Microstructure and mechanical properties of titanium (Grade 4) processed by high-pressure torsion

doi:10.1016/j.msea.2007.08.084

Materials Science and Engineering: A

Volume 493, Issues 1–2, 15 October 2008, Pages 190-194

https://doi.org/10.1016/j.msea.2007.08.084 Get rights and content

Abstract

The influence of high-pressure torsion (HPT) at various temperatures on microstructure and mechanical properties of commercially pure (CP) titanium (Grade 4) has been studied using transmission electron microscopy and tensile tests. The temperature range of 70–450 °C for isothermal HPT processing has been used to fabricate nanocrystalline samples with diameter 20 mm under a pressure of 6 GPa. Using the new installation, we demonstrated that the ultimate tensile strength of CP Ti (Grade 4) after HPT processing achieves 1600 MPa as a result of significant grain refinement. At the same time, ductility of the HPT samples has been affected strongly by processing and annealing temperature.

Introduction

It is known that the application of severe plastic deformation (SPD) results in strong grain refinement and high strength of pure metals. High-pressure torsion (HPT) processing is one of the most widely used among the SPD techniques, contributing to maximum grain refinement in the structure of pure metals to the size of grains less than 100 nm. Particularly, recent studies [1], [2], [3] were devoted to the investigation of the HPT effect on structure and properties of commercially pure (CP) Ti VT1-0. These investigations revealed an unusual simultaneous increase in strength and ductility of the HPT processed samples after annealing at 300 °C; in Ref. [1], this increase was attributed to a grain-boundary effect. Another explanation may be based on the segregation of impurities and precipitation of secondary phase particles as a result of aging. CP Ti VT1-0, being the analog of Ti Grade 2, contains up to 1% of impurities, which could become the source of impurities segregation along the grain boundaries at heating.

There is Ti Grade 4 among CP Ti materials, which contains higher level of impurities as compared to VT1-0 and Grade 2. Thus, application of HPT to process Ti Grade 4 can lead to even stronger grain refinement due to the accumulation of enhanced density of dislocations as well as to pronounced aging effect at subsequent heating due to higher level of impurities.

Therefore, CT Ti Grade 4 has been selected to study the effect of HPT on structure and mechanical properties in the present work. Main attention was paid to the comparison of HPT regimes, which could affect the average grain size as well as kinetics of hardening at subsequent annealing of the samples. In particular, the samples subjected to isothermal HPT processing at the temperature range 70–450 °C and the samples obtained by HPT at ambient temperature with subsequent annealing at the same temperatures were studied in the present work.

Section snippets

Experimental

In the present work coarse-grained CP Ti Grade 4 with a mean grain size 40 μm was used as initial material (Fig. 1a). It is known that HPT processing can lead to inhomogeneity of microhardness distribution along diameter of discs [4]. Especially this effect can be expected after enlarging the disc's diameter up to 20 mm used in the given investigation (Fig. 1b). Therefore, to improve the structure homogeneity and for preliminary grain refinement the equal channel angular pressing (T = 450 °C, angle

Results and discussions

The grain size in the material prior to HPT was 300 nm in cross-section and 2–3 μm in longitudinal direction. TEM studies have shown that the application of the HPT processing at room temperature results in further grain refinement of CP Ti Grade 4 and the average grain size amounted to 105 nm, 110 nm and 120 nm, at the edge, radius center and samples’ center, correspondingly (Fig. 2).

The microhardness value of CP Ti Grade 4 averaged over the samples’ diameter made 4460 MPa with a S.D. of 140 MPa.

Conclusions

It is established that HPT processing leads to strong grain refinement of CP Ti Grade 4 from 40 μm to 105–120 nm and enhancement of ultimate tensile strength up to 1600 MPa. Ductility of HPT samples amounted to 7%, and was affected by the temperature of HPT processing as well as by the temperature of subsequent annealing that can be connected with relaxation of internal stresses at heating of CP Ti Grade 4.

Acknowledgements

This work was supported in part by program “Development of Potential of Higher School” of the Russian Ministry for Education and Science, and project #3208p by the International Science and Technology Center (IPP-DOE program).

References (5)

R.Z. Valiev et al.
Scripta Mater.
(2003)
A.A. Popov et al.
Scripta Mater.
(1997)

There are more references available in the full text version of this article.

Cited by (92)

Combined severe plastic deformation processing of commercial purity titanium enables superior fatigue resistance for next generation implants
2024, Biomaterials Advances
Commercial purity titanium (cp-Ti) is considered for replacing Ti64 as an implant material in various applications, due to the potential toxicity associated with the release of Al and V ions. However, the mechanical properties of cp-Ti, particularly fatigue resistance, are inadequate for this purpose. In this study, cp-Ti grade 4 rods were processed using a combination of equal channel angular pressing and rotary swaging (ECAP/RS). Tensile and fatigue tests were conducted, along with detailed microscopy and evaluation of corrosion resistance and biocompatibility. An average yield strength of 1383 MPa was obtained while maintaining moderate ductility of 10 %. This represents the highest strength ever recorded for cp-Ti, even exceeding that of Ti64. Additionally, fatigue endurance limit increased by 43 % up to 600 MPa, almost obtaining that of Ti64. Strengthening mechanisms were attributed to the ultrafine-grained (UFG) microstructure generated by ECAP/RS, along with strong crystallographic texture and formation of sub-grain structure. Furthermore, the corrosion resistance and biocompatibility of cp-Ti were largely unaffected, potentially easing regulatory transition in future medical devices. Thus, these results demonstrate high potential of combined ECAP/RS processing to manufacture UFG cp-Ti grade 4 materials that prospectively allow for the substitution of questionable alloys and downsizing of medical implants.
Exceptional strength-plasticity synergy in β-Ti alloy via HPT and short-period annealing
2023, Journal of Alloys and Compounds
A new metastable β-type Ti-3.5Al-5Mo-4 V (Ti-B20) alloy with a high strength has been recently developed. However, the plasticity of the alloy after solution or aging treatment is poor due to its large initial β grain size, and conventional deformation-induced grain refinement is quite limited. In the present work, a high-pressure torsion (HPT) method was used to manufacture a Ti-B20 alloy with large deformed grains. The Ti-B20 alloy subjected to HPT was annealed in a β-phase field (850 ℃) for different periods. The deformation mechanism during HPT processing and the recrystallization behavior of the deformed alloy were systematically revealed. The results show that the deformation mode during HPT processing is mainly dislocation slip and shear band splitting of the original grains and martensitic phase (α″). During recrystallization annealing, the shear bands and high-density dislocations generated by the HPT deformation of the Ti-B20 alloy significantly promote nucleation and growth of recrystallized grains, which shortens the time to complete recrystallization. After recrystallization annealing, exceptional strength-ductility synergy with a high tensile strength of 941.9 MPa and a high plasticity of 39.7% is achieved in the fine-grained (14.68 µm) alloy, which is the highest elongation reported in the Ti-B20 alloy at present. The fine grains with a low dislocation density provide more space for dislocation slip and accumulation while maintaining high strength. This work provides a new possibility for preparing fine-grained β-titanium alloys with synergistic strength and plasticity.
Fracture mechanism of electrically-assisted micro-tension in nanostructured titanium using synchrotron radiation X-ray tomography
2023, Scripta Materialia
A coarse-grained (CG) Ti with an average grain size of ∼50 μm was processed by high-pressure torsion (HPT) under a pressure of 6.0 GPa through 10 turns at room temperature (RT) to produce nanostructured titanium (Nano-Ti) with an average grain size of ∼95 nm. Electrically-assisted (EA) micro-tensions of CG-Ti and Nano-Ti were performed using pulsed unidirectional current with a current density of 750 A/mm², pulse width of 10⁻⁴ s. The stress drop is higher for Nano-Ti compared with CG-Ti. The results of fractograph observations show many cleavage steps in Nano-Ti while the CG-Ti is dominated by dimples. The spatial distribution of voids in tensile specimens was revealed after testing using synchrotron radiation X-ray tomography. The results demonstrate that applying a pulse current can effectively reduce the number and volume of voids before fracture. A model is presented describing the fracture mechanism of CG-Ti and Nano-Ti during EA micro-tension.
Phase transitions and mechanical behavior of Ti-3wt.%Nb alloy after high pressure torsion and low-temperature annealing
2022, Materials Science and Engineering: A
The influence of microstructure and phase transitions on tensile strength, plasticity and microhardness of a Ti-3wt.%Nb alloy subjected to high pressure torsion (HPT) followed by annealing is studied in this work. The initial state of the alloy was the α-solid solution with a small amount of the β-precipitates. The characterization of microstructure and phase transitions has been performed using scanning and transmission electron microscopy as well as X-ray diffraction. During HPT, the grain refinement took place and the α-phase partially transformed into ω-phase. The tensile test of the HPT-deformed sample showed an almost two-fold increase of ultimate strength in comparison with the initial state, as well as lower plasticity. During the short-term annealing of the deformed state up to 250 °C, the grain size slightly increased, the density of the crystal structure defects decreased and ω-phase transformed back to the α-phase. Such microstructural changes led to an important growth of plasticity and a slight decrease in ultimate strength of the HPT-deformed alloy.
Surface modification and twinning behavior in gradient graphene-based TiC/Ti6Al4V composite
2022, Applied Surface Science
Gradient materials have significant potential to break strong plastic tradeoffs. Graphene with a strong affinity for titanium alloys has an influential application value for material modification. In this study, microstructure evolution and deformation behavior of graphene-based TiC/Ti6Al4V composites processed by friction stir processing (FSP) have been investigated. Electron backscattered diffraction (EBSD) reveals uniform microstructure in the stir zone (SZ). Transmission electron microscopy (TEM) observations reveal distinct microstructures at different depths from the processed surface. The SZ includes nano/micro grains and TiC nanoplates. Twin structure exists in both α matrix and TiC. Stress-induced martensitic transformation is suppressed. As depth increases, TiC gradually disappears and the FSP-induced texture {-2116} < 2–1–11 > becomes slightly stronger. Moreover, there exists special crystallographic orientation relation: (1 1 1)TiC//(0001)Ti. In the base metal (BM), larger grains are observed, and dislocation structure becomes the dominant defect feature. Nanoindentation results show that hardness decreases first and then increases from the processed surface to the bulk metal. The distribution of hardness is the result of combined action of strengthening effect of TiC twins and deformation adaptation effect of α twins.
Fracture of severely plastically deformed titanium
2022, Materials Letters
The mechanical properties with emphasis on the fracture behaviour of ultrafine-grained Ti processed by high pressure torsion were evaluated. Through this process a substantial refinement of the microstructure was induced. As a consequence, a pronounced increase of tensile strength was achieved compared to conventional Ti while the ductility exhibited only a minor reduction after processing. The fracture toughness in terms of the critical stress intensity factor and crack tip opening displacement was evaluated in different crack growth directions and showed a pronounced orientation dependency, which can be related to the typically elongated microstructure induced by this deformation method.

View all citing articles on Scopus

View full text