An electron microscope study of Mo2C precipitates in molybdenum—I. Crystallographic relationships

Kumar, Aniruddha; Eyre, B.L.

doi:10.1016/0001-6160(78)90109-8

Cited by 23 publications

(3 citation statements)

References 11 publications

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“…Combining the present results with those of an earlier study of carbide precipi tation in molybdenum (Kumar & Eyre 1978), we can now attribute the effect of carbon in improving the ductility of recrystallized molybdenum to three main effects. First, it is very effective in reducing oxygen segregation to grain boundaries, thus increasing the intergranular fracture stress and thereby promoting plastic defor mation.…”

Section: Structure O F Fracture Surfacessupporting

confidence: 79%

“…However, little work has been carried out with surface analysis techniques to positively identify segregated species at grain boundaries. The reasons for the beneficial effects of carbon, in contrast to the deleterious effects of the gaseous interstitial impurities* are not properly understood, although one im portant factor could be the morphology of the precipitates (Kumar & Eyre 1978).…”

Section: Introductionmentioning

confidence: 99%

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Grain boundary segregation and intergranular fracture in molybdenum

Kumar

Eyre

1980

Proc. R. Soc. Lond. A

141

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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.

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Section: Structure O F Fracture Surfacessupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Grain boundary segregation and intergranular fracture in molybdenum

Kumar

Eyre

1980

Proc. R. Soc. Lond. A

141

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“…However, its practical application is severely hindered due to a strong grain boundary embrittlement. This could be partially compensated by the inclusion of carbon, which has a beneficial effect of strengthening [7][8][9][10][11][12] the metal. Detailed experimental [13][14][15] and theoretical studies [16][17][18][19] have been carried out on the 5 symmetrical tilt grain boundary (STGB) in pure Mo.…”

Section: Previous Studies Of the Moc X Precipitatementioning

confidence: 99%

Ab initioprediction of the critical thickness of a precipitate

Sampath

Janisch

2013

J. Phys.: Condens. Matter

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Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface.Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations.

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The origin of Mo2C precipitate morphology in molybdenum

Lang¹,

Dahmen²,

Westmacott³

1983

Phys. Stat. Sol. (a)

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A detailed electron metallographic analysis is conducted on the morphology and interface structure of Mo2C precipitates in a quenched and aged MoC alloy. The precipitates are found to be plate‐shaped and to lie on{301} matrix planes. The approximate Burgers orientation relationship,(0001) ‖ (110), [1210] ‖ [111], is confirmed. Characteristic striations along 〈113〉 matrix directions in the interface are analyzed as slip traces of shear loops on the basal plane. The overall features of the precipitate morphology are explained in terms of a model based on the invariant line concept. The invariant plane strain condition of a planar interface is met by the periodic nucleation of shear loops to relieve the developing stresses. An observed small misorientation between the close‐packed planes is a consequence of the restraints imposed on the precipitate by the surrounding matrix. Good agreement is also found between the model and independent observations on other refractory metal interstitial alloy systems.

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An electron microscope study of Mo2C precipitates in molybdenum—I. Crystallographic relationships

Cited by 23 publications

References 11 publications

Grain boundary segregation and intergranular fracture in molybdenum

Grain boundary segregation and intergranular fracture in molybdenum

Ab initioprediction of the critical thickness of a precipitate

The origin of Mo2C precipitate morphology in molybdenum

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