Reaction mechanism in high Nb containing TiAl alloy by elemental powder metallurgy

1 Introduction High Nb containing TiAl alloys have attracted much attention owing to their low densities and potential applications at high-temperature environments[1,2]. It has been found that Nb is the essential and effective element improving their me…     

Reaction mechanism in high Nb containing TiA1 alloy by elemental powder metallurgy

High Nb containing TiAl alloy was fabricated in argon atmosphere by reactive hot pressing process. Reaction mechanism was investigated by means ofmicrostructural analyses and thermodynamic calculations. The results show that it is feasible to prepare high Nb containing TiAl alloy with fine lamellar colonies by reactive hot pressing process. The reaction between Ti and Al powders is dominant in Ti-Al-Nb system. Nb powders dissolve into the Ti-Al matrix by diffusion. Pore nests are formed in situ after Nb powders diffusion. The hot pressing atmosphere is optimized by thermodynamic calculations. Vacuum or argon protective atmosphere should be adopted.     

Isothermal oxidation behavior of powder metallurgy beta gamma TiAl–2Nb–2Mo alloy

A new TiAl–2Nb–2Mo beta gamma alloy was synthesized by powder metallurgy process. HIP’ed and vacuum heat treated specimens were isothermally oxidized at 800 °C and 900 °C in air up to 500 h. The TiAl–2Nb–2Mo alloy oxidized parabolically up to 500 h at both 800 °C and 900 °C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer, and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of lamellar colonies formed a diffusion zone just below the oxide/substrate interface consisting of γ-TiAl matrix and dispersed beta phases which contained high concentration of Nb and Mo. The oxidation rate of the TiAl–2Nb–2Mo alloy is more sensitive to temperature than those of the Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.     

Cyclic oxidation behavior of a beta gamma powder metallurgy TiAl–4Nb–3Mn alloy coated with a NiCrAlY coating

A new beta gamma TiAl–4Nb–3Mn alloy was synthesized by powder metallurgy (PM) and coated with NiCrAlY powder by Air Plasma Spray (APS). The cyclic oxidation behavior of the coated beta gamma alloy was investigated between room temperature and 1000 °C for up to 800 cycles. During cyclic oxidation testing, a TiN layer formed at the interface between NiCrAlY and TiAl–4Nb–3Mn and the thickness of the TiN layer increased with the number of cycles. The outward Ti diffusion also resulted in the formation of titanium oxides on the coating top surface after 630 cycles. Inward diffusion of Ni led to the formation of an inner diffusion zone containing NiAlTi and a mixture of NiAlTi + TiAl. Due to the spallation of coating scales on the circumferential surface, weight loss was observed after 120 cycles although both the top and bottom surfaces of the specimens remain in contact with the substrate until 800 cycles. Due to inward and outward diffusion, half of the NiCrAlY coating was consumed after 800 cycles suggesting the need for diffusion barrier coating at the interface between the NiCrAlY coating and the TiAl–4Nb–3Mn alloy.     

Characterization and formation mechanism of nanocrystalline W–Al alloy prepared by mechanical alloying

Tungsten and aluminum elemental powders with composition W–20 wt.% Al were mechanical alloyed in high energy planetary ball mill. Structural and morphological changes of powder particles after different milling times were studied by X-ray diffractometer, scanning electron microscopy and microhardness measurements. Mechanical alloying of this system led to the formation of W–Al alloy as a result of formation of W/Al layered microstructure having faceted interface between layers. This alloy indicated high microhardness value of about 570 Hv.     

The crack and pore formation mechanism of Ti–47Al–2Cr–2Nb alloy fabricated by selective laser melting

In this study, a broad range of parameter combinations (laser power: 100–400 W; scanning speed: 10–90 mm/s) were used to fabricate Ti-47Al-2Cr-2Nb alloy at the layer thickness of 100 μm by selective laser melting (SLM). The preparation of the TiAl-single track by SLM was prone not only to balling and irregularity but also to cracking. Although the optimized process parameters were used to fabricate TiAl specimens, many pores and cracks still existed and a low density was achieved. To understand the mechanism for the crack and pore formation, the connections among the cracks, pores, and the process parameters were investigated in addition to the variation in the crack propagation with an increase in the number of deposition layers. The results indicated that the cracks originated in the third layer, because of the accumulation of residual stresses and the changes in the composition of Ti-47Al-2Cr-2Nb deposition layers. Additionally, the frequency of cracks constantly increased with an increase in the number of deposition layers. Preheating the substrate to 200 °C improved the degree of cracking to a certain extent, as the initiation layer for the cracks increased from the third layer to the fifth layer. Despite the achieved improvement, it was not possible to produce crack-free specimens on the SLM machine used for this study. Finally, there was a good metallurgical bond between the Ti-6Al-4 V substrate and the Ti-47Al-2Cr-2Nb deposition layers that was free of pore and crack defects. These findings suggest that using SLM to fabricate Ti-6Al-4 V/TiAl intermetallic laminate composites may potentially eliminate cracking and improve the properties of TiAl alloys.     

Microstructure,mechanical properties and oxidation behavior of powder compacts of the Nb–Si–B system prepared by spark plasma sintering

Powder compacts with eight different compositions in the Nb–Si–B system were prepared by spark plasma sintering and their microstructure, mechanical properties and oxidation behavior were investigated. Oxidation resistance of Nb₅Si₃B₂ compacts is better than that of Nb₅Si₃ compacts, but extremely poorer than that of NbSi₂ compacts. However, since the oxidation experiment was of short duration, details of the oxidation behavior of Nb–Si–B compacts have yet to be investigated. Compacts were prepared via the following three different routes: (i) elemental powders were mixed in a rotational ball mill and then consolidated by spark plasma sintering method, (ii) elemental powders were mechanically alloyed and consolidated similarly to (i) and (iii) pulverizing compacts prepared via route (i) and consolidated once again similarly to (i). Compacts prepared via the third route exhibit more homogeneous microstructures and contain a smaller amount of SiO₂ than compacts prepared via the other two routes. Compacts with compositions around the line of Nb₅Si₃–Nb₅Si₃B₂–NbB₂ exhibit a high hardness at room temperature and a high compressive strength at high temperatures in comparison to those with compositions away from the line. The strength of compacts containing NbSi₂ decreases with increasing the volume fraction of NbSi₂ phase.     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

Reaction behavior and formation mechanism of ZrC and ZrB2 in the Cu–Zr–B4C system

In this work, the formation mechanism of ZrC and ZrB₂ in the Cu–Zr–B₄C system was studied by differential scanning calorimeter and X-ray diffraction. Moreover, the effect of heating rate on the reaction behavior was also investigated. The results revealed that the heating rate did hardly influence the reaction process and product in the range of 10–30 °C/min. The formation mechanism of ZrC and ZrB₂ in the Cu–Zr–B₄C system could be ascribed to the solid-state reaction between Zr and B₄C particles, and the replacement reactions of B₄C with the Cu–Zr liquid and copper zirconium compounds. The addition of Cu in the Cu–Zr–B₄C reactants can change the phase evolution route via producing various Cu–Zr intermediate phases and promote the formation of ZrC and ZrB₂.     

Corrosion behavior of Zr–Nb–Cr cladding alloys

Zr-Nb-Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corro- sion resistance of ZrxNb0.1Cr （x = 0.2, 0.5, 0.8, 1.1; wt%） alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zrl.lNb alloy, which is ascribed to Zr（Cr,Fe,Nb）2 precipitates with a much larger size than β-Nb.     

Comparison of Cu–Al–Ni–Mn–Zr shape memory alloy prepared by selective laser melting and conventional powder metallurgy

This work aimed to investigate and critically analyze the differences in microstructural features and thermal stability of Cu−11.3Al−3.2Ni−3.0Mn−0.5Zr shape memory alloy processed by selective laser melting (SLM) and conventional powder metallurgy. PM specimens were produced by sintering 106−180 µm pre-alloyed powders under an argon atmosphere at 1060 °C without secondary operations. SLM specimens were consolidated through melting 32−106 µm pre-alloyed powders on a Cu−10Sn substrate. Mechanical properties were measured through Vickers hardness testing. Differential scanning calorimetry was conducted to assess the martensitic transformation temperatures. X-ray diffraction patterns were collected to identify the metallurgical phases. Optical and scanning electron microscopy was used to analyze the microstructural features. β′₁ martensite was found, irrespective of the processing route, although coarser martensitic variants were present in PM-specimens. In conventional powder metallurgy samples, intergranular eutectoid constituents and stabilized austenite also formed at room temperature. PM-specimens showed similar average hardness values to the SLM-specimens, albeit with high standard deviation linked to the porosity. The specimens processed by SLM showed reversible martensitic transformation (T₀=171 °C). PM-processed specimens did not show shape memory effects.     

Ductilization of Mo–Si solid solutions manufactured by powder metallurgy

Mo–1.5 at.% Si alloys with additions of either Y₂O₃ or Zr were manufactured by mechanical alloying. The Y₂O₃ particles reduced the grain size and increased the room temperature strength, but did not alleviate the brittleness of previously investigated Mo–1.5 at.% Si without Y₂O₃. Additions of Zr, on the other hand, resulted not only in a fine grain size and an extremely high bend strength (～2 GPa), but also in limited bend ductility at room temperature. Zr additions are seen to be beneficial for three reasons. First, Zr reduces the grain size. Second, Zr getters detrimental oxygen by forming ZrO₂ particles (which in turn help to pin the grain boundaries). Third, in situ Auger analysis shows that Zr reduces the concentration of Si segregated at the grain boundaries. This is thought to enhance the grain boundary cohesive strength and thus leads to the observed ductility.     

Effect of synthesis parameters on structural and mechanical properties of Ti3Al–Nb–Mo intermetallic compound obtained by powder metallurgy techniques

The influence of microstructure, heat treatment and alloying addition on mechanical and fracture properties of Ti₃Al-based intermetallic at room and elevated temperatures was studied. Ti₃Al–11Nb–1Mo (mole fraction, %) alloy was consolidated via powder metallurgy processing by mechanical alloying (MA) and hot pressing (HP). MA powders were characterized using XRD and SEM-EDS. Optimum MA duration was 25 h and HP conditions of 1350 °C, 2 h, 35 MPa. After HP, solution treatment at 1050 °C for 1 h and water quenching α₂+β Widmanstätten microstructure is present, while subsequent aging at 800 °C during 24 h induces small content of O-phase. High fraction of β-phase is a direct consequence of Mo. Compression tests were performed from room temperature to 750 °C in vacuum. The yield strength of compacts increases with temperature up to 250 °C (pyramidal slip systems activation), after which it decreases. Ductility increases throughout the whole temperature range. The presence of O phase contributed to ductility increase in aged alloys, while negligibly lowering yield strength. Registered drop in the yield strength of aged alloys compared with non-aged ones was mostly influenced by precipitation of α″₂ particles. Mixed fracture modes are operative at all temperatures.     

Mechanical alloying behavior of Nb–Ti–Si-based alloy made from elemental powders by ball milling process

Nb-Ti-Si-based alloy powders were prepared by mechanical alloying(MA) of elemental particles.The evolutions of morphology,size,phase constituents,crystallite size,lattice strain,composition and internal microstructure,etc.,of the alloy powders were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM),energy-dispersive spectroscopy(EDS),laser particle size analyzer and transmission electron microscope(TEM) analyses.The alloy particles are gradually refined and their shapes become globular with the increase in milling time.The diffraction peaks of Nb solid solution(Nbss) phase shift toward lower29 angles during ball milling from 2 to 5 h,and after that Nbss diffraction peaks shift toward higher 29 angles with the increase in milling time from 5 to 70 h,which is mainly attributed to the alteration of the lattice parameter of Nbss powders due to the solution of the alloying element atoms into Nb lattice to form Nbss.During ball milling process,the decrease in crystallite size and increase in lattice strain of Nbss powders lead to continuous broadening of their diffraction peaks.A typical lamellar microstructure is formed inside the powder particles after ball milling for 5 h and becomes more refined and homogenized with the increase in milling time.After 40-h-ball milling,the typical lamellar microstructure disappears and a very homogeneous microstructure is formed instead.This homogeneous microstructure is proved to be composed of only supersaturated Nbss phase.     

Thermal explosion synthesis of ZrC particles and their mechanism of formation from Al–Zr–C elemental powders

ZrC particles were fabricated by thermal explosion (TE) from mixture of Al, Zr and C elemental powders. Without the addition of Al, the synthesized ZrC particles had irregular shape of ~ 4.0 μm in average. Increasing Al content up to 30 wt.%, however, refined significantly them down to < 0.2 μm with regularly square morphology. The Al effect of reaction mechanism promoted the ZrC formation as diluents in the course of TE, which was clarified using differential thermal analysis and X-ray diffraction technique. The melting of Al favored the reaction with Zr to generate ZrAl₃, and then the dissolution of C into the Al–Zr liquid resulted in precipitation of ZrC. Meanwhile, the exothermic effect prompted C atoms dissolving into Zr–Al liquid and eventually led to precipitation of ZrC out of the supersaturated liquid. The Al addition inhibited particle growth, but also promoted the TE reaction.     

Diffusion behavior of Nb element in high Nb containing TiAl Alloys by reactive hot pressing

Diffusion behavior of Nb in elemental powder metallurgy high Nb containing TiAl alloys was investigated. The results show that Nb element dissolves into the matrix by diffusion. Pore nests are formed in situ after Nb diffusion. With the increase in hot pressing temperature, the diffusion of Nb will be more sufficient, and the microstructure is more homogeneous. Nb element diffuses completely at 1400℃. Meanwhile, compression deformation and agglomeration phenomena of pores are observed in some pore nests. Hot isostatic pressing （HIP） treatment can only efficiently decrease but not eliminate porosity completely.     

Wetting of porous titanium carbonitride by Al–Mg–Si alloys

Wetting of porous TiC_0.17N_0.83 by six alloys from the Al–Mg–Si system (pure Al, pure Mg, Al–15 at.% Mg, Al–10 at.% Si, Mg–5 at.% Si, and Al–10 at.% Mg–10 at.% Si) in an argon atmosphere was studied using the sessile drop experiment. The contact angle of the liquid drops on TiC_0.17N_0.83 substrates was measured as a function of temperature. Aluminium, Al–10 at.% Si, and Al–10 at.% Mg–10 at.% Si did not wet TiC_0.17N_0.83 in the studied temperature range. Magnesium always wetted TiC_0.17N_0.83 with a minimum contact angle of ≈44° at 900°C, and alloying with Mg significantly lowered the contact angle of Al on TiCN. Alloying with Si deteriorated the wetting of TiCN by Mg. A comparative study between the systems was conducted, based on the results and on data available in the literature. The improvement of the wetting of TiCN by Al due to alloying with Mg can be explained by the segregation of Mg to the interface with TiCN, where it lowers the interface energy. The addition of Si to pure Mg or to Al–Mg results in an increase in the contact angle on TiCN.     

Corrosion behavior of three Ag–50Cu alloys prepared by different processes in NaCl solutions

Corrosion behavior of two nanocrystalline bulk Ag–50Cu alloys and one coarse-grained counterpart prepared by liquid-phase reduction(LPR), mechanical alloying(MA) and powder metallurgy(PM) methods,respectively, were investigated in Na Cl solutions. They were finished by means of PARM273 A and M5210 electrochemical apparatus through potentiodynamic polarization method and electrochemical impedance spectroscopy(EIS) technique. The results show that corrosion rates of three Ag–50Cu alloys increase with the increment of Na Cl solution concentrations. Corrosion rates of LPRAg–50Cu alloy are a little higher than those of PMAg–50Cu alloy,but evidently lower than those of MAAg–50Cu alloy. The difference in corrosion rates is attributed to the large reduction in the grain size and homogeneous microstructure of nanocrystalline alloys. Passive current densities decrease and afterward increase for PMAg–50Cu alloy,decrease for MAAg–50Cu alloy, and increase for LPRAg–50Cu alloy with the increment of Na Cl solution concentrations. After the grain sizes are refined, passive current densities become lower.