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Oxidation and embrittlement of Ti-6Al-2Sn-4Zr-2Mo alloy

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Abstract

Ti-6Al-2Sn-4Zr-2Mo titanium alloy is a candidate material for multiwall thermal protection system concepts for advanced space transportation system vehicles. The total oxidation kinetics for this alloy, exposed to laboratory air in the 593–760°C range, were monitored by thermogravimetric analysis. The oxide thickness was measured by microscopy and the substrate contamination was estimated from microhardness measurements. Tensile elongation was determined for selected foil specimens after exposure to simulated space shuttle reentry conditions. The variation of total weight gain with time was found to have two distinct parabolic stages separated by a transient region. This transient was due to a process which involved an increase in the parabolic growth rate constant for the oxide and a simultaneous increase in oxygen solubility at the oxide metal interface. The time dependent increase in oxygen solubility at the interface was from about 7 at. % in stage 1 to about 18 at. % in stage 2. The diffusion coefficient for oxygen in the alloy was determined as a function of temperature using the difference between the total weight gain in stage 1 and the corresponding weight gain due to oxide growth. A model for the total oxidation kinetics, accounting for the two individual components namely oxide growth and solid solution formation, is proposed. The activation energy for the diffusion of oxygen in the alpha-solid solution is shown to be roughly equal to the activation energy for the degradation of tensile elongation for the alloy in the foil gage condition.

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Abbreviations

A s :

Area underC s vsX profile, atomic per cent oxygen times cm

C s :

Concentration of oxygen in the alpha-Ti solid solution (see Fig. 1), atomic per cent

C z :

Concentration of oxygen in the oxide (see Fig. 1), atomic per cent

D s :

Diffusion coefficient of oxygen in the alpha-Ti solid solution, cm2s−1

D z :

Diffusion coefficient of oxygen in the oxide, cm2s−1

erfc:

Error function complement

KHN:

Knoop hardness number (subscripts 5g and 15g represent indenter loads)

LID:

Specimens cleaned and heat-treated using liquid interface diffusion technique

R :

Gas constant,=1.987 cal g·mole−1 deg K−1

r :

Oxide growth constant defined by z=rt 1/2, cm s−1/2

STS:

Space transportation system

T :

Temperature in Kelvin (K)

TGA:

Thermogravimetric analysis

t :

Time in seconds (s)

W if :

Total weight gain per unit area from initial and final weights, g cm−2

W s :

Weight gain per unit area due to solid solution formation, g cm−2

W z :

Weight gain per unit area due to oxide growth, g cm−2

X :

Distance from oxide-metal interface, cm

z :

Oxide thickness, cm

ε :

Per cent tensile elongation

e :

Effective value

if :

Obtained from initial and final weights of specimen

sl :

Solubility limit in the solid solution, see Fig. 1

so :

Base oxygen level in the solid solution, see Fig. 1

zl :

Upper solubility limit in the oxide, see Fig. 1

zo :

Lower solubility limit in the oxide, see Fig. 1

1:

At the end of stage 1, see Fig. 4

2:

At the beginning of stage 2, see Fig. 4

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Authors and Affiliations

  1. Analytical Services and Materials, Inc., 103 Winder Road, 23602, Tabb, Va

    R. N. Shenoy, J. Unnam & R. K. Clark

  2. NASA Langley Research Center, 23665, Hampton, Va

    R. K. Clark

Authors
  1. R. N. Shenoy
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  2. J. Unnam
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  3. R. K. Clark
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Formerly with Vigyan Research Associates, Inc., 28 Research Drive, Hampton, Va 23666.

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Shenoy, R.N., Unnam, J. & Clark, R.K. Oxidation and embrittlement of Ti-6Al-2Sn-4Zr-2Mo alloy. Oxid Met 26, 105–124 (1986). https://doi.org/10.1007/BF00664276

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  • DOI: https://doi.org/10.1007/BF00664276

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