Human tissue is structured mainly of self-assembled polymers (proteins) and ceramics (bone minerals), with metals present as trace elements with molecular scale functions. However, metals and their alloys have played a predominant role as structural biomaterials in reconstructive surgery, especially orthopedics, with more recent uses in non-osseous tissues, such as blood vessels. With the successful routine use of a large variety of metal implants clinically, issues associated with long-term maintenance of implant integrity have also emerged. This review focuses on metallic implant biomaterials, identifying and discussing critical issues in their clinical applications, including the systemic toxicity of released metal ions due to corrosion, fatigue failure of structural components due to repeated loading, and wearing of joint replacements due to movement. This is followed by detailed reviews on specific metallic biomaterials made from stainless steels, alloys of cobalt, titanium and magnesium, as well as shape memory alloys of nickel–titanium, silver, tantalum and zirconium. For each, the properties that affect biocompatibility and mechanical integrity (especially corrosion fatigue) are discussed in detail. Finally, the most critical challenges for metallic implant biomaterials are summarized, with emphasis on the most promising approaches and strategies.

Metallic implant biomaterials

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Creep resistance is a major requirement for the use of magnesium in automotive powertrain components that are currently made of aluminium or cast iron. This paper summarises various new magnesium alloys that have been developed in recent years for elevated temperature applications. The alloy development basis, mechanical properties, creep resistance, bolt-load retention characteristics, and microstructure of seven new alloy systems (Mg–Al–RE, Mg–Al–Ca, Mg–Zn–Al–Ca, Mg–Al–Ca–RE, Mg–Al–Sr, Mg–Si, and Mg–RE–Zn; where RE represents mischmetal rare earth elements) are critically reviewed. The potential applications of these alloys in automotive powertrains are also discussed.

Recent magnesium alloy development for elevated temperature applications

Texture analysis in hexagonal materials

Development of the as-cast microstructure in magnesium-aluminium alloys

Creep properties of specimens taken from the core of AZ91D magnesium alloy (9% Al–1% Zn) ingots were investigated in the temperature range 120–180°C and stress range 40–115 MPa. Creep tests were performed under constant and varying loads, solution treated creep specimens were also tested. Observed creep rates are about three orders of magnitude lower than those of pure magnesium and elongations to fracture are at least twice those of pure magnesium. No steady-state stage is observed, however a minimum creep rate is reached after approximately two thirds of the creep life of the specimens. The stress exponent is ≈11 and is independent of temperature and stress. The activation energy decreases with increasing temperature, from 220 to 94 kJ mol −1 . Fractography studies show that the fracture is intergranular.

Creep studies of coarse-grained AZ91D magnesium castings

Creep properties of pressure die cast AZ91D magnesium alloy (9% Al-1% Zn) were investigated in the temperature range of 150–180 °C and load range of 30–100 MPa. The pressure die cast material is characterized by a small grain size of about 10 μm. Creep tests were performed under constant load and under varying load. Creep rates were found to be a few orders of magnitude lower than these of pure magnesium but higher than these found by the authors in previous study for ingot casting. Elongations to fracture were found to be at least 1.5 times than these of pure magnesium. No steady state stage was observed, however a minimum creep rate was reached after approximately one third of the creep life of the specimens. The stress exponent was found to be 6.9 for 150 °C and 5.4 for 180 °C and was independent of temperature and stress. Transmission electron microscopy (TEM) contrast experiments show dislocation existence on other planes than the basal planes.

Creep studies of AZ91D pressure die casting

Creep properties of specimens taken from the core of AZ91D magnesium alloy ingots (9 pct Al, 1 pct Zn) were examined in the temperature range of 120 °C to 180 °C and stress range of 40 to 115 MPa. Solution-treated and aged creep specimens were also tested. The creep rates observed were about three orders of magnitude lower than those of pure magnesium, and elongations to fracture were seen to be at least twice those of pure magnesium. A minimum creep rate was reached after approximately 2/3 of the creep life of the specimens (except for the aged specimens, in which the minimum creep rate appears at the beginning of the test). A qualitative model for the creep process in proposed on the basis of the creep tests and optical, scanning electron, and transmission electron microscopy. This model proposes that dislocation motion on additional slip systems is the primary creep mechanism and that cracking acts as a stress relief mechanism. Structural instability dictates the amount of hardening and, hence, creep resistance.

Qualitative model for creep of AZ91D magnesium alloy

The available studies of the creep behaviour of AZ91 alloy produced by die casting, ingot casting, or thixoforming are reviewed in the present paper. Differences in microstructure and creep response are analysed and discussed with a view to providing a unitary, comprehensive description of the creep behaviour of this material. In particular, the minimum creep rate dependence on applied stress is described by means of a modified form of power law, which takes into consideration the effects of grain size and of intragranular precipitates interacting with dislocations. Analysis of the data obtained from the die cast alloy indicates that, in this material, creep is controlled by climb. Based on this observation and on theoretical considerations, as well as on the microstructural similarities, it is concluded that creep is controlled by climb also in thixoformed and ingot alloys. The effect of grain size is then quantified, and the strengthening effect of intragranular precipitates is described by intr...

Review of creep behaviour of AZ91 magnesium alloy produced by different technologies

A qualitative model of the creep process of AZ91D was proposed in the past by the authors on the basis of continuous creep tests, optical and SEM metallography and TEM study. In view of this model, creep resistance in the case of interrupted creep tests is expected to be inferior compared with conventional creep test under the same load and temperature. The results of interrupted creep tests performed in the recent investigation, described in this paper, support the prediction and can be explained on the basis of the above mentioned creep model. The differences between the results of the continuous and interrupted creep test stems from microstructural instability of the alloy. This fact has been verified by heat treating of the alloy and stabilizing the microstructure. The creep behavior of the heat-treated specimens was found to be comparable to that of the as-cast pressure die casting specimen subjected to continuous creep.

M. Regev

Papers

Creep studies of coarse-grained AZ91D magnesium castings

Creep studies of AZ91D pressure die casting

Qualitative model for creep of AZ91D magnesium alloy

Review of creep behaviour of AZ91 magnesium alloy produced by different technologies

Continuous versus interrupted creep in AZ91D magnesium alloy