As-cast State Research Articles

Influence of the Chromium Content on the Characteristics of the Matrix, the Tantalum Carbides Population, and the Hardness of Cast Co(Cr)-0.4C-6Ta Alloys

The mechanical and chemical behaviors of cast cobalt-base superalloys are governed by the carbides and by a reactive element, which is often chromium. The content of this later element, which is efficient in resisting hot oxidation and also hot corrosion, may have consequences on the melting temperature, microstructure, and mechanical properties at high temperatures and at room temperature. Seemingly, the effect of chromium content on the microstructure and properties of cast equi-axed Co-Cr-Ta-C superalloys containing TaC as single reinforcing carbide and in high-enough quantities to achieve a high level of creep resistance has not been the subject of previous investigations. The present work is devoted to the exploration of this influence of Cr content on the as-cast microstructure of a model alloy in this category, as well as on its microstructure transformations at high temperatures. The work aims to help rate the Cr content to achieve the best characteristics in machinability and high-temperature properties. This is of great importance for fabricability (production cost) and sustainability in service (long enough lifetime performance). A series of six alloys derived from a rather well-known alloy and presenting various Cr contents were thus elaborated by casting. Their microstructures were investigated in their as-cast state as well as in an aged state resulting from a 4-day stage at 1400 K. Vickers indentation was also carried out to study how hardness may evolve with Cr content. It was seen that the higher the Cr content, the lower the solidus temperature, the coarser the TaC population, the harder the alloy, and the higher the risk of brittleness. In order to reach the best compromise, the preferred Cr weight content range, as identified by this work, is 20–30%; indeed, for such Cr contents: (1) the matrix is austenitic, then dense, and then hard and not brittle, and thus is mechanically resistant and tough; (2) the TaC carbides are script-shaped and resistant against morphology changes at high temperatures, and thus efficiently preserve interdendritic cohesion for a long time, and, consequently, (3) the alloys are machinable, have expected good toughness, and can be resistant against creep deformation as well as oxidation and corrosion at high temperatures thanks to the Cr content, allowing for chromium-forming behavior.

Characterization of Al Alloys Injected through Vacuum-Assisted HPDC and Influence of T6 Heat Treatment

AlSi12(Fe), AlSi10Mg(Fe), AlSi10MnMg, and AlMg4Fe2 die-casting alloys were produced by high-pressure die casting (HPDC) and vacuum-assisted high-pressure die casting (VADC) under a vacuum level of 200 mbar. The chemical composition, hardness, gas and shrinkage porosity, and mechanical properties were analyzed. The parts under study were subjected to a T6 heat treatment. The VADC led to a decrease in the percentage of defects in the as-cast state for all the alloys, due to a reduction in the amount of gas porosities. After heat treatment, the quantity of gas and shrinkage porosities increased. The efficiency and level of vacuum used were not sufficient to improve the mechanical properties in the as-cast state. The ductility of AlSi10Mg(Fe) and AlSi10MnMg alloys was improved after heat treatment; however, the YS and UTS of AlSi10Mg(Fe) did not increase. The primary aluminum alloys presented higher elongation values than the secondary aluminum alloys due to the reduced amount of the needle-like β-Al5FeSi phase.

Influence of Nd Substitution on the Phase Constitution in (Zr,Ce)Fe10Si2 Alloys with the ThMn12 Structure

Iron-based compounds with a ThMn12-type structure have the potential to bridge the gap between ferrites and high performance Nd2Fe14B magnets. From the point of view of possible applications, the main advantage is their composition, with about 10 wt.% less rare earth elements in comparison with the 2:14:1 phase. On the other hand, the main issue delaying the development of Fe-rich alloys with a ThMn12-type structure is their structural stability. Therefore, various synthesis methods and stabilizing elements have been proposed to stabilize the structure. In this work, the influence of increasing Nd substitution on the phase constitution of Zr0.4-xNdxCe0.6Fe10Si2 (0 ≤ x ≤ 0.3) alloys was analyzed. X-ray diffraction and 57Fe Mössbauer spectrometry were used as the main methods to derive the stability range and destabilization routes of the 1:12 structure. For the arc-melted samples, an increase in the lattice parameters of the ThMn12-type structure was observed with the simultaneous growth of bcc-(Fe,Si) content with increasing Nd substitution. After isothermal annealing, the ThMn12-type structure (and the coexisting bcc-(Fe,Si)) were stable over the whole composition range. While the formation of a 1:12 phase was totally suppressed in the as-cast state for x = 0.3, further heat treatment resulted in the growth of about 45% of the ThMn12-type phase. The results confirmed that the stability range of ThMn12-type structure in the Nd-containing alloys was well improved by other substitutions and the heat treatment, which in turn, is also needed to homogenize the ThMn12-type phase. After further characterization of the magnetic properties and optimization of microstructure, such hard/soft magnetic composites can show their potential by exploiting the exchange spring mechanism.

Influence of Hafnium Addition on the Microstructure, Microhardness and Corrosion Resistance of Ti20Ta20Nb20(ZrMo)20-xHfx (where x = 0, 5, 10, 15 and 20 at.%) High Entropy Alloys.

The presented work aimed to investigate the influence of the hafnium/(zirconium and molybdenum) ratio on the microstructure, microhardness and corrosion resistance of Ti20Ta20Nb20(ZrMo)20-xHfx (where x = 0, 5, 10, 15 and 20 at.%) high entropy alloys in an as-cast state produced from elemental powder and obtained via the vacuum arc melting technique. All studied alloys contained only biocompatible elements and were chosen based on the thermodynamical calculations of phase formation predictions after solidification. Thermodynamical calculations predicted the presence of multi-phase, body-centered cubic phases, which were confirmed using X-ray diffraction and scanning electron microscopy. Segregation of alloying elements was recorded using elemental distribution maps. A decrease in microhardness with an increase in hafnium content in the studied alloys was revealed (512-482 HV1). The electrochemical measurements showed that the studied alloys exhibited a high corrosion resistance in a simulated body fluid environment (breakdown potential 4.60-5.50 V vs. SCE).

Enhanced Solvent and Thermal Stability in Cross-Linkable Conjugated Statistical Copolymers for Organic Field-Effect Transistors.

The ability to enhance both the solvent and thermal stability of semiconducting π-conjugated polymers is highly desired for various device-related applications. Herein, a series of poly(3-hexylthiophene)-stat-poly[3-(6-hydroxy)hexylthiophene] (P3HT-stat-P3HHT) statistical copolymers with thermally cross-linkable hydroxyl groups is synthesized and their crystalline structures in three different states, solvent and thermal stability for use in organic field-effect transistors (OFETs) are elucidated. Importantly, these initial P3HT-stat-P3HHT thin films in their as-cast state crystallize well in an edge-on orientation. During annealing at 150 °C, these P3HT-stat-P3HHT occur cross-linked and retain edge-on orientation with increased crystallinity and ordering. In contrast, after high-temperature annealing at 300 °C, their edge-on orientation is significantly destroyed due to the cross-linking of hydroxyl groups at melted state. The correlation between different P3HT-stat-P3HHT and their charge mobilities is scrutinized. These cross-linked P3HT-stat-P3HHT exhibit good solvent resistance property and improved thermal stability in OFETs. Conceptually, such side-chain functionalization approach to improve the stability of P3HT-stat-P3HHT can be conveniently extended to other conjugated polymers for diverse optoelectronic applications.

Effect of yttrium content on microstructure and tensile properties of as-cast and as-solutionized Al-Zn-Mg-Cu alloys

Four kinds of 7xxx series aluminum alloys with different Y elementcontents were obtained by ordinary gravity casting. The effect of Y content on the microstructure and mechanical properties of as-cast and as-solutionized Al-7.9Zn-3Mg-2.4Cu-0.13Zr (wt%) alloys were investigated by means of x-ray diffraction (XRD), electron backscattering diffraction (EBSD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and room-temperature tensile tests. The results shows that, in as-cast condition, Y element can refine the grain and reduce the content of Mg(Zn, Cu, Al)2 lamellar phases at the interdendritic. (Al, Zn)8Cu4Y block-shaped phases form in interdendritic regions. After solution treatment, the undissolved Mg(Zn, Cu, Al)2 phases evolved from lamellar to bulk-like which distribution in interdendritic, but no obvious change in (Al, Zn)8Cu4Y phase. The tensile testing results shows that the optimal yttrium content is 0.45 wt%. At 0.45 wt% Y, the ultimate tensile strength and elongation are 267 MPa and 2.4% in as-cast condition and 420 MPa and 3.6% in as-solutionized condition.

Understanding the yield strength difference in dual-phase eutectic high-entropy alloys

Face-centered cubic (FCC)/ordered body-centered cubic (B2)-structured Al–Co–Cr–Fe–Ni system eutectic high-entropy alloys (EHEAs) have a favorable combination of high strength and ductility in the as-cast state. However, the mechanical properties of this type of alloy vary significantly with composition. In this study, we attempted to determine the yield strength differences in FCC/B2 EHEAs. Based on reported Al0.8CrFeNi2.2, we successfully prepared three low-cost Co-free EHEAs: Al0.82Cr0.8Fe1.2Ni2.18, Al0.83Cr0.6Fe1.4Ni2.17, and Al0.85Cr0.4Fe1.6Ni2.15. All the EHEAs are shown to exhibit the same phase constituent (FCC/B2) and complete eutectic microstructure, with similar lamellar thickness but different tensile properties. We constructed a theoretical calculation model based on the Hall-Petch regime which fits the measured yield strength σ0.2. The yield differences for current FCC/B2 EHEAs results mainly from different intrinsic strengths of their soft FCC phases.

Development of a high strength, low density and corrosion resistant novel FeCrMoNb1.5Ti0.5 complex concentrated alloy

The present work deals with the microstructure, mechanical properties and corrosion behavior of a FeCrMoNb1.5Ti0.5 complex concentrated alloy (CCA). The aforementioned alloy was fabricated by vacuum arc melting technique. The observed density and lattice parameters of the as-cast alloy are 7.821 g/cm3 and 3.179 ± 0.003 Å, respectively, which follow the rule of mixture. The phase evolution, chemical composition, and microstructure in the as-cast condition, as well as after annealing treatment at 800°C, 1000°C, and 1200°C, were investigated through XRD, TEM, and SEM-EDS. The XRD and TEM analyses confirmed the presence of BCC structure with minor amount of TiCr2 type and NbCr2 type C14 Laves phases. Instrumented micro-indentation and corrosion testing were performed in the as-cast and annealed specimens. The microhardness of the as-cast alloy was 825 HV and it increased to 1102 HV after annealing at 1200ºC for 20 h. This peak hardness was achieved after annealing at 1200ºC due to the presence of a large volume fraction of Laves phase. Potentiodynamic polarization tests of the as-cast and as-annealed specimens revealed outstanding corrosion resistance of the CCA compared to stainless steel. This is attributed to better microstructural homogeneity and reduced elemental segregation after annealing.

Non-Metallic Alloying Constituents to Develop a Wear-Resistant CrFeNi-BSiC High-Entropy Alloy for Surface Protective Coatings by Thermal Spraying and High-Speed Laser Metal Deposition

Compositional alterations to high-entropy alloys (HEAs) allow further evolution of these materials by adjusting their property profiles. This way, they can be used for coating technologies and surface-protection applications. In the present work, minor quantities of the non-metallic alloying constituents, BSiC, were added to the CrFeNi base system. The alloy development was carried out in an electric arc furnace in comparison with the nickel-based alloy Ni-600. With regard to the BSiC-free variant, the wear resistance can be significantly increased. The powder was manufactured by inert gas atomization and characterized, followed by processing via high-velocity oxy-fuel spraying (HVOF) and high velocity laser metal deposition (HS-LMD). Depending on the manufacturing conditions, the proportion and shape of the precipitates within the microstructure differ. Compared to both the reference system and the as-cast condition, the coating systems demonstrated comparable or improved resistance to wear. The evaluation of the process–structure–property relationships confirmed the great potential of developing load-adapted HEA systems using non-metallic alloy constituents in the field of surface engineering.

Enhanced mechanical properties of a Ni–W–Al alloy in the as-cast state

An as-cast Ni81W10Al9 (at.%) alloy with remarkable mechanical properties was developed by adjusting the alloying element with an appropriate enthalpy of mixing. A yield strength of ∼640 MPa was achieved for the as-cast Ni81W10Al9 alloy, which is approximately three times larger than that of its Al-free counterpart. Extensive L12 phase nanoscale precipitations were generated in the Ni81W10Al9 alloy, which provided strong strengthening by blocking dislocation sliding. The contribution of precipitation shearing to the strength is evaluated to be 366.4 MPa. The Taylor lattices as well as microbands were formed owing to the dislocation slipping, resulting in significant strain hardening in the plastic deformation region. The alloying method for strengthening the material in the as-cast state serves as an effective way to shorten the material preparation process.

Impact of aging temperature on the metallurgical and dry sliding wear behaviour of LM25 / Al2O3 metal matrix composite for potential automotive application

The current study exhibits the influence of aging temperatures on the metallurgical, hardness, and dry-sliding wear behaviour of LM25 (Al-6.6Si-0.2Mg) alloy reinforced with Al2O3 particles. The LM25 alloy reinforced with 10 wt% of alumina particles was fabricated using the liquid metallurgy route followed by solutionizing and aging. The baseline LM25 alloy and its composite were solutionized at 538 °C for 8 h and were aged at 155, 165, and 175 °C for 12 h. Optical, FESEM, EDS, and X-ray diffraction analysis were done on the fabricated alloy and its composite in all conditions. The microstructure revealed the formation of the Mg2Si phase in the baseline alloy and the MgAl2O4 spinel generated at the composite interface of the aluminium matrix. The heat-treated alloy and composites were tested for their hardness on the Vickers microhardness tester. It was concluded that the aging temperature of 155 °C displayed significant enhancement in hardness values for tested samples. The heat-treated alloy and composite samples displayed an increment of 96% and 55% in hardness values relative to LM25. The wear rate and friction coefficient for the fabricated samples were analyzed using the pin-on-disc tribometer under dry sliding conditions. The hardness value increased from the as-cast state to samples aged at 155 °C and then decreased at 165 and 175 °C. Based on the wear study, a 14% and 25% decrease in the wear rate values for heat-treated alloy and composites were noted when sliding velocity was increased from 1 m/s to 3 m/s. However, the coefficient of friction (COF) decreased by 23% and 13% for the specimens in the same conditions. Furthermore, a similar trend was displayed by age-hardened LM25 alloy and the composite when subjected to varying load (5, 10, 15 N) condition. Lastly, the worn-out surface mechanisms were examined using FESEM analysis. Amongst the investigated samples, LM25/10 wt% Al2O3 composite aged at 155 °C revealed the least wear rate when subjected to an external load of 5 N and sliding velocity of 2 m/s. Therefore, it can be suggested to manufacture components in the automotive industry.

Effect of precipitation hardening treatment on hardness and tensile behaviour of stir cast LM4 hybrid composites through TEM and fractography analysis

LM4 hybrid composites reinforced with TiB2 and Si3N4 are fabricated using a two-stage stir casting method and are subjected to precipitation hardening treatment. The efficacy of TiB2, Si3N4, and precipitation hardening treatment on LM4 was investigated. When compared to as-cast LM4, all hybrid composites had displayed better hardness and UTS values. This increase in mechanical properties of hybrid composites may be attributed to the presence of hard reinforcement particles, appropriate stirring parameters, which resulted in uniform reinforcement distribution free of porosity and agglomeration. It was observed that in as-cast condition hybrid composites with a higher wt.% of TiB2 had displayed improved hardness (12.5% higher) and UTS (4.4% higher) values than composites with a higher wt.% of Si3N4, because Si3N4 particles are 3.8 times larger than TiB2 particles. Precipitation hardening treatment enhanced the mechanical properties of hybrid composites, with multistage solutionizing (MSHT) and artificial aging at 100 °C producing the best results. Peak aged (MSHT + aging at 100 °C) hybrid composites exhibited 160, 178, and 202% improvement in hardness and 46, 34, and 30% improvement in UTS when compared to as-cast LM4. This increase in age hardened specimens properties is due to the formation of metastable phases, which were validated using XRD and TEM. Fracture analysis of these as-cast and peak aged hybrid composites revealed a mixed mode of fracture-mostly brittle type, because of the presence of reinforcements, intermetallic phases, and strong interface bonding between matrix and reinforcements.

Effect of MWCNTs and heat treatment on coefficient of friction and wear characteristics of MWCNTs reinforced Al6082 composites under dry sliding condition

Herein, MWCNTs/Al6082 composites were produced using a stir casting technique, and their tribological behavior was studied in as-cast and T6 heat-treated conditions using a pin-on-disk tribometer at a sliding speed of 2 m/s (fixed) and varying applied load (40, 60, and 80 N) and sliding distance (500–3000 m). Morphology of composites and worn-out surfaces were studied using a scanning electron microscope (SEM). Linear increment in wear rate was observed with sliding distance up to transition limit and then upsurged drastically beyond that limit. At 40 N, a protective layer started forming between the disk and surface of pin, which reduced coefficient of friction (COF), and wear mechanism was abrasion and oxide. Annihilation of protective layer at higher load increased COF and observed wear mechanism was adhesion and delamination. Wear resistance of alloy and nanocomposites was enhanced by heat treatment due to the strengthening of matrix. Wear rate of Al matrix was reduced with MWCNTs due to their self-lubricating property.

Microstructure, mechanical properties and fatigue behaviour of a new high-strength aluminium alloy AA 6086

This study presents the comprehensive experimental investigation of the microstructure, mechanical and fatigue properties of a new high-strength aluminium alloy AA 6086, which was developed from a commercial aluminium alloy AA 6082. The new alloy possesses a higher content of Si, and, it also contains Cu and Zr. The alloy was characterised in the as-cast condition after homogenisation, extrusion, and T6 heat treatment. Light microscopy, scanning and transmission electron microscopy with energy dispersive spectrometry were used to analyse the microstructure and the fractography of broken specimens. The quasi-static and fatigue tests were performed on the MTS Landmark 100 kN servo-hydraulic test machine, controlled with a mechanical extensometer with a 25 mm gauge length. The quasi-static strength of the analysed aluminium alloy AA 6086 was found to be significantly higher if compared to some other AA 6xxx alloys, while the ductility was kept almost the same. The experimental results of the comprehensive fatigue tests in a Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) regime showed a good fatigue resistance, and represent a good basis for engineering design applications of the newly developed aluminium alloy AA 6086.

Origin of hard magnetism in Fe-Co-Ni-Al-Ti-Cu high-entropy alloy: Chemical shape anisotropy

Some non-equiatomic magnetic high-entropy alloys (HEAs), e.g. Fe2CoNiAlCu0.4Ti0.4 (at.%), have recently been found to yield appealing hard magnetic properties even at as-cast state, showing great potential in developing advanced hard magnets. However, the underlying origin remains unclear, restricting the effective manipulation towards better magnetic performance. By detailed microstructural and chemical investigations, in this work, it was found that the as-cast Fe2CoNiAlCu0.4Ti0.4 (at.%) alloy has a hierarchically heterogeneous microstructure, where the nano-arrays enriched or depleted with ferromagnetic element Fe exhibit large aspect ratio (defined as chemical shape anisotropy, CSA), penetrating through the cellular nanostructure consisting of A2 (disordered body-centered cubic, bcc) and L21 (ordered bcc) phases. The aspect ratio and elemental concentration of the nano-arrays can be further manipulated by magnetic annealing (MA), significantly enhancing the coercivity. These results suggest that the nanoscale CSA is responsible for the hard magnetism. Further study showed that the CSA is not strengthened continuously with MA time because over-annealing creates interfacial precipitations of hexagonal ω and face-centered-cubic (fcc) Cu-rich nanoparticles, deteriorating both coercivity and saturation magnetization. As a result, a ∼49% enhancement in coercivity can be achieved by balancing the strengthened CSA and the detrimental interfacial precipitates. Consequently, this study not only uncovers that the nanoscale CSA accounts for the hard magnetism of Fe-Co-Ni-Al-Ti-Cu HEAs, but also provides a feasible approach to manipulate CSA towards better magnetic performance.

Optimization of T5 heat treatment in high pressure die casting of Al–Si–Mg–Mn alloys by using an improved Kampmann-Wagner numerical (KWN) model

The increased use of lightweight aluminum die castings in the automotive industry leads to new die casting alloys and their heat treatment processes to meet performance requirements. In this paper, an integrated precipitation and strengthening framework based on an improved Kampmann-Wagner numerical (KWN) model was developed to predict the hardness and yield strength (YS) of high pressure die casting (HPDC) Al–11Si-xMg (x = 0.3 and 0.6) alloys during T5 aging (artificial aging after casting) process. The hardness and YS of the alloys were measured and compared with the predication results, for T5 treatment at 190 °C, 210 °C, 225 °C and 240 °C, respectively. The increased Mg content (from 0.3 to 0.6%) leads to the formation of Mg2Si phase during solidification, which effectively increases the hardness and YS in as-cast conditions. Also, the higher Mg content results in increased peak strengths because of higher precipitate density of needle-like Mg5Si6-β″ phase. This improved KWN model showed a good accuracy with experimental results. Mechanical property maps were generated to guide the design and optimization of T5 process window for these two important die casting aluminum alloys.

Investigation of the Superlattice Phases Formed in Ta72Ru28.

The Ta-Ru binary phase diagram has not been fully investigated, but shows potential for a two-phase region of A2 + B2. Given the high melting points of both Ta and Ru, such an alloy would have the potential for high temperature strength. A Ta72Ru28 alloy was arc melted and investigated in the as-cast and aged (at 1000 °C) states. The as cast alloy was composed of A2 and B2, albeit not in a superalloy-like morphology. A third phase was found in the aged alloy, which has not been reported before, and which is also a coherent superlattice phase of the Ta BCC matrix. The structure of this phase was found to be consistent with the tetragonal Cr2Al prototype structure, with lattice parameters of (a, a, 3a), where a is the Ta BCC lattice parameter.

A novel AlMoNbHfTi refractory high-entropy alloy with superior ductility

Refractory high-entropy alloys (RHEAs) have attracted considerable attention in the recent pastdue to their high yield strength (YS) and superior high-temperature mechanical properties. However, poor ductility at room temperature and high density are still major challenges in their processing and industrial application. Here, two novel single-phase body-centered cubic (BCC) structured Al5MoxNb36Hf13Ti46-x (x = 5, 7 at%, marked as Mo5 and Mo7) RHEAs with excellent room-temperature ductility and low density (7.53–7.64 g/cm3) were designed and prepared by tailoring the Mo content. These two RHEAs have a good combination of ambient ductility, high YS, and low density. The as-cast state Mo5 and Mo7 alloys show ∼ 20%, ∼ 15% tensile ductility and 680, 820 MPa YS, respectively. The YS increased to 892 MPa for the Mo5 alloy and up to 900 MPa for the Mo7 alloy after cold rolling and heat treatment, respectively. The peak strength at 873 K increased from 666 MPa of the Mo5 alloy to 710 MPa of the Mo7 alloy, and the peak strength of the Mo7 alloy at 1073 K was higher than 450 MPa. According to the solid solution strengthening model, the high strength can be attributed to the atomic radius misfit and shear modulus misfit of the alloy components, as well as high atomic-level pressure. Increasing the Mo content will introduce a solid solution strengthening effect into the RHEAs. The large tensile ductility arises owing to the dense dislocation slip and no twins or the second phase was detected in the tensiled sample. This study provides more helpful information to design RHEAs with superior ambient tensile ductility.

Influence of constituent particles on fracture of aluminum alloys under high-triaxiality loading

Single-edge notch bending tests are conducted to study the influence of constituent particles on the fracture resistance of aluminum alloys 6061, 6063, and 6110 under high-constraint loading conditions. The alloys are tested in the as-cast state after homogenization and artificial aging to temper T6. Each alloy type was delivered with two different volume fractions of constituent particles to enable a quantitative assessment of its impact on the toughness of these aluminum alloys. One variant corresponds to the commercial alloy, whereas the other variant is tailor made with an increased amount of constituent particles by adding Fe and Si to the commercial alloy. All alloys exhibit a dendritic structure with particles clustered at grain boundaries and dendrite arm boundaries. The increased content of constituent particles in the tailor-made alloys is shown to be purely detrimental for the toughness and reduces relevant fracture energy parameters by more than 50% in the alloys tested herein. In the plane-strain-dominated regions of the specimens where the stress triaxiality is highest, crack propagation was found to take place on grain boundaries and dendrite arm boundaries due to void nucleation, growth, and coalescence from the constituent particles. Differences in toughness between the alloys are primarily related to variations in the content, size, and spacing of the constituent particles. A comparison between the three different alloy types, i.e. 6061, 6063, and 6110, shows that strength affects the toughness, but it does not follow the commonly reported trade-off between strength and ductility.

Effects of Sn on the densification and microstructure of a Ti-48Al-2Nb-0.7Mn-0.3Si-1Sn alloy fabricated from cold-pressed powders through vacuum melting

The density and microstructure of a modified second-generation γ-TiAl intermetallic alloy with a nominal composition of Ti-48Al-2Nb-0.7Mn-0.3Si-1Sn (at. %) were investigated and compared with similar low-Nb containing alloys reported in the literature. The alloy was successfully fabricated from blended and uniaxial cold-pressed precursor powders and subsequently arc re-melted under vacuum and heat-treated. From the obtained results, the alloy showed improved castability as evidenced by the absence of shrinkage cavities and the highest relative density of 99.87%. In as-cast condition, the microstructure comprised fine-grained dendritic structures of γ-phase and γ + α2 lamellae with a mean grain size of about 49 μm which transformed into a well-developed duplex structure consisting of γ grains and γ + α2 lamellae, and traces of Ti5Si3 precipitates after heat treatment. The alloy exhibited a more significant grain refinement in as-cast condition than its counterparts reported previously.