Mechanical Properties and Corrosion Behavior of Powder Metallurgy Biodegradable Metals

Mahdi Dehestani, Purdue University

Abstract

Successfully designing biodegradable metallic orthopedic implants that support bone remodeling after injury, while gradually and safely being resorbed would be the next step in the development of transient biomedical devices. Such an advancement would eliminate the need for a second surgery to remove traditional permanent implants and allow some orthopedic trauma patients to live prosthetics free after bone healing is complete. Candidate materials for this area of research include iron (Fe) and magnesium (Mg) based alloys and composites. This thesis is focused on the Fe-based materials realm and is presenting results extracted from two main research avenues. One area is on powder metallurgy (PM) design and fabrication of iron-hydroxyapatite (Fe–HA) composites, while a second one investigates the potential of iron-manganese alloys (Fe–30wt%Mn). In short, the thesis critically assessed and analyzed the impact of powder particle size on mechanical properties and in vitro degradation rates of Fe-based materials prepared by PM. The microstructural design approach, based on the relative particle sizes of the constituents in PM-derived biodegradable metals, taken in this thesis shows potential for producing materials with tailored mechanical strength and degradation rate for desired implant application. This opens the possibility for constructing material selection platforms and processing windows, aiding in enhancing future resorbable implant designs and performance. In the first part of this thesis, it was demonstrated that certain critical parameters (particle size, morphology, and distribution patterns of HA as a bioactive phase in the Fe matrix), determine the mechanical properties and in vitro degradation rates of Fe–HA composites. Nine Fe–HA composites were fabricated by varying the amount (2.5, 5, 10 wt%) and particle size (< 1 µm, 1–10 µm, 100–200 µm) of HA. Yield strength, tensile strength, and ductility of the composites decreased with increasing HA content and decreasing HA particle size, whereas their corrosion rates (CR) increased. The strongest composite was Fe–2.5 wt% HA (100–200 µm) with σy = 81.7 MPa, σ u = 130.1 MPa, fracture strain of 4.87%, and CR = 0.23 mmpy. The weakest composite was Fe–10 wt% HA (< 1µm) which did not exhibit plastic deformation, fractured at σu = 16.1 MPa with 0.11% strain, and showed the highest CR of 1.07 mmpy. In the second part of the thesis, microstructures, mechanical properties and in vitro degradation rates of PM-derived Fe–Mn alloys were investigated with respect to the particle size of the iron (Fe) powder and extent of Mn diffusion and alloying during sintering. Additionally, by applying different heat treatments on sintered Fe–30wt%Mn alloy, a phase transformation (γ → &egr;) for this composition and its influence on mechanical and corrosion properties were studied. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) were used to characterize the transformation and identify the austenite (γ) and epsilon martensite (&egr;) phases in the system. Microstructures and tensile fracture surfaces were examined by Scanning Electron Microscope (SEM). The results showed that the Fe particle size affects the overall Mn alloying significantly, i.e., coarse Fe particles (30-200 µm) result in Fe–Mn alloys with σ y = 48.2 MPa, σu = 73.6 MPa, fracture strain of 2.42% and CR = 1.36 mmpy, while ultrafine particle size ( < 44 µm) leads to σy = 134.2 MPa, σu = 215.8 MPa, fracture strain of 10.9% and CR = 0.29 mmpy. The striking difference is due to the different degrees of interparticle Fe–Mn contact areas that act as physical diffusion paths and determine Mn diffusion and microstructural homogeneity. Heat treatments increased the hardness of Fe–30wt%Mn alloy due to some martensite formation, but tensile properties did not change noticeably. Also, formation of martensite increased the corrosion rate of the alloy slightly.

Degree

Ph.D.

Advisors

Stanciu, Purdue University.

Subject Area

Engineering|Materials science

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