Metallic Materials PDF

Ch4 Metallic Materials D r. N u r N a b i l a h S h a h i d a n BF 2.18  FR Outline Metallic Materials: Stainless Steels Co-Based Alloys Ti and Ti-Based Alloys Corrosion of Metallic...

Ch4 Metallic Materials D r. N u r N a b i l a h S h a h i d a n BF 2.18  FR Outline Metallic Materials: Stainless Steels Co-Based Alloys Ti and Ti-Based Alloys Corrosion of Metallic Implants  FR Introduction N. Grant and K. McIntosh, Austrailian National University  FR Introduction Strength of pure metals versus alloys N. Grant and K. McIntosh, Austrailian National University  FR Introduction Most metals used for manufacturing implants (e.g., Fe, Cr, Co, Ni, Ti, Ta, Mo, and W) can be tolerated by the body in small amounts. Alloy: are metals containing 2 or more elements Sometimes those metallic elements, in naturally occurring forms, are essential in cell functions (Fe), synthesis of a vitamin B12 (Co), and crosslinking of elastin in the aorta (Cu) but cannot be tolerated in large amounts in the body. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Introduction The biocompatibility of implant metals is of considerable concern because they can corrode in the hostile body environment. Biocompatible: the ability of material to perform within an appropriate host response in a specific application. The consequences of corrosion include loss of material, which will weaken the implant, and probably more important, that the corrosion products escape into tissue are potentially cytotoxic. Cytotoxic: a substance that has a toxic effect on certain cells results in cell damage or death.  FR Introduction N. Grant and K. McIntosh, Austrailian National University FR Classification of metal alloy Al, Ti, Co, Mg  Stainless Steel Add a footer 9  FR Compositions of Steel Following is a list of some important chemical elements used in structural steels: Carbon (C) Next to iron, carbon is by far the most important chemical element in steel. Increasing the carbon content produces a material with higher strength and lower ductility. Structural steels, therefore, have carbon contents between 0.15 to 0.30 percent; if the carbon content goes much higher, the ductility will be too low, and for magnitudes less than 0.15 percent the strength will not be satisfactory. Design of Steel Structures MIT Department of Civil and Environmental Engineering , http://web.mit.edu/1.51/www/pdf/chemical.pdf  FR Compositions of Steel Nickel (Ni) In addition to its favorable effect on the corrosion resistance of steel, nickel enhances the low- temperature behavior of the material by improving the fracture toughness. It is used in structural steels in varying amount; for example, certain grades of ASTM A514 have Ni contents between 0.30 and 1.50 percent; some types of A588 have nickel contents from 0.25 to 1.25 percent. Design of Steel Structures MIT Department of Civil and Environmental ASTM A588 Weathering Steel Engineering , http://web.mit.edu/1.51/www/pdf/chemical.pdf ASTM A515 low carbon quenched : outdoors machinery  FR Compositions of Steel Vanadium (V) The effects of this chemical element are similar to those of Mn, Mo, and Cb (Columbium). It helps the material develop a finer crystalline microstructure and gives increased fracture toughness. Vanadium contents of 0.02 to 0.15 percent are used in ASTM grades A572 and A588, and in amounts of 0.03 to 0.08 percent in A514. Design of Steel Structures MIT Department of Civil and Environmental Engineering , http://web.mit.edu/1.51/www/pdf/chemical.pdf  FR Compositions of Steel Molybdenum (Mo) Molybdenum has effects similar to manganese and vanadium and is often used in combination with one or the other. It particularly increases the strength of the steel at higher temperatures and also improves corrosion resistance. Typical amounts of molybdenum are 0.08 to 0.25 percent for certain grades of A588 steel, and 0.15 to 0.65 percent for various types of A514. Design of Steel Structures MIT Department of Civil and Environmental Engineering , http://web.mit.edu/1.51/www/pdf/chemical.pdf  FR Compositions of Steel Chromium (Cr) Chromium is present in certain structural steels in small amounts. It is primarily used to increase the corrosion resistance of the material, and for that reason often occurs in combination with nickel and copper. Stainless steel will typically have significant amounts of chromium. Thus, the well-known “18-8” stainless steel contains 18 percent of nickel and 8 percent of chromium. Design of Steel Structures MIT Department of Civil and Environmental Engineering , http://web.mit.edu/1.51/www/pdf/chemical.pdf  FR Stainless Steels 18% chromium and 8% nickel. First stainless steel : 18-8 (type 302). 18-8 is stronger and resist corrosion then steel. 18-8Mo : contain Molybdenum. Here, Mo helps to improve the corrosion resistance in salt water. This alloy became known as type 316 stainless steel. The carbon content of 316 stainless steel was reduced from 0.08 % to 0.03 % maximum for better corrosion resistance to chloride solution, and it became known as 316L. They are corrosion resistance than any others stainless steel. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Compositions of Stainless Steel Adapted from Biomaterials, sujata V. Bhat Carbon must be reduced compared to the level in cast alloys (0.05% versus approximately 0.25% or higher). Low carbon contents mean that less strengthening is produced by carbides. To enhance fabricability (to shape/form), chromium contents generally are reduced and nickel added. Design of Steel Structures MIT Department of Civil and Environmental Engineering , http://web.mit.edu/1.51/www/pdf/chemical.pdf  FR Medical device applications for stainless steels Handbook of Materials for Medical Devices J.R. Davis, editor, p21-50  FR Medical device applications for stainless steels Handbook of Materials for Medical Devices J.R. Davis, editor, p21-50  FR Types and Composition of Stainless Steels Chromium is a major component of corrosion-resistant stainless steel. The minimum effective concentration of chromium is 11% Stainless steel can be classified as: 1. Austenitic 2. Martensitic 3. Ferritic 4. Duplex stainless steels 5. Precipitation-hardenable (PH) stainless steels Handbook of Materials for Medical Devices J.R. Davis, editor, p21-50  FR Austenitic Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. This structure gives these steels their characteristic combination of weldability and formability. Austenitic stainless steels offer excellent formability, and their response to deformation can be controlled by the nickel content (i.e., higher nickel contents result in improved formability). Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel. Austenitic stainless steels find applications in medical devices where good corrosion resistance and moderate strength are required, for example, canulae, dental impression trays, hypodermic needles. Handbook of Materials for Medical Devices J.R. Davis, editor, p21-50  FR Martensitic These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic. Handbook of Materials for Medical Devices J.R. Davis, editor, p21-50  FR Martensitic Martensitic stainless steels are essentially Fe-Cr-C alloys. They are ferromagnetic, hardenable by heat treatments, and generally resistant to corrosion only in relatively mild environments. Chromium content is generally in the range of 10.5 to 18%, and carbon content can exceed 1.2%. Elements such as niobium, silicon,tungsten, and vanadium can be added to modify the tempering response after hardening. Small amounts of nickel can be added to improve corrosion resistance. The high hardness of martensitic stainless steels makes them ideally suited or dental and surgical instruments. Handbook of Materials for Medical Devices J.R. Davis, editor, p21-50  FR Austenitic vs Martensitic Stainless Steel Composition and properties? Can you list them? Add a footer 23  Co- Based alloy Add a footer 24  FR Co- based alloys These materials are usually referred to as cobalt-chromium alloys. There are basically two types: one is the CoCrMo alloy, which is usually used to cast a product, and the other is CoNiCrMo alloy, which is usually wrought by (hot) forging. The castable CoCrMo alloy has been in use for many decades in dentistry and in making artificial joints. The wrought CoNiCrMo alloy has been used for making the stems of prostheses for heavily loaded joints (such as the knee and hip). Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Co- based alloys Castable: In all casting processes, a metal alloy is melted and then poured or forced into a mold where it takes the shape of the mold and is allowed to solidify. Once it has solidified, the casting is removed from the mold. Wrought: Wrought iron is iron that has been heated and then worked with tools Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Types &Composition of Co-Based Alloys ASTM lists four types of Co-based alloys that are recommended for surgical implant applications: 1. cast CoCrMo alloy (F75), 2. wrought CoCrWNi alloy (F90), 3. wrought CoNiCrMo alloy (F562) 4. wrought CoNiCrMoWFe alloy (F563). At the present time only two of the four alloys are used extensively in implant fabrications — the castable CoCrMo and the wrought CoNiCrMo alloy. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Chemical Compositions of Co-Based Alloys (ASTM, 2000) Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Chemical Compositions of Co-Based Alloys The two basic elements of Co-based alloys form a solid solution of up to 65 % Co and the remainder is Cr, as shown in previous table. Molybdenum is added to produce finer grains, which results in higher strength. One of the most promising wrought Co-based alloys is the CoNiCrMo alloy originally called MP35N (Standard Pressed Steel Co.), which contains approximately 35 % Co and Ni each. The alloy has a high degree of corrosion resistance to seawater (containing chloride ions) under stress. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Chemical Compositions of Co-Based Alloys In order to produce wrought cobalt-chromium alloys, carbon must be reduced compared to the level in cast alloys (0.05% versus approximately 0.25% or higher). Low carbon contents mean that less strengthening is produced by carbides. To enhance fabricability, chromium contents generally are reduced and nickel added. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Ti and Ti-Based Alloys Attempts to use titanium for implant fabrication date to the late 1930s. It was found that titanium was tolerated in cat femurs, as was stainless steel and Vitallium® (CoCrMo alloy). The lightness of titanium (4.5 g/cm3 compared to 7.9 g/cm3 for 316 stainless steel, 8.3 g/cm3 for cast CoCrMo, and 9.2 g/cm3 for wrought CoNiCrMo alloys). Good mechanochemical properties are important features for implant application. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Chemical composition of Ti and Ti-Based Alloys There are four grades of unalloyed titanium for implant applications as given in table below. The impurity contents distinguish them; oxygen, iron and nitrogen should be controlled carefully. Oxygen in particular has a great influence on ductility and strength. Composition: 98.9–99.5% titanium, with up to 0.50% iron, 0.40% oxygen, 0.08% carbon, 0.05% nitrogen, and 0.015% hydrogen. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Chemical composition of Ti and Ti-Based Alloys One titanium alloy (Ti6Al4V) is widely used to manufacture implants. The main alloying elements of the alloy are aluminum(5.5–6.5 %) and vanadium (3.5–4.5 %). Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Mechanical properties of Ti and Ti-Based Alloys high aluminum content of alloys makes for excellent strength characteristics and oxidation resistance. Good mechanochemical Biomaterials, Sujata V.Bhat Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Mechanical properties of Ti and Ti-Based Alloys The strength of the Ti alloys is similar to 316 stainless steel or the Co-based alloys. From table below, the higher impurity content leads to higher strength and reduced ductility. Titanium, nevertheless, has poor shear strength, making it less desirable for bone screws, plates, and similar applications. It also tends to gall or seize when in sliding contact with itself or another metal. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Corrosion of Metallic Implants Titanium derives its resistance to corrosion by the formation of a solid oxide layer. Under in vivo conditions the oxide (TiO2) is the only stable reaction product. The oxide layer forms a thin adherent film and passivates the material. Biomaterials, An Introduction, Joon Park R.S. Lakes  FR Corrosion of Metallic Implants N. Grant and K. McIntosh, Austrailian National University  Passivity FR Is the phenomenon that demonstrate how the corrosive is inhibited in any given environment. Oxide Film Theory Metal oxide is formed This oxide separate metal from environment. Slow down the rate of reaction.  FR Passivity When titanium is exposed to oxygen, thin layer of oxide film naturally forms in titanium’s surface. This layer is a strong and stable layer that grows spontaneously in contact with air and prevents the diffusion of the oxygen from the environment providing corrosion resistance.