The solubility of hydrogen in liquid binary Al-Li alloys

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-~ log

-2113 + 2.568 - - -

AI-2 pct Li: log

- -~ log

-2797 + 3.329 - - -

A1-3 pct Li: log

- ~- log

-2889 + 3.508 = ~

1

r/K

r/K r/K

where S ~ is a standard value of solubility equal to 1 cm 3 of diatomic hydrogen measured at 273 K and 101,325 Pa per 100 g of metal, and po is a standard pressure equal to 101,325 Pa. Added lithium progressively increases the solubility of hydrogen in liquid aluminum, due more to its effect on the entropy of solution of hydrogen, through its influence on the liquid metal structure than to an increase in the solute hydrogen atom binding enthalpy.

I.

INTRODUCTION

LITHIUM-containing aluminum alloys have been developed to provide improved materials for aircraft structures, t-4 Commercial versions of these alloys also contain copper and/or magnesium and have high strength, high modulus, low density, and adequate fracture toughness. The inherent suitability of the alloys for their intended purpose has been validated by exhaustive research, but since the intended application is highly critical, the integrity of the manufactured products is also of paramount importance. Potentially damaging factors such as the influence of hydrogen absorbed during manufacture must be characterized and quantified. It became apparent at an early stage in the development of suitable ingot casting techniques that the behavior of hydrogen absorbed in AI-Li alloys differs in several respects from that for all other aluminum alloys. 3'4'5 Unless care is taken, high hydrogen contents can be introduced into the alloy via lithium addition because the solubility of the gas in liquid pure lithium is high, e.g., >70 cm3/100 g* *The unit of hydrogen concentration used here will be cm 3 STP/100 g. 1 cm3/100 g = 8.922 • 10 5 wt pct H2.

at 473 K and 101,325 Pa, 6 and liquid lithium readily yields hydrogen by reaction with moisture in its environment, except under high vacuum or in very dry atmospheres. Standard industrial degassing procedures which are effective in reducing the hydrogen content of other liquid aluminum alloys to

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