M5B3 Borides Research Articles

Cr-rich nanosize precipitates in a standard heat-treated Inconel 738 superalloy

An analytical transmission electron microscopy study was performed on the nature of nanosize secondary phase precipitates that form in standard heat-treated commercial nickel base superalloy IN 738. In addition to M5B3 borides and M23C6 carbides, the precipitates were found to also consist of particles of M2B-type boride phase, which is previously unreported in the alloy and rarely reported in nickel base superalloys. The M2B borides exhibited a dual crystallographic structure: body centered tetragonal and face centered orthorhombic. Analyses of electron diffraction patterns and high-resolution transmission electron microscopy images indicated that both crystal structures can co-exist in a single M2B particle as an intergrowth with the formation of planar defects. Crystallographic relationship between microconstituent phases in the heat-treated alloy were elucidated and the influence of the borides on resistance to grain boundary liquation cracking during high-temperature material processing, such as laser beam treatment, is discussed.

Improvement in Laser Weldability of INCONEL 738 Superalloy through Microstructural Modification

Microstructural study of laser-beam-welded IN 738 superalloy was carefully performed to better understand the causes of heat-affected zone (HAZ) cracking and to determine an improved approach of alleviating the weldability problem. The HAZ cracks in the alloy were intergranular liquation cracks that resulted from the liquation reaction of both secondary solidification products (MC carbides and γ-γ′ eutectic) and solid-state reaction products (γ′ particles) present in the preweld material. In contrast to the expectation based on Chadwick’s equation, a reduction of grain boundary liquid film thickness did not produce a decrease in HAZ cracking owing to increased base alloy hardness that accompanied a preweld heat treatment designed to reduce the intergranular liquation. Moreover, a major factor limiting the effectiveness of an existing preweld heat treatment with low base alloy hardness in reducing HAZ cracking was found to be the formation of intergranular M5B3 boride particles during the heat treatment. These borides can widen the HAZ brittle temperature range (BTR) during weld cooling and increase the propensity for cracking. Based on the results, a new preweld heat treatment that induces a moderate hardness and precludes grain boundary boride formation was found and was shown to produce a significant reduction in HAZ cracking in the welded alloy compared to the most effective pre-existing preweld heat treatment.

Microstructural Response of Directionally Solidified René 80 Superalloy to Gas-Tungsten Arc Welding

The microstructural response of directionally solidified Rene 80 (DS Rene 80) superalloy to gas-tungsten-arc (GTA) welding was investigated. Rapid heating during welding resulted in a significant grain-boundary liquation of solid-state reaction product γ′ precipitates, intergranular elemental segregation induced M5B3 borides, and secondary solidification constituents MC carbides and sulfocarbides, which were all present in the preweld heat-treated alloy. Liquation of these particles embrittled the grain boundaries in the heat-affected zone (HAZ) and caused microfissuring along the liquated grain boundaries. Nevertheless, contrary to the generally observed increase in HAZ cracking in superalloys with an increase in Ti and Al concentration, due to increase in the alloy’s hardness, significantly reduced cracking was observed in DS Rene 80 compared to the conventionally cast IN738 welded under the same conditions, despite its hardness being higher than that of IN738. This was related to the nature of base-metal grain- boundary intersections at the fusion-zone boundary in these materials.

Microstructural Analysis of Laser-Beam-Welded Directionally Solidified INCONEL 738

The laser welding of a directionally solidified INCONEL 738 superalloy (DS IN738) was investigated. Microstructural analysis of the fusion zone (FZ) and the heat-affected zone (HAZ) revealed that cracking occurred mainly in HAZ with only some cracks being extended into the FZ. However, the frequently observed centerline cracking in FZ of DS and single crystal (SX) alloys did not occur, which was attributed to the presence of negligible volume fraction of γ-γ′ eutectic in FZ. Constitutional liquation of secondary solidification constituents (MC carbides, M3B2 borides, M2SC sulphocarbides, and γ-γ′ eutectic) and γ′ precipitate particles was found to be the major cause of grain boundary liquation and the resultant intergranular microfissuring in the HAZ. The extent of HAZ microfissuring was, however, observed to be smaller in samples welded along the transverse direction (perpendicular to solidification direction) than when welding was done along the longitudinal direction (solidification direction). Nonetheless, more severe HAZ cracking occurred in samples of similarly welded conventionally cast (CC) alloy, and the present results indicate that the severity of HAZ liquation cracking in IN738 superalloy can be reduced by using a DS version of the alloy.

Eta Phase and Boride Formation in Directionally Solidified Ni-Base Superalloy IN792 + Hf

A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN792 + Hf. Both eta and Cr-rich phases were found to be the final solidification products developed from the remaining liquid after γ/γ′ eutectic reaction. The (Ti + Ta + Hf)/Al ratio in the residual liquid played a significant role in the nucleation of eta phase. During the solidification of γ/γ′ eutectic, the continual increase of (Ti + Ta + Hf)/Al ratio in the residual liquid eventually led to the completion of γ/γ′ eutectic reaction and caused the nucleation of eta phase. The results of electron probe microanalysis and transmission electron microscopy revealed that the Cr-rich phase was Cr, Mo, and W containing M5B3 and M3B2 type borides. The formation of these boride phases was found to be strongly influenced by the formation of γ/γ′ eutectic. Because of the limited solubility of Cr, Mo, and W in γ′ phase, these elements were enriched in the residual liquid during the solidification of γ/γ′ eutectic. In addition, boron would preferentially segregate into liquid due to its very limited solubility in both γ and γ′ phases so that the possibility of boride formation in the residual liquid ahead of the γ/γ′ eutectic was increased. A modified Scheil model was adopted to explain the influence of solidification rate on the formation of eta phase and borides, and the results were discussed.

Effects of Mo/B Atomic Ratio on the Mechanical Properties and Structure of Mo2NiB2 Boride Base Cermets with Cr and V Additions

The effects of Mo/B atomic ratio ranging from 0.8 to 1.5 (Mo contents) on mechanical properties and phase formation were studied for cermets with the composition of Ni–5.0B–xMo– 3.5Cr–11.5V (wt%). A strong correlation between mechanical properties and phase formation was observed in the cermets. The cermets with somewhat higher stoichiometric Mo content, i.e., Mo/B ratios of 1.1 and 1.2, showed a very fine ideal two-phase microstructure with a homogeneous distribution of a tetragonal Mo2NiB2 (M3B2)-type complex boride in a solution-strengthened Ni base binder and hence exhibited an excellent transverse rupture strength (TRS) of more than 2.7 GPa and hardness of about 88 RA. In the cermets with low and high Mo/B ratios, another type of a complex boride, which was an orthorhombic M3B2 or a tetragonal M5B3 boride, was detected and was considered to deteriorate TRS due to a coarse-grained microstructure.

Structure and Phases in Nickel-Base Self-Fluxing Alloy Coating Containing High Chromium and Boron

The structure of a nickel-base, self-fluxing alloy coating, containing chromium and boron thermal sprayed and fused, was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). A lumpy M6C carbide, a rodlike M3B2 boride of tetragonal structure, a rodlike M7C3 carbide of hexagonal structure, and a Ni-Ni3B eutectic phase formed in the coating after fusing. Metals of M6C, M3B2, and M7C3 phases are composed of chromium, molybdenum, and nickel; chromium and molybdenum; and mainly chromium, respectively. The nickel phase in the coating has the L12 type superlattice structure.

ニッケル基自溶合金溶射皮膜の組織と生成相

The structure and constituents in two types of nickel-base self fluxing alloy coatings thermal sprayed and subsequently fused were precisely investigated by use of X-ray diffraction analysis, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy. JIS-MSFNi1 alloy coating was composed of primary nickel solid solution and Ni3B phase between primary nickel grains with a small amount of Ni-Ni3B eutectic. In case of JIS-MSFNi4 alloy coating, lumpy crystals and two kinds of rod-like crystals appeared in addition to Ni-Ni3B eutectic. It was found that the lumpy crystal is M6C carbide containing chromium, molybdenum and nickel. The rod-like crystals are M3B2 boride of tetragonal structure and M7C3 carbide of hexagonal structure. Metal constitution of M3B2 are chromium and molybdenum and M7C3 contains mainly chromium. The nickel phase in the Ni-Ni3B eutectic of MSFNi4 alloy coating has the superlattice structure.

Formation of carbide and boride phases in KhN65VMTYu alloy during heat treatment

1. Homogenization at 1200°C provides practically complete solution of carbides and borides. At 1160°C MC carbides and M5B3 borides are preserved and at 1100°C additional formation of M6C carbide is observed. 2. During subsequent aging at 1000°C M6C and M3B2 phases are precipitated and at 800–900°C M2 3C6 phase. MC carbide is stable only in the first steps of heat treatment. 3. The reaction M5B3→M3B2, the intensity of occurrence of which is determined by the conditions of heat treatment, established in KhN65VMTYu alloy.

Ｆｅ２Ｂほう化物系硬質合金の焼結機構と機械的性質

By utilizing the excellent properties of a boride, a new type of sintered hard alloy has been developed. This alloy, categorized as a cermet, is produced by liquid phase sintering and comprises a M2B (M: metal) boride as a hard phase and a ferrous binder phase containing Cr, Ni and other alloying elements.In the present investigation, the sintering phenomena of Fe-(4-8) wt%B-(0-30) wt%Cr-0.3 wt%C alloys are studied by means of TG-DTA, dilatometric measurements and x-ray diffractometry. In addition, the mechanical properties of the sintered alloys are discussed.The results can be summarized as follows:(1) The hard alloy comprises an Fe2B type boride as a hard phase and a ferrous binder phase containing Cr and C. When the hard alloy contains more than 10 wt% Cr, both Fe2B and Cr2B type borides coexist.(2) The liquid phase sintering of the alloy is attained by two liquid forming reactions, namely, γ-Fe +Fe3 (B, C)+M23(B, C)6 L1 and γ-Fe+Fe2B L2, the latter reaction occurring in a higher temperature range.

Effects of B, C, and Zr on the structure and properties of a P/M nickel base superalloy

The boron and carbon levels of a P/M nickel base superalloy were systematically varied in order to determine the mechanisms by which these elements strengthen the alloy, and their optimum concentration. Carbon levels were reduced to 20 ppm while the boron level was varied from 0.02 to 0.10 wt pct. Carbon levels of 0.002 and 0.05 wt pct were also studied, while maintaining a boron concentration of 0.02 wt pct. Zirconium levels were maintained at 0.06 wt pct. The resulting alloys were subjected to identical heat treatments and examinedvia SEM, TEM, and STEM microscopy. The alloys were also subjected to tensile, creep, stress-rupture, and fatigue crack growth tests. Results show that both carbon and boron have a strong influence on the formation of grain boundary precipitates, as expected. Carbon was present as the MC and M23C6 type carbides, while boron combined to form an intergranular M3B2 boride. Boron and zirconium were observed to be critical to the alloys' mechanical properties, although boron levels above the solubility limit resulted in no further improvement or debit in strength. Carbon additions resulted in no improvement in properties, indicating the feasibility of a carbon-free P/M superalloy. The role of the minor element additions is discussed in terms of both microstructural features and related strengthening mechanisms.

Boride particles in a powder metallurgy superalloy

Using optical and electron metallography, the composition, morphology, and distribution of M3B2 borides in as–hipped (hot isostatically pressed) samples of the powder metallurgy superalloy Nimonic AP1 have been determined. Two types of boride are present depending on the HIP temperature. Hipping below the boride solvus results in low–aspect ratio particles, distributed both inter– and intragranularly. Hipping above the boride solvus produces high–aspect ratio particles which are exclusively intergranular. A small difference in both lattice parameter and composition has been measured. Electron energy loss spectroscopy of the particles has confirmed the presence of boron, and laser ion–induced mass analysis has indicated a low carbon level. The higher susceptibility to edge cracking during forging of material hipped above the boride solvus is related to the boride morphology. Studies of the subsequent recrystallization of the forged samples have indicated that necklace formation is neither inhibited nor accelerated by the presence of grain boundary borides. Intragranular boride particles have a similar influence to cuboidal γ′ in controlling the subgrain size within the warm worked grains.MST/210

Boride particles in a powder metallurgy superalloy

AbstractUsing optical and electron metallography, the composition, morphology, and distribution of M3B2 borides in as–hipped (hot isostatically pressed) samples of the powder metallurgy superalloy Nimonic AP1 have been determined. Two types of boride are present depending on the HIP temperature. Hipping below the boride solvus results in low–aspect ratio particles, distributed both inter– and intragranularly. Hipping above the boride solvus produces high–aspect ratio particles which are exclusively intergranular. A small difference in both lattice parameter and composition has been measured. Electron energy loss spectroscopy of the particles has confirmed the presence of boron, and laser ion–induced mass analysis has indicated a low carbon level. The higher susceptibility to edge cracking during forging of material hipped above the boride solvus is related to the boride morphology. Studies of the subsequent recrystallization of the forged samples have indicated that necklace formation is neither inhibited...

Boride phases in nickel-base alloys

1. The characteristic boride phases in multi-alloyed nickel-base alloys are M3B2 and M5B3. 2. The type of boride phase is determined by the chromium, molybdenum, and tungsten content in the alloy. 3. In alloys in which borides of two types are observed a type M5B3→M3B2 boride reaction is possible during heat treatment and subsequent aging.