Lecture 4 Chem 442/Mase 542 Metals as Biomaterials cont.

Metals Used as Biomaterials • Stainless Steels • Cobalt- based alloys – Always mixed with chromium • Titanium • Titanium- based alloys – Very light with relatively high strength – Oxygen content affects strength – Relatively pure • Noble metals- Au, Au, Pt, Pd, Ir – Expensive and poor material properties – Used in electrodes- highly corrosion resistant Fracture Fixation Plates Hip Prostheses Heart Pacemaker Housings Hip Prostheses Alloying is usually to modify corrosion and mechanical properties

Metals as Biomaterials Alloying is usually to modify corrosion and mechanical properties

Dislocation Motion & plastic deformation Metals - plastic deformation occurs by slip – an edge dislocation (extra half-plane of atoms) slides over adjacent plane half-planes of atoms. So we saw that above the yield stress plastic deformation occurs. But how? In a perfect single crystal for this to occur every bond connecting tow planes would have to break at once! Large energy requirement Now rather than entire plane of bonds needing to be broken at once, only the bonds along dislocation line are broken at once. If dislocations can't move, plastic deformation doesn't occur! Adapted from Fig. 7.1, Callister & Rethwisch 8e.

Deformation Mechanisms: Slip plane Slip plane - plane on which easiest slippage occurs Highest planar densities (and large interplanar spacings) FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed) total of 12 slip systems in FCC For BCC & HCP there are other slip systems. Strength is increased by making dislocation motion difficult!

Slip Motion in Polycrystals 300 mm • Polycrystals stronger than single crystals – grain boundaries are barriers to dislocation motion. • Slip planes & directions change from one grain to another. • The grain with the largest shear stress yields first. • Other (less favorably oriented) grains yield later. Adapted from Fig. 7.10, Callister & Rethwisch 8e. (Fig. 7.10 is courtesy of C. Brady, National Bureau of Standards [now the National Institute of Standards and Technology, Gaithersburg, MD].) WHEN RESOLVED SHEAR STRESS CREATED UNDER TENSİLE STRESS IS LARGER THAN THE CRITICAL SHEAR STRESS AT WHICH MATERIAL YİELDS, THEN SLIP OCCURS

Reducing grain size increases the strength… • Grain boundaries are barriers to slip. • Barrier "strength" increases with Increasing angle of misorientation. • Smaller grain size: more barriers to slip. • Adapted from Fig. 7.14, Callister & Rethwisch 8e. (Fig. 7.14 is from A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)

Precipitation Increases Strength • Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum). Dislocation “advances” but precipitates act as “pinning” sites .

Impact of Cold Work As cold work is increased • Yield strength (sy) increases. • Tensile strength (TS) increases. • Ductility (%EL or %AR) decreases. low carbon steel Adapted from Fig. 7.20, Callister & Rethwisch 8e.

Strain Hardening Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point (cold working). The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work). The average distance between dislocations then decreases and dislocations start blocking the motion of each one.

Cold Work (Strain Hardening) • Deformation at room temperature (for most metals). • Common forming operations reduce the cross-sectional area: Adapted from Fig. 11.8, Callister & Rethwisch 8e. -Forging A o d force die blank -Rolling roll A o d -Drawing tensile force A o d die -Extrusion ram billet container force die holder die A o d extrusion

Dislocation Structures Change During Cold Working Dislocation structure in Ti after cold working. • Dislocations entangle with one another during cold work. • Dislocation motion becomes more difficult. Fig. 4.6, Callister & Rethwisch 8e. (Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.)

Grain Size Influences Properties Metals having small grains – relatively strong and tough at low temperatures Metals having large grains – good creep resistance at relatively high temperatures

Fracture mechanisms Ductile fracture Accompanied by significant plastic deformation Brittle fracture Little or no plastic deformation Catastrophic

Ductile vs Brittle Failure Very Ductile Moderately Brittle Fracture behavior: Large Moderate %AR or %EL Small • Classification: Adapted from Fig. 8.1, Callister & Rethwisch 8e. • Ductile fracture is usually more desirable than brittle fracture! Ductile: Warning before fracture Brittle: No warning

1. Stainless Steel Main advantage: corrosion resistance Most common: 316L Mixture of Fe (60-65%), Cr (17-20%), and Ni(12-14%) Chromium oxidizes to limit corrosion (Cr2O3 formation on surface) But, high chromium content reduces strength (increases BCC over FCC)

Stainless Steels Low carbon content (L) : less than 0.03% to reduce corrosion If High C content: Carbides (ex. Cr23C6 ) ppt at grain boundries Prevents protective Cr2O3 formation Corrosion assisted fracture at grain boundries

Stainless Steels Ni: added to increase strength Stabilizes austenite (FCC) Mo: increases resistance to pitting corrosion N: increases mechanical strength

Microstructure and Properties 316L : single phase FCC grain size 100 micron or less Smaller grain means more strength Grain size: Manufacturing history Plastic deformation within grains Cold worked metal versus annealed Higher tensile and fatigue strength Decreased ductility (not a problem in implants) Texture Preferred orientation of grains Typical microstructure of cold-worked 316L stainless steel, ASTM F138,

Microstructure and Properties Texture Preferred orientation of grains Evidence of textured grain structure in 316L stainless steel ASTM F138, as seen in a longitudinal section through a cold-worked bone screw. The long axis of the screw is indicated by the arrow.

Corrosion Even the 316L stainless steels may corrode inside the body under certain circumstances in a highly stressed and oxygen depleted region, such as contact under screws or fracture plates. Thus, stainless steels are suitable to use only in temporary implant devices, such as fractures plates, screws and hip nails.

Corrosion Corrosion Leaching into body Crack formation and fatigue Mechanism – Lowest energy state is oxidized state – Metal atoms ionize, go into solution, and combine with oxygen – Similar to rusting of iron: metal just flakes off Degradation : oxides, hydroxides, and other compounds Biological fluids contain water, dissolved oxygen, ions, etc – Very aggressive mixture Tendency to corrosion based on electrochemical series

Corrosion – Different regions of the body affect corrosion levels differently – Wounds and infections can also change pH dramatically • Corrosion and fatigue can be synergistic effects • Rubbing on surfaces can disrupt passivation • Pitting is corrosion in local area • General problem is assuring metals in screws and plates are identical • Surgeons must be careful not to scratch metals or leave metals in tissues

Cobalt-Based Alloys Four main alloys ASTM F 75 – Co-28Cr-6Mo ASTM F 90 – Co-20Cr-15W-10Ni ASTM F 799 – Co-28Cr-6Mo ASTM F 562 – Co-35Ni-20Cr-10Mo

Co-alloys Strength > Stainless steel Lost wax method (femoral stems, oral implant, dental bridge, etc) Grain size can be controlled

Cobalt-Based Alloys Large grain size ( 4mm) Casting defects Microstructure of as-cast Co–Cr–Mo ASTM F75 alloy Large grain size ( 4mm) Casting defects HIP (Hot isotatic pressing): 8 micron grain size Higher yield strength and better ultimate and fatigue properties Lost-wax process Molten @ 1350-1450°C Pressurized into ceramic molds

Cobalt-Based Alloys Most widely used Co-Cr-Mo: F75 Corrosion resistant in chloride environment (Cr2O3) Aerospace and biomedical use Uses Femoral stems, oral implants, dental bridges Microstructure Co-rich matrix Carbides @ grain boundry Dendritic structure Lost-wax process Molten @ 1350-1450°C Pressurized into ceramic molds

Scanning electron micrographs of the fracture surface from a cast F75 subperiosteal dental implant Macrophoto of the fracture surface of the same Co–Cr–Mo ASTM F75 hip stem

Ti & Ti Alloys High strength/weight ratio Can be strengthened by alloying and deformation processing Excellent biocompatibility – little to no reaction with surrounding tissue Corrosion resistance derives from stable TiO2 surface oxide layer

For biomedical applications CP-Ti Ti-6Al-4V Ti-6Al-4V ELI

CP-Ti: Commercially Pure ASTM F 67 Essentially all α (hcp) Relatively low strength, high ductility Excellent corrosion resistance Yield strength varies widely (25 ~ 70 ksi) depending upon interstitial & substitutional levels Used for : pacemaker cases, ventricular-assist devices, implantable infusion drug pumps, dental implants, craniofacial implants, screws and staples for spinal surgery

Alpha-beta allays used for Ti-6Al-4V is an alpha-beta alloy (V – beta stabilizer) Contains both alpha and beta phases Also contains alpha stabilizer and beta stabilizer Solution heat treated and quenched, followed by aging to precipitate alpha phase Ti-6Al-4V and Ti-6Al-4V ELI (ASTM F 136) extra-low interstitial: ELI used for total joint replacement arthroplasty hips and knees Arthroplastjoint surgery

Alpha-beta allays Ti-6Al-7Nb (ASTM F 1295) : Aluminum Niobium allow used for femoral hip stems, fracture fixation plates, spinal components, fasteners, nails, rods, screws and wire Ti-3Al-2.5V (ASTM F 2146) used for tubing Arthroplastjoint surgery

Callister: page 413

Metal Implant Reliability depends largely on the: corrosion, Structural integrity Host reaction wear, Reaction of body to wear debris Longevity of implant fatigue resistance of the materials

Cobalt-chromium-molybdenum Ti-6Al-4V ELI Titanium AlloyInterface

Fatigue Surface condition, e.g., smooth vs rough Loading type & characteristics (place of the implant) Microstructure (grain size, allignment, phases) Site of use

Friction & Wear Passive & tenacious TiO2 film remains under low loads and slow sliding in articulating conditions Wearing away of TiO2 film can lead to high wear rates of mating surfaces Wear debris leads to adverse tissue reactions Wear of the mating surfaces leads to loosening of implant

Shape memory Allos NITINOL nickel±titanium alloy of near-equiatomic composition high elastic deformation self-expanding stents Exceptionally flexible short and long-term biocompatibility chemical (degradation,corrosion, and dissolution) stability