7: Titanium Alloys

  • Introduction to Titanium Alloys

    • Titanium alloys are a superior engineering material due to various characteristics.
    • Key characteristics include:
    • Low density (~4.5 g/cm³)
    • High modulus (~15 GPa)

  • Specific Moduli Comparison

    • Specific modulus for titanium is approximately 25-26.
    • Aluminum alloys have a slightly higher specific modulus.
    • Beryllium, though not widely used, has the highest specific modulus at 63, but has significant drawbacks such as:
    • Extremely high cost.
    • Difficult machining processes.
    • Low ductility and poor formability.
    • Toxicity, making it unsafe for widespread use.

  • Strength and Yield Stress

    • Titanium has the highest specific strength on a strength-density basis.
    • Grade 5 titanium (Ti-6Al-4V):
    • Yield stress in solution-treated and aged condition: nearly 1,000 MPa.
    • Can exceed 1,100 MPa in cold-worked conditions.

  • Strength vs. Density

    • Ashby diagram shows titanium's strength surpasses all other engineering alloys for tensile applications.
    • Magnesium alloys may surpass titanium in bending applications.

  • Temperature and Creep Resistance

    • Titanium has a high melting temperature of 1668 °C, providing strength stability at high temperatures.
    • It is resistant to creep, which is beneficial for engineering applications.

  • Corrosion Resistance

    • Titanium demonstrates excellent corrosion resistance:
    • Resistant to seawater and chloride solutions, alongside oxidizing acid environments.
    • Reliable in cryogenic conditions with no ductile-to-brittle transition temperature.
    • Good biocompatibility, making it ideal for medical implants.

  • Cost Considerations

    • High cost of titanium alloys hinders broader use:
    • Second only to tungsten alloys in terms of approximate cost per unit volume.
    • When considering cost per unit mass, titanium is the most expensive due to its low density.

  • Comparison vs Other Alloys

    • Titanium alloys rank below various stainless steels, aluminum, and magnesium in strength versus cost ratio.

  • High-Temperature Strength

    • Thermal softening curves for Grade 5 titanium demonstrate its strength at elevated temperatures, comparable to some superalloys:
    • At 538 °C, Grade 5 titanium's strength approaches that of low-strength superalloy Inconel 601.
    • Strength remains competitive with high-strength superalloy Inconel 750 at mid-elevated temperatures.

  • Common Titanium Alloys

    • Commercially Pure Titanium (Grades 1-4, 7, 11, 12):

    • Grades 1-4 are the most frequently used, with Grade 1 being the purest.

    • Increased impurities boost strength but reduce ductility.

    • Ti-6Al-4V (Grade 5 Titanium):

    • Represents nearly 70% of titanium alloy usage.

    • Alpha-beta alloy, allowing hardening through heat treatment.

    • Yield stress in hardened condition can reach 965 MPa.

    • Maintains a yield strength of 700 MPa even at 540 °C.

    • Its good biocompatibility leads to its use in biomedical implants, alongside commercially pure titanium.

  • ELI Grade Titanium:

    • Grade 23:
    • Enhanced ductility and fatigue resistance for cryogenic applications, specifically for aerospace.

  • Conclusion

    • Titanium alloys offer exceptional properties but are tempered by cost and process challenges.
    • Their attributes make them essential in high-performance applications across various industries.