B-rich Phase Research Articles

Growth kinetics and microstructure transition of undercooled Fe7(CoNi)75B18 eutectic muti-principal element alloy

The mechanism of rapid solidification behavior of eutectic muti-principal element alloys (EMPEAs) is an eye-catching field. In this study, the novel Fe7(CoNi)75B18 EMPEA was undercooled via rapid solidification and B2O3 glass purification method and the maximum undercooling up to 190 K was achieved. A clear microstructure evolution path was identified with the undercooling: fully regular eutectic → primary dendrite and regular eutectic → primary dendrite and divorced eutectic. The orientation relationship (OR) between the primary (Fe, Co, Ni) phase and secondary B-rich phase was revealed both at low and medium undercoolings by electron backscatter diffraction (EBSD) analysis. The results show that the OR disappears at high undercooling (190 K) owing to the microstructure fragment and rotation during the post-recalescence. Furthermore, the growth velocity exponentially raised with the increase of undercooling. The maximum growth velocity of the primary phase was 1.152×10−1 m/s, which is significantly lower than binary alloys due to the sluggish kinetics effect of EMPEAs. This study helps to understand the solidification kinetics and microstructure transition of the novel EMPEA and has important implications for controlling the solidification process of more complex alloys.

Study on Ce2Fe14B-based sintered magnets with Ce/RE> 80%

Up to now, the development of Ce/RE> 80% sintered permanent magnets still faces a challenge, owing to their seriously deteriorated microstructure. In this article, the Ce25.5∼31.5Nd0∼5Febal.B1.25M1.15 (wt.%) sintered magnets with Ce/RE > 80% were investigated, the roles of minor Nd substitution were explored. The typical island-like phase common existed in the grain-boundary (GB) regions of Ce2Fe14B-based SC alloys (which destroys the continuity of RE-rich GB phase), was confirmed as a tetragonal B-rich phase (fct-RE5Fe18B18), and probably generated by the peritectic reaction of “L + Ce2Fe14B → CeFe2 + B-rich” at 797 °C. It was found that: the deteriorated microstructures of high Ce sintered magnets were hardly improved (Nd element was not enriched in the main phase as it was expected), and the coercivity increments were far below expectations by directly adding 3–5% Nd in alloy designs. However, the GB phase distribution was more uniform and continuous, Nd-rich shells formed, and magnetic properties were remarkably promoted by blending 2% NdHx powders into the Ce27.5Nd3Febal.B1.25M1.15 (Ce27.5Nd3) magnet during JM milling, compared with the Ce25.5Nd5Febal.B1.25M1.15 (Ce25.5Nd5) magnet with similar composition. The good comprehensive magnetic properties of Hcj = 1.714 kOe, Br = 9.395 kG, and (BH)max = 11.16 MGOe have been reached in the Ce27.5Nd3+2% (NdHx) magnet (Ce accounted for 84.5 wt% of total rare earth). The present work deepens our understanding of metallurgical behavior, process/microstructure designing for Ce/RE> 80% sintered magnets, and shed a light on the large-scale utilization of Ce element in a permanent magnet.

Microstructure and coercivity of Al-diffused Sm(Fe,Co,B)12 film

Fe-rich SmFe12-based compounds are considered as potential next generation permanent magnets due to their excellent intrinsic magnetic properties. However, their reported coercivities remain low, only ∼12 % of the magnetic anisotropy field of the SmFe12-based compound. In this work, we successfully increased the coercivity of B-doped Sm(Fe0.8Co0.2)12-based thin films from 1.1 T to 1.8 T by diffusion of Al into the grain boundaries. Detailed microstructural characterization using scanning transmission electron microscopy (STEM) and atom probe tomography (APT) revealed that the high coercivity is due to the segregation of Al into ∼4 nm thin B-rich amorphous intergranular phase, resulting in a reduction of its magnetization. This compositional modification of the intergranular phase can prevent the nucleation of reversed magnetic domains in the triple junctions and increase the pinning force of the grain boundary region against the magnetic domain wall motion. Based on detailed microstructural observations, the mechanism of coercivity enhancement is discussed.

Effect of boron content on microstructure evolution and crystallization behavior of Fe7(CoNi)93-xBx (x=15, 18, 21) eutectic high-entropy alloys

The effect of boron content on microstructure evolution, glass-forming ability and crystallization behavior of Fe7(CoNi)93-xBx (x = 15, 18, 21) eutectic high-entropy alloys (EHEAs) was investigated. Bulk Fe7(CoNi)93-xBx EHEAs and ribbons were prepared by vacuum frequency induction melting furnace and melt-spinning method, respectively. The results show that the microstructure of as-cast hypoeutectic Fe7(CoNi)78B15 alloy consists of primary (Fe, Co, Ni) dendrite and lamellar eutectic. The eutectic Fe7(CoNi)75B18 alloy shows a fully lamellar eutectic structure consisting of (Fe, Co, Ni) phase and B-rich phase. The microstructure of hypereutectic Fe7(CoNi)72B21 alloy exhibits primary B-rich dendrite and lamellar eutectic. The as-spun Fe7(CoNi)75B18 and Fe7(CoNi)72B21 alloys show an amorphous structure. By comparison, the hypereutectic Fe7(CoNi)72B21 alloy shows an excellent glass-forming ability and thermal stability, which is attributed to the increase of crystallization activation energy induced by B addition. In addition, the increase of B addition inhibits the crystallization behavior, which is caused by the formation of stable covalent bonds between boron and other elements. This work provides an alternative route for the preparation of metallic glass by introducing the non-metallic boron component.

Effect of wire diameter on structure and electrical properties of (Al + Al2O3)-sheathed MgB2 with Nb barrier

Effect of wire diameter and annealing on the properties of internal magnesium diffusion (IMD)-processed MgB2 composite wires is studied by establishing a correlation with the characteristics of the interfacial reaction zones developed at Mg − B and barrier (Nb) – sheath (Al + Al2O3) interfaces. The MgB2 superconductor phase grows at the Mg − B interface along with the B-rich MgB4 phase. Formation of MgB2 prior to MgB4 is anticipated owing to a relatively lower Gibbs free energy at annealing temperatures studied. An intermetallic NbAl3 phase has developed at the barrier – sheath interface whose thickness decreases with an increase in the wire diameter. Development of NbAl3 further provides mechanical reinforcement and aids at yielding a dense superconductor phase. Resistivity – voltage characteristics of wire indicates the formation of MgB2 through a huge drop in resistivity values across the core of the wire. An annealing temperature of 645 °C near the melting point of pure Mg (650 °C) and annealing time of 120 min results in highest critical current of the MgB2 wire. Annealing time beyond 120 min has shown an impediment to the flow current due to crack propagation possibly because of accumulation of stresses within the developed superconductor phase due to grain growth. A significantly greater resistance for the wire with the smallest diameter is attributed to the formation of a thicker NbAl3 phase. However, highest engineering current density has also been attained by the wire with the smallest diameter annealed at 645 °C for 60 min.

Interfacial reactions and critical current density of Cu-sheathed Cu-doped MgB2 wire with Ti diffusion barrier

Development of reaction zones at the core Mg(Cu) – boron (B) and barrier (Ti) – sheath (Cu) interfaces of an internal magnesium diffusion (IMD) manufactured MgB2 composite wire has been studied at varying temperatures of 560–665 °C for 30 min and varying time periods of 30–180 min at 560 °C. MgB2 has predominantly formed at the Mg(Cu) – B interface with little solubility of Cu of 1–3 at%. A second phase enriched with Cu, Mg2Cu, has also formed at random locations within the MgB2 phase matrix, indicating the advantage of doping Cu in the Mg rod. However, a layer of another B-rich phase, MgB4, has also formed at a higher temperature of 665 °C with negligible solubility of Cu at the MgB2 – B interface. At the Ti – Cu interface, the formation of CuTi2, CuTi, Cu4Ti and Cu4Ti3 intermetallic phases took place. However, the Cu4Ti and Cu4Ti3 phases develop after annealing the wire at a relatively higher temperature of 620 °C and at 560 °C with progress in annealing time. Critical current density (Jc) decreases with annealing time; however, the sensitivity of Jc towards annealing time is low. Critical current density is found to be strongly dependent on the annealing temperature of the composite wires for the formation of the MgB2 superconductor phase. A maximum critical current density of ∼ 2.5 × 104 A/cm2 at 5 T and 4.2 K is attained for the Cu-doped MgB2 composite wire when annealed at 665 °C for 30 min. A maximum shift of only 0.5 K is measured for MgB2 when formed at the highest annealed temperature compared to the lowest one in this study.

Effect of grain boundary reconstruction and regenerated main phase shell on magnetic properties in high-abundance (NdLaCeY)-Fe-B magnets

The (NdLaCeY)-Fe-B magnets with 45–50 wt% high-abundance rare earth substitution were systematically investigated by co-mixing high boron content matrix phase and Pr–Fe alloy powders (0, 1, 3, 5, 7, and 9 wt%) during the preparation process. The coercivity of magnets increased successively with the addition of Pr70Fe30 (wt.%) and reached 9.32 kOe at the addition amount of 9 wt% from the initial 7.33 kOe without addition, while the remanence had a very small decrease of only 0.13 kGs. The analyses showed that the B-rich phase in the high-boron powders reacted with Pr70Fe30 to form the main phase shell, forming a core-shell structure with a Pr-rich shell and a Y-rich core. Meanwhile, the grain boundary was significantly widened with the introduction of Pr70Fe30. Then, just the improved anisotropy of the main phase grain surface, accompanied with the enhanced magnetic de-coupling effect between the main phase grains, resulted in a great coercivity increment. Notably, on the remanence, the beneficial effect of increased main-phase proportion by regenerated grain shell had a competitive relationship with the deteriorating effect of the magnetic dilution by the non-magnetic material introduction, thus under the combined effects, the remanence firstly increased and then decreased with the addition of Pr–Fe.

Microstructure and high temperature tribological behaviour of self-lubricating Ti-TiBx composite doped with Ni[sbnd]Bi

It is a well-established fact, that the tribological performance of titanium and its alloys at high temperatures is poor, especially for self-mated titanium‑titanium pairs. Previous work has shown a significant improvement in mechanical and high temperature tribological behaviour (700–900 °C) for pure Ti sintered with TiB2 ceramic, occasioned by the in-situ formed boric acid lubricant. The current work explores the possibility to extend the lubrication temperature range (below 700 °C) of Ti-TiB2 composites through the incorporation of a nickel‑bismuth mixture (NiBi) as a solid lubricant using laser melting. To this end, phase equilibrium diagrams have been calculated for the Ti-B-NiBi quaternary system using the CALPHAD method to evaluate the possibility of the formation of different phases due to dilution between the substrate and deposit, as well as their possible interactions. The self-lubricating NiBi samples feature a formation of Bi-Ni-Ti-TiBx functional surface. The functional surface demonstrates a compact and unique microstructure of a hard B-rich phase encapsulated inside a lubricous Bi-rich phase. A two-fold increase in hardness of the surface in comparison to the underlying substrate was noted for NiBi laser-melted samples. Further, the NiBi samples were studied under a reciprocating sliding configuration against a Ti-alloy flat-pin counterbody at temperatures including RT, 400 and 600 °C. The resulting wear scars were analysed utilizing SEM, XRD and 3D profilometry to understand the prevalent wear mechanisms. A significant decrease in coefficient of friction and wear rate was observed for the NiBi laser-melted samples sliding at 400 °C and 600 °C due to the formation of a self-lubricating tribo-oxide layer rich in Bi-compounds. The results are reported in comparison to the unmodified reference Ti-TiB2 composite.

Revisiting the origins of super-hardness in TiB2+z thin films – Impact of growth conditions and anisotropy

Hexagonal transition metal diborides embody promising material systems for the purpose of protective thin films. Here, we focus on DC magnetron sputtered TiB2+z coating materials, comprehensively revisiting the impact of the stoichiometry on the structure-mechanical properties, from nearly stoichiometric TiB2.07 (B: 67 at. %) up to super-stoichiometric TiB4.42 (B: 82 at. %). The structural analysis confirmed the apparent correlation between the deposition pressure and the preferred {0001} orientation, which is essential to gain super-hardness (>40 GPa). In contrast, the hardness decreases for >10 GPa for 101¯1 and 1000 oriented thin films, underlining the pronounced anisotropy of TiB2+z. The broad stoichiometry variation revealed no predominant hardness effect based on a B-rich tissue phase. The excess B contributes to a decreasing column size correlating with a decreasing hardness of ≈ 7 GPa (B/Ti ratios >2.5) due to column boundary sliding events. Micro-cantilever bending experiments proved a declining fracture toughness from 3.02 ± 0.13 MPa√m for TiB2.43 to 2.51 ± 0.14 MPa√m for TiB4.42 to be column size dependent.

Oxidation resistance and mechanical properties of sputter-deposited Ti0.9Al0.1B2-y thin films

Direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) were used to deposit understoichiometric Ti1-xAlxB2-y diboride coatings by sputtering from a segmented TiB2-AlB2 target using Ar and Kr as sputtering gas. For films with a fixed Al/(Ti + Al) ratio of x = 0.1 (Ti0.9Al0.1B2-y), the B content was varied with y ∈ (0.1, 0.6 and 0.7). For films with a fixed y = 0.7 (Ti1-xAlxB1.3), the Al content was varied with x ∈ (0.1, 0.4 and 0.7). Evaluation of the mechanical properties of the Ti1-xAlxB1.3 samples showed a reduction in both hardness and elastic modulus with increasing Al concentration, while the Ti0.9Al0.1B2-y samples showed a hardness increase with decreasing B content. Thus, Ti0.9Al0.1B1.3 films exhibited a superior hardness of 46.2 ± 1.1 GPa and an elastic modulus of 523 ± 7 GPa, compared to the values for Ti0.9Al0.1B1.4 and Ti0.9Al0.1B1.9, showing a hardness of 44 ± 1 GPa and 36 ± 1 GPa, and an elastic modulus of 569 ± 7 GPa and 493 ± 6 GPa, respectively. The oxidation behavior of the mechanically most promising Ti0.9Al0.1B2-y sample series was investigated through air-annealing at 600 °C for durations from 1 h to 10 h. All films formed a mixed non-conformal Al2O3-TiO2 oxide scale which acts as an inward and outward diffusion barrier, significantly reducing the oxidation rate compared to TiBz films, which form an oxide scale consisting of porous TiO2. The thinnest oxide scale after 10 h was found in the B-deficient samples, Ti0.9Al0.1B1.3 and Ti0.9Al0.1B1.4, at ~200 nm, which is significantly below that for Ti0.9Al0.1B1.9 at 320 nm. The enhanced oxidation resistance of highly understoichiometric films is due to the elimination of the B-rich tissue phase that is present at the grain boundaries for higher B content, where the latter has been shown to enhance the rate of oxidation in borides.

Design and microstructural development of a new eutectic Fe-Co-Ni-B multi-principal element alloy system

A new eutectic Fe-Co-Ni-B multi-principal element alloy system was designed. Microstructure evolution and mechanical properties of the Fe-Co-Ni-B alloys with varied content of Fe and B were investigated. For the Fex(CoNi)84−xB16 alloys, it is found that the eutectic structure transforms from anomalous to lamellar, further to rod, and then to faceted dendrite, which was attributed to the transition of crystal structure of B-rich intermetallic phase from orthogonal to tetragonal as the Fe content increases. For the Fe7(CoNi)93−yBy alloys, the primary phase changes from (Fe, Co, Ni)-rich FCC solid solution to B-rich intermetallic with the increase of B content. The Fe7(CoNi)75B18 shows fully lamellar structure consisted of (Fe, Co, Ni)-rich phase and boron-rich phase with cubic and orthogonal crystal structures respectively. It is also found that the compressive strength and hardness of Fe7(CoNi)93−yBy alloys increase significantly but the plasticity decreases as B increases. The introduction of non-metallic element B as principal component provides a new idea for the design of eutectic high entropy alloy.

Wetting behavior of a colloidal particle trapped at a composite liquid-vapor interface of a binary liquid mixture.

A partially miscible binary liquid mixture, composed of A and B particles, is considered theoretically under conditions for which a stable A-rich liquid phase is in thermal equilibrium with the vapor phase. The B-rich liquid is metastable. The liquids and the thermodynamic conditions are chosen such that the interface between the A-rich liquid and the vapor contains an intervening wetting film of the B-rich phase. In order to obtain information about the large-scale fluid structure around a colloidal particle, which is trapped at such a composite liquid-vapor interface, three related and linked wetting phenomena at planar liquid-vapor, wall-liquid, and wall-vapor interfaces are studied analytically, using classical density functional theory in conjunction with the sharp-kink approximation for the number density profiles of the A and B particles. If in accordance with the so-called mixing rule the strength of the A-B interaction is given by the geometric mean of the strengths of the A-A and the B-B interactions, and similarly the ratio between the wall-A and the wall-B interaction, the scenario, in which the colloid is enclosed by a film of the B-rich liquid, can be excluded. Up to six distinct wetting scenarios are possible, if the above mixing rules for the fluid-wall and for the fluid-fluid interactions are relaxed. The way the space of system parameters is divided into domains corresponding to the six scenarios, and which of the domains actually appear, depends on the signs of the deviations from the mixing rule prescriptions. Relevant domains, corresponding, e.g., to the scenario in which the colloid is enclosed by a film of the B-rich liquid, emerge, if the ratio between the strengths of the wall-A and the wall-B interactions is reduced as compared to the mixing rule prescription, or if the strength of the A-B interaction is increased to values above the one from the mixing rule prescription. The range, within which the contact angle may vary inside the various domains, is also studied.

Microstructure and materials properties of understoichiometric TiBx thin films grown by HiPIMS

TiBx thin films with a B content of 1.43 ≤ x ≤ 2.70 were synthesized using high-power impulse magnetron sputtering (HiPIMS) and direct-current magnetron sputtering (DCMS). HiPIMS allows compositions ranging from understoichiometric to overstoichiometric dense TiBx thin films with a B/Ti ratio between 1.43 and 2.06, while DCMS yields overstoichiometric TiBx films with a B/Ti ratio ranging from 2.20 to 2.70. Excess B in overstoichiometric TiBx thin films from DCMS results in a hardness up to 37.7 ± 0.8 GPa, attributed to the formation of an amorphous B-rich tissue phase interlacing stoichiometric TiB2 columnar structures. We furthermore show that understoichiometric TiB1.43 thin films synthesized by HiPIMS, where the deficiency of B is found to be accommodated by Ti-rich planar defects, exhibit a superior hardness of 43.9 ± 0.9 GPa. The apparent fracture toughness and thermal conductivity of understoichiometric TiB1.43 HiPIMS films are 4.2 ± 0.1 MPa√m and 2.46 ± 0.22 W/(m·K), respectively, as compared to corresponding values for overstoichiometric TiB2.70 DCMS film samples of 3.1 ± 0.1 MPa√m and 4.52 ± 0.45 W/(m·K). This work increases the fundamental understanding of understoichiometric TiBx thin films and their materials properties, and shows that understoichiometric films have properties matching or going beyond those with excess B.

A comparison of field assisted hot pressing and hot isostatic pressing for gas atomized Ti–22Al–26Nb(at.%) and Ti–22Al–26Nb–5B(at.%) powders

Ti–22Al–26Nb(at.%) and Ti–22Al–26Nb–5B(at.%) compacts were processed from gas atomized prealloyed powders by field assisted hot pressing (FAHP) for between 600 and 1200 s at temperatures between 725 and 1070 °C and the resulting microstructures were compared. The average powder diameters ranged between 50 and 100 μm, where both the B-containing powders and resulting FAHP compacts contained elongated B-rich needles. The microstructure in the FAHP compacts depended on the processing temperature. For processing temperatures less than or equal to 960 °C, porosity was evident on the FAHP compact, where greater porosity was observed with decreasing temperature. Processing temperatures greater than or equal to 980 °C resulted in minimal porosity and three-phase microstructures containing a mixture of the orthorhombic, α2, and body-centered-cubic (BCC) phases, where longer processing times led to microstructural coarsening. The microstructures of the FAHP compacts were compared to those from compacts consolidated at 1027 °C for 4 h using hot isostatic pressing (HIP). For both HIP and FAHP processing, the B-containing compacts exhibited finer prior-BCC grain sizes. Compositional analysis of the B-rich phase within the Ti–22Al–26Nb HIP compact suggested that it could be the B27 orthorhombic structure with a B2TiNb stoichiometry. Overall, FAHP was shown to be a viable powder metallurgy processing technique for intermetallic Ti2AlNb based intermetallic alloys.

Structural regulation of hard NbB2 films by DC magnetron sputtering: From amorphous to crystalline

NbB2 with the AlB2-type structure exhibits a wide homogeneity range, providing more possibilities for studying the evolution of its structure and properties. Adjusting the growth conditions to induce the structural transformation, could be carried out to optimize its mechanical and tribological properties. Here, DC magnetron sputtering was used to deposit NbB2 thin films at different temperature, aiming to regulate the microstructure. Further investigations were carried out on chemical bonding states, mechanical and tribological properties by XPS, nanoindentation and tribometer. With the deposition temperature increasing, the crystallinity of the film is improved, while the amorphous B-rich phase is reduced, which is confirmed by the changing of chemical bonding states. The hardness increases first and reaches its maximum (29.90 ± 1.53 GPa) at 400 °C, and then decreases in the higher temperature. The tribochemical product (H3BO3), the hardness and the wear track morphology affect the tribological behavior of NbB2 films. It is worth noting that the samples deposited in middle temperature (200–600 °C) with the encapsulated structure exhibit the superior performance, reaching a higher hardness (>25 GPa) and a lower friction coefficient (~0.2).

Influence of the Al concentration in Ti-Al-B coatings on microstructure and mechanical properties using combinatorial sputtering from a segmented TiB2/AlB2 target

A series of (TixAl1-x)B2+y coatings with compositions in the range of x = 0.01–0.94 and y = 1.70–2.92 has been synthesized using magnetron sputtering from a segmented TiB2/AlB2 target. The coatings are amorphous at x ≤ 0.05 while a (TixAl1-x)B2+y solid solution forms for x > 0.05. As a consequence of the sputtering process, the B/(Ti + Al) atomic ratio varied with the metal content resulting in the formation of under-stoichiometric coatings at x < 0.35 and over-stoichiometric coatings at x > 0.35. Surplus Al segregates to grain boundaries of the under-stoichiometric coatings whereas the over-stoichiometric coatings have a tissue phase containing mainly B and some Al. The B-rich tissue phase restrains grain growth in the in-plane direction while an increase in Ti content promotes the growth of columnar structured coatings with a pronounced (001) texture up to x = 0.84. The combination of such preferred orientation and tissue phase results in the highest hardness of 39 GPa for the (Ti0.79Al0.21)B2.70 coating. The Young's modulus, on the other hand, increases continuously from 262 GPa for the most Al-rich coating to 478 GPa for the most Ti-rich coating. Comparing to calculated values of Young's modulus, good agreement is observed for the close to stoichiometric coatings (x = 0.40–0.50). For the off-stoichiometric coatings, the experimental values are lower due to the existence of the tissue phase.

Reactive wetting and filling of boron nitride nanotubes by molten aluminum during equilibrium solidification

Interactions between long boron nitride nanotube (BNNT) fibrils and molten aluminum (Al) pool are probed in this study to assess the feasibility of fabricating composite materials by solidification route. BNNTs were found to survive high temperature and reactive conditions present in molten aluminum. Very limited interfacial reaction was observed, resulting in the formation of AlN, AlB2 and AlB10 in trace amounts. AlN was the principal reaction product, resulting in improved interfacial wetting. Calculations based on surface energies revealed improved work of interfacial adhesion due to AlN formation. BNNTs were found to be well integrated in the aluminum matrix, signifying AlN induced excellent wetting. We also report capillarity-induced high temperature filling of BNNT by molten Al. The filling was promoted by AlN formation. In addition, formation of B-rich AlB10 phase inside the nanotube was observed. Nanotube filling by metal and subsequent reaction to form nano-ceramic phases is expected to alter mechanical properties of the cast Aluminum-BNNT composites. This study establishes the suitability of solidification route for developing high strength Al-BNNT composites in future.

Transient liquid-phase bonding of superalloy K465 using a Ni–Cr–Fe–B–Si amorphous interlayer alloy

A kind of Ni–Cr–Fe–B–Si system amorphous alloy was used as interlayer in transient liquid-phase bonding (TLP bonding) of polycrystalline superalloy K465. The bonding behavior, microstructure feature and the tensile properties of the joints were investigated. There are B-rich phase and Si-rich phase formed in the center of the seam after bonding at 1210 °C for 30 min. The isothermal solidification is complete after bonding at 1210 °C for 4 h. The relationship of the average width of the remnant eutectic zone and bonding time at 1210 °C is nonlinear. The tensile strength of the bonded joint at room temperature and 900 °C is comparable to that of K465 alloy.

Solidification and phase formation of alloys in the hypoeutectic region of the Fe–C–B system

In this work, alloys from the hypoeutectic iron-rich region of the iron–carbon–boron (Fe–C–B) system were investigated with respect to the solidification and the phase formation. Laboratory melts with a constant carbon content of 0.6mass% and boron contents of 0.2mass%, 0.6mass%, and 1.8mass% were fabricated and metallographically examined. In addition the microstructures were investigated by CALPHAD method in the state of equilibrium and by multiphase-field (MPF) method to reproduce the non-equilibrium process of the technical solidification. The results were analyzed with respect to the effect of boron on the solidification paths, microstructural crystallization processes as well as the morphological and chemical characteristics of the solidified phases.The investigated alloys undergo primary crystallization of austenite (γ-Fe). Due to the low solubility of B in the primary phase γ-Fe, B is strongly segregated in the melt and the solidification paths are deviated to high B contents. Therefore, as the B content increases, the eutectic solidification sequence starts with the B-rich Fe2B phase and continues with the formation of the B-rich Fe3(B,C) phase in the latter process. The B content of the melt thus decreases during the eutectic reaction, and the eutectic Fe3(B,C) phase exhibits a decreasing B gradient in the direction of growth. Consequently, the low-melting phase of the Fe–C–B system is the Fe3(B,C) phase with a low B content and a composition closest to its low-melting B content of 14.10at.% B. Increasing B/(C+B) ratios of the alloy composition raise the average B content of the Fe3(B,C) phase (up to >20at.% B) and hence at the same time increase the solidus temperature of the alloy. These findings revealed consistency with experimental results for chemical composition (WDX), phase analysis (diffraction with synchrotron radiation, EBSD), and thermal analysis (DTA).

Characterization of magnetron sputtered Cr–B and Cr–B–C thin films for electrical contact applications

We have deposited Cr–B and Cr–B–C thin films by co-sputtering from chromium boride and carbon targets. The binary Cr–B films consist of nanocrystalline and substoichiometric CrB2−x grains (B/Cr atomic ratio≤1.5) with a (101)-texture, where B segregates to the grain boundaries forming a B-rich tissue phase. A hardness of 25GPa is measured for these films. They have a low wear resistance, attributed to a (101)-texture and limited adhesion. As a consequence, wear debris in the CrB2−x wear track from delaminated film and steel-to-steel contact between the exposed substrate and the counter surface result in a high friction (0.52–0.78 against stainless steel) making the Cr–B films unsuitable as sliding electric contacts. Cr–B–C films, on the other hand, form a two phase amorphous structure at >17at.% C consisting of an amorphous Cr-rich phase containing both B and C and an amorphous matrix phase containing mainly B and C. The addition of C improves the adhesion and tribological properties and a coefficient of friction of 0.12 is obtained at 38at.% C. The improved tribological properties are explained by the formation of the matrix phase, which acts as a solid lubricant forming a graphite-like tribofilm during ball-on-disc test. However, the formation of an amorphous structure is not beneficial for the electrical contact resistance, which increases from 0.5Ω for the Cr–B film to 1.5 and 2.3Ω for the Cr–B–C films containing 17 and 26at.% C, respectively. Finally, the importance of a chemical analysis of the chromium boride sputtering target composition is discussed.