Elsevier

Materials & Design

Volume 50, September 2013, Pages 613-619
Materials & Design

Effect of pore structure on the compressive property of porous Ti produced by powder metallurgy technique

https://doi.org/10.1016/j.matdes.2013.02.082Get rights and content

Highlights

  • Porous Ti with various pore size and porosity was made using PMMA as space holder.

  • Compressive strength and modulus vary with the porosity and macro-pore size.

  • The failure of the struts on porous Ti is controlled primarily by macro-pores.

  • Compressive fractography shows the brittle cleavage fracture mainly.

Abstract

Porous Ti with an average macro-pore size of 200–400 μm and porosity in the range of 10–65% has been manufactured using polymethyl methacrylate (PMMA) powders as spacer particles. The compressive strength and elastic modulus of resultant porous Ti are observed in the range of 32–530 MPa and 0.7–23.3 GPa, respectively. With the increasing of the porosity and macro-pore size, the compressive strength and modulus decrease as described by Gibson–Ashby model. The failure due to cracking (complete fracture) of the struts on porous Ti is controlled primarily by macro-pores. Fractography shows evidence of the brittle cleavage fracture mainly, but containing a few fine shallow dimples and a small amount of transcrystalline fracture of similarly oriented laths. The failure mechanism has been discussed by taking the intrinsic microstructural features into consideration.

Introduction

Ti and its alloys are nowadays the most attractive metallic biomaterials due to their excellent mechanical properties, wonderful biocompatibility and the good corrosion resistance [1], [2], [3]. Unfortunately, most bulk metallic implant materials continue to suffer from problems of interfacial stability with host tissues, biomechanical mismatch of elastic modulus. For instance, the elastic modulus of pure Ti (110 GPa) is much higher than that of human bone (1.5–26 GPa) [4], [5]. Consequently, porous Ti in the form of pore structure can be potentially used as the bone implants because that not only can it provide a favorable environment for the new-bone ingrowth and the transport of the body fluids, but also match mechanical properties to those of the surrounding bone, which would be expected to reduce the extent of stress shielding and resist handling during implantation and in vivo loading [6], [7], [8].

However, the pores in porous metallic materials could make the stress concentration higher than the theoretical value calculated by effective area [9]. Li et al. [10] reported that the pores (236 μm average size) in A356-T6 alloys are more likely to initiate a fatigue crack and accumulate plastic strain. Similarly, Ti matrix can be weakened by the pores in porous Ti. As the porosity of porous Ti decreases, the strength [1], [11] and stiffness [12] increase, and the plateau region in stress–strain curve starts to disappear (owing to the increasing of the closed cellular pores) [12], [13]. Spoerke et al. [14] found that the elastic modulus and yield strength of porous Ti with elongated and aligned pores are larger in the longitudinal direction than in the transverse direction. Shen et al. [15], [16], [17] analyzed the effect of microstructure on the uniaxial compressive response in a porous Ti with 12% porosity by 2D and 3D finite element models and suggested that the orientation and arrangement (random vs periodic microstructure) of pores are very important for mechanical response, and the arrangement of pores impacts more on “local properties” like stress concentration than “macroscopic properties” like Young’s modulus. According to the finite element predictions and the experimental data on the porous Ti with 50% porosity, Li et al. [18] showed that the randomization of pore size and spatial distribution have significant influence on both “local properties” and “macroscopic compressive properties”. Zou et al. [13] investigated the compressive properties of porous Ti (with a porosity ranging from 35% to 84% and the pore size from 150 μm to 600 μm) manufactured by sintering Ti fibers and found that the inhomogeneous microstructure and the spiral structure of the porous Ti bring about the large difference between the experimental data and the calculated data by Gibson–Ashby model.

Our previous study [19] on the microstructure evolution and tensile fracture behavior of porous Ti produced by powder metallurgy technique using polymethyl methacrylate (PMMA) as space holder has shown that, the porous Ti has two kinds of pores: open interpenetrated macro-pores with the size of 30–260 μm and micro-pores (with the average size of 9 ± 2 μm and the porosity of 8 ± 3%) on the ligaments of porous Ti. The latter is the same as the pores on solid Ti prepared by the same processing but without space holders in green body of Ti powders. The stress–strain curves of solid and porous Ti both show the linear-elastic relation in tension right up to fracture at small strains (<3%). Porous Ti fails mainly by the brittle cleavage, although, a few small shallow dimples and a small amount of transcrystalline fracture of similarly oriented laths in a colony are observed on the fracture surface. In contrast to porous Ti, the mixed failure mechanism of solid Ti involves cleavage fracture with laths and voids, local ductile failure by dimples of varying size and shape, and transcrystalline fracture along the colonies. Additionally, porous Ti fails with the formation of shear bands at 45° to the tensile axis and the cracking of the struts on porous Ti is controlled primarily by the macro-pores.

The present paper reports the compression behavior and the fracture mechanism of porous Ti produced by powder metallurgy technique using PMMA as space holder. Through the finite-element modeling method, the stress and strain distribution related to the macro-pores under load-bearing conditions has been analyzed. The influence of the porosity, pore size, pore morphology and distribution on the compressive behavior in porous Ti has been investigated and compared with theoretical prediction.

Section snippets

Materials and methods

Porous Ti was prepared by powder metallurgical method using PMMA with the size of 71–100 μm, 100–154 μm, 154–200 μm, 200–315 μm, 315–400 μm and 400–630 μm as the space holders. The mixtures containing 10–70 vol% PMMA and CP-Ti powders were compacted as a cylindrical rod (∅10 mm × 10 mm). The detailed preparation method was based on that in Ref. [19]. For comparative purpose, similar measurements have been carried on the solid Ti prepared by the same processing but without space holders in green body of Ti

Compression behavior and fracture mechanism of porous Ti

The compressive stress–strain curves of three porous Ti samples with the porosity of 50% and average macro-pore size of 300 μm, shown in Fig. 1a, exhibit three stages: a linear elastic region (marked I); a long plateau stage (marked II) with nearly constant flow stress to a large strain; and a densification stage (marked III), where the flow stress increases sharply, which are typical compression stress–strain curves of elastic–plastic foams, similar to porous bone [21] and porous Ti produced by

Conclusions

Porous Ti with the porosity of 10–65% and an average macro-pore size of 200–400 μm was prepared by powder metallurgical method using PMMA as the space holder. The resultant compressive strength and elastic modulus of the porous Ti vary in the range of 32–530 MPa and 0.7–23.3 GPa, respectively. A study of the influence of the pore structure on the compressive deformation and final fracture behavior provides the following key findings:

  • (1)

    The stress–strain curves of the porous Ti show the

Acknowledgements

This work was supported by research project under Grant No. 20060103 from Education Office of Liaoning Province in China.

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