Mo Solid Solution Research Articles

Age hardening of Ni–P–Mo electroless deposit

A Ni–P–Mo autocatalytic coating was developed in order to study the structural changes according with the molybdenum codeposition and the effects of aging treatment. Results show that heat treatment induces structural changes that allow for total crystallisation of amorphous regions and the reordering of crystalline phases, to produce a Ni–Mo solid solution with precipitation of Ni3P and MoNi4 particles. Hardness of the as plated condition is similar to high phosphorous Ni–P deposits as mentioned in the literature, however the present work revealed that subsequent heat treatment was beneficial in enhancing the hardness of the metallic coating with a maximum of 1450 HV(100 g) being measured for coatings treated at 500°C for 2 h. This hardening can be interpreted as a consequence of amorphous to crystalline phase transformation and the aging processes of Ni–P–Mo ternary alloy. This enhancement is further reinforced by the precipitation of MoNi4 intermetallic particles.

Crack Growth Behavior in a Two-Phase Mo-Si-B Alloy

AbstractMo-rich Mo-Si-B multiphase alloys are currently being explored for their potential as high-temperature structural materials for components in hot sections in aircraft engines. In this paper, we present crack growth behavior in one such two-phase alloy consisting of a Mo solid solution matrix in which is dispersed approximately 40 volume percent of the Mo5SiB2 (T2) phase. Crack growth under monotonic and cyclic loading is considered over a temperature range spanning 20°C to 1400°C. The effects of loading rate (in monotonic loading) and dwell times at maximum stress (in cyclic loading) at high temperatures on crack growth were examined to understand the contribution from creep. Results confirm a gradual increase in fracture toughness upto 1000°C, beyond which the increase is more substantial with temperature; fatigue susceptibility was also observed in excess of 900°C and crack-tip-stresses-driven microstructural instability is evident at 1400°C. At this temperature, slow loading rates or dwell times at maximum stress lead to crack-tip recrystallization and creep cavitation that together degrade the material's properties.

High Temperature Deformation Behavior of a Mechanically Alloyed Mo Silicide Alloy

AbstractA 3-phase Mo-Si-B alloy consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) was manufactured employing mechanical alloying (MA) as the crucial processing step. After consolidation via cold compaction, sintering in hydrogen atmosphere and final hot isostatic pressing (HIP) at 1500°C, one obtains an ultra-fine microstructure with a nearly continuous Mo(ss) matrix and the sizes of all phases being in the 1 micron range. Tensile tests were carried out in vacuum at initial strain rates ranging from 10-4 to 10-2 s-1 and the temperature varied between n1200 an 1400 °C. With a stress exponent of about 2 and the activation energy being close to that of Mo-self diffusion the material exhibits superplasticity at temperatures as low as 1300°C and tensile strain to failures up to 400%, thus, making sound wrought processing on industrial-scale facilities at temperatures typical for refractory metals and alloys feasible. To enhance creep resistance at high temperatures the alloys were annealed at 1700°C for 10h for a coarsening of the microstructure. While, still, the average sizes of all phases were below 10 microns, a considerable reduction in minimum creep rate was noted. This finding also demonstrates the extraordinary high thermal stability of this 3-phase Mo-silicide alloy.

Metallographic study and wear resistance of a high-C wrought Co-based alloy Stellite 706K

The new Co Cr Mo C type wrought alloy Stellite 706K is an improved alloy over the Co Cr W C type wrought alloy Stellite 6K in wear and corrosion resistance. The microstructure of Stellite 706K is characterized by Co Cr Mo solid solution matrix with banding and occasional twinning and various carbide phases. Pin-on-disc wear resistance of Stellite 706K at room temperature is evaluated. Solution treatment at temperatures from 1235 to 1250 °C on the mill annealed Stellite 706K decreases the bulk hardness and the hardness of Co Cr Mo matrix while increases the wear resistance. The excellent properties of Stellite 706K come from the combination of the high volume fraction of carbide particles and the tough Co Cr Mo matrix due to the alloying addition of Mo.

Oxidation protective silicide coating on Mo-Si-B alloys

A MoSi2 coating was successfully formed on a Mo-9Si-18B alloy, consisting of Mo5SiB2 (T2) and Mo solid solution (Moss) phases, using pack cementation with Si. Isothermal and cyclic oxidation tests of pack-cemented Mo-9Si-18B alloys were performed at 1300 °C and 1500 °C. Steady-state oxidation rates at both temperatures are almost equal to those of pure MoSi2. The MoSi2 layer is completely transformed into Mo5Si3 (T1) containing B after oxidation at 1500 °C for 24 hours. Thermal expansion of the T1 phase is anisotropic, but a [001] texture in the growth direction for the columnar grains in the T1 layer reduces thermal stresses generated around the phases. Evolution of T1 layers during oxidation between 1300 °C and 1500 °C was investigated; their growth rate constants and the interdiffusion coefficient of Mo and Si in the Mo-Si-B system have been evaluated and compared with those in the binary Mo-Si system. Furthermore, we have studied phase transformations in a simpler system MoSi2vs T2 using MoSi2/T2 diffusion couples. Layers of T1 and MoB + T1 were formed in the diffusion zone during oxidation at temperatures between 1400 °C and 1600 °C. This behavior is different from that of the pack-cemented Mo-9Si-18B alloy. Pack-cemented T2 single crystals show a diffusion structure similar to that of MoSi2/T2 diffusion couples, but the ratio of layer thickness is different. Based on these diffusion results, a method for extending the lifetime of the MoSi2 layer is proposed.

Multiphase equilibria in the metal-rich region of the Mo–Ti–Si–B system: thermodynamic prediction and experimental validation

Multiphase composite alloys based on the phases of Mo solid solution (Bcc), Mo 5SiB 2(T2), Mo 5Si 3(T1), Ti 5Si 3 (D88), and Mo 3Si (A15) in the Mo–Ti–Si–B system are candidate materials for ultra-high temperature applications. Determination of the phase relationship in a quaternary system exclusively from experiments is very time-consuming and expensive. A strategy using thermodynamic modeling to aid the selection of key alloy compositions was used in this study. The key alloys selected with the guidance of the calculated phase diagram were prepared by arc-melting and then subjected to homogenization at 1600 °C for 150 h and 1200 °C for 1200 h. The microstructure of these alloys was characterized by means of scanning electron microcopy, electron probe microanalysis, X-ray diffraction and electron backscatter diffraction. The experimental results obtained from this study were then used to validate the thermodynamic modeling. The phase relationship among Bcc, T2, T1, A15 and D88 at 1600 and 1200 °C is clearly defined, which can be used as a road map for alloy design and processing control.

High-temperature compression behavior of Mo–Si–B alloys

The compression behavior of a two-phase (Mo solid solution + T2) Mo–6.1Si–7.9B (at.%) alloy is examined as a function of strain rate in the 1000–1400 °C range and compared to that of commercially available powder-metallurgy-processed TZM (often referred to as MT104). A limited number of tests was also conducted on a three phase alloy with a composition of Mo–8.6Si–8.7B (at.%) that lies in the three phase field, Mo solid solution + T2 + Mo 3Si. These compression studies confirmed that deformation in the temperature-strain rate space evaluated is matrix-controlled, yielding an activation energy of ∼415–445 kJ/mol. As a consequence, the response of the three-phase material overlaps that of the two-phase material. In all instances evaluated, the Mo–Si–B alloys exhibit superior flow stress relative to their TZM counterpart. Examination of the deformed microstructure illustrates that recovery and in some instances, recrystallization occurs in the Mo solid solution matrix whereas the T2 phase either cracks or deforms plastically depending on the temperature and strain rate used. Finite element analysis assuming an elastic–plastic matrix and an elastic second-phase illustrates strain localization in the matrix, the extent being more severe when the work-hardening rate in the matrix decreases (i.e., increasing temperature and decreasing strain rate), while the T2 particles are highly stressed. However, if plastic deformation is permitted in the T2 particles, strain distribution is homogenized substantially and the level of stress build up in the T2 particles diminishes by an order of magnitude. The interplay between matrix and T2 properties and their dependence on temperature and strain rate are used to explain the observed deformed microstructure.

Nucleation of (Mo) Precipitates on Dislocations During Annealing of a Mo-rich Mo5SiB2 Phase

ABSTRACTUpon annealing an as-cast Mo-10Si-20B alloy at high temperatures, a Mo solid solution phase precipitates within a supersaturated Mo5SiB2 phase. The precipitation behavior of the Mo solid solution was investigated by means of transmission electron microscopy and X-ray diffractometry. It is found that the Mo5SiB2 phase in a Mo-10Si-20B alloy contains a significant amount of structural vacancies in the as-cast state. The excess vacancies are removed to form dislocations during annealing, which provides the heterogeneous nucleation sites for the (Mo) precipitates.

Thermal expansion, strength and oxidation resistance of Mo/Mo 5SiB 2 in-situ composites at elevated temperatures

Four Mo/Mo 5SiB 2 in-situ composites with different volume fractions of Mo solid solution, V f Mo, were produced by ingot metallurgy and their microstructure, thermal expansion, compressive strength and oxidation behavior were investigated relating to V f Mo. Mo-8.5Si-13.2B annealed at 2073 K for 24 h has nearly eutectic microstructure consisting of Mo solid solution and Mo 5SiB 2. For higher V f Mo, there are micro-cracks in Mo 5SiB 2 phase probably induced by thermal stress during the cooling process. Mo-8.5Si-13.2B shows the highest yield stress among the four Mo/Mo 5SiB 2 in-situ composites, and about 1 GPa even at 1773 K. For lower V f Mo than that of Mo-8.5Si-13.2B, the steady state oxidation behavior is observed at temperatures between 1173 and 1473 K, indicating that the oxidation resistance is relatively good, whereas the oxidation resistance is catastrophic for higher V f Mo than that of Mo-8.5Si-13.2B.

Theoretical description of the interdiffusion of Al in the U–Mo solid solution

The Bozzolo–Ferrante–Smith (BFS) method for alloys was applied to the analysis of Al interdiffusion in the U–Mo solid solution system as a function of Mo concentration. The binary Al/U and Al/Mo systems show opposite behavior, which in the ternary case Al/U–Mo, translates into the role of regions rich in Mo acting as interdiffusion barriers to Al, in excellent agreement with experimental evidence.

High temperature strength of Mo–Mo 3Si–Mo 5SiB 2 molybdenum silicides

Ternary and quaternary intermetallic alloys consisting of the phases Mo 3Si, Mo 5SiB 2 (“T2”) and α-Mo (Mo solid solution) were fabricated by melting and casting as well as powder-metallurgy (PM). A newly developed PM technique allowed the fabrication of microstructures with a nearly continuous α-Mo matrix. The creep strengths were determined by slow compression testing at an initial strain rate of 10 −5 s −1 at temperatures ranging from 1200 to 1400 °C. Alloying with niobium improved the creep strength much more than alloying with tungsten. This result is rationalized in terms of the atomic size difference between Mo and Nb. Not only the composition but also the topology and scale of the microstructure had a pronounced influence on the creep strength. As expected, coarse microstructures with isolated α-Mo particles resulted in higher creep strengths than microstructures with a continuous α-Mo matrix.

Advances in niobium silicide in situ composites

The compression behavior of a two-phase (Mo solid solution + T2) Mo–6.1Si–7.9B (at.%) alloy is examined as a function of strain rate in the 1000–1400 °C range and compared to that of commercially available powder-metallurgy-processed TZM (often referred to as MT104). A limited number of tests was also conducted on a three phase alloy with a composition of Mo–8.6Si–8.7B (at.%) that lies in the three phase field, Mo solid solution + T2 + Mo3Si. These compression studies confirmed that deformation in the temperature-strain rate space evaluated is matrix-controlled, yielding an activation energy of ∼415–445 kJ/mol. As a consequence, the response of the three-phase material overlaps that of the two-phase material. In all instances evaluated, the Mo–Si–B alloys exhibit superior flow stress relative to their TZM counterpart. Examination of the deformed microstructure illustrates that recovery and in some instances, recrystallization occurs in the Mo solid solution matrix whereas the T2 phase either cracks or deforms plastically depending on the temperature and strain rate used. Finite element analysis assuming an elastic–plastic matrix and an elastic second-phase illustrates strain localization in the matrix, the extent being more severe when the work-hardening rate in the matrix decreases (i.e., increasing temperature and decreasing strain rate), while the T2 particles are highly stressed. However, if plastic deformation is permitted in the T2 particles, strain distribution is homogenized substantially and the level of stress build up in the T2 particles diminishes by an order of magnitude. The interplay between matrix and T2 properties and their dependence on temperature and strain rate are used to explain the observed deformed microstructure.

Structure and hyperfine interactions in mechanosynthesized iron–molybdenum alloys

X-ray diffraction and Mössbauer spectroscopy were used to study the structure and hyperfine interactions in Fe80Mo20 and Fe50Mo50 prepared by mechanical alloying. Two solid solutions, i.e. Fe(Mo) and Mo(Fe) with b.c.c. lattice were formed during milling of Fe80Mo20. Mössbauer spectra revealed different magnetic arrangements in these solid solutions. In the case of Fe50Mo50 no amorphisation was observed, as literature data suggest. During mechanosynthesis of Fe50Mo50 a paramagnetic Mo(Fe) solid solution was probably formed.

Oxidation behavior of Mo 5SiB 2-based alloy at elevated temperatures

A Mo 5SiB 2-based alloy having composition of Mo–12.3 mol% Si–24.9 mol% B was produced by arc-melting in an Ar atmosphere, and its oxidation behavior was investigated at temperature between 973 and 1673 K. At and above 1273 K, transient and steady state oxidation stages were clearly observed. The occurrence of the transient and steady state oxidation is interpreted in terms of rapid volatilization of MoO 3 and B 2O 3 under ambient O 2 pressure at the initial stage and the passive oxidation after completely sealing the substrate by silicate glass. Development of two layers onto the substrate, i.e. SiO 2 glass scale and Mo solid solution interlayer including SiO 2 dispersions, strongly supports the interpretation. Dissolution of B into the SiO 2 scale was not confirmed because of low B concentration that was under a detectable limit of EPMA and TEM-EDS. It is suggested that the SiO 2 glass scale formed on the Mo 5SiB 2-based alloy is more protective than as expected.

A Mo–Si–B intermetallic alloy with a continuous α-Mo matrix

A novel technique for fabricating high-temperature molybdenum–silicon–boron intermetallic composites is described. Vacuum annealing of silicide particles resulted in a loss of Si, and consequently a Mo solid solution layer, at the surfaces. Subsequent consolidation produced a microstructure of silicide particles bonded by a continuous Mo matrix. The α-Mo imparted significant room temperature fracture toughness.

Microstructures and fracture toughness of directionally solidified Mo-ZrC eutectic composites

Microstructures and fracture toughness of arc-melted and directionally solidified Mo–ZrC eutectic composites were investigated in this study. Two kinds of directionally solidified composites were prepared by spot-melting and floating zone-melting. Microstructure of the arc-melted composite (AMC) consists of equiaxed eutectic colonies, in which ZrC particles are dispersed. The spot-melted composite (SMC) exhibits spheroidal colony structure, which is rather inhomogeneous in size and morphology. ZrC fibers in the eutectic colonies are aligned almost parallel to the growth direction. Well aligned, homogeneous columnar structure with thin ZrC fibers evolves in the floating zone-melted composite (FZC). Texture measurement by X-ray diffractometry revealed that the growth direction of Mo solid solution (MoSS) in FZC is preferentially 〈100〉, while that of SMC is scattered. Fracture toughness Kq evaluated by three point bending test using the single edge notched beam method is >13 MPa m1/2 for AMC, 20 MPa m1/2 for SMC and 9 MPa m1/2 for FZC. Intergranular fracture along colony boundaries is often observed in AMC. In contrast, transgranular fracture is dominant in SMC and FZC, although significant gaps caused by intergranular fracture are occasionally observed in SEM micrographs of SMC. Fracture surface in FZC is wholly flat. Pull-out of ZrC occurs owing to Mo/ZrC interfacial debonding in intergranularly fractured regions of AMC and SMC.Coarse elongated colonies in SMC and FZC induce transgranular fracture instead of intergranular fracture. Intergranular fracture and interfacial debonding in AMC and SMC causes frequent crack deflection accompanied by ligament formation and crack branching, which is responsible for the high fracture toughness of the composites. Preferred 〈100〉 growth of MoSS phase in FZC leads to brittle 〈100〉 cleavage fracture associated with low fracture toughness.

Optimization of Mo-Si-B Intermetallics

ABSTRACTMo-Si-B intermetallics consisting of the phases Mo3Si, Mo5SiB2, and α-Mo (Mo solid solution) can be designed to exhibit some degree of oxidation resistance, fracture toughness, and creep strength, but not necessarily all of these at the same time. For example, microstructures that enhance the oxidation resistance are typically associated with low fracture toughness. Examples will be given illustrating the oxidation resistance, fracture toughness, and creep strength of Mo-Si-B intermetallics as a function of their phase volume fractions as well as the topology and length scale of their microstructures. Microstructures containing either individual α-Mo particles or a continuous α-Mo matrix will be described. The examples provide possible ways to control the composition and microstructure of Mo-Si-B alloys such as to optimize the desired balance of properties.

Processing and Characterizing RuAl Eutectics

ABSTRACTEutectic alloys in the Ru-Al-Mo system were directionally solidified by a cold crucible Czochralski technique. The solidification microstructures and composition of the phases were examined and used to estimate a partial liquidus projection. Preliminary results from compression and hardness testing indicate that the hardness of RuAl varies significantly with composition. The Ru-rich alloys in which RuAl is in equilibrium with the hcp (Ru,Mo) solid solution were found to have the lowest hardness values.

Brillouin scattering investigation of elastic properties of Cu–Mo solid solution thin films

Brillouin scattering has been used to investigate the elastic properties of CuxMo1−x thin films with thicknesses varying from 200 up to 430 nm, elaborated by ion beam sputtering. Two compositions around Cu30Mo70 and Cu70Mo30 and pure Cu and Mo films have been studied. The samples are essentially randomly oriented polycrystals, leading to an isotropic effective symmetry. By fitting all the Brillouin spectral lines, two independent effective elastic constants of these films have been determined, allowing the calculation of their elastic moduli. Results indicate that pure copper thin films’ elastic properties are similar to the bulk copper ones whereas pure molybdenum thin films' Young's modulus is weaker (about 10%) than the bulk Mo one. Concerning Cu–Mo solid solutions, elastic constants values lie between pure copper and molybdenum ones. Finally, the effect of annealing at 760 K on the mechanical properties of the solid solutions in relation with the microstructural evolution is discussed. The annealed solid solutions show a hardening of the elastic moduli with respect to the as-deposited samples, which is supposed to reflect their full demixing.

Mechanical Properties of Mo–Nb–TiC <I>In-situ</I> Composites Synthesized by Hot-Pressing

Mo-40 mol%TiC and Mo-20 mol%Nb-40 mol%TiC in-situ composites were synthesized by hot-pressing mixed Mo, Nb and TiC powders. Both composites consist of two phases, Mo solid solution (A2) and TiC (B1), after hot-pressing at 2073 K and 70 MPa for 2 h followed by annealing at 2073 K for 24 h. Porosity of hot-pressed compacts decreased by Nb addition to Mo-40 mol%TiC. The composites have different microstructural features. Clusters consisting of fine bcc particles are observed in sizes similar to original Nb powder in Mo-20 mol%Nb-40 mol%TiC, while there are no clusters in Mo-40 mol%TiC. These composites show excellent strength superior to monolithic TiC at high temperatures. The addition of Nb to Mo-40 mol%TiC suppresses effectively intergranular fracture above 1473 K. Furthermore, fracture toughness of these composites is higher than that of monolithic TiC. Fracture toughness of Mo-20 mol%Nb-40 mol%TiC is slightly lower than that of Mo-40 mol%TiC. The obtained results are discussed in relation to microstructural characteristics.