Implant Materials.
Titanium Titaniu m – 6% Al Alum umin inum um – 7% Niobium.
Second Edition
John Disegi Second Edition November 2008
About the Cover A portion of the Periodic Table depicts various major implant alloying elements.
Alpha and beta crystal structure of Ti-6Al-7Nb. Acknowledgement The author wishes to acknowledge the technical contributions of Professor S. Steinemann, University of Lausanne, Switzerland.
Table of Contents
2
Introduction
Basic Metallurgy
Properties
1. Composition
3
2. Microstructure
4
1. Physical
5
2. Tensile
6
3. Fatigue
7
4. Corrosion
8
5. Biocompatibility
11
6. Surface
12
References
13
Glossary
16
Introduction
Titanium – 6% Aluminum – 7% Niobium is a relatively new titanium alloy that has been selected by Synthes for the next generation of advanced fracture fixation devices. The alloy was conceived in 1977 by a team of researchers at Sulzer Bros., Winterthur, Switzerland.1 The new alloy was introduced by 1985 following six years of intensive development and testing. A total hip replacement (THR) prosthesis fabricated from Ti-6Al-7Nb was originally marketed by Protek as Protasul® 100 and has been in clinical use since early 1986. The mechanical properties of Ti-6Al-7Nb alloy are very similar to Ti-6Al-4V alloy which has been used as a biomaterial for many years. The major difference between the alloys is the replacement of vanadium by niobium. The Ti-6Al-7Nb composition is in agreement with the principle of utilizing only non-toxic elements for implant devices as outlined in U.S. Patent 4,040,129 assigned to Institute Straumann, Waldenburg, Switzerland.2 Niobium was discovered 3 while chemist C. Hatchett was analyzing a black stone near Connecticut in 1801. The element was originally named columbium and was assigned atomic number 41 in the Periodic Table. Columbium was later renamed niobium which was derived from Niobe, the goddess of tears in Greek mythology. The word niobium was preferred in Europe while the word columbium continued to be used in the United States. Various niobium compounds are extracted from ore concentrates by a chlorination process followed by metallic reduction to niobium metal.3 The metal is then purified by electron beam melting into ingots. The ingots are alloyed with specific metals and further refined in a vacuum consumable arc melting furnace to produce an intermediate raw material known as a niobium master alloy. The niobium master alloy is blended with pure titanium in the correct proportion and a cylindrical electrode is formed for melting under high vacuum. Ti-6Al-7Nb alloy is double or triple vacuum arc melted to provide an ingot composition that is very uniform and homogeneous. The ingot is hot pressed, hot rolled, and finished into round and flat bar products using conventional titanium alloy processing methods.
2
Basic Metallurgy
1. Composition
Composition requirements for Ti-6Al-7Nb alloy are included in ISO 5832-11 4 and ASTM F 1295 specification 5. The composition limits for Ti-6Al-7Nb alloy are listed as follows: Ti–6Al–7Nb composition limits Element
Composition (%)
Aluminum
5.50 to 6.50
Niobium
6.50 to 7.50
Tantalum
0.50 max
Iron
0.25 max
Oxygen
0.20 max
Carbon
0.08 max
Nitrogen
0.05 max
Hydrogen
0.009 max
Titanium
Balance
The product tolerance limits for chemical check analysis must meet the requirements in AMS 2249C specification. 6 Product analysis tolerance limits do not broaden the specified heat analysis requirements but cover variations between laboratories in the measurement of chemical content. Hydrogen content must be kept very low in Ti-6Al-7Nb alloy to avoid hydrogen embrittlement. Hot working operations in the titanium mill are typically performed in air and this can increase the hydrogen content because of the reactive nature of the alloy at elevated temperature. Surface protective coatings and special thermal treatments are used to minimize the pick-up of residual hydrogen during high temperature processing. Ti-6Al-7Nb cleaning operations which use nitric plus hydrofluoric acid solutions are carefully controlled to eliminate hydrogen absorption during pickling. A ratio of 10 parts nitric acid to 1 part hydrofluoric acid is recommended.7 Hydrogen analysis is typically performed on the finish mill product after all high temperature and cleaning operations have been completed.
3
Basic Metallurgy continued
2. Microstructure
The room temperature microstructure of Ti-6Al-7Nb consists of a dual phase alpha + beta structure that is similar to Ti-6Al-4V alloy. An alpha + beta globular microstructure is typically obtained for Ti-6Al-7Nb alloy after solution annealing at 700° C, 1 hour, air cool or water quench. 1 Hot pressing studies 8 have shown that Ti-6Al-7Nb alloy compositions with 6.0% to 7.0% niobium contain 10% to 12% beta phase. This is comparable to Ti-6Al-4V alloy which contains 9% to 12.5% beta phase. The temperature at which the alpha-beta phase transformation occurs is known as the beta transus. A beta transus of 1010° C ± 15° C has been reported for Ti-6Al-7Nb alloy.1 The actual beta transus temperature is dependent on composition and, hence, can be used as a measure of compositional uniformity. Optimized heat treating operations may also be established on the basis of beta transus temperature. The alpha + beta microstructure of hot rolled and annealed Ti-6Al-7Nb bar is shown in the following transverse photomicrograph at 200X magnification. The photomicrograph was provided by Dr. L. Zardiackas, Division of Biomaterials, University of Mississippi Medical Center.
Annealed bar microstucture
ASTM F 1295 microstructure requirements specify a fine dispersion of alpha+ beta phases resulting from processing in the alpha plus beta field. No alpha case, coarse elongated alpha platelets, or continuous alpha network at prior beta grain boundaries are permitted. ISO requirements specify that the material must meet micrographs A1 to A9 in ETTC 2 standard 9 for annealed material.
4
Properties
1. Physical
The physical properties of Ti-6Al-7Nb 1 have been compared to unalloyed titanium Grade 1 through Grade 4 and Ti-6Al-4V alloy.10
Material
Density (gm/cc)
Modulus of elasticity in tension (GPa)
Ti-6Al-7Nb
4.52
105
Ti-6Al-4V
4.43
114
Ti Grade 1
4.51
103
Ti Grade 2
4.51
103
Ti Grade 3
4.51
103
Ti Grade 4
4.51
104
The density of Ti-6Al-7Nb is nearly identical to Synthes unalloyed titanium and Ti-6Al-4V. Titanium implants weigh about 45% of 316L stainless steel and cobalt base implants 10 and this may represent a patient comfort factor especially when large-sized devices are compared. Modulus of elasticity is a physical property that describes the stress per unit strain in the elastic region. A low modulus of elasticity is desirable because stress shielding is minimized and increased stress will be transferred to the bone. The relative importance of stress shielding increases as the size of the implant increases. The modulus of elasticity of Ti-6Al-7Nb, Ti-6Al-4V, and unalloyed titanium are similar and the values are significantly lower than 187 GPa for 316L stainless steel and 248 GPa for cast Co-Cr-Mo biomaterials.10 The magnetic permeability of low permeability materials may be measured with an instrument known as a High Sensitivity Low-Mu Permeability Indicator or Severn Gauge.11 The lowest calibrated measurement probe is equal to 1.01. Magnetic permeabilities of less than 1.01 have been recorded 12 for Ti-6Al-7Nb alloy. The titanium alloy demonstrates negligible residual magnetism and Ti-6Al-7Nb implants may be routinely scanned with Magnetic Resonance Imaging equipment.13
5
Properties continued
2. Tensile
Minimum tensile properties for Ti-6Al-7Nb bar up to 100 mm diameter, Ti-6Al-4V ELI bar from 4.75 to 44.45 mm in diameter or thickness, and commercially pure unalloyed titanium Grade 4 bar have been compared as follows: Minimum tensile properties for bar in the annealed condition
Material
ASTM Spec.
Min. UTS (MPa)
Min. 0.2% y. S. (MPa)
Min. Elong. (%)
Min. Reduction in area** (%)
Ti-6Al-7Nb
F 1295
900
800
10*
25
Ti-6Al-4V ELI
F 136
860
795
10**
25
Ti Grade 4
F 67
550
483
15**
25
* gage length = 5.65 X √S, where S is the original cross-sectional area, in mm ** gage length = 4D, where D is the machined diameter in mm
The minimum UTS (Ultimate Tensile Strength) and minimum 0.2% Y.S. (Yield Strength) of Ti-6Al-7Nb is similar to Ti-6Al-4V ELI alloy and significantly higher than Ti Grade 4. The minimum % elongation for both titanium alloys is less than Ti Grade 4 and this is expected, because of the significantly higher strength characteristics. The minimum % reduction of areas are identical. Mechanical property limits indicate these titanium materials have similar levels of tensile ductility. A mechanical test known as a notched tensile test is commonly used to evaluate the relative notch sensitivity of various materials. The test is performed by comparing the tensile strength of a notched cross section to a smooth cross section. The notch is produced by precision machining a bar specimen so that the notch geometry matches a specific factor of stress concentration (Kt). Notched tensile results for a Kt value of 3.2 have been reported by S. Steinemann 14 as follows: Notch tensile sensitivity of titanium materials
Material
Condition
NTS (MPa)
Ti-6Al-7Nb
Annealed
1387
1024
1.35
Ti-6Al-4V
Annealed
1598
1076
1.49
Ti Grade 4
Cold Worked
1387
785
1.77
6
UTS (MPa)
NTS/UTS
Materials which have a NTS/UTS ratio of 1.10 or greater in this test are classified as notch insensitive, i.e. they are not notch sensitive. Results clearly demonstrate that Ti-6Al-7Nb along with Ti-6Al-4V and Ti Grade 4 are not considered notch sensitive materials when evaluated according to notch tensile strength criteria. 3. Fatigue
M. Semlitsch and the group at Sulzer 8 have reported the rotating bending fatigue strength of shot peened Ti-6Al-7Nb alloy in various hot worked conditions.
Rotating bending fatigue strength of hot worked Ti-6Al-7Nb
Fatigue 700 Strength 650 (MPa) 600 550 500 450 400 350 300 Extruded and Forged
Pressed and Forged
Hot Rolled
The fatigue strength increases as the total amount of hot deformation increases. This is related to the homogeneous structure and grain refinement that result from cumulative hot working operations. Hot rolled Ti-6Al-7Nb bar typically has a fatigue strength in excess of 50% of the ultimate tensile strength which is very desirable for a titanium alloy. Rotating bending fatigue tests 14 have also been performed with 5.68 mm round hourglass shaped specimen at 6,000 rpm. Samples were prepared by vibratory finishing followed by electropolishing. Comparative results generated in this series of fatigue tests showed the following trends:
7
Properties continued
Fully reversed rotating fatigue
Stress Amplitude (MPa) at Specified Cycles
Material
Condition
104
107
>10
Ti-6Al-7Nb
Annealed
810
540
540
Ti-6Al-4V
Annealed
—
540
540
Ti Grade 4
Cold Worked
670
430
430
The stress amplitude at 10 7 cycles is known as the endurance limit. This is the maximum stress below which a material can presumably endure an infinite number of stress cycles. The Ti-6Al-7Nb endurance limit is equivalent to Ti-6Al-4V alloy and is substantially better than cold worked Ti Grade 4. 4. Corrosion
Polarization curves 15 in 2 molar hydrochloric acid at 37° C are shown in the following diagram. µA / cm2 Pitting Potential 103
102
Active/Passive Transition Ti–6Al–4V Ti Ti–6Al–7Nb
101
-500
0
1000
2000
3000
4000
5000
6000
7000
Millivolts vs. SCE The unalloyed titanium and Ti-6Al-7Nb exhibit a classic active-passive transition at -500 mV with stable passive behavior up to at least +3000 mV. The very similar anodic polarization curves for Ti-6Al-7Nb and unalloyed titanium also suggest that galvanic corrosion would not be experienced. Unalloyed titanium and Ti-6Al-7Nb resist breakthrough 8
but not Ti-6Al-4V, which exhibits a strong current increase for a pitting potential of 6000 mV. Anodic polarization results confirm the improved corrosion resistance of Ti-6Al-7Nb when compared to Ti-6Al-4V in a reducing acid environment. Pitting potential results 16 versus a Saturated Calomel Electrode (SCE) in a severe reducing acid environment of 2 molar HCl at 37° C are shown in the following graph. Breakthrough results indicate that the pitting potential of Ti-6Al-7Nb is above +10.0 volts which is identical to unalloyed titanium and substantially better than the + 5.9 volts exhibited by Ti-6Al-4V alloy. This suggests that the Ti-6Al-7Nb passive film is more resistant to breakdown than the Ti-6Al-4V in an aggressive reducing acid environment. Pitting potential in 2 molar HCI at 37°C
10
>10.0
>10.0
8 Pitting Potential (+ volts)
6
5.9
4 2 0 Ti–6Al–7Nb
Ti–6Al–4V
Ti
The corrosion fatigue strengths of Ti-6Al-7Nb and Ti-6Al-4V total hip stems 17 have been compared by Semlitsch. Corrosion fatigue strengths were similar when tested for 5 million cycles in Ringer’s solution at 37° C and a frequency of 6 Hertz. Exposure of Ti-6Al-7Nb alloy to sodium chloride, ferric chloride, and amino acids 18 have shown that the alloy is chemically inert under the test conditions investigated. Samples have also been tested by four-point bend testing at 80% of the 0.2% Y.S. in NACE TM0177-86 solution for 760 hours.19 The test solution is very aggressive and contains sodium chloride, glacial acetic acid, and is saturated with bubbling hydrogen sulfide. 9
Properties continued
The specimen size was 70 mm x 10 mm x 2 mm. No cracked or failed specimens were found after exposure to the NACE solution at 24° C for 760 hours under high bending loads and very strong reducing conditions. Slow strain rate stress corrosion cracking (SCC) tendency has been evaluated by constant extension rate testing in 3% NaCl or deionized water at 40° C as reported by Simpson. 19
Constant extension rate testing at 2 x 10-7 sec-1 Deionized Water
76
3% NaCl
74 Reduction in Area (%)
72 70 66 64 62 60 Ti–6Al–7Nb
Ti–6Al–4V
A material is considered to be susceptible to SCC if the ductility properties in an aggressive media (3% NaCl) are inferior to the properties in a non-aggressive media (deionized water). Reduction of area or elongation ratios for sodium chloride/water that are below 90% may indicate SCC problems. Comparison of the results in NaCl with those in water indicated no problem is to be expected with SCC because the reduction of area ratios were virtually identical regardless of the test environment. This trend was also observed when the test was repeated at a strain rate of 1 x 10 -6 sec -1 and when the % elongation ratios were used as a criteria.
10
5. Biocompatibility
In vitro organ culture tests evaluated the growth inhibition of organ cultured embryonic rat femurs in the presence of soluble metal chlorides for 10 days and solid metal implants for 7 days.20 Results indicated that vanadium salts are an order of magnitude more toxic than nickel, cobalt, and copper salts. The same experiments were also performed with niobium and aluminum oxide salts that were saturated in the test electrolyte. No toxic reactions occurred.21 No growth inhibition was observed in femurs with niobium and titanium metal wire implants.20 These metals were well tolerated as evidenced by the appearance of normal cartilage cells near the wire implants. However, other metals such as nickel, cobalt, copper, and iron showed marked corrosion, growth inhibition, and cell damage. Aluminum metal was not tested in this study. Work at Lausanne University, Institute for Experimental Physics, 22 has concluded that the oxides or hydroxides of Ti, Al, and Nb are at saturation in biological tissue. The dissolved metals will not be ionized or transported in vivo. The elements in Ti-6Al-7Nb do not create a bioburden and this accounts for the excellent localized biocompatibility observed for this alloy. In vivo screening tests 23 were performed with grooved cylinders implanted subcutaneously in the backs of mice for 1, 3, and 9 weeks. This test is identical to the subcutaneous screening method specified in ASTM F 1408.24 Histological analysis revealed that the Ti-6Al-7Nb implants did not create adverse tissue tolerance reactions. The number of giant cell nuclei was smaller for vanadium-free alloys. Synthes bone screws machined from unalloyed titanium and various titanium alloys were implanted in dog femora for up to 2 years. 25 Testing was performed according to ASTM F 981 standard.26 Histological evaluation indicated that direct bone attachment was observed at the Ti-6Al-7Nb bone screw surface. Overall biocompatibility results were excellent with no adverse cellular reactions observed histologically. Removal torques at 26 and 52 week retrieval periods were similar for anodized screws fabricated from Ti-6Al-7Nb and Ti-6Al-4V. All unalloyed titanium and titanium alloy screws evaluated in the study demonstrated similar removal torque from dog femora bone. Mechanically polished Ti-6Al-7Nb discs implanted in the backs of mice 27 were retrieved for histological and surface chemistry analysis at the University of Lausanne. Desorption studies indicated that the amount of adherent soft tissue was similar for Ti-6Al-7Nb and unalloyed
11
Properties continued
titanium implants. Visible corrosion products were not present on the surface of the implants after the surrounding tissue was removed. Retrieval analyses of human hip joint prostheses 28 have concluded that Ti-6Al-7Nb is extremely biocompatible as evidenced by osseous ingrowth at the implant surface. Histological examination indicated that osseous tissue formation along the entire stem length provided secondary implant stabilization. 6. Surface
The surface oxide of Ti-6Al-7Nb has been characterized 18 by X-Ray Photoelectron Spectroscopy analysis. The surface study found the oxide film was a mixture of TiO2, Al2O3, and Nb2O5. This study concluded that the mixed oxide film formed on Ti-6Al-7Nb alloy is more chemically stable than the TiO 2 oxide layer formed on unalloyed titanium. Ti-6Al-7Nb implants may be shot peened for improved fatigue life but the shot peening media must be carefully selected to avoid iron contamination at the surface of the implant. Final surface treatment after machining, or after machining and shot peening, consists of electrochemical anodizing, although nitric acid passivation may also be used. 29 Synthes Ti-6Al-7Nb implants have an anodized surface finish that is produced by immersing the implants in an electrochemical solution for a specified time and voltage. The color that is produced is a function of the mixed oxide film thickness which is controlled in the anodizing process. Visible light diffraction within the oxide produces a distinct color. No pigments or organic coloring agents are present in the anodized film. The standard Synthes anodizing treatment creates a gold appearance that is a distinguishing feature of Synthes titanium implants. The anodizing process is capable of creating a variety of colors, depending on the thickness of the oxide film. Surface analysis of anodized CP titanium 30 has shown that typical anodizing treatments increase the oxide thickness and alter the oxide chemistry. Corrosion studies concluded that the anodized film exhibited reduced corrosion rates when compared to unanodized specimens. Extrapolation of the CP titanium results suggest that anodizing improves the corrosion resistance of the mixed oxide film that is normally present on the surface of Ti-6Al-7Nb alloy. Recent studies in aerated 3% sodium chloride 31 indicated that anodized films on Ti-6Al-7Nb and other titanium alloys have higher polarization resistance than CP titanium. The results confirmed increased stability and better corrosion resistance for an anodized oxide film in a chloride-containing corrosive environment.
12
References
1. Surgical Implant Alloy IMI 367, Product Bulletin, IMI Titanium Limited, Birmingham, England. 2. Steinemann, S., and Perren, S., Surgical Implant and Alloy for Use in Making an Implant, U.S. Patent 4,040, 129, August 9, 1977. 3 Niobium, Product Brochure, Teledyne Wah Chang, Albany, OR, pp. 3-4. 4. ISO 5832-11: Implants for Surgery – Metallic Materials, Part II: Wrought Titanium-6 Aluminum-7 Niobium Alloy, International Organization for Standardization. 5. ASTM F 1295: Standard Specification for Wrought Titanium-6 Aluminum-7 Niobium Alloy for Surgical Implant Applications, American Society for Testing and Materials, Philadelphia, PA. 6. AMS 2249C: Chemical Check Analysis Limits, Titanium and Titanium Alloys, Society of Automotive Engineers, Warrendale, PA. 7. ASTM B 600, Standard Recommended Practices for Descaling and Cleaning Titanium and Titanium Surfaces, American Society for Testing and Materials, Philadelphia, PA. 8. Semlitsch, M., et.al., “Titanium-Aluminum-Niobium Alloy, Development for Biocompatible High Strength Surgical Implants,” Sonderduck aus Biomedizinische Technik 30, (1985), 12, S. 334-339. 9. Technical Committee of European Titanium Producers, ETCC2 Monograph, IMI Titanium Ltd., Birmingham, England, Kynoch Press. 10. Disegi, J., “AO/ASIF Unalloyed Titanium Implant Material,” Second Edition, AO/ASIF Technical Publications, SYNTHES (USA), July 1991. 11. Severn Engineering Company, Inc., Annapolis, MD. 12. Disegi, J., Internal Correspondence, SYNTHES (USA), Paoli, PA. 13. Disegi, J., “Magnetic resonance imaging of AO/ASIF stainless steel and titanium implants,” Injury, AO/ASIF Scientific Supplement, Vol. 23, Supplement 2, 1992. 14. Steinemann, S., et.al., “Beta-Titanium Alloy for Surgical Implants,” Seventh World Conference on Titanium, San Diego, CA, June 28-July 2, 1992. 15. Protasul® 100 (Ti-6Al-7Nb) Vanadium-Free, High Strength Titanium Alloy, Technical Report, Protek, Inc., Indianapolis, IN, 1988. 13
References continued
16. Simpson, J., “The Electrochemical Behavior of Titanium and Titanium Alloys With Respect to Their Use as Surgical Implant Materials,” Biological and Biomechanical Performance of Biomaterials, Elsevier Science Publishers, Amsterdam, pp. 63-68, 1986. 17. Semlitsch, M., et.al. “Development of a Vital, High-Strength Titanium-Aluminum-Niobium Alloy for Surgical Implants,” Biological and Biomechanical Performance of Biomaterials , Elsevier Science Publishers, Amsterdam, pp. 69-74, 1986. 18. Maeusli, P., et.al., “Surface Characterization of Titanium and Ti-Alloys,” Biological and Biomechanical Performance of Biomaterials, Elsevier Science Publishers, Amsterdam, pp. 57-62, 1986. 19. Simpson, J., “Re: ASTM-Norm Titanium Alloy Ti-6Al-7Nb” Internal Report, Sulzer Innotec, Winterthur, February 3, 1990. 20. Gerber, H., and Perren, S., “Evaluation of Tissue Compatibility of in vitro Cultures of Embryonic Bone,” Evaluation of Biomaterials , John Wiley and Sons, pp. 307-314, 1980. 21. Gerber, H., et.al., “Bioactivity of metals: Tissue tolerance of soluble solid metal tested on organ cultured embryonic bone rudiments,” Technical Principles, Design and Safety of Implants , G.H. Buchhorn, H.G. Wiliert (eds.), Hogrefe & Huber, Toronto/Bern. 22. Steinemann, S., and Maeusli, P., “Titanium Alloys for Surgical Implants – Biocompatibility from Physiochemical Principles, ” Sixth World Conference on Titanium, France, pp. 535-540, 1988. 23. Perren, S., et.al, “Quantitative Evaluation of Biocompatibility of Vanadium Free Titanium Alloys,” Biological and Biomechanical Performance of Biomaterials , Elsevier Science Publishers, Amsterdam, pp. 397-402, 1980. 24. ASTM F 1408: Practices for Subcutaneous Screening Test for Implant Materials, American Society for Testing and Materials, Philadelphia, PA. 25. Olmstead, M., and Pohler, O., “Report on Long Term Compatibility Testing of New Titanium Alloys,” AO Research Grant 1987/88, Stratec Medical, Waldenburg, January 17, 1990. 26. ASTM F 981: Assessment of Compatibility of Biomaterials (Nonporous) for Surgical Implants with Respect to Effect of Materials on Muscle and Bone, American Society for Testing and Materials, Philadelphia, PA. 27. Gold, J., et.al., “XPS Study of Retrieved Titanium and Ti Alloy 14
Implants,” Advances in Biomaterials, Volume 9, Clinical Implant Materials, Heimke, G., ed., Elsevier Science Publishers, Amsterdam, pp. 69-74, 1990. 28. Zweymuller, K., et.al., “Biologic Fixation of a press-Fit Titanium Hip Joint Endoprosthesis,” Clinical Orthopaedics, Volume 235, pp. 195-206, October 1988. 29. Solar, R., et.al., “Titanium Release from Implants: A Proposed Mechanism,” Corrosion and Degradation of Implant Materials , ASTM STP 684, B. Syrett and A. Acharya, American Society for Testing and Materials, Philadelphia, PA, p. 172, 1979. 30. Lucas, L., et.al., “Corrosion and Auger Surface Chemistry Analyses of Surface Modified Porous Titanium,” Transaction of the 17th Annual Society for Biomaterials, p. 201, May 1-5, 1991. 31. Frey, N., et.al., “Properties of Surface Oxides on Titanium and Some Titanium Alloys,” Seventh World Conference on Titanium, San Diego, CA, June 28-July 2, 1992.
15
Glossary
ALLOY. A metallic substance composed of two or more elements at least one of which is metal. ALLOYING ELEMENT. An element, added to and remaining in a metal, that changes the metal’s structure and properties. ALPHA. The low temperature form of titanium with a hexagonal close-packed (hcp) crystal structure. ALPHA + BETA STRUCTURE. A microstructure containing alpha and beta as the principal phases at ambient temperatures. ALPHA CASE. The oxygen, nitrogen, or carbon enriched alpha stabilized surface resulting from elevated temperature exposure. ANNEALING. A metal softening operation in which the metal is heated to and held at a specified temperature, followed by cooling at a controlled rate. ANODIC REACTION. An oxidation reaction that produces electrons at the anode of an electrochemical cell. When dissimilar metals are coupled, the anode usually experiences increased corrosion. ANODIZING. An electrolytic process that increases the thickness of the protective oxide film on titanium. BETA. The high temperature form of titanium with a body-centered cubic (bcc) crystal structure. BETA TRANSUS. The minimum temperature at which 100% beta phase can exist. BODY-CENTERED CUBIC. A unit cell which consists of atoms arranged at cube corners with one atom at the center of the cube. BRITTLENESS. The tendency of a material to fracture without first undergoing significant permanent deformation. CATHODIC REACTION. A reduction reaction that consumes electrons at the cathode of an electrochemical cell. When dissimilar metals are coupled, the cathode usually undergoes reduced corrosion. COLD-WORKED MICROSTRUCTURE. A microstructure resulting from cold working the material. COLD WORKING. Permanently deforming a metal or alloy at room temperature to increase its strength. CRYSTAL. A solid composed of atoms that repeat in a pattern of regular intervals in three dimensions.
16
DESCALING. Chemically or mechanically removing the thick oxide layer that is formed on metals during high temperature processing. DUCTILITY. The ability to permanently deform before fracturing. ELECTRODE. A cylindrical metal compact that is suitable for vacuum arc melting or a metal ingot that is suitable for remelting. ELONGATED ALPHA. A fibrous type of microstructure that results from unidirectional cold working of unalloyed titanium. ELONGATION. A term that describes ductility by measuring the amount of extension that a material undergoes during tensile testing. EQUIAXED STRUCTURE. A microstructure feature that consists of polygonal shaped grains with equal dimensions in all directions. FATIGUE. The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. FATIGUE LIFE. The number of cycles of stress or strain of a specified character that a given specimen sustains before failure of a specific nature occurs. FATIGUE STRENGTH. The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated. FRETTING CORROSION. An accelerated form of corrosion that can occur when the protective passive film is mechanically abraded. The relative motion of the underside of a bone screw head with the contact surface of a bone plate is a typical example. GPa. Gigapascal equals 1000 MPa. HEXAGONAL CLOSE-PACKED. A unit cell which consist of a hexagonal arrangement of atoms in a plane surrounding an atom followed by three atoms in the next horizontal plane. HOT-WORKED MICROSTRUCTURE. A microstructure resulting from hot working the material. HOT WORKING. Permanently deforming metal at an elevated temperature that is usually above recrystallization temperature. INCLUSION. A particle of foreign material in a metallic microstructure that is usually considered undesirable. INGOT. A metal casting that is suitable for remelting or hot working.
17
Glossary continued
INTERMETALLIC COMPOUND. A phase in an alloy system that has a well-defined composition and limited solubility. LONGITUDINAL . Parallel to the principal direction of hot or cold working. MASTER ALLOY. An alloy rich in one or more elements that is added to a melt to raise the percentage of a desired constituent. MICROSTRUCTURE. The structure of metals as revealed by microscopic examination of a specimen. MODULUS OF ELASTICITY. A measure of the stress per unit strain in the elastic region before permanent deformation occurs. PASSIVATION. The process of changing the chemical activity of a metal surface to a less reactive state, usually to increase the corrosion resistance. PICKLING. Chemical removal of the thick oxide layer that is formed on metals during high temperature processing. POLYGONAL STRUCTURE. A closed planar shape bound on at least three sides. RECRYSTALLIZATION. A change from one crystal structure to another that occurs during heating or cooling through a critical temperature range. REDUCTION IN AREA. A tensile testing measure of ductility that equals the original area minus the area after fracture divided by the original area, expressed as a percentage. SOLUBILITY. A measure of the amount of a substance that can be dissolved in a metal or alloy. STRAIN. Change in length per unit length in the direction of the applied stress. STRESS. Forcer per unit area. STRESS CORROSION CRACKING. Failure of metals by cracking under combined action of corrosion and stress. TRANSVERSE. Perpendicular to the principal direction of hot or cold working. TWINNING. A microstructure feature that describes mirror-image positions across a planar interface.
18
ULTIMATE TENSILE STRENGTH. In tensile testing, the maximum load at fracture divided by the original cross-sectional area. UNALLOYED TITANIUM. Single phase titanium metal that does not contain major alloying additions. VACUUM ARC REMELTING. A melting process in which an electric arc is used to remelt an electrode inside a vacuum chamber. YIELD STRENGTH. In tensile testing, the stress at which the stress-to-strain ratio exhibits a specified deviation, usually designated as 0.2% offset.
19
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