium alloys or β titanium alloys may not exactly represent the properties of different phases in a two-phase titanium alloy. Therefore, a direct and accurate measurement of the property of the individual phase is more attractive. Luo [15], [16] studied the microstructure and strengthening mechanisms in pure titanium and found that the correlation between the equiaxed α grain sizes and yield strengths follows the well-known Hall–Petch relationship. Thus, the contributions of the matrix and the boundary to the integrated strength of the pure titanium were determined. However, apart from the equiaxed α, lamellar α also presents in a two-phase titanium alloy. The coexistence of the equiaxed α boundary and lamellar α interface makes the determination of boundary strengthening effect more difficult, which needs further investigation.

In this study, nanoindentation tests were used to examine the properties of microstructural features in TA15 titanium alloy. Vickers hardness tests were used to determine the integrated hardness of the alloy. The measured hardness was used to analyze the contributions of microstructural features to the integrated property of the alloy. It will provide a basis to optimize the microstructure for obtaining desired property.

Section snippets

Microstructures preparation

The experimental material in the current study is a near-α TA15 titanium alloy with the chemical composition (wt%) of 6.69 Al, 2.26 Zr, 1.77 Mo, 2.25 V, 0.14 Fe, and balance Ti. The microstructure of the as-received hot forged plate is shown in Fig. 1. It consists of approximately 50% equiaxed primary α (α_p) within the transformed β matrix (β_t). The measured β-transus temperature of the alloy is about 985 °C.

Since near-β compression combined with subsequent heat treatment provides a feasible way

Microstructures analysis

The obtained microstructures are shown in Fig. 3. In these micrographs, the blacker and the whiter phase correspond to the α phase and the β phase, respectively. Fig. 3(a–c) shows the bi-modal microstructures containing α_p and β_t with α_s and β_r. Fig. 3(d–f) shows the tri-modal microstructures consisting of α_p, α_s, α_t and β_r. The integrated structure of α_s, α_t, and β_r in the tri-modal microstructure is also treated as β_t in the present study.

The quantitatively measured microstructural parameters

Strengthening by α plates-residual β interface

Grain boundaries in titanium alloy serve as strong barriers for dislocation transmission, since its hexagonal crystal structure has limited number of independent slip modes. Hence, titanium alloy exhibits a very strong grain boundary strengthening. For the bi-modal and tri-modal microstructures of TA15 titanium alloy, such boundaries consist of α_GB and the α plates-residual β interface (β_GB) within β_t. The strengthening effect of β_GB can be obtained by analyzing the nanoindentation hardness of β

Conclusions

Based on the nanoindentation and Vickers hardness tests, the following conclusions are drawn with respect to the contributions of microstructural features to integrated hardness:

(1)
The hardness of α_p varies slightly with the compression and heat treatment temperatures. The hardness of β_t decreases while the hardness of integrated microstructure increases with increasing compression temperature. Besides, the hardness values of β_t and integrated microstructure are larger for tri-modal

Acknowledgments

The authors would like to gratefully acknowledge the support of the National Natural Science Foundation of China (51175428) and 111 Project (B08040).

References (34)

J.C. Williams et al.
Acta Mater.
(2003)
L.J. Huang et al.
Mater. Sci. Eng. A
(2008)
M.T. Jovanović et al.
Mater. Des.
(2006)
G. Lütjering
Mater. Sci. Eng. A
(1998)
P.W. Peng et al.
J. Alloy. Compd.
(2010)
Y. Sun et al.
Mater. Sci. Eng. A
(2011)
R.C. Picu et al.
Mater. Sci. Eng. A
(2002)
P. Luo et al.
Mater. Sci. Eng. A
(2012)
P. Luo et al.
Scr. Mater.
(2012)
X.G. Fan et al.
Mater. Sci. Eng A.
(2012)

P.F. Gao et al.

Mater. Sci. Eng. A

(2012)

G.B. Viswanathan et al.

Acta Mater.

(2005)

W.D. Nix et al.

J. Mech. Phys. Solids

(1998)

I. Manika et al.

Acta Mater.

(2006)

E. Harvey et al.

Mech. Mater.

(2012)

D. Goldbaum et al.

Mater. Sci. Eng. A

(2011)

M. Delincé et al.

Acta. Mater.

(2006)

Cited by (26)

Microstructure evolution, mechanical response and strengthening models for TA15 titanium alloy during thermal processes: A brief review
2024, Journal of Materials Research and Technology
TA15 titanium alloy is widely used in aerospace industry in terms of its high specific strength, good thermal stability, and excellent corrosion resistance. To further improve its mechanical properties, thermal deformation combined with heat treatment is always used, during which the microstructure evolution and mechanical response are absolutely complex leading to the difficulty in control of final mechanical properties. Therefore, in-depth research on the relationship between microstructure and mechanical response of TA15 titanium alloy during these processes is of great significance. At present, less relevant review has been reported. In this article, the effects of process parameters such as heating temperature, strain rate, deformation amount, deformation mode, deformation path, cooling method and soaking time on the microstructure and mechanical properties of TA15 alloy, namely tensile performance and damage tolerance properties have been discussed. In addition, some strengthening models suitable for TA15 titanium alloy have been analyzed. In conclusion, different microstructure morphologies formed in various processing processes will display diverse mechanical performances in which the direct relationship between the yield strength and microstructure morphology can be established through distinct calculation models but with some limitations. So, in future works, microstructure and mechanical properties should be optimized further through adjusting processing parameters. Moreover, strengthening models remains to be modified by adopting more accurate statistical methods and considering more factors. Finally, establishing the relationship between damage tolerance properties such as fracture toughness, creep performance and LCF property and microstructure features should be considered more for TA15 alloy.
Influence of Ti60/Ti6554 ratio on the microstructure and properties of Ti alloys fabricated by laser directed energy deposition
2023, Materials Characterization
To develop high-strength and high-temperature Ti alloy, a series of new Ti alloys with different Ti60/Ti6554 ratios were designed under the guidance of an advanced cluster model, and then these alloys were fabricated by laser-directed energy deposition. The effects of Ti60/Ti6554 ratio on the microstructure and properties of the as-deposited alloys were investigated. The results show that all as-deposited alloys exhibit typical morphological characteristics of basket-weave microstructure. The difference is that the fraction and size of α lath gradually decrease with the increase of Ti60/Ti6554 ratio, which leads to a decreased room-temperature strength and an increased room-temperature ductility. However, the high-temperature strength and ductility show a diametrically opposite trend to room-temperature strength and ductility, having obviously dynamic softening characteristics. Increasing Ti60/Ti6554 ratio can effectively improve the high-temperature oxidation resistance. As predicted by the cluster model, the as-deposited alloy with Ti60/Ti6554 ratio of 5:5 has the best balance of room- and high-temperature mechanical properties and oxidation resistance among the studied alloys, being a promising candidate as high-strength and high-temperature Ti alloy.
Micro-cantilever testing of microstructural effects on plastic behavior of Ti–6Al–4V alloy
2021, Materials Science and Engineering: A
To evaluate the effects of microstructural factors on the plastic behavior of Ti–6Al–4V alloys, micro-cantilevers featuring several grains were machined using a focused ion beam. The equiaxed structure (a mixture of equiaxed α phase and intergranular β phase) and bimodal structure (a mixture of an equiaxed α phase and lamellar phase) were examined to verify the microstructural effects. To determine the slip plane and direction, the crystal structure was evaluated using electron backscattered diffraction, following bending tests of the microcantilever. In addition, the elastic modulus and hardness of each grain were determined via nanoindentation testing. In the equiaxed microstructure, the β-phase deformation often occurred first; then, slips were activated inside the equiaxed α phase. It is concluded that the β phase deforms more easily than the α phase because the elastic modulus and hardness of the former are lower, as indicated by the nanoindentation testing. In the bimodal microstructure, β-lath deformation occurred along the acicular direction in the lamellar-phase region, and slips occurred inside the α grain. The deformation of the lamellar structure was associated with β-phase deformation and affected by the high local stress at the grain boundary and phase interface, owing to the difference in grain orientation within the lamellar structure. The results of this study suggest that the plastic behavior of Ti–6Al–4V alloys depends not only on the slip system of the α grains but also on the deformability of the β phase and the misorientation of adjacent grains in polycrystalline Ti–6Al–4V alloy.
Reprint: Boron modified titanium alloys
2021, Progress in Materials Science
Titanium and its alloys are extensively used in a variety of high performance applications with the α + β alloy, Ti-6Al-4V, being the most popular. Conventionally Ti alloys in the as-cast condition possess highly coarse prior β grains, whose size is in the order of ~mm. Such large grain sizes not only reduce the strength of the alloy, but also impair its workability. The addition of about 0.1% wt.% B can result marked reduction in the grain size. The advantages offered by microstructural refinements, thus induced by trace addition of B in Ti alloys, are reviewed in this paper. Processing response of the as-cast alloys improved as a result of grain refinement due to B addition, leading to the possibility of removal or minimization of primary ingot breakdown steps. This can significantly bring down the cost of the finished Ti alloys components. Microstructural refinements with B addition on the mechanical performance of the alloys both at room and elevated temperatures are reviewed with emphasis on the microstructure-property correlations.
Multi-laser powder bed fusion of Ti-6.5Al-2Zr-Mo-V alloy powder: Defect formation mechanism and microstructural evolution
2021, Powder Technology
Citation Excerpt :
A direct correlation between the hardness and the laser beam number can be seen. The very fine martensite α' in this work is regarded as the α boundary, the kH is taken to be 122.4 Hv·μm0.5 [60]. The calculated Hall-Petch relationship based on Eq. (4) is presented in Fig. 15.
Multi-laser powder bed fusion (ML-PBF) has attracted much attention due to its advantage of directly building complex-structured and large-size components. In this work, this technique was conducted to fabricate Ti-6.5Al-2Zr-Mo-V (TA15) parts using sequential and synchronized scan strategies. The defect formation mechanism and the effect of the laser beam number (up to four) on density, microstructure, and microhardness were clarified. We find that the densification gradually deteriorates as increasing the number of involved laser beams. The laser-switching induced pores are dominated in the sequential scan strategy, which cause a directional pore distribution along the overlap lines. The synchronized scan strategy has a similar phenomenon. Moreover, the laser intersection induced by the synchronized scan strategy leads to a potential transition of molten pool mode and an increase of recoil pressure, which significantly deteriorates the densification. ML-PBF processed Ti-6.5Al-2Zr-Mo-V parts are composed of near-full acicular martensite α'. ML-PBF induces the low angle grain boundary to transfer to the high angle grain boundary due to the heat accumulation effect. Consequently, slow grain coarsening of acicular martensite α' occurs with more laser beams involving in. This leads to the microhardness of single-, dual-, and quadruple- L-PBF processed samples value from ~423 Hv to ~388 Hv. The microstructure-property correlation in ML-PBF is in good agreement with the Hall-Petch relationship.
Whole-process modeling of titanium disc forming for gradient distributions of temperature and deformation
2020, Chinese Journal of Aeronautics
Gradient distributions of temperature and deformation (GDTD) are crucial for achieving dual-performance discs of titanium alloys which is required by the service environment of aeroengine. However, heating, cooling and deforming sequence in the whole process of the titanium disc forming, which leads to difficulties for achieving GDTD due to a lot of parameters. To solve this problem, a whole-process model of the titanium disc forming for GDTD has been established. In the model, heating and cooling via heat radiation, conduction and convection, and deforming by local loading with mold chilling are all considered. Experiments on heating and cooling as well as deforming were carried out by using a furnace and the Gleeble-3500 machine. The experimental data are used to determine thermal parameters and constitutive relations of the IMI834 titanium alloy, and then to verify the reliability of the model. Then the model was used to simulate the evolution rules of temperature and deformation of the titanium disc. The results show that the heating surface, furnace temperature, billet profile and loading rate play the core role for the control of GDTD, and thus a set of parameters were determined. Therefore, this work provides a base for developing a new forming technology of the dual-performance titanium discs with the approach of local heating and local loading.

View all citing articles on Scopus

View full text

Contributions of microstructural features to the integrated hardness of TA15 titanium alloy

Abstract

Introduction

Section snippets

Microstructures preparation

Microstructures analysis

Strengthening by α plates-residual β interface

Conclusions

Acknowledgments

Acta Mater.

Mater. Sci. Eng. A

Mater. Des.

Mater. Sci. Eng. A

J. Alloy. Compd.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Scr. Mater.

Mater. Sci. Eng A.

Mater. Sci. Eng. A

Acta Mater.

J. Mech. Phys. Solids

Acta Mater.

Mech. Mater.

Mater. Sci. Eng. A

Acta. Mater.