Hydrogen enbrittlement & LME

Question 1

What are some sources of hydrogen in metals (picked up by the enviroment)

Answer 1

• Electrochemical processes, when hydrogen develops as reaction product (e.g. pickling or descaling, electroplating)
• Environmental absorption from air humidity by liquid metals (in foundries, during welding)
• Electrochemical reactions related to corrosive processes

Question 2

What does the amount of captured hydrogen depend on?

Answer 2

1. solubility
2. diffusivity
3. entrapment in specific sites of the lattice (hydrogen trapping)

Question 3

How can the solibility of hydrogen in metals change?

Answer 3

• when moving from liquid to solid phase of metals (higher solubility in liquid)
• Lattice distortion (entrapment in spesific sites of the lattise)

Question 4

What happens to the solubility of hydrogen in a metal under tension stresses?

Answer 4

The lattice will be distorted (widening of the lattice atomic spacing) -> larger solubility

Question 5

How does the amount of dislocations in a structure affect hydrogen solubility?

Answer 5

Increased amount of dislocations leads to a higher solubility of hydrogen

Question 6

When are accelerated diffusion conditions encountered?

Answer 6

• When atoms can move with dislocations
  during plastic deformation
• When preferential diffusion path are available along grain boundaries, dislocation lines, or any other kind of surfaces.

Question 7

What is the goal of hydrogen entrapment?

Answer 7

To prevent hydrogen to go from the metal to the enviroment (leading to permanent harmful effects)

Question 8

How does hydrogen entrapment work?

Answer 8

When an atom jumps from a general site to a trap site, then the possibility to make a further jump out of it is drastically reduced (due to difference in activation energy)

Question 9

What is the most common effect of hydrogen in metals?

Answer 9

Brittle fracture by intergranular
decohesion

Question 10

What are the most common hydrogen induced damage mechanisms?

Answer 10

• Hydrogen-induced blistering
• Hydrogen-induced cracking
• Hydrogen attack
• Cracking from hydride formation
• Hydrogen embrittlement

Question 11

How does hydrogen-induced blistering happen?

Answer 11

• Hydrogen absorbed from surfaces
  collects at trap sites, especially at interfaces
  between inclusions and matrix and along
  elongated GBs.
• Hydrogen recombines to form H2 molecules
• The amount of H2 increases the pressure inside these pre-existing defects , and plastically deform the surrounding metal.

Question 12

How does hydrogen-induced cracking happen?

Answer 12

• Hydrogen clusters at voids or pores and recombines as H2
• Internal pressure form small cracks
• Crack growth does not always occur up to fracture, but generally cracks stop
  when they reach a certain size and the internal pressure of hydrogen drops to low values

Question 13

Where and under what conditions are hydrogen attacks typically encountered?

Answer 13

In industrial plants when steel parts are exposed to high temperatures and high hydrogen pressures

Question 14

How does hydrogen attacks happen?

Answer 14

Absorbed hydrogen interacts with alloying or impurity elements to form insoluble (generally gaseous) phases

Question 15

What does the general case of hydrogen enbrittlement depend on?

Answer 15

• absorption
• temperature
• strain rate
• alloy composition
• structure

Question 16

What does liquid metal enbrittlement lead to?

Answer 16

Loss of ductility (leading to anticipated fracture) or in the embrittlement (intergranular brittle fracture) of otherwise ductile metals

Question 17

When does liquid metal enbrittlement happen?

Answer 17

When metals are exposed or held in contact with liquid (solid) metals or metal vapors and
mechanically stressed

Question 18

In which metals can liquid metal enbrittlement be found?

Answer 18

In Cu alloys, Al alloys and some steels in contact with several low melting point metals

Question 19

What are the phenomenas that can rule the
failure mechanism in LME?

Answer 19

• Reduction of surface energy at GBs due to absorption and diffusion of the embrittling metal atoms
• Decrease of the cohesive strength of the lattice
• Accelerated emission of dislocations
• Diffusion of liquid metals along GBs and dissolution/corrosion of
  solid metal lattice accelerated by stress field and capillarity effects

Question 19

What are the different forms of failure LME operates with?

Answer 19

1. Instantaneous failure of a LME-material
   once the load is applied, or even without
   load, if residual stresses are high enough
2. Dissolution of grain boundaries due to
   infiltration of the liquid metal
3. Delayed fracture even at loads
   significantly lower than the static
   strength

Question 20

Why does liquid metal reduce the binding energy in the solid metal lattice?

Answer 20

Due to adsorption of elements at crack tip

Question 21

What does liquid metal do to the binding energy in a solid metal lattice?

Answer 21

liquid metal acts as a reducer of the binding energy of the solid metal lattice due to
adsorption of elements at crack tip.

Question 22

What is happening here? L are liquid metal atoms.

Answer 22

Atoms L reduce the cohesive strength
of the S-S bonds, the crack grows by
the progressive diffusion of L
elements along the crack path

Question 23

The extension of the embrittlement depends on the diffusion rate of atoms L along
the future crack path in S. What are the two different options?

Answer 23

• “spontaneous” adsorption of L atoms and their diffusion along grain boundaries
  or other microstructural features, leading to general embrittlement of the volume
• Adsorption stimulated by the stress field at crack tip, leading to embrittlement
  only in regions affected by notch effects

Question 24

How does LME affect ductile metals during a fracture process?

Answer 24

Reduction of metal plasticity

Question 25

How can LME reduce the fracture elongation in ductile materials?

Answer 25

With the adsorption of atoms at cracks, a reduction of the ideal shear stress
value can be detected. This would lead to easy emission of dislocations at lower stress values (hence lower extension of plastic zone) and to anticipated nucleation and growth of voids.

Question 26

What type of fracture does LME (generally) lead to?

Answer 26

Intergranular fracture