As-atomized Powder Research Articles

Microstructural stability and precipitate evolution of thermally treated 7075 aluminum alloy fabricated by cold spray

In this work, the effects of post-deposition heat treatment on microstructural stability of cold sprayed AA7075 aluminum coatings were thoroughly characterized. The as-atomized powder and annealed coatings were investigated using scanning electron microscopy, electron back-scattered diffraction, and microhardness measurements. Scanning transmission electron microscopy was further employed to evaluate grain/subgrain structure near the particle interface region. Precipitation in the coating was studied through differential scanning calorimetry and X-ray diffraction. The feedstock powder revealed dendritic/cellular-like structure accompanied by microsegregations of the main alloying elements at dendrite boundaries. The high-pressure Cold Spray process led to formation of lamellar subgrains and UFG grains via dynamic recovery/recrystallization mechanisms. GP zones present in the AA7075 powder continuously reprecipitated into more stable η´/η phases owing to propellant gas heating and extensive plastic strains induced during cold spraying. The applied annealing resulted in progressive softening of the AA7075 coatings and gradual transformation of LAGBs into HAGBs. The solute-rich grain boundary network observed in powder microstructure was fully retained after CS deposition but decomposed into coarse η and S precipitates upon high-temperature annealing. The CS AA7075 coating showed decent thermal stability up to 300 °C and rapid grain growth at 400 °C. The relationship between precipitate size and thermal stability was further discussed in terms of the Zener pinning theory.

Extremely low coercivity of powder Co-rich amorphous alloy obtained by gas atomisation

A novel Co-base soft magnetic powder with ultra-low coercivity was produced by inert gas atomisation. The as-atomised powder is fully amorphous owing to its high glass forming ability and the proper selection of atomisation conditions; it exhibits a very low coercivity (0.130 Oe) mainly due to the low magnetostriction coefficient of Co-based alloys. After production, the powder was annealed at different temperatures between 300 and 600 ºC for 30 minutes. Crystallisation started at around 575 ºC, which is the temperature of the first exothermic peak detected by differential scanning calorimetry. Annealing the powder at temperatures below 550 ºC develops only structural relaxation of the amorphous structure. The smallest coercivity (0.056 Oe) was found in the sample annealed at 400 ºC. On the other hand, an increment of the coercivity of four orders of magnitude occurred after full crystallisation at 600 ºC (394.320 Oe). From the analysis of the curves of anisotropy field distribution, it can be concluded that this Co-base alloy shows lower average anisotropy field, a more gaussian shape and a wider distribution than Fe-base alloys. This is explained by a lower magnetoelastic anisotropy and a more homogeneous distribution of the magnetic anisotropy. The powder exhibits a spherical shape that makes it very suitable as ferromagnetic phase to manufacture powder cores with extremely low hysteresis loss for medium-high frequency applications.

On the transformation temperatures of Ti-6Al-4V: Effect of oxygen pick-up during Laser Powder Bed Fusion

Ti-6Al-4V (Ti64) parts manufactured by Laser Powder Bed Fusion (L-PBF) exhibit a fine martensitic microstructure and significant residual stresses that are detrimental for their mechanical performance. Consequently, post-processing heat treatments are necessary to relieve these internal stresses and achieve a microstructure adequate for the mechanical requirements. Given the variations in powder compositions and processing parameters, there is a large scattering of physical properties of both as-built and heat-treated parts. In this context, this study quantifies the link between interstitial contents (specifically oxygen and nitrogen) and the transition temperatures of as-built parts, using as-atomized pre-alloyed powders for reference. The martensite decomposition temperature and beta transus temperature are assessed using high-temperature X-Ray diffraction and differential scanning calorimetry for both the powder and as-built parts. It was found that the martensite that composes the as-built microstructure can be decomposed into a stable α+β structure at temperatures exceeding 490 °C. Oxygen and nitrogen pick-up occurs during the L-PBF process, leading to an elevated beta transus temperature in as-built parts relative to the original powder. This insight provides a guide for optimizing post-processing heat treatments for Ti64 as-built components.

Effect of gas Mach number on the flow field of close-coupled gas atomization, particle size and cooling rate of as-atomized powder: Simulation and experiment

Gas velocity is a key parameter regulating the particle size and the cooling rate of the gas atomized powder applied in additive manufacturing, metal injection molding, thermal spraying, and soft magnetic composites. In this paper, on basis of the well-designed close-coupled nozzles with different gas Mach numbers at the outlet, the gas field structure was simulated by Computational Fluid Dynamics (CFD) software, and the process of cooling and solidification of Fe-6.5 wt% Si metal droplets was calculated by finite difference method. The results show that with the increase of Mach number, both the gas velocity downstream and the pressure at the base of melt delivery tube tip rise, whereas the mass flow rate of the melt decreases. The nozzles with high Mach number can produce finer powder with higher cooling rate. The median diameter of the powder prepared by the nozzle with Mach numbers of 1.0, 1.5, 2.0, and 2.5 is 44.9, 39.0, 32.5, and 29.1 μm, respectively, and the corresponding cooling rate of the metal droplet with a diameter of 80 μm is 2.85 × 104, 2.98 × 104, 3.32 × 104, and 3.50 × 104 K/s, respectively. This work provides new ideas and suggestions for the preparation of metal powder with small particle size at high cooling rate.

Evaluation of a laser powder bed fusion designer Al-Mg-Zr-Si alloy for cold spray additive manufacturing

Metal additive manufacturing (AM) is a constantly developing approach utilized in many novel applications. Most metal AM techniques utilize alloys designed for traditional processing conditions, such as casting, whereas implementing alloys designed for AM may prove useful. Given the proven utility of the Al-Mg-Zr-Si Addalloy 5T® in melt-driven AM, this study investigated the use of this alloy in cold spray (CS), as minimal research has explored alloy design for use in solid-state processing. The as-atomized powder and CS samples were evaluated microstructurally and mechanically to determine suitability for CS. Premature brittle fracture were seen in the deposits, necessitating processing improvements for enhanced deposit performance. Feedstock thermal pre-processing was explored, guided by thermodynamic modeling, to improve spray-ability. Heat treatments resulted in generalized powder softening, suggesting improved processability using a CS gas-particle dynamics model. This model allowed for theoretical tuning of processing parameters, predicting improved deposition using a specific configuration with He gas. The through-process model-informed experimental approach emphasizes the potential of using Addalloy 5T® powder in CS when coupling heat treatment with processing parameter optimization. Results suggest that the CS community may benefit from alloy design specifically for solid-state processing, tailoring powder chemistry and microstructure for enhanced properties and processability.

Evolution of Fe-Rich Phases in Thermally Processed Aluminum 6061 Powders for AM Applications.

Gas-atomized powders are frequently used in metal additive manufacturing (MAM) processes. During consolidation, certain properties and microstructural features of the feedstock can be retained. Such features include porosity, secondary phases, and oxides. Of particular importance to alloys such as Al 6061, secondary phases found in the feedstock powder can be directly related to those of the final consolidated form, especially for solid-state additive manufacturing. Al 6061 is a heat-treatable alloy that is commonly available in powder form. While heat treatments of 6061 have been widely studied in wrought form, little work has been performed to study the process in powders. This work investigates the evolution of the Fe-containing precipitates in gas-atomized Al 6061 powder through the use of scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDS). The use of coupled EDS and thermodynamic modeling suggests that the as-atomized powders contain Al13Fe4 at the microstructure boundaries in addition to Mg2Si. After one hour of thermal treatment at 530 °C, it appears that the dissolution of Mg2Si and Al13Fe4 occurs concurrently with the formation of Al15Si2M4, as suggested by thermodynamic models.

Thermal Preprocessing of Rapidly Solidified Al 6061 Feedstock for Tunable Cold Spray Additive Manufacturing

In this work, the influence of thermal pre-processing upon the microstructure and hardness of Al 6061 feedstock powder is considered through the lens of cold spray processing and additive manufacturing. Since solid-state cold spray processes refine and retain microstructural constituents following impact-driven and high-strain rate severe plastic deformation and bonding, thermal pre-processing enables application-driven tuning of the resultant consolidation achieved via microstructural and, therefore, mechanical manipulation of the feedstock prior to use. Microstructural analysis was achieved via X-ray diffraction, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and differential thermal calorimetry. On the other hand, nanoindentation testing and analysis were relied upon to quantify pre-processing effects and microstructural evolution influences on the resultant hardness as a function of time at 540 °C. In the case of the as-atomized powder, β-Mg2Si-, Al-Fe-, and Mg-Si-type phases were observed along polycrystalline grain boundaries. Furthermore, after a 60 min hold time at 540 °C, Al-Fe-Si-Cr-Mn- and Mg-Si-type intermetallic phases were also observed along grain boundaries. Furthermore, the as-atomized hardness at 250 nm of indentation depth was 1.26 GPa and continuously decreased as a function of hold time until reaching 0.88 GPa after 240 min at 540 °C. Finally, contextualization of the observations with tuning cold spray additive manufacturing part performance via powder pre-processing is presented for through-process and application-minded design.

An Al-5Fe-6Cr alloy with outstanding high temperature mechanical behavior by laser powder bed fusion

This research aims to study the laser powder bed fusion ( L -PBF) processability, the microstructure and the tensile mechanical properties of an AlFeCr alloy at a wide range of temperatures. With this goal, Al-5Fe-6Cr (wt%) pre-alloyed powder was produced by casting and gas atomization. The microstructure of the as-atomized powders and the as-built specimens was characterized via scanning electron microscopy, x-ray diffraction, and transmission electron microscopy. Following a parameter optimization study, dense as-built specimens with a high relative density of 99.8% and a Vickers hardness at room temperature of 192 HV were fabricated. In addition, the newly developed AlFeCr alloy exhibits yield strength values of 273 ± 5 MPa and 179 ± 3 MPa at 300ºC and 400ºC, respectively. The exceptional strength of this alloy at high temperature is attributed to the homogeneous precipitation during processing of large density of nanoscale icosahedral (i-phase) and intermetallic phases, which are endowed with high thermal stability. These values are significantly higher than those corresponding to the conventional wrought Al7075 aluminum alloy in the T6 condition and to all Al alloys manufactured by L -PBF to date. The current research sheds guidelines for the design of high strength aluminum alloys containing transition alloying elements that are suitable to be processed by L -PBF for high temperature structural applications in a cost effective way.

Crystallization kinetics of Cu50Zr40Ti10 amorphous powder

The non-isothermal and isothermal crystallization behaviors are systematically investigated for the gas-atomized and as-milled amorphous alloy powders. The results show that the crystallization phase is Cu10Zr7 phase for the gas-atomized and as-milled amorphous alloy powders in non-isothermal and isothermal crystallization modes. The glass transition temperature, onset crystallization temperature, and crystallization peak temperature are larger for the as-atomized alloy powders than for the as-milled ones. The activation energies for the glass transition and crystallization are lower for the as-atomized alloy powders than for the as-milled ones. The crystallization mechanism of the gas-atomized and as-milled amorphous alloy powders nearly depends on the heating rates and the annealing temperatures. The detailed crystallization mechanism of non-isothermal and isothermal modes are discussed.

Oxidation Characteristics of Nickel-Based Superalloy Powders Exposed at Ambient Condition

The existence of adsorbed oxygen and oxides on the surface of initial powders has serious effects on the microstructure and mechanical properties of the powder metallurgy alloys. However, the powder surface is inevitably oxidized immediately after the powder preparation. In this work, the oxidation characteristics for the argon atomized powders of a Ni-based superalloy containing Cr, Co, W, Mo, Nb, Ti and Al after exposure at ambient condition for various time were investigated in detail. It is found that various gases can be absorbed on the powder surface, but most of them can be removed by low temperature (<151.5 °C) outgassing procedure. The thermodynamic calculation shows that the oxidation reaction occurs firstly with the alloying elements rather than Ni matrix, whether at room temperature or elevated temperature. The kinetic measurement indicates that the oxygen content on the powder surface approaches a saturation value after 24 h exposure and remains almost stable after 720 h. The oxygen content increases with the decrease of particle size after exposure. X-ray photoelectron spectroscopy characterized that, except the formed oxides, adsorbed oxygen also exists on the powder surface of the as-atomized initial fine powders with particle size <30 μm and the powders with size >18.7 μm after exposure, which may be caused by the internal stress and surface energy of the initial atomized powder. All alloying elements except Ti can form stable oxides directly on the powder surface. For the element of Ti, the metastable TiO forms on the initial powder surface after preparation and it transforms into stable TiO2 or Ti2O3 during exposure. The results provide a deep understanding of absorbed gases and oxide on the surface of powders under treatment and possible desorption approach.

Development of novel Fe based nanocrystalline FeBNbPSi alloy powder with high B s of 1.41T by forming stable single amorphous precursor

The possibility of the powderization of precursor with a single amorphous phase was investigated in FeBNbP nanocrystalline alloy particles and FeBNbPSi nanocrystalline alloy powders. Fe90−xBxNb7P3 (x=9, 10), Fe79.5B9.5Nb7P3Si1 and Fe82−x B9Nb6P3Six (x=2, 3) powders were prepared by the gas atomization method using high pressure water for rapid quenching. The as-atomized particle as a precursor exhibiting low Hc of 141 A/m with a single amorphous phase was observed from a cross sectional image and SAED pattern in the Fe80B10Nb7P3 nanocrystalline alloy powder. In addition, the stability of amorphous phase in the FeBNbP nanocrystalline alloy was also significantly improved by the addition of Si. Therefore, the as-atomized Fe79B9Nb6P3Si3 nanocrystalline alloy powder with Si as a precursor powder of nanocrystalline alloy was achieved to exhibit excellent magnetic softness of low Hc of 53 A/m compared to the ordinary Fe73Si11B11Cr3C2 amorphous and Fe73.5Si13.5B9Nb3Cu1 nanocrystalline alloy powders. In addition, the Fe79B9Nb6P3Si3 powder after nanocrystallization at 873K achieved both high Bs of 1.41T and low Hc of 37 A/m compared to ordinary amorphous, and nanocrystalline alloys.

Tailoring powder strengths for enhanced quality of cold sprayed Al6061 deposits

As verified by literature, heat-treatments of as-atomized Al-alloy powders before cold spraying, result in microstructural homogenization and deposition efficiency increment. So far, a straightforward correlation between powder strength and consequences for the performance in cold spraying and deposit properties is still missing. This work thus provides reliable analyses of powder strengths in as-atomized and annealed states to the calculation of critical velocities and deposit quality parameter η, as well as the associated influences of powder strength on single-particle adhesion and deposit microstructures and properties. By annealing of as-atomized powder, its strength is reduced by about 60%, which allows decreasing the critical velocity for a successful deposition. Experimental results demonstrate that powder strength-based calculation of quality parameter η allows for a more realistic description of microstructural characteristics and deposits properties. The single-particle impact morphologies as well as the detachment features of adhering splats by cavitation tests visualize the respective deposition characteristics and bonding behaviors. The lower critical velocities by annealing contribute to better single splat adhesion, lower porosity, higher electrical conductivity, as well as improved tensile strength of deposits. The direct correlation of powder pre-annealing and strength in combination with cold spraying parameter variation allows defining effective strategies for improving deposit properties.

Defect and satellite characteristics of additive manufacturing metal powders

Metal additive manufacturing (AM) requires high-quality metal powders to three-dimensionally (3D) print metallic components with complex and customizable geometries. The lack of quantification of AM metal powders creates quality control challenges for 3D printed components, increases the uncertainty of printing reliability and net cost of inspected and certified printed components, and reduces the recyclability of used powders. However, critical characteristics of AM metal powders that are decisive factors for the 3D printing process, such as internal porosity, contamination, and satellite feature, remain ambiguous. Here, we developed a novel approach to 3D quantify key characteristics of AM metal powders down to individual particles by using high-resolution synchrotron x-ray computed tomography. Empowered by the penetrative capability of high-energy x-ray, internal porosity and contamination within as-atomized metal powders from high-entropy alloys to nickel-based superalloys were evaluated. Additionally, the newly-developed dispersion method enables the homogeneous separation of individual particles, and consequently, results in the implementation of 3D particle shape analysis. To resolve a major challenge of identification and quantification of satellite-feature particles in as-atomized AM metal powders, the satellite features were quantitated by modeling and analyzing the shape parameter of local thickness variance. Furthermore, the 3D analytical methods of particle assessment in this study can be applied to other materials systems like rock, food, and pharmaceutical particles, and provide insights for process optimization across powder metallurgy, concrete, food and pharmaceutical manufacturing, and AM industries.

Influence of the Zr content on the processability of a high strength Al-Zn-Mg-Cu-Zr alloy by laser powder bed fusion

The objective of this research is to study the effect of the Zr on the L-PBF processability, microstructure, and microhardness of an AlZnMgCu-Zr alloy. Two AlZnMgCu-0.5 and 1.5 wt% Zr pre-alloyed powders were produced by casting followed by gas atomization. In addition, an excess of Mg and Zn was added to the target compositions to compensate for vaporization during L-PBF. The as-atomized powders and the as-built and heat treated specimens were characterized via scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Crack-free samples with a relative density of 99.0 ± 0.1% were obtained in the alloy containing 1.5 wt%Zr. The as-built microstructure of this alloy consisted of small equiaxed grains without preferred grain orientation near the melt pool boundary and slightly columnar grains between adjacent melt pools. After a solution and aging heat treatment (T6), the newly developed AlZnMgCu-1.5Zr alloy has an outstanding Vickers micro-hardness of 223 ± 3 HV, which is significantly higher than that of the wrought Al7075-T6 aluminum alloy. This excellent mechanical behavior is attributed to the presence of large Al3Zr nucleant particles of up to 500 nm in size that inhibit grain growth during solidification and the subsequent heat treatment, and of a high density of nanoscale second phases (MgZn2 and Al3Zr) located within the aged α-Al grains.

Effects of Gas Mach Number on the Flow Field of Close-Coupled Gas Atomization, Particle Size and Cooling Rate of As-Atomized Powder: Simulation and Experiment

Effects of Gas Mach Number on the Flow Field of Close-Coupled Gas Atomization, Particle Size and Cooling Rate of As-Atomized Powder: Simulation and Experiment

Sol–Gel Encapsulation of ZnAl Alloy Powder with Alumina Shell

Additive manufacturing (AM), for example, directed energy deposition (DED), may allow the processing of self-healing metal–matrix composites (SHMMCs). The sealing of cracks in these SHMMCs would be achieved via the melting of micro-encapsulated low melting point particulates (LMPPs), incorporated into the material during AM, by heat treatment of the part during service. Zn-Al alloys are good candidates to serve as LMPPs, for example, when the matrix of the MMC is made of an aluminum alloy. However, such powders should first be encapsulated by a thermal and diffusion barrier. Here, we propose a sol–gel process for encapsulation of a custom-made ZA-8 (Zn92Al8, wt.%) core powder in a ceramic alumina (Al2O3) shell. We first modify the surface of the ZA-8 powder with (12-phosphonododecyl)phosphonic acid (Di-PA) hydrophobic self-assembled monolayer (SAM) in order to prevent extensive hydrogen evolution and formation of non-uniform and porous oxide/hydroxide surface layers during the sol–gel process. Calcination for 1 h at 500 °C is found to be insufficient for complete boehmite-to-γ(Al2O3) phase transformation. Thermal stability tests in an air-atmosphere furnace at 600 °C for 1 h result in melting, distortion, and sintering into a brittle sponge (aggregate) of the as-atomized powder. In contrast, the core/shell powder is not sintered and preserves its spherical morphology, with no apparent “leaks” of the ZA-8 core alloy out of the ceramic encapsulation.

Effect of nanostructure on phase transformations during heat treatment of 2024 aluminum alloy

In this work, we studied the transformations involving the coherent (θ”/S”), semicoherent (θ’/S’), and incoherent (θ/S) precipitates (θ = Al 2 Cu, S = Al 2 CuMg) formed during the heat treatment of an annealed nanostructured powder of Al 2024 produced by cryogenic milling. These precipitates form over a wide temperature range (300–500 °C) depending on the chemical composition. They influence grain growth during sintering and may facilitate the formation of a fully dense nanostructured material. Mechanical milling was performed for 20 h at a cryogenic temperature to obtain nanostructured particles. The structural evolution and morphology of the particles during the heat treatment were investigated for three particle size ranges (<25, 25–45, and 45–90 μm). Heat treatment of the milled powder was performed at three different temperatures (475, 500, and 525 °C) for 5, 10, 15, and 20 min. Morphological analysis of the as-milled particles using scanning electron microscopy indicated that the milling process was not completed. X-ray diffraction analysis yielded the θ and S volume fractions for the heat-treated and as-milled powders. DSC analysis indicated the precipitation and dissolution of GP (Guinier-Preston zone), coherent, and incoherent precipitates during heating of the as-atomized and milled powders. Accumulation of the deformation energy during milling is suggested to lead to an early transformation of the S and θ phases in the finer powder.

Investigation of phase constitution and stability of gas-atomized Al0.5CoCrFeNi2 high-entropy alloy powders

In this study, Al0.5CoCrFeNi2 high-entropy alloy powders with uniform elements distribution were prepared by gas atomization process. The effects of heat treatment on the microstructure, phase transformation and mechanical properties were discussed. The FCC phase was observed in the as-atomized powder. The In-situ XRD results indicated that phase transform occurred at around 900 °C. Therefore, the annealing treatment was conducted at 1000 °C to observe the phase transformation. From the XRD of the different annealing times, additional diffraction peaks began to generate after 24 h. With SEM and EDS analyses, the results indicated the Cr element precipitated along the grain boundaries. The longer the annealing time, the more the precipitates. Afterward, EBSD was carried out to identify the crystal orientation of the phase. It was found that the FCC phase was aligned with the Ni3Al crystal structure. On the other hand, the Cr-rich phase formed after the annealing process tended to form the BCC crystal structure of NiAl and Fe, observed from the results of EDS and EBSD. Furthermore, the average hardness of as-atomized powder was 4.46 GPa. After the annealing process, the lattice distortion was eliminated. As a result, the hardness of FCC matrix was significantly decreased to 2.6 GPa. Whereas the hardness of segregated Cr-rich phase was 12.74 GPa. Thus, the average hardness of annealed powder was increased to 6.93 GPa due to the precipitation strengthening resulting from Cr-rich region.

Precipitation phenomena in a powder-processed quasicrystal-reinforced Al-Cr-Mn-Co-Zr alloy

The thermal behavior of a powder-processed Al-2.5Cr-1.35Mn-0.25Co-0.31Zr alloy (in at.%) has been studied by comparing hardness and SEM data from as-consolidated and heat-treated samples of the bulk alloy with data from in situ TEM heating experiments on sections through individual particles of the as-atomized powder. The starting microstructure consisted of an Al matrix with I-phase quasicrystalline dispersoids, and the distribution of the I-phase depended on the powder particle size. Most of the powder (≈ 70% by volume) exhibited a cellular dendritic Al microstructure with minority phases at the cell boundaries and I-phase at the powder particle surfaces. Upon heat treatment, the consolidated alloy retained the initial hardness and microstructure up to about 400 °C. At this point the alloy hardened by about 6%, and then softened significantly at higher temperatures. The main microstructural change was the precipitation of a plate-like ternary Al11(Cr,Mn)2 phase within the cellular Al matrix. In situ TEM observations on powder particles with the cellular dendritic microstructure at temperatures of 400–450 °C revealed the kinetics of the precipitation for this ternary phase. The kinetic data gave Avrami exponents of approximately 3 in the nucleation and growth regime. The temperature dependence of the rate constants gave an activation energy of 277 kJ/mol, which is consistent with Cr diffusion in the Al matrix being the rate-limiting process for the precipitation of Al11(Cr,Mn)2. The onset of the precipitation at 350–400 °C should not restrict the application of such alloys, but it may limit the processing if the decomposition of the supersaturated solid solution is to be avoided.

Effect of neodymium content and niobium addition on grain growth of Nd-Fe-B powders produced by gas atomization

Nd-Fe-B powders of different compositions were produced by gas atomization. These powders were annealed between 1000 and 1150 °C for several times to study the microstructural evolution. Differential scanning calorimetry was used to determine the thermal transitions on the as-atomized powders and after slow solidification. The microstructure was studied by high resolution scanning electron microscopy at each stage. Electron back scattering diffraction was used to measure grain size and confirm that gas atomized powders are isotropic. It was observed the formation of necks between the particles, densification, and grain growth due to liquid phase sintering. Grain growth and densification occur in parallel by a dissolution-reprecipitation mechanism. The effect of Nd content and Nb addition on the microstructural changes was analyzed in detail, particularly on grain growth. The degree of sintering increases with Nd content, as this element enhances the formation of the liquid phase. Nb addition leads to the formation of precipitates that delay densification and grain growth at 1000 °C, but promote abnormal grain growth at 1100 °C.