The effect of Nb addition on microstructure and mechanical properties of Ni3(Si,Ti) alloy

The alloying behavior, the microstructure and the high-temperature tensile properties of the Nb-added Ni₃(Si,Ti) alloys were investigated. The solubility limit of Nb element in the L1₂ Ni₃(Si,Ti) phase at 1273 K was shown to be approximately 2.7 at%, and thermodynamically discussed. The second-phases (dispersions) formed beyond the solubility limits were identified as D0_a-type Ni₃Nb and Ni₁₂Si₄TiNb₂ compounds, and contained some lattice defects with incoherency with the L1₂ Ni₃(Si,Ti) matrix. The Nb-containing second-phase dispersions in the L1₂ Ni₃(Si,Ti) phase matrix resulted in strengthening over a wide range of temperature, and also an improvement of the high-temperature tensile elongation.     

Unconventional synthesis of Mg x C y Ni3: Synergic combination of mechanical alloying, SHS and isothermal heating

Several methods have been tested in order to prepare the perovskite type compound Mg _x C _y Ni₃ applying the mechanical alloying (MA), the self-propagating high-temperature synthesis (SHS) and the isothermal heating techniques in the different steps of preparation. These methods may be summarized as follows: method MCN-1) synthesis of the Mg _x C _y Ni₃ phase through MA of Mg₂Ni (previously synthesised by isothermal heating) and selected amounts of graphite and Ni, followed by isothermal treatment; method MCN-2) synthesis of the Mg _x C _y Ni₃ phase applying the SHS technique using powder compacts; method MCN-3) synthesis of an eutectic sample composed of Ni and MgNi₂ by means of isothermal heating, subsequent MA with graphite and final synthesis of Mg _x C _y Ni₃ by means of SHS. The methods MCN-2 and MCN-3 proved their validity to the synthesis of the desired compound with two main important results: complete conversion of the reactants into Mg _x C _y Ni₃ and control of the stoichiometry in the final product. For instance method MCN-1 shows instead a very low degree of conversion of the reactants. All the phases obtained after each preparation step (MA, SHS, isothermal heating) have been characterized by means of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) coupled with electron dispersive spectroscopy (EDS).     

TiC/Ni3Al Composites Manufactured by Self-Propagating High-Temperature Synthesis and Hot Isostatic Pressing

TiC–20 wt% Ni₃Al and TiC–40 wt% Ni₃Al composite materials were produced by self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP). In the SHS method the reacted powders were compacted by uniaxial pressing immediately after the reaction. The microstructure of the materials produced by SHS consisted of spherical carbides embedded in the Ni₃Al matrix, whereas the microstructure of the materials produced by HIPing was more irregular. A maximum hardness of 2010 HV₁ was measured for the material produced by HIP and a maximum fracture toughness of 10.5 MPa m^1/2 was measured for materials produced by SHS. High-temperature resistance was investigated by exposing the materials to 800°C in air for 110 h. The results obtained showed that the TiC + Ni₃Al composite materials can be recommended for use in environments consisting of oxidizing atmosphere at temperatures around 800°C where high wear resistance is required.     

Effect of mechanical alloying beyond the completion of glass formation for Ni-Zr alloy powders

Elemental powders of nickel and zirconium were mechanically alloyed over a wide concentration range 10 to 90 at % Zr. The amorphous single phase was formed over the range 20 to 80 at % Zr. The effect of the excessive mechanical alloying on the glass formation was studied by continuing ball-milling beyond the completion of the glass formation for the powders with the average compositions Ni₃₀Zr₇₀, Ni₅₀Zr₅₀ and Ni₇₀Zr₃₀. A partial crystallization took place in all three cases and its initiation was the fastest in Ni₃₀Zr₇₀ and was delayed with decreasing zirconium content. The critical factor for triggering the crystallization was attributed to the oxygen contamination for the zirconium-rich Ni₃₀Zr₇₀ powders and to the reduction in glass-forming ability for the nickel-rich Ni₇₀Zr₃₀ powders. The latter conclusion is drawn from the facts that the impurity concentrations arising from the debris of the stainless steel balls and the vial are gradually accumulated with increasing milling time and that the effective zirconium concentration is reduced below the critical concentration of approximately 20 at % as a result of alloying with the elements iron, chromium and nickel in the stainless steel.     

Mathematical and experimental investigation of the self-propagating high-temperature synthesis (SHS) of TiAl3 and Ni3Al intermetallic compounds

One-dimensional mathematical modeling was used to describe the self-propagating high-temperature synthesis (SHS) process for preparing TiAl₃ and Ni₃Al intermetallics. The kinetic parameters (activation energies and pre-exponential factors) for the two compounds were obtained by matching experimental measurement and the numerical solution. The results thus obtained were compared with rate parameters obtained using different methods. The activation energy was 483 and 283 kJ mol^?1 for the formation of TiAl₃ and Ni₃Al, respectively. The temperature profiles calculated using the mathematical model were compared with experimental measurements for both aluminides which indicated reasonable agreement. Fine particle size and moderate preheating increase the SHS rates.     

Synthesis and formation mechanisms of molybdenum silicides by mechanical alloying

The synthesis and formation of MoSi₂, Mo₅Si₃, and Mo₃Si compounds by the mechanical alloying of MoSi powder mixtures has been investigated. Ball-milling experiments were conducted for the composition range of 10–80 at.% Si. The formation of molybdenum silicides, especially MoSi₂, during mechanical alloying and the relevant reaction rates markedly depended on the powder composition. The spontaneous formation of MoSi₂ during mechanical alloying at 67 at.% Si (MoSi₂ stoichiometry) proceeded by a mechanically-induced self-propagating reaction (MSR), the mechanism of which is analogous to that of the self-propagating high-temperature synthesis (SHS). At the compositions of 54 and 80 at.% Si, however, the formation of MoSi₂ proceeded by the gradual formation of both the and /gb phases instead of the MSR mode. The formation of Mo₅Si₃ during mechanical alloying was characterized by a slow reaction rate as the reactants and product coexisted over a long period. The milling of Mo-rich powder mixtures up to 150 h did not lead to the direct formation of Mo₃Si. The Mo₃Si phase appeared only after brief annealing at temperatures of 800°C and above.     

Microstructure and tensile properties of Ni3 (Si,Ti) alloys containing second phase dispersions

Abstract

The alloying behaviour, microstructure, and high temperature mechanical properties of quaternary polycrystalline Ni₃ (Si,Ti), which was alloyed with transition elements V, Nb, Zr, and Hf beyond their maximum solubility limits, were investigated. The solubility limits of the quaternary elements in the L1₂ Ni₃ (Si,Ti) phase were determined to be ranked in the sequence of Nb > V > Hf > Zr, and correlated with the size misfit parameter between Si and the quaternary element X, and with the difference in formation enthalpy between Ni₃ Si and Ni₃ X. The second phases (dispersions) formed beyond the solubility limit were identified as a face centred cubic type Ni solid solution for the V containing Ni₃ (Si,Ti) alloy and Ni₃ X type compounds of the Nb, Zr, and Hf containing Ni₃ (Si,Ti) alloys. The second phase dispersions in the L1₂ phase matrix resulted in strengthening over a wide range of temperatures. The high temperature tensile elongation was improved by the introduction of the second phase dispersions. Among the quaternary Ni₃ (Si,Ti) alloys observed in the present study, the Nb containing Ni₃ (Si,Ti) alloy with the Nb containing second phase dispersion was shown to have the most favourable mechanical properties.     

Formation of amorphous Ni-Zr alloy powder by mechanical alloying of intermetallic powder mixtures and mixtures of nickel or zirconium with intermetallics

Mechanical alloying was used to synthesize Ni_xZr_1–x alloys from mixtures of intermetallic compound powders, and also from mixtures of intermetallic compound powders and pure elemental powders. The mechanically alloyed powders were amorphous in the range 0.24 x 0.85. This range is larger than amorphous alloys produced by the melt-spinning technique and mechanical alloying of elemental crystalline powders. Two-phase mixtures of the amorphous phase and the corresponding crystalline terminal solid solution were formed in the range 0.10 x 0.22, and x=0.90. It is found that the morphological development during mechanical alloying of these powders is different from mechanical alloying using only pure ductile crystalline elemental powders. The thermal stability has been investigated. The enthalpy and activation energy of crystallization for Ni-Zr amorphous powders prepared by mechanical alloying are lower than those for melt-spun samples of the same composition. The crystallization temperature of the mechanically alloyed Ni-Zr amorphous powders is higher than that of meltspun samples in the composition range Ni₂₀Zr₈₀ to Ni₃₃Zr₆₇ and Ni₄₀Zr₆₀ to Ni₆₀Zr₄₀. The presence of tiny crystallites as nucleation centres and high oxygen levels in the mechanically alloyed amorphous alloys might be responsible for the differences in crystallization behaviour. A new crystalline metastable phase was observed during crystallization studies of Ni₂₄Zr₇₆ amorphous powder.     

The sequence of phase formation during mechanical alloying of chromium and silicon powders

The sequence of phase formation during mechanical alloying of chromium and silicon powders has been studied using high-energy ball milling of mixtures of elemental powders with different Si/Cr atomic ratios. X-ray diffactometry and transmission electron microscopy have been utilized to identify the phases and to characterize the microstructure of the powders. All four equilibrium phases in the Cr-Si system can form. With a Si/Cr atomic ratio equal to or higher than 3/5, CrSi₂ is always the first phase to form, and then CrSi₂ can react with chromium to form CrSi or Cr₅Si₃, depending on the Si/Cr atomic ratio. This is similar to the sequence of phase formation during annealing of multilayer chromium and silicon thin films. However, with low Si/Cr atomic ratio close to 1/3, Cr₃Si is the first and only phase to form during mechanical alloying.     

Mechanochemical Synthesis and SHS of Diborides of Titanium and Zirconium

Some properties of titanium diboride (TiB₂) obtained by explosive mechanochemical synthesis and self-propagated high-temperature synthesis (SHS) have been investigated. The properties of zirconium diboride (ZrB₂) obtained by SHS have also been studied. There is a general opinion that explosive mechanochemical synthesis proceeds by a SHS mechanism. For that reason, it is of interest to compare the properties of a product synthesized from the same reagents, by both mechanochemical synthesis and SHS. In order to elucidate the peculiarities of mechanochemical synthesis, the changes in shape and size of the titanium particles occurring during their mechanical treatment up to the moment of synthesis have been examined. Titanium and zirconium powders with particles differing drastically in shape and size have been used for the synthesis of TiB₂ and ZrB₂ by SHS. It has been shown that irrespective of the difference in properties of the reagents, the products obtained have some common properties characteristic of the synthesis method and important with respect to the practical applications of the borides of titanium and zirconium.     

Effect of high dilution on the in situ synthesis of Ni–Zr/Zr–Si(B,C) reinforced composite coating on zirconium alloy substrate by laser cladding

High dilution of transition metals was employed as a new idea for in situ synthesis of Ni–Zr/Zr–Si(B, C) reinforced composite coatings by high power diode laser (HPDL) cladding Ni–Cr–B–Si powders on zirconium substrate. Microstructure, phase composition, the mechanism of in situ synthesis reinforcement and the microhardness of coatings were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and micro-sclerometer. The results reveal that the morphologies and phase constituents are related to the content of alloying elements in powders. In low alloy coatings, the matrix was mainly composed of intermetallic compounds including NiZr and Ni₁₀Zr₇, while the reinforcements consisted of Zr₅Si₄, β-ZiSi, α-ZrSi and ZrC. At the top of high alloy coatings, the matrix was partially comprised of Zr-based amorphous phase with the reinforcements containing ZrB₂. It is thermodynamically favorable for ZrB₂ ceramic reinforcement to form compared to ZrC phase. The microstructure evolution was dependent on the contribution of the high dilution zirconium alloy substrate to the in situ reinforcement synthesis. The microhardness of the coating showed clear improvement compared with zirconium alloy substrate, although high variability was also found.     

Formaton of nanocrystalline Mg2Si and Mg2Si dispersion strengthened Mg-Al alloy by mechanical alloying

Formation of Mg₂Si via mechanical alloying of elemental Mg and Si powders has been investigated. The formation of Mg₂Si occurs after 10 hours of mechanical alloying. Nanocrystalline structure of Mg₂Si with grain size of 22 nm obtained after 50 hours of milling was found to be stable upon heating to about 390 °C. Sudden increase in crystalline size to 157 nm after annealing at 520 °C was observed. Although the reaction between Mg and Si could be completed after about 50 hours of mechanical alloying, thermal assisted reaction starting at as low as 190 °C could promote the formation of Mg₂Si at a short milling duration and hence reduce Fe contamination. Mg-Al alloy reinforced by Mg₂Si was prepared by milling Mg, Si and Al powders. Intermediate phase of Al₁₂Mg₁₇ has been detected after 5 hours of mechanical alloying. This intermediate phase was observed to disappear to form equilibrium solid solution of Mg-Al alloy after annealing at 300 °C.     

Nanostructured TiC-TiB2 composites obtained by adding carbon nanotubes into the self-propagating high-temperature synthesis process

The synthesis of nanostructured TiC-TiB₂ by self-propagating high-temperature synthesis (SHS) has been investigated by using carbon nanotubes as precursor materials in partial substitution of graphite according to the following reaction: 6Ti + B₄C + (3−x)C + x CNT → 4TiC + 2TiB₂.Different amounts of CNTs addition have been studied in order to achieve structural refinement of the SHS products. The CNT molar content was varied in order to define the optimal composition, which allows to obtain nanostructured TiC-TiB₂ powders morphologically homogenous.The optimized composition has been chosen for the further densification step. The Pressure Assisted Fast Electric Sintering (PAFES) technique gave bulk composites with ultrafine grained microstructure. The mechanical characterization showed very high hardness and good fracture toughness values if compared to literature data.     

Reaction mechanism in SiO2-MnO2-Fe-Al system for producing ferrosilicomanganese powder

In this research, the ferrosilicomanganese powder was synthesized via mechanical alloying and SHS methods. Silicon oxide, manganese oxide, iron and aluminum powders were used as starting materials. SHS process was initiated by oxyacetylene flame. The activated and inactivated powder mixtures were used for producing ferrosilicomanganese grade 26%Si-53%Mn-21%Fe. The powder mixtures were characterized by X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. Thermodynamic analysis showed that aluminothermic reduction of MnO₂ is more thermodynamically favored as compared with aluminothermic reduction of SiO₂. Adiabatic temperature of MnO₂ and SiO₂ reduction by Al was calculated about 2946 K. It was found that no reaction took place during mechanical alloying. After annealing and SHS processes, first MnO₂, then SiO₂ oxides were reduced by Al and ferrosilicomanganese and Al₂O₃ phases were produced. Due to the high adiabatic temperature, all products were formed in liquid state, leading to produce sintered ferrosilicomanganese and isolated Al₂O₃ particles.     

Evolution of manufacturing parameters in Al/Ni3Al composite powder formation using blending and mechanical milling processes

Aluminu–matrix composites produced by Ni₃Al intermetallic particles are increasingly used in aerospace and structural applications because of their outstanding properties. In manufacturing of metal–matrix composites using powder metallurgy blending and milling are important factors. They control the final distribution of reinforcement particles and porosity in green compacts which in turn, strongly affect the mechanical properties of the produced PM materials. This paper studies different conditions for producing composite powders with uniform dispersion of Ni₃Al particles in aluminum powders and improved physical and mechanical properties. The results indicated that an intermediate milling time for fabrication of composite powder, better than prolonged and shortened ones, causes better microstructure and properties. It was shown that addition of 5 vol.% Ni₃Al particles, produced by 15 h mechanical alloying to aluminum powders, and then 12 h blending operation provides an appropriate condition for producing Al–Ni₃Al composite powder.     

Reaction-sintered hot-pressed TiAl

Titanium aluminide intermetallic alloys and composites were formed from elemental titanium and aluminium powders by self propagating, high-temperature synthesis in an induction-heated hot-press. The crystal phases, density, transverse rupture stress, and hardness of the reaction-sintered compacts, were observed to be controlled by hot-pressing conditions. The principal phase formed was TiAl together with a significant second-phase concentration of Ti₃AI. The transverse rupture strength (TRS) of the intermetallic composites was observed to vary directly with compact density. Under selected high-temperature synthesis hot-pressing conditions, TRS values were comparable to those obtained for fully dense TiAl. Titanium aluminide composites were formed by adding boron, carbon, silicon and Al₂O₃, and SiC powders and whiskers to the Ti-Al powders before reaction sintering. Changing the alloying additions did not have as strong an effect on properties of the composite compacts as did varying hot-pressing conditions.     

Alloy design for Al addition on microstructure and mechanical properties of Ni3(Si,Ti) alloy

The effect of Al addition on the microstructure and tensile properties of Ni₃(Si,Ti) alloys with an L1₂ ordered structure, which were fabricated through thermomechanical processing from arc-melted ingots, was investigated. Al was added to a Ni₃(Si,Ti) alloy by using two methods such that Al substituted for (1) only Ti and (2) both Ni and Ti along a Ni₃(Si,Ti)-Ni₃Al pseudo-binary line. In the case of the alloys prepared by the former method, the addition of more than 4 at.% Al resulted in a two-phase microstructure consisting of disordered fcc Ni solid solution dispersions in the L1₂ matrix, while in the case of the alloys prepared by the latter method, the addition of 4 at.% Al retained the L1₂ single-phase microstructure. In the case of the 4 at.% Al-added alloys, the room-temperature tensile properties were similar and independent of the alloying methods, whereas the high-temperature yield stress was higher in the alloys prepared by the latter method than in the case of the alloys prepared by the former method. These results suggest that a single-phase microstructure consisting of an entire L1₂ structure is favorable for obtaining high-temperature tensile properties.     

Effect of initial composition on phase selection in Ni–Si powder blends processed by mechanical alloying

The effect of initial powder blend composition on the synthesis and formation mechanism of nickel silicide phases was investigated by mechanical alloying in Ni-60 and Ni-66.7?at.% Si powder blends. It was noted that the equilibrium NiSi phase started to form in the early stages of milling and that the amount of the NiSi phase in the milled powder increased with increasing milling time. Even though, under equilibrium conditions, a mixture of both the NiSi and NiSi₂ phases was expected to be present in the Ni-60?at.% Si composition and the stoichiometric NiSi₂ phase in the Ni-66.7?at.% Si composition, the NiSi phase was present in both the compositions investigated. However, while only the NiSi phase was present homogeneously in the Ni-60?at.% Si powder blend, both the NiSi phase and a very small amount of unreacted Si were present in the powder blend of Ni-66.7?at.% Si composition. This unexpected phase constitution in the milled powders was attributed to a partial loss of Si during mechanical alloying of the powder blends, confirmed by energy dispersive X-ray spectrometer analyses, and explained on a thermodynamic basis.     

Preparation of ultrafine and nanosized MoSi2 particles by self-propagating high-temperature synthesis with a reduction step

We have developed technological principles of the preparation of ultrafine and nanosized MoSi₂ particles by self-propagating high-temperature synthesis (SHS) with a reduction step. The effect of synthesis conditions (starting-mixture composition, relative amounts of reactants, and the presence and amount of an inert diluent) on the composition, structure, and particle size of the powders has been studied. The results demonstrate that inert additives reduce the adiabatic temperature. The crystallite size of MoSi₂ decreases with increasing additive concentration. The MoSi₂ powders obtained by SHS with a reduction step have the form of agglomerates consisting of spherical particles ranging widely in size: from large (several microns) to ultrafine and nanosized. The composition of the powders was checked by chemical analysis, microstructural examination, and X-ray diffraction.     

Influence of benzene on the Ni3Fe nanocrystalline compound formation by wet mechanical alloying: An investigation combining DSC,X-ray diffraction,mass and IR spectrometries

Nanocrystalline Ni₃Fe powders were obtained via wet mechanical alloying using benzene as surfactant. The differential scanning calorimetry (DSC) measurements showed the presence of an exothermic peak which does not correspond to any phase transformation or phase formation as was proved by X-ray diffraction measurements. The exothermic peak was observed neither for the dry milled samples nor for the wet milled and subsequently annealed powders at 350 °C for 4 h. The infra-red (IR) spectra registered for the wet milled samples showed a series of vibration bands corresponding to C₆H₆ and also to a series of fragments resulting from benzene decomposition. The results obtained by IR investigation were confirmed by thermogravimetry and mass spectrometry (TG + MS) investigations. The main fragments resulting from the benzene decomposition on the surface of the nanocrystalline Ni₃Fe powders are: CO₂, CO and C. The evolution of the particle size distribution versus the milling time has been determined for the wet mechanical milling process of nanocrystalline Ni₃Fe powders. The DSC analysis reveals a displacement of the exothermic peak onset towards lower temperatures and an increase of the surface of this peak attributed to the changes in the particles specific surface and to the quantity of benzene added in the milling experiments.