C.L. White

Effect of boron on grain-boundaries in Ni3Al†☆

Abstract The effects of boron additions (up to 0.4 wt% B) on grain-boundary chemistry and tensile properties of Ni3Al containing 24–26 at.% Al were studied. Room-temperature ductility and fracture behavior of B-doped Ni3Al depended critically on deviation from alloy stoichiometry. As the aluminum content of B-doped Ni3Al is decreased below 25 at.%, the ductility increases dramatically and the fracture mode changes from intergranular to transgranular. Auger studies indicate that the intensity of boron segregation to grain boundaries increases and the concentration of grain-boundary aluminium decreases significantly with decreasing bulk aluminum concentration. These results suggest that alloy stoichiometry strongly influences grain-boundary chemistry which in turn, affects the grain-boundary cohesion. Boron exhibits an unusual segregation behavior in Ni3Al, i.e. it has a strong tendency to segregate to the grain boundaries but not to cavity (free) surfaces. On the other hand, sulfur, an embrittling impurity, tends to segregate more strongly to free surfaces than to grain boundaries. The beneficial effect of boron is in agreement with existing theories of solute segregation effects on grain-boundary cohesion. The yield stress of B-doped Ni3Al decreases with increasing grain size produced by long-term annealing at 1000°C The yield stress obeys the Hall-Petch relation: σ y = α o,y + k y d 1 2 with σy = 163 MPa and ky = 8.2 MPa cm 1 2 . The tensile elongation was initially independent of grain size, and showed only a moderate decrease from about 50–40% with grain diameters larger than 110μm.

Dynamic embrittlement of boron-doped Ni3Al alloys at 600°C☆

Boron-doped Ni3Al alloys, with and without 0.5at.%Hf, were tensile tested in vacuum and in oxidizing environments at 600°C. Tensile ductility was found to be strongly dependent on test environment, with much lower ductilities observed in air than in vacuum. The loss in ductility is accompanied by a change in fracture mode from transgranular to intergranular. The severity of this environmental effect on elevated-temperature ductility is also affected by preoxidation in air as well as by the aluminum content of the aluminide. Tests of preoxidized specimens indicate that the embrittlement is due to a dynamic effect simultaneously involving localized stress concentrations, elevated temperature, and gaseous oxygen. The oxygen embrittlement becomes less severe with a decrease in aluminum concentration from 24 to 21 at.%.

Surface and grain boundary segregation in relation to intergranular fracture: Boron and sulfur in Ni3Al☆

Unlike most solutes that segregate to grain boundaries, we have observed that boron segregates more strongly to grain boundaries than to free surface in Ni/sub 3/Al. This observation, along with the previously reported beneficial effect of boron segregation on grain boundary strength, is in qualitative agreement with a theory of grain boundary cohesion first put forward by Rice. If similar studies of other beneficial grain boundary segregants bear out this initial agreement with Rices theory, our understanding concerning the nature of segregation effects on grain boundary cohesion could be significantly enhanced.

High temperature embrittlement of NI and Ni-Cr alloys by trace elements

The effects of Sb, Sn, and Zr additions on the creep properties of Ni and Ni + 20 pct Cr are reported. Antimony and tin additions (~1 wt pct) induce extensive grain boundary cavitation in nickel, while smaller antimony additions had little effect on Ni + 20 pct Cr. Addition of 0.11 pct Zr to Ni + 20 pct Cr greatly inhibited grain boundary cavitation and reduced its Coble creep rate. Auger electron spectroscopy of cavitated specimens provided direct evidence of impurity segregation to cavity surfaces. Residual sulfur segregated most strongly, and was observed on cavity surfaces in all cavitated specimens. Tin segregated somewhat less intensely than sulfur, and antimony segregated only slightly. Segregation of antimony and sulfur to uncavitated portions of Ni + 1 pct Sb grain boundaries was also observed. These results are discussed in terms of segregation effects on energetic and transport properties of the grain boundaries and cavity surfaces.

Effect of test environment on ductility and fracture behavior of boron-doped Ni3Al at 600° C

The work reported here was a preliminary effort to identify the factors responsible for the marked difference between ductility reported by Liu and Taub in the temperature range above 400 degrees C. The authors found that tensile ductility of boron-doped Ni3Al alloys is a strong function of test environment at 600 degrees C, with much lower ductilities observed in air than in vacuum.

Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys

A scanning Auger microprobe study has been made of the segregation of substitutional alloying elements to austenite grain boundaries in Fe-C-X alloys (where X = Mn, Ni, Si, Co, and Mo). The grain boundary enrichments in X are considerably smaller than those previously estimated from the thermal grooving method but appear consistent with other SAM results in the literature. Mo exhibits the highest enrichment factor, those for Si and Mn are intermediate, and no appreciable grain boundary enrichment of either Co or Ni is observed. In view of the special relevance of this information to nucleation kinetics of austenite decomposition products at austenite grain boundaries, the reductions in grain boundary energy attending the measured enrichments are evaluated using the model of interactive segregation of interstitial and substitutional solutes formulated by Guttmann and McLean. These calculations were performed under two different (limiting) conditions: (i) equilibrium segregation of both solutes is fully achieved at the isothermal reaction temperatures, and (ii) the boundary concentration of X is fully inherited from the austenitizing temperature and only paraequilibrium segregation of carbon is achieved. Various characteristics of interactive segregation are also discussed in terms of the interaction and binding energies of each solute.

The effect of trace element additions on the grain boundary composition of Ir + 0.3 pct W alloys

The grain boundary fracture surfaces of several Ir + 0.3 pct W alloys have been examined using Auger electron spectroscopy. Dopant additions (between 10 and 80 wt ppm) of Al, Fe, Th, Ni and Rh are shown to result in thorium enriched grain boundaries. Inert ion sputtering experiments suggest that the thorium enriched region at the grain boundaries is probably only a few atom layers thick. The other dopants (Al, Fe, Ni and Rh) do not appear to influence the segregation of thorium, and their function (if any) in improving the high temperature impact properties of this alloy is unclear at this time.

Grain boundary cohesion and fracture in ordered intermetallics

This paper reports that there is indirect experimental evidence (based on the unusual segregation behavior of boron), as well as direct theoretical evidence, to support the notion that increases GB cohesion in Ni{sub 3}Al. Whether that is enough to account for the dramatic ductilizing effect of boron is still unclear, although it is worth remembering that embrittling impurities can have similarly large effects on fracture mode and ductility (in the opposite direction) through changes principally in the interfacial energies. There is also indirect evidence (based on the lowering of the Hall-Petch slope by boron) to support the notion that boron facilitates slip transfer across GBs. However, as discussed in the text, there are factors other than the presence of boron which could affect the magnitude of the Hall-Petch slope, making its interpretation uncertain. Even if boron really does facilitate slip transfer, the details of exactly how this happens are still unclear. Of the two mechanisms suggested so gar, the one which suggests that boron facilitates slip transfer by increasing the mobility of GB dislocations appears to have been discounted recently by TEM experiments.

The intergranular segregation of boron in Ni3Al: Equilibrium segregation and segregation kinetics

Abstract The grain boundary B content of high-purity Ni-24 at.% Al alloys containing 0.048, 0.144, 0.240 and 0.480 at.% B (100, 300, 500, 1000 ppm mass) has been determined for samples aged from 1323 to 873 K for sufficient times to attain equilibrium. The B content was derived from Auger electron spectra of the intergranular fracture facets. Many facets were exposed during fracture at ≈ 300 K, and additional facets were formed upon fracturing following hydrogen charging after heat treatment. For each alloy sample, about 25 facets were analyzed. The grain boundary B contents were in the range of 0.5–2.5 at.%. The grain boundary B content increased with decreasing temperature and with increasing bulk B content in the alloys. The energy of binding of a B atom to the grain boundary was calculated using McLeans segregation theory and assuming a unique binding energy for each alloy. The values were in the range of 0.15–0.45 eV/atom, and increased with increasing temperature and with decreasing bulk B content. These results have been rationalized in terms of a spectrum of binding energies for a given alloy. However, when the entropy of adsorption was taken into account, an enthalpy of adsorption of B to the grain boundary of 0.13 eV/atom was obtained, independent of temperatire and bulk B content. This is interpreted to mean that the spectrum of binding energies is quite restricted. The grain boundary B content of these alloys has also been measured as a function of annealing time at 773, 873, 973 and 1173 K. The diffusion coefficient of B in Ni 3 Al at 773 K is about 5 × 10 −21 m 2 /s, and the equilibrium grain boundary B content is attained at about 3000 s. The diffusion coefficient at 973 K is between 10 −16 and 10 −17 m 2 /s. The activation energy for diffusion of B in Ni 3 Al is between 200,000 and 300,000 J/mol.

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni5Zr and ZrO2 inclusions, a Zr4C2S2 carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.