Grain boundary segregation and intergranular fracture in molybdenum

Abstract

The refractory group VIA metals generally exhibit intergranular brittleness when they are in the recrystallized condition. This causes severe problems in their fabrication and places major limitations on their practical application. The phenomenon, generally referred to as recrystallization embrittlement, results in large increases in the ductile-to-brittle transition temperature and a change in fracture mode in the lower shelf regime from cleavage to intergranular with a significant decrease in ductility. The embrittlement is widely considered to be associated with interstitial impurities but there have been few systematic studies to elucidate their effects. The present paper reports results from a systematic study of segregation and intergranular embrittlement in binary molybdenum-oxygen and ternary molybdenum-oxygen-carbon alloys. The experiments were carried out on ‘bamboo’ specimens containing a series of identical single grain boundaries traversing their cross-sections. Measurements have been made of the activation energy for oxygen segregation to grain boundaries in the binary molybdenum-oxygen alloys. The influence of carbon additions on the level of oxygen segregation has also been determined. In addition, the influence of oxygen segregation on the energy to fracture has been studied and this has involved quantitative measurements of the work of fracture and the contribution made by plastic deformation. Results from metallographic studies are also presented, showing the effects of segregation on fracture surface topography and dislocation structures immediately adjacent to the fracture surfaces. In discussing the results we consider the thermodynamics of oxygen segregation to grain boundaries and the role played by carbon in inhibiting segregation. It is proposed that carbon either increases the effective solubility of oxygen in molybdenum or acts as a trap for oxygen atoms. In either case the effect is to reduce the driving force for segregation. We also consider the influence of segregation on the work of fracture and show that the reduction in oxygen segregation resulting from the addition of carbon produces small increases in fracture energy. This increases the local stress to propagate a crack sufficiently to promote plastic deformation which blunts the crack tip. The consequent change in geometry reduces the stress concentration at the crack tip, thereby resulting in a large increase in the applied fracture stress and the work to fracture.

Fracture Behavior and Delamination Toughening of Molybdenum in Charpy Impact Tests

“…Especially, oxygen segregation weakens the grain boundaries. 6,11,12 On the other hand, it is believed that the embrittlement is an intrinsic property of molybdenum. 7,8,32 Intergranular fractures occur more often in microstructures with many random high-angle grain boundaries.…”

“…[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.…”

“…[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. 4,11,16 However, molybdenum suffers from severe delamination, a typical splitting of grains boundaries, in the transition and upper shelf regions due to the anisotropic grain geometry which makes its processing challenging.…”

Mullite/Mo interfaces formed by intrusion bonding

“…It is well known that the fracture strength of molybdenum depends strongly on factors such as grain boundary character and impurity segregation. Kumar and Eyre [12] demonstrated that under certain conditions, extremely small amounts of oxygen (about 6 ppm) could cause grain-boundary embrittlement.…”

Change of Mechanical Property and Fracture Mode of Molybdenum by Carbon Addition

“…[1][2][3][4][5][6][7][8] A small amount of carbon can significantly enhance the fracture strength of pure molybdenum, and consequently, its ductility. The latter effect is characterized by the relative decreasing in the ductile-to-brittle transition temperature (DBTT).…”