Intergranular Fracture in BCC Metals

Abstract

The intergranular fracture (IGF) in high purity molybdenum and high purity iron are discussed together with the effects of interstitial and substitutional solutes on it.Recrystallized molybdenum is inherently susceptible to IGF. Purification does not improve it. Grain boundary segregation of carbon increases the grain boundary cohesion of general boundaries and that of oxygen decreases it.IGF occurs in recrystallized commercial high purity iron about 99.995% pure, but does not in specially prepared iron of about 99.999% or better purity. Segregation of oxygen decreases the boundary cohesion, but the effect is much smaller than usually believed. Segregation of carbon increases the grain boundary cohesion and reduces IGF caused by segregation of other impurities, such as hydrogen, phosphorus and sulfur. Molybdenum and chromium reduce IGF caused by the segregation of phosphorus as far as the alloy is in the solid solution. These elements interact with phosphorus to reduce the detrimental effect of phosphorus and also increase the boundary cohesion of iron. Nickel also reduces IGF, but the reduction is caused by the solid solution softening due to nickel.Also discussed are the chemical state of the segregated phosphorus and the dependence of the phosphorus segregation on the grain boundary character. (Received May 9, 1988) Keywords: intergranular fracture, high purity iron, high purity molybdenum, effect of carbon and oxygen, iron-phosphorus-molybdenum alloys, iron-phosphorus-chromium alloys, iron-phosphorus-nickel alloysThe intergranular fracture (IGF) is not rare in BCC metals and alloys, and causes serious technical probelems. Phosphorus and sulfur are common impurities in iron and steel and are known to embrittle them severely. In most cases IGF is caused by the segregation of impurities to grain boundaries. IGF caused by an impurity is promoted or suppressed by other kinds of impurities and alloying elements. An example is the temper-embrittlement in low alloy steels; when low alloy steels with high strength are heated at temperatures between 770 and 870K after quenching and tempering, impurities (typically phosphorus) segregate to grain boundaries and cause IGF. On the other hand, if a plain carbon steel containing a similar amount of phosphorus is subjected to the same heat treatment, no temper-embrittlement is observed. Thus, in the discussion of IGF due to impurity segregation, we have to consider not only the impurity directly responsible for the IGF, but also other impurities and alloying elements. Furthermore, effects of one kind of solute on a specific impurity may be modified by other kinds of solutes. Hence, we have to employ high purity materials with controlled compositions to investigate the behavior of a specific impurity and the effects of a specific solute without interference from other solutes.The grain boundary segregation is measured fairly accurately with Auger electron spectroscopy (AES); specimens are fractured and the revealed surface is analyzed under an ultra high vacuum without bei...

Thermochemical Interactions Versus Site Competition in Grain Boundary Segregation and Embrittlement in Multicomponent Systems

“…It should also be remembered that the very concept of intergranular cohesion per se is notorious1 y insufficient to account for intergranular fracture in metals, since the latter is determined not only by the strength of atom bonds in the interface region but also by dislocation behaviour in this region and in the adjacent crystals. All the solutes known to have an influence on intergranular fracture of Fe have also been shown to affect its bulk plastic behaviour in a complex manner [52]. Minute amounts of these elements interact with screw dislocations and modify the formation and mobility of kink pairs.…”

“…Minute amounts of these elements interact with screw dislocations and modify the formation and mobility of kink pairs. This can either strengthen or soften the solid solution as a whole [52], and modify the arrangement of dislocations [53], notably in the vicinity of grain boundaries. All these effects deeply alter the ductile-brittle transition behaviour, the stress concentrations on grain boundaries and the fracture mode.…”

Fracture Behavior and Delamination Toughening of Molybdenum in Charpy Impact Tests

“…[4][5][6][7][8][9] Nevertheless, recrystallization annealing of the as-deformed and stress-relieved condition of molybdenum leads to a significant increase of the DBTT and even more to intergranular embrittlement, which is believed to be caused by the larger grain size and segregation enrichments at the grain boundaries. [4][5][6][10][11][12][13] Recent atom probe investigations revealed phosphor, nitrogen and oxygen impurities at the grain boundaries of technically pure molybdenum 14,15 of which especially segregated oxygen is assumed to promote intergranular failure. 6,11,12 By adding small amounts of carbon and boron, the ductility of molybdenum can be improved.…”

Magnetic Properties and Recrystallization Texture Evolutions of Phosphorus-bearing Non-oriented Electrical Steel Sheets

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