B-doped Alloy Research Articles

Improving shape memory effect in Fe-Mn-Si-based alloys by reducing annealing twin boundaries through trace boron doping

Previous work has shown that the shape memory effect (SME) exhibited a strong negative dependence on the density of annealing twin boundaries (ATBs) in Fe-Mn-Si-based alloys. Reducing the density of ATBs has thus been an effective way to improve the SME of Fe-Mn-Si-based alloys. This study investigated the effects of trace B doping on the density of ATBs and the resultant SME in solution-treated Fe-Mn-Si-based alloys. Results indicated that doping 89 ppm B effectively reduced the density of ATBs in the solution-treated Fe-Mn-Si-based alloy. Consequently, its SME was remarkably improved. The maximum shape recovery strain reached 2.7 % in the B-doped alloy, 1.3 % higher than in the alloy without B doping. This work demonstrates a new avenue to improve the SME by doping trace B, which is of significance for the fabrication of Fe-Mn-Si-based shape memory alloys in a more cost-effective manner.

Effect of B doping on magnetic and mechanical properties of all-d-metal Heusler alloys

The effect of trace doping on the magnetocaloric effect and mechanical properties of Ni36·5Mn35·5Co13Ti15Bx (x = 0, 0.1, 0.2, 0.3) are investigated. It is found that a small amount of B doping not only reduces the thermal hysteresis of the alloys, but also increases the magnetocaloric effect and mechanical properties. With increasing B doping, the transformation temperature of Ni36·5Mn35·5Co13Ti15Bx (x = 0, 0.1, 0.2, 0.3) increases from 273 K to 297.5 K. The alloy Ni36·5Mn35·5Co13Ti15B0.1 has large magnetocaloric effect with the thermal hysteresis of 7.5 K, isothermal magnetic entropy change and adiabatic temperature change of 37.8 J kg−1K−1 and 7.9 K under μ0H = 5 T, respectively. When x increases from 0 to 0.3, the maximum compressive strength increases by 29.8 %, from 1023 MPa of x = 0–1328 MPa of x = 0.3. It is also observed a large number of dimples and ligaments in the fracture surface for sample x = 0.1, indicating that the B-doped alloys have better toughness.

Towards novel in-situ composites: MAB nanolaminate-reinforced FCI quasicrystal in multi-phase aluminum alloys

A range of new in situ composites with prospective application for low-density ballistic shielding was investigated. The beneficial mechanical properties will come from the co-existence of two phases: impact-resistant face-centered icosahedral (FCI) and Fe2AlB2. The aim of the current study was the investigation of phase equilibria in Al-Cu-Fe-B system. Each of the elements in Al-Cu-Fe base alloy was doped with boron and the alloys were produced by means of suction casting. The composition impact on microstructure, accompanying phases identification and hardness were discussed in detail. The compositional FCI phase stability area was found to be smaller than predicted from atomic size constraint imposed by Hume-Rothery criteria for ternary system. Primary phases were Fe2AlB2 or Fe4Al13 in B-doped alloys. It was found that atomic positions of B in Fe2AlB2 phase can be occupied by Cu, what opens new route of tailoring its properties. B:Fe and B:Cu substiution in the 4–10 at% range promotes the growth of AlB2 phase from small tabular precipitates to long needles, what is correlated with the reduction of overall hardness. B:Al substitution leads to progressive increase in thickness of Fe2AlB2 lamellae that accompanies the hardness improvement. It was found that dominant factor contributing to overall alloy hardness was the shape of reinforcing Ψ and ζ grains rather than volume fraction of constituent phases.

Electrochemical delithiation of the binary LiAl, Li3Al2, Li9Al4 and boron–doped phases

The process of electrochemical delithiation of LiAl (structure type (ST) NaTl, space group (SG) Fd m ), Li 3 Al 2 (own ST, SG R m ), Li 9 Al 4 (own ST, SG C 2/ m ) intermetallic compounds and doped by boron LiAl 1- y B y , Li 3 Al 2- y B y , Li 9 Al 4- y B y phases was investigated by X-ray powder diffraction and scanning electron microscopy. The samples were synthesized by arc melting of pressed pellets of pure components under an argon atmosphere and further annealing of the alloys at 200 ºC for 1 month in sealed evacuated silica tubes without quenching. Electrochemical delithiation of the phases, which were used as anode materials, was carried out in the two-electrode prototype of the battery “Swagelok-cell”. LiCoO 2 (ST NaFeO 2 ) was used as cathode material. The anode and cathode materials were separated by pressed cellulose to avoid contact between them. An electrolyte for batteries consisted of 1 M Li[PF 6 ] solution and a mixture of aprotic solvents ethylene carbonate and dimethyl carbonate (1:1 vol. ratio). X-ray phase analysis was carried out on powder data obtained on automatic diffractometer DRON-2.0M (Fe K α -radiation). Investigated alloys contained the expected phases and small amount of other phases. Solid solutions of the substitution had homogeneity ranges about 5 at. % of boron and were characterized by reduced unit cell parameters. All observed binary and ternary phases demonstrated electrochemical ability. The unit cell parameters of the binary and ternary phases reduced after the delithiation. The lithium mobility of the electrodes on the basis of the LiAl, Li 3 Al 2 , and Li 9 Al 4 alloys was 0.16 Li/f.u., 0.52 Li/f.u., and 1.6 Li/f.u., respectively. The better results were obtained for the electrodes based on the B-doped alloys: 0.18 Li/f.u. for the LiAl 1- y B y , 0.61 Li/f.u. for the Li 3 Al 2- y B y , and 1.8 Li/f.u. for the Li 9 Al 4- y B y phase. In all cases the surface morphology of the electrodes after 50 cycles of delithiation was changed and became more porous. The small particles (80‒500 nm) aggregation increased as a result of material amorphization and interaction of the electrode surface with electrolyte. Keywords: synthesis, solid solution of the substitution, electrochemical delithiation, Li-ion batteries.

Effects of Precipitate Element Addition on Microstructure and Magnetic Properties in Magnetostrictive Fe83Ga17alloy

The <100> oriented <TEX>$Fe_{83}Ga_{17}$</TEX> alloys with various contents of NbC or B were prepared by directionally solidification method at the growth rate of <TEX>$720mm{\cdot}h^{-1}$</TEX>. With a small amount of precipitates, the columnar grains grew with cellular mode during directional solidification process, while like-dendrite mode of grains growth was observed in the alloys with higher contents of 0.5 at% due to the dragging effect of precipitates on the boundaries. The NbC precipitates disperse both inside grains and along the boundaries of <TEX>$Fe_{83}Ga_{17}$</TEX> alloys with NbC addition, and the Fe2B secondary phase particles preferentially distribute along the grain boundaries in B-doped alloys. Precipitates could affect grain growth and improved the <100> orientation during directional solidification process. Small amount of precipitate element addition slightly increased the magnetostrictive strain, and a high value of 335 ppm under pre-stress of 15 MPa was achieved in the alloys with 0.1 at% NbC. Despite the fact that the effect on magnetic induction density of small amount of precipitates could be negligible, the coercivity markedly increased with addition of precipitate element for <TEX>$Fe_{83}Ga_{17}$</TEX> alloy due to the retarded domain motion resulted by precipitates.

Effect of Boron Doping on Cellular Discontinuous Precipitation for Age-Hardenable Cu–Ti Alloys

The effects of boron doping on the microstructural evolution and mechanical and electrical properties of age-hardenable Cu–4Ti (at.%) alloys are investigated. In the quenched Cu–4Ti–0.03B (at.%) alloy, elemental B (boron) is preferentially segregated at the grain boundaries of the supersaturated solid-solution phase. The aging behavior of the B-doped alloy is mostly similar to that of conventional age-hardenable Cu–Ti alloys. In the early stage of aging at 450 °C, metastable β′-Cu4Ti with fine needle-shaped precipitates continuously form in the matrix phase. Cellular discontinuous precipitates composed of the stable β-Cu4Ti and solid-solution laminates are then formed and grown at the grain boundaries. However, the volume fraction of the discontinuous precipitates is lower in the Cu–4Ti–0.03B alloy than the Cu–4Ti alloy, particularly in the over-aging period of 72–120 h. The suppression of the formation of discontinuous precipitates eventually results in improvement of the hardness and tensile strength. It should be noted that minor B doping of Cu–Ti alloys also effectively enhances the elongation to fracture, which should be attributed to segregation of B at the grain boundaries.

Microstructural stability and mechanical properties of a boron modified Ni–Fe based superalloy for steam boiler applications

Ni–Fe based superalloys are being considered as boiler materials in 700°C advanced ultra-supercritical (A-USC) coal fired power plants due to their excellent oxidation and hot corrosion resistance, outstanding workability and low cost. In this paper, the microstructural stability and mechanical properties of a boron (B) modified Ni–Fe based superalloy designed for 700°C A-USC during thermal exposure at 650–750°C for up to 5000h were investigated. The results show that adding boron has no apparent influence on the major precipitates, including spherical γ′ and blocky MC. However, the amount of M23C6 decreases markedly after standard heat treatment. During long-term thermal exposure, the addition of boron has no influence on γ′ coarsening, η phase precipitation and primary MC degeneration, but decreases the growth rate of M23C6 along grain boundary. The stress rupture life and ductility are obviously improved after the addition of B. Meanwhile, the yield strength of B-doped alloy almost keeps the same level as that without boron addition. The fracture surface characterization exhibits that the dimples increase significantly after adding boron. During long-term thermal exposure, the elongation of the alloy with B addition increases slightly, but, for the alloy without B addition, the elongation obviously increases. The improvement of the stress rupture life and ductility can be attributed to the increase of grain boundary strength and the optimization of M23C6 carbide distribution at grain boundary.

Effect of boron on the oxidation behavior of NiCrAlYHfTi in H2O and CO2 environments

Cast NiCrAl alloys, co-doped with Y, Hf, Ti, and/or B, were evaluated at 1100°C and 1150°C in dry air, wet air (10 or 50% H2O), and CO2–10% H2O, in order to study the effect of boron additions on alumina scale growth and adhesion. After 200, 1-h cycles at 1100°C, all of the alloys with Y and Hf showed good scale adhesion. By increasing the test temperature to 1150°C, the addition of ~0.3at.% (0.07wt.%) B was shown to improve alumina scale adhesion during 1-h cycles in air with 10% H2O. Analytical transmission electron microscopy showed no effect of B on the alumina scale microstructure and only minor effects on the depth of internal oxidation at 1100°C. Combined Ti and B additions did not produce an additional benefit and Cr–B precipitates were detected in both B-doped alloys. Exposures in O2-buffered CO2–H2O did not result in any detrimental effect at 1100°C.

Catalytic Reactivity of Reaction Synthesized Ni3Al Alloy (Free and Boron-Doped)

Micro-alloyed nickel aluminide Ni3Al and boron-doped Ni3Al, containing 0.15 wt% B, were processed via powder metallurgy. The effect of the boron on the crystalline structure, porosity, surface area, and toluene catalytic hydrogenation was studied. The results indicate that the doping of boron decreases the percentage total porosity of micro-alloyed nickel aluminide. Consequently, slight grain growth in the case of B-doped alloy occurred. Such changes in the alloy structure had a negative influence on the catalytic activity of the nickel aluminide as a catalyst where the selectivity of the alloy toward the hydrogenation of toluene decreased from ~95 to ~64%.

Effects of boron on the microstructure and thermal stability of directionally solidified NiAl–Mo eutectic

Microalloying with 0.01 at.% B decreases the range of growth speeds over which a well-aligned fibrous eutectic microstructure can be obtained in directionally solidified NiAl–Mo. Compared to the undoped alloy, the size/spacing of the Mo fibers is larger, and the fiber density smaller, in the B-doped alloy. Annealing at 1400 °C coarsens the fibers by a mechanism involving fault migration and annihilation driven by diffusion along the fiber–matrix interface. The coarsening kinetics, given by the decrease in Mo fiber density with time, is exponential, and microalloying with B decreases the coarsening rate.

Effect of processing and Microalloying Elements on the Thermal Stability of Cr-Cr3Si and NiAl-Mo eutectic alloys

AbstractThe thermal stability of multiphase intermetallics at temperatures to 1400°C was investigated by studying two model eutectic systems: Cr-Cr3Si having a lamellar microstructure and NiAl-Mo having a fibrous microstructure. In drop cast Cr-Cr3Si, coarsening was found to be interface controlled. The coarsening rate could be reduced by microalloying with Ce and Re, two elements which were chosen because they were expected to segregate to the Cr-Cr3Si interfaces and decrease their energies. Similarly, directional solidification, which is also expected to lower the Cr-Cr3Si interfacial energy, was found to dramatically decrease the coarsening rate. In the case of NiAl-Mo, coarsening was found to occur by fault migration and annihilation. Microalloying with B was found to significantly decrease the coarsening rate. The fiber density in the B-doped alloy was smaller than in the undoped alloy, suggesting that B affects the coarsening rate by lowering the fault density.

Continuous-cooling α-to-γ transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy

The continuous-cooling transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy was quantitatively measured using a real-time resistivity–temperature–time measurement apparatus operating under a high vacuum. The addition of a small amount of B does not significantly alter the α–γ-phase equilibria but significantly raises the α–γ lamellar start temperature of Ti–45.5 at.% Al alloy at most cooling rates. Furthermore, it markedly increases the critical cooling rate for the ordering reaction. The effect of B addition, which greatly stabilizes the lamellar structure up to a fast cooling rate, is to accelerate the lamellar formation kinetics; the lamellar spacing was nevertheless distinctively larger in a B-doped alloy. This is because lamellae in B-doped alloy nucleate heterogeneously on titanium borides at the grain boundary; the borides are effective nucleation sites particularly since local Ti depletion can occur near the interface of the growing titanium borides during cooling. In the absence of B addition, the lamellar structure starts to form only at temperatures below T 0, suggesting that a large undercooling is required for the nucleation of lamellae even at the grain boundaries. On the other hand, the B addition greatly retards the kinetics of the α-to-α2 ordering reaction by markedly increasing its critical cooling rate without a large change in the ordering temperature. This is believed to be due to its tendency to segregate strongly to the antiphase boundaries.

Effect of boron microalloying on microstructure, tensile properties and creep behavior of Ti–22Al–20Nb–2W alloy

The effect of 0.2 at.% boron addition on the microstructure, tensile properties, and creep behavior of Ti–22Al–20Nb–2W were investigated. A TiB phase was observed in the B-doped alloy, which shows that the solubility of boron in this alloy is very low. The addition of 0.2 at.% boron was very effective in reducing the prior β-grain size of the Ti–22Al–20Nb–2W alloy, which is thought to be due to a strong boron segregation at the grain boundaries. The decrease in grain size due to the addition of boron is effective in improving the alloy's room temperature ductility. The addition of a small amount of boron was beneficial in lowering the steady-state creep rate at 650°C/310 MPa, but did not improve the creep behavior at 750°C/310 MPa.

The effect of boron addition on brittle-to-ductile transition temperature and its strain rate sensitivity in gamma titanium aluminide

Temperature dependence of tensile properties ofTi–47Al–2Mn–2Nb–0.8TiB2 alloy was investigated andbrittle-to-ductile transition temperature (TBD) wasevaluated accordingly within the strain rate range from 10−5 to10−1 s−1. TBD and its strain rate sensitivity inTi-47Al-2Mn-2Nb-0.8TiB2 alloy were compared withthose in Ti-47Al-2Mn-2Nb alloy. It is found that theminor addition of 1.0 at% boron reduces TBD by more than100 K and that TBD in both alloys shows a positivesensitivity to the strain rate. But the B-doped alloy has a lower BDTactivation energy (256 kJ/mol) than that of B-free alloy(324 kJ/mol). The effect of boron on TBD and its strain ratesensitivity is attributed to the reduction in the grain size.

Compression properties of B-doped Ir-15Nb two-phase refractory superalloys

Refractory superalloys based on platinum group metals (PGMs), which have coherent fcc-L1{sub 2} two-phase structure and high melting temperature, have good potential as structural materials for use at temperatures above 1,800 C. The authors previously made an extended investigation on iridium (Ir) and rhodium (Rh) binary systems and found that Ir-based two-phase alloys, such as Ir-15at%Nb and Ir-15At%Zr alloys, were superior in high-temperature strength and oxidation resistance. However, their investigation also showed that polycrystalline Ir-15at%Nb two-phase alloys exhibited intergranular fracture with limited ductility in compression tests, such as in pure Ir and its single-phase alloys. An interpretation based on improved grain-boundary cohesion by boron (B) segregation has been at least partially successful in accounting for enhanced ductility in alloys prone to intergranular fracture, such as in many intermetallic alloys and other alloy systems. However, limited data are available for the effect of B on the ductility and fracture behavior of Ir-based two-phase refractory superalloys. Recently, Wolff and Sauthoff reported that the addition of 0.5at%B could raise the strength and ductility of the Ir-16at%Nb alloy at lower temperature but caused a rapid decrease in strength above 1,100 C. However, no detailed fracture behavior for this B-doped alloy has been reported. Themore » effect of boron on the strength and fracture behavior of Ir-based two-phase refractory superalloys still needs to be systematically studied. The purpose of this work is to examine the mechanical properties of Ir-15at%Nb two-phase alloys doped with various B contents, with emphasis on high temperature strength and fracture behavior.« less

Room-temperature mechanical behavior of FeAl: effects of stoichiometry, environment, and boron addition

The intrinsic ductility of FeAl (in ultrahigh vacuum) decreases with increasing Al content, from around 16% in Fe–37Al to zero in Fe–48Al. The sharpest decline occurs around the composition where the fracture mode changes from transgranular to intergranular. Boron shifts this ductile–brittle transition to higher Al levels by segregating to the grain boundaries and suppressing grain-boundary fracture. However, its effectiveness decreases with increasing Al concentration, even though the amount of B segregating to the grain boundaries remains the same, independent of alloy stoichiometry. Consequently, even the B-doped alloys become brittle and fracture intergranularly as the stoichiometric composition is approached. Low-pressure hydrogen embrittles FeAl, although not as severely as atmospheric moisture. Environmental embrittlement is most noticeable in Fe-rich FeAl; with increasing Al concentration, the grain boundaries become intrinsically weak, and brittle fracture persists even after environmental effects are eliminated.

Ductility and Fracture of FeAl: Effects of Composition and Environment

ABSTRACTIn ultrahigh vacuum (UHV), the ductility of FeAl decreases with increasing Al content and fracture becomes increasingly intergranular. Boron improves ductility by segregating to the grain boundaries and suppressing grain-boundary fracture. However, with increasing Al concentration, even the B-doped alloys become brittle and fracture intergranularly. Hydrogen gas at low pressures embritties FeAl, although not as severely as atmospheric moisture. Ductility is highest in UHV followed by that in O2, vacuum, and air.

Toughening Mechanism of Ni3Al Alloys by B-doping

Since Aoki and Izumi found out the effectiveness of B-doping for improving the ductility of Ni3Al alloys [1], the ductilizing and toughening mechanisms of the doping have been actively discussed [2±5]. In the early stage of the studies, the segregation of B on the grain boundaries was found to have a bene®cial effect on enhancing the grain boundary cohesion and to relieve the intrinsic grain boundary brittleness [2, 3]. Recently, the suppression of environmental embrittlement by the doping is believed to be the main factor for the toughening of Ni3Al alloys [4]. Because of the diffusivity of atomic hydrogen, which, when generated by the surface reaction of moisture in air, is reduced [5], the alloys are consequently insensitive to the environmental effect by the doping. However, there are some problems that cannot be explained except by the suppression of environmental effects such as the alloy composition dependence of the ef®cacy of B [2]. In the present work, the toughening mechanism of stoichiometric and Ni-rich Ni3Al alloys by the doping will be brie y discussed. Ni3Al alloys, with and without the doping of 0.1 at % B, were fabricated by reactive hot-pressing [6] at 1573 K with an applied pressure of 30 MPa. The fracture toughness of the alloys was measured by the single-edge chevron notched beam (SECNB) method in air and an oil bath (silicone oil SRX310: Toray Dow Corning Silicone Co., Ltd) at 300 K. The chevron notched (notch length a0: 4 mm) testing bars with 8 mm 3 4 mm 3 35 mm dimensions were used for the evaluation of the fracture toughness. The three-point bending test of the notched bars with a span length of 30 mm was conducted at a loading rate of 10y2 y 10 MPa m secy1. The tensile test of the alloys was also performed in air by using the specimens with a gage section of 0:5 mm 3 1 mm 3 8 mm. The strain rate for the tensile test was 8:3 3 10y4 secy1. The fracture surface of the specimens was observed by scanning electron microscopy (SEM). Fig. 1 shows the loading rate dependence of fracture toughness for the Ni3Al alloys measured by SECNB method. The fracture toughness of the alloys without the doping decreases with the decrease of the loading rate, although a similar behavior is not observed in the oil bath. Because the moistureinduced embrittlement is expected to be suppressed in the oil bath, this loading rate dependence in air is caused by the environmental effect. However, the fracture toughness measured at a loading rate of 10 MPa m secy1 in air is equal to that measured in the oil bath. Therefore, the fracture toughness unaffected by the environmental effect (intrinsic toughness) can be estimated above 10 MPa m secy1. The intrinsic toughness values for the nondoped 24 and 25 at % Al alloys are estimated to be 19 and 16 MPa m, respectively. The environmental embrittlement of Ni-25 at % Al alloy is successfully inhibited by the 0.1 at % Bdoping because the fracture toughness of the doped alloy in air is independent of the loading rate and is equal to that in the oil bath. Concurrently with the suppression of the environmental effect, its intrinsic fracture toughness is improved to be 30% larger than that of the non-doped alloy by the doping. On the other hand, the chevron-notched testing bars of the B-doped Ni-24 at % Al alloy cannot be fractured validly in the present conditions due to its extensive ductility. At least, the toughness of the doped Ni24 at % Al alloy is much larger than the intrinsic value of the non-doped one. If the doped B only plays a role for the suppression of environmental effect, the fracture toughness of the B-doped Ni24 at % Al alloy is improved until it has the intrinsic toughness of the non-doped alloys. However, the fracture toughness of the B-doped alloy is clearly improved much more. Hence, another effect of B is necessary to interpret the extensive improvement of

Effect of low-pressure hydrogen on the room-temperature tensile ductility and fracture behavior of Ni3Al

Abstract The effect of low-pressure (≤1.3 × 10 3 Pa) hydrogen gas on the ductility and fracture behavior of polycrystalline Ni 3 Al (23.4 at% Al) was investigated. Room-temperature tensile ductilities remained high over the entire pressure range tested: from 41% elongation to fracture at 5.7 × 10 −8 Pa pressure to 31% at 1.3 × 10 −3 Pa. Over this pressure range, the amount of transgranular fracture also remained quite high and scaled with the tensile ductility, increasing from ~60% in the samples with 31% ductility to ~70% in the specimen with 41% ductility. The ionization gage—used to measure hydrogen pressure—had a dramatic (deleterious) effect on the ductility of Ni 3 Al: at any given hydrogen pressure, the ductility measured with the ion gage on was about half to a quarter of that measured with the ion gage turned off. Accompanying this decrease in ductility was a change in fracture mode from predominantly transgranular to predominantly intergranular. The role of the ion gage is believed to be hot-filament-assisted dissociation of molecular H 2 into atomic H, which is quickly absorbed and embrittles the crack-tip regions. In the absence of any H-induced embrittlement (either by filament-assisted dissociation of H 2 or by Al-induced reduction of H 2 O), polycrystalline Ni 3 Al is found to be quite ductile, with tensile elongations exceeding 40% and predominantly (>70%) transgranular fracture. Since these ductilities are similar to those of the most ductile B-doped alloys, the main role of boron is to suppress environmental embrittlement. Our results indicate that, at room temperature, low-pressure H 2 does not dissociate very efficiently into atomic H on the surfaces of Ni 3 Al and that, at comparable pressures, hydrogen is not as harmful to ductility as moisture (H 2 O).

Dislocation-grain boundary interactions in Ni 3Al; effects of structure and chemistry

This paper reports that one of the critical issues in developing an understanding of the response of Ni{sub 3}Al to stress is the interaction between dislocations and grain boundaries. Even in simple metals this issue is complex, depending as it does on the nature and orientation of the grain boundary and the character of the interacting dislocations. In the case of intermetallic alloys, the issue is even more complex, as the additional factors of grain boundary chemistry, local order, etc. play an important role. This si clearly illustrated in the specific case of Ni{sub 3}Al which exhibits brittle intergranular fracture at low temperatures, except for hypo-stoichiometric alloys containing small amounts of Boron. The B-doped alloy exhibits extensive plastic deformation before undergoing ductile rupture. The effect of B is clearly on the response of grain boundaries to the stresses caused by slip dislocations as both ductile and brittle alloys show extensive slip within the grains prior to fracture. Two mechanism have been proposed to account for this effect of B on the fracture process. In one, it is postulated that B segregation to the grain boundaries increases the cohesive energy of the grain boundaries to the extent that stress concentrations atmore » the boundaries are relieved by initiation of slip rather than fracture. This is consistent with the thermodynamic treatment of fracture as it has been observed that B does segregate more strongly to grain boundaries than to external surfaces.« less