LME

Question 1

What is the effect of LME?

Answer 1

Embrittling/loss in ductility of otherwise ductile solid materials when they are exposed to liquid metals or metal vapors AND mechanically stressed. (Weakening and mechanical degradation can also happen)

Question 2

In which metals do LME usually occur?

Answer 2

LME occurs in Cu alloys, Al alloys and some steels, when in contact with low-melting point metals. (Several low-melting point metals possible)

Question 3

Different phenomenas can promote the embrittlement and failure. Can you list some?

Answer 3

1. Reduction of surface energy at Grain Boundaries (due to absorption and diffusion of the embrittling metal atoms)
2. Decreasement of the cohesive strength (Sigma_id)
3. Accelerated emission of dislocations
4. Diffusion (infiltration) of liquid metals along Grain Boundaries
5. Dissolution of Grain Boundaries

Question 4

What is the time-dependence of the delayed fracture (in connection to LME) ruled by?

Answer 4

The rate the Liquid Metal can infiltrate the solid. This depends again on the Temperature, as there occurs “wetting” of the Grain boundary if the temperature passes a transition temperature.

Question 5

Can the embrittling metal be solid?

Answer 5

Yes. Usually it is liquid, and therefore the term LME, but the same phenomena can happen, but at a much lower crack growth rate.

Question 6

How does liquid metals reduce the cohesive strength of a lattice?

Answer 6

Liquid metals acts as a reducer of the binding energy at the crack tip, due to absorption of liquids atoms. The extension of the embrittlement depends on the diffusion rate of the liquid atoms along the crack path.

If the cohesive strength reduction is significant, then cleavage fracture becomes possible also in very ductile metals (such as FCC metals).

Question 7

Explain two different ways Liquid Metal can reduce the cohesive strength at the crack tip?

Answer 7

1. Spontaneous absorption of liquid atoms, and diffusion of these along grain boundaries.
   -> General embrittlement of the volume
2. Absorption stimulated by the stress field at the crack tip.
   -> Embrittlement only in regions affected by notch effects

Question 8

List two different forms of failure that can happen when a metal is subjected to liquid metal attack.

Answer 8

1. Instantaneous failure of a liquid-metal embrittled material once the load is applied (or even without load if residual stresses are high enough)
2. Delayed fracture even at loads significantly lower than the static strength

Question 9

What is the “wettability” of a metal?

Answer 9

Wettability is the ability of a liquid to spread over a surface. It can be measured by the contact angle between the liquid and the surface.

Question 10

Conditions for LME, that need to satisfied to induce a significant embrittlement effect.

Answer 10

• The solid metal has to be plastically strained in service.
• LME needs to be originated at liquid-solid metal interface
• The diffusion of the liquid metal to the advancing crack tip is a pre-requisite for growth. When diffusion requires significant periods of time, delayed fracture evidence are recorded.

Additional facts:
- LME Fracture is generally (but not always) intergranular
- The crack growth stage is usually very fast and can start at very low stress intensity factor values
- If limited diffusion are able to take place, the crack can only propagate according to other, more conventional mechanisms.
- Fracture surfaces can be covered (and hence recognized) by a layer of metal (the former liquid metal) with a thickness ranging from a few millimeter to a few atoms

Question 11

Can you list som eamples of liquid metal embrittlement

Answer 11

In zinc (immersion (nedsenking)) coatings, tin and cadmium coatings, LME phenomena may occur either during coating process and during service.

Cracks can grow during hot plastic deformation of metals containing low-melting point constituents (Cu-containing steels. A Cu-rich layer accumalated during hot rolling, and can melt at rolling temperatures and infiltrate through steel grain boundaries with deleterious effects under the rolling stresses.)

Welding processes combine high temperature and residual stresses (thermal shrinkage) that can promote LME when low-melting point metals are present.

Brazing an soldering (lodding), by definition implies the use of a low-melting filler metal to join different parts.

In airports, the use of devices containing mercury (e.g. old fashion thermometers) is strictly forbidden to avoid accidental damage of the aluminium structure of aircrafts.