Elsevier

Materials & Design (1980-2015)

Volume 54, February 2014, Pages 786-791
Materials & Design (1980-2015)

Mechanical properties enhancement in Ti–29Nb–13Ta–4.6Zr alloy via heat treatment with no detrimental effect on its biocompatibility

https://doi.org/10.1016/j.matdes.2013.09.007Get rights and content

Highlights

  • Diverse heat treatment schemes have been applied on a new Ti-biomedical alloy.

  • Mechanical properties and microstructures are investigated.

  • Superior mechanical properties without detriment to biocompatibility are attained.

  • We propose liquid nitrogen quenching as the optimum procedure.

Abstract

The present investigation deals with the microstructural evolution and mechanical properties optimization of a new titanium based TNTZ (Ti–29Nb–13Ta–4.6Zr) alloy through applying a proper heat treatment strategy. The TNTZ alloy was subjected to solution treatment and cooling procedure via different medias including liquid nitrogen, water, oil, static air and vacuum furnace. The slow cooling in vacuum furnace and static air resulted in precipitation of ω phase. The formation of this phase secures an increase in the strength and hardness value, however stands detrimental to the ductility. The α″ martensite is formed due to accelerated cooling in liquid nitrogen, water and oil. This phase remains neutral to the ductility of the alloy, while its excessive formation increases the hardness and shear strength. This study proposes the liquid nitrogen quenching as a process that optimizes the mechanical properties by rendering the highest strength while preserving the ductility.

Introduction

The life expectancy of human beings has constantly and extraordinarily increased over the past few decades. This has led to a vast increasing demand for implants application and a necessity for their continuous improvement [1], [2]. The ideal biomaterial for implant applications is expected to exhibit excellent properties such as high mechanical strength, fatigue resistance, low-density, no adverse tissue reactions and corrosion resistance [3], [4], [5], [6], [7], [8]. Another prime key in applicability of bio-implants is the usage of materials with low Young’s modulus near that of human hard tissue. This is due to the fact that the insufficient load transfer from implant to adjacent bone may cause its absorption and eventual failure of the implant. This is often referred to as the stress-shielding phenomenon [9], [10].

Currently, titanium based alloys are of the most favorable groups of material in medical and dental applications. To meet the aforementioned properties in the optimum combination, TNTZ series, meta-stable β titanium alloys consisting nontoxic elements, namely Nb, Ta and Zr have been developed [3], [11]. Among them, Ti–29Nb–13Ta–4.6Zr (wt%) is believed to possess the lowest Young’s modulus [11].

Generally TNTZ alloy is potentially consisted of four phases of β, α, ω, and α″. Apart from the prior beta, each of these phases would be attained applying different thermal and mechanical treatments. Metastable ω, which is an intermediate phase in β→α transformation, may form during slow cooling from the β region or during isothermal aging below the β transus temperature [12], [13]. It is proved by Bowen [14] and Cui and Guo [15] that the existence of ω phase in the matrix results in high mechanical strength but a reduced elongation. Moreover Akahori et al. has established that the precipitation of this phase would increase the Young’s modulus of TNTZ alloy [12]. The α″ martensitic phase, however, can be obtained through rapid quenching from β region and by mechanical deformation below the β transus temperature (a temperature, above which the β phase is thermodynamically stable) [16]. In an investigation by Hao and Niinomi the effect of α″ formation on mechanical properties of TNTZ has been investigated [17]. They have proved that the existence of α″ will not deteriorate the biocompatibility of the alloy because this phase possesses a low Young’s modulus near that of the initial β phase. In conclusion, it is well established that the final mechanical properties of these alloys are affected by the volume fraction of the constituent phases as well as their state, including their morphology and distribution.

Accordingly, a variety of cooling media have been employed in present work to produce different microstructural characteristics and mechanical properties. The authors aim to achieve the best mechanical properties without deteriorating the biocompatibility of the alloy, needless of mechanical deformation. These studies would in due course result in simpler production of superior implants applied in human body.

Section snippets

Experimental details

The experimental material used in this study was as-forged TNTZ alloy, the chemical composition of which is presented in Table 1. The as-forged material was subjected to a solution treatment at 845 °C (above β transus temperature) for 45 min to remove the inhomogeneity of the alloying elements in particular Ta and Zr. The solution treatments were performed by a direct drive oiled sealed diffusion vacuum pump under a vacuum pressure of 0.00001 mbar. This was followed by rapid water quenching (WQ),

Microstructural evaluation

Fig. 2, Fig. 3 present the related XRD pattern and optical microstructures of the heat treated specimens, respectively. The microstructural characteristics have been significantly affected by the cooling procedure. As is observed all the heat treated specimens comprise equiaxed β grains, however, the higher the cooling rate the smaller the grain size. Accordingly the specimen quenched in liquid nitrogen and the one cooled in furnace hold the smaller and larger grain sizes, respectively. This is

Conclusions

The microstructural characteristics and mechanical properties of TNTZ alloy subjected to different heat treatment procedures have been investigated. The results are summarized as follows:

  • 1.

    The highest strengths have been obtained through liquid nitrogen quenching and furnace cooling. However the latter procedure results in β→ω transformation, which deteriorates the ductility of the alloy.

  • 2.

    Since the α″ martensite phase possesses proper ductility, the cooling schemes that result in the formation of

References (24)

Cited by (29)

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    One of the first TNZT alloys was Ti-29Nb-13Ta-4.6Zr characterized by low elastic modulus (65 GPa [5,10]). Its mechanical properties could be improved even further by proper heat treatment without detrimental effects to its biocompatibility [12]. A different low-modulus (48 GPa) gum metal alloy having the composition Ti-35Nb-3Zr-2Ta was proposed by Guo et al. [13,14].

  • The room temperature tensile deformation behavior of thermomechanically processed β-metastable Ti-Nb-Ta-Zr bio-alloy: the role of deformation-induced martensite

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