Present Alloy System Research Articles

Establishing a process route for additive manufacturing of NiCu-based Alloy 400: an alignment of gas atomization, laser powder bed fusion, and design of experiments

Alloy 400 is a corrosion-resistant, NiCu-based material which is used in numerous industrial applications, especially in marine environments and the high-temperature chemical industry. As conventional manufacturing limits geometrical complexity, additive manufacturing (AM) of the present alloy system promises great potential. For this purpose, a robust process chain, consisting of powder production via gas atomization and a design of experiment (DoE) approach for laser powder bed fusion (LPBF), was developed. With a narrow particle size distribution, powders were found to be spherical, flowable, consistent in chemical composition, and, hence, generally applicable to the LPBF process. Copper segregations at grain boundaries were clearly detected in powders. For printed parts instead, low-intensity micro-segregations at cell walls were discovered, being correlated with the iterative thermal stress applied to solidified melt-pool-near grains during layer-by-layer manufacturing. For the production of nearly defect-free LPBF structures, DoE suggested a single optimum parameter set instead of a broad energy density range. The latter key figure was found to be misleading in terms of part densities, making it an outdated tool in modern, software-based process parameter optimization. On the microscale, printed parts showed an orientation of melt pools along the build direction with a slight crystallographic [101] texture. Micro-dendritic structures were detected on the nanoscale being intersected by a high number of dislocations. Checked against hot-extruded reference material, the LPBF variant performed better in terms of strength while lacking in ductility, being attributed to a finer grain structure and residual porosity, respectively.

The dispersion-strengthening effect of TiN evoked by in situ nitridation of NiCu-based Alloy 400 during gas atomization for laser powder bed fusion

Alloy 400 is a widely used material being known for its excellent corrosive resistance. Within the chemical industry and in contrast to conventional manufacturing processes, Laser Powder Bed Fusion (LPBF) of Alloy 400 opens up for functional components that withstand harsh environments. On the basis of a holistic process route, the present work focusses on modifying the chemical composition of the base material with Titanium in order to allow the formation of TiN nanoparticles during powder production and LPBF, respectively, as well as documenting their influence on the mechanical properties. Parameter optimization for gas atomization and LPBF is carried out and the microstructure of both powders and parts is examined. It was found that besides Cu segregations on grain boundaries and dislocation formation on cell walls, TiN successfully formed in both powders and parts. The Ti-enriched parts resulted in enhanced mechanical properties in terms of hardness, tensile and creep due to these homogeneously distributed dispersoids. Hence, nanoparticle integration proved to be feasible and effective for the present alloy system.

Sc doping induced the mechanical property improvement of dual-phase high-entropy alloy

To tailor the generation and proportion of precipitated phase in high entropy alloys (HEAs) to achieve mechanical property improvement, Sc, known as a rare-earth element, is introduced into an equimolar AlCoCrFeNi HEA with a dual-phase structure. Hence, (AlCoCrFeNi)100-xScx HEAs (x = 0, 0.5, 1 and 2, at.%) are fabricated, and also the phases, microstructures, compressive behaviors and strengthening mechanisms are studied in detail. When introducing the Sc element, HEA microstructure evolves from a single BCC structure to a hypoeutectic structure. The hypoeutectic structure contains the primary BCC phase and eutectic region, in which the eutectic region is comprised of BCC phase and Laves phase. With an increase of Sc addition, the yield strength exhibits an improvement from ∼1293 to ∼1441 MPa due to solid solution strengthening and precipitated phase strengthening. However, the fracture strain decreases from 32.5% to 23.6%, and meanwhile compressive strength lowers from ∼2352 to ∼2018 MPa due to high stress/strain concentration stemming from the incompatibility of elastic and plastic deformation between precipitated eutectic phase and primary BCC phase. Moreover, the brittle fracture mechanism of the present alloy system is identified.

Microstructural Constituents and Mechanical Properties of Low-Density Fe-Cr-Ni-Mn-Al-C Stainless Steels.

Metallic material concepts associated with the sustainable and efficient use of resources are currently the subject of intensive research. Al addition to steel offers advantages in view of lightweight, durability, and efficient use of high-Fe scrap from the Al industry. In the present work, Al was added to Fe-12Cr-(9,12)Ni-3Mn-0.3C-xAl (x = 0.1–6) (wt.%) stainless steels to assess its influence on microstructure and mechanical properties. According to density measurements based on Archimedes’ principle, densities were between 7.70 and 7.08 g/cm3. High-energy X-ray diffraction estimations of the lattice parameter indicated that nearly 31% of density reduction was caused by the lattice expansion associated with Al addition. Depending on Al concentration, austenitic and duplex matrix microstructures were obtained at room temperature. In the presence of up to 3 wt.% Al, the microstructure remained austenitic. At the same time, strength and hardness were slightly enhanced. Al addition in higher quantities resulted in the formation of duplex matrix microstructures with enhanced yield strength but reduced ductility compared to the austenitic alloys. Due to the ready formation of B2-(Ni,Fe)Al intermetallics in the ferrite phase of the present alloy system, the increase in strength due to the presence of ferrite was more pronounced compared to standard duplex stainless steels. The occurrence of B2 intermetallics was implied by dilatometry measurements and confirmed by electron microscopy examinations and high-energy X-ray diffraction measurements.

Influence of C and N on Strain-Induced Martensite Formation in Fe-15Cr-7Mn-4Ni-0.5Si Austenitic Steel.

In this study, the effect of interstitial contents on the mechanical properties and strain-induced martensite formation in an austenitic stainless steel was investigated. The mechanical properties of solution annealed Fe-15Cr-7Mn-4Ni-0.5Si-(0.01-0.2)N-(0.01-0.2)C concentrations in weight percent stainless steels were studied using room temperature tensile tests. All three alloys used in the present study have a sum content of C + N of about 0.2 wt.%. To verify the influence of C and N on deformation behavior, microstructural investigations are performed using light optical microscopy, scanning electron microscopy, and magnetic and hardness measurements. Moreover, strain-induced α′-martensite nucleation was characterized by scanning electron microscope using EBSD. In the present alloy system, carbon provides a stronger austenite stabilizing effect than nitrogen. Hence, the smallest amount of strain-induced α′-martensite was formed in the steel alloyed with 0.2 wt.% C. It also exhibited the optimal mechanical properties, including the highest ultimate tensile strength (1114 MPa), uniform elongation (63%), and total elongation (68%). Moreover, the interstitial content influences the occurrence of dynamic strain aging (DSA), which was only observed in the steel alloyed with carbon. With increasing C content, the triggering strain for DSA decreases, which can be confirmed by in situ magnetic measurements during tensile testing.

The effects of misfit and diffusivity on γʹ rafting in Re and Ru containing Nickel based single crystal superalloys—details in thermodynamics and dynamics

Rafting is one of the prominent features of Nickel single crystal superalloys in high temperature creep deformation, which has significant influence on creep rupture property. Some detailed comparisons were carried out to illustrate the different effects of misfit and diffusion on the rafting behavior. Its thermodynamic and kinetic processes were also discussed in the present study. Firstly, comparing with the lattice misfit, atom diffusion shown more remarkable influence on γʹ rafting in the present alloy system. Second, the misfit stress release made Gibbs free energy of γ and γʹ phases move in the opposite direction, which provided driving force for the dissolution of γʹ in horizontal channel. Thirdly, external loading could not change the fraction of γʹ phase, the dissolved part of γʹ must re-precipitated in the adjacent vertical channel powered by rafting driving force. At last, the atoms migration by diffusion were mainly occurs in the disordered γ channel, the pipe diffusion along interfacial dislocations accelerated the efficiency of atom migration for rafting.

Phase formation and magnetic properties of high-entropy metallic glasses in (Fe, Co, Ni)-P-B alloy system with non-equiatomic ratio

In this paper, a series of Fe25+2xCo27.5-xNi27.5-xP14B6 (x = 0, 2.5, 5, 7.5 and 9) high entropy alloys (HEAs) were fabricated via melt-spinning and copper-mold casting, and their phase formation, thermal stability and magnetic properties were investigated. It was found that different phase structures were formed in this alloy system, depending on the variation of composition and cooling rate (Ra). Either a single amorphous phase or multiple phases containing solid-solution phase + compound phase complex were formed in these HEAs depending on Ra. Meanwhile, for Fe30Co30Ni30P7B3 alloy, only a simple solid-solution phase was formed under the fabrication of this work, indicating its insensitivity to Ra. This phenomenon provides a clue to the selection of the desirable phase structure and tailoring magnetic properties by adjusting the composition and Ra. Among this present alloy system, Fe25Co27.5Ni27.5P14B6 alloy ribbon exhibits the lowest coercivity (Hc) of 1.42 A/m, while Fe43Co18.5Ni18.5P14B6 alloy ribbon has the highest magnetic saturation (Bs) of 1.0 T. For Fe30Co30Ni30P7B3 alloy, the Hc decreases from 429.84 A/m to 114.50 A/m by rapid solidification. This work suggests a new route to overcome the restrictions of compositional design by equiatomic ratio to improve soft magnetic properties of HEAs in a wide compositional space.

Effect of alloyed Ca on the microstructure and corrosion behavior of extruded Mg-Bi-Al-based alloys

Microstructural characteristics and corrosion behavior of extruded Mg-1Bi-1Al-xCa (x = 0, 0.6 and 1.2 wt%) alloys were investigated. It was found that the average grain size, texture intensity and corrosion resistance of the present low-alloyed alloy system initially increased and then decreased with increasing Ca content. Intergranular corrosion was observed in the present alloy system, which was related to the higher energy of the grain boundaries. Furthermore, the corrosion mode changed from pitting corrosion in the Ca-free alloy to the mixed pitting and filiform corrosion in the Ca-containing alloys in the initial stage of immersion based on the analysis in terms of micro-galvanic corrosion and the formation of protective film. As the immersion time increased, the filiform corrosion dominated in the Mg-1Bi-1Al-0.6Ca alloy. However, the corrosion product film of the Mg-1Bi-1Al-1.2Ca alloy was quickly destroyed due to the shedding of the coarse Mg2Bi2Ca2 phase, which resulted in severe corrosion.

Improved crystallographic compatibility and magnetocaloric reversibility in Pt substituted Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy

We present here the improved crystallographic/geometric compatibility and magnetocaloric reversibility by measurement of magnetic entropy change using different protocols in 10% Pt substituted Ni2Mn1.4In0.6 magnetic shape memory alloy. The substitution of Pt reduces the thermal hysteresis about 50% to the Ni2Mn1.4In0.6. The origin of the reduced thermal hysteresis is investigated by the crystallographic compatibility of the austenite and martensite phases. The calculated middle eigenvalue of the transformation matrix turned out to be 0.9982, which is very close to 1 (deviation is only 0.18%) suggests for the crystallographic compatibility between the austenite and martensite phases in Ni1.9Pt0.1Mn1.4In0.6. A very small thermal hysteresis and crystallographic compatibility between two phases in this alloy system indicate a stress-free transition layer (i.e. perfect habit plane) between the austenite and martensite phase, which is expected to give reversible martensite phase transition and therefore reversible magnetocaloric effect (MCE) as well. The calculated value of the isothermal entropy change (ΔSiso) using the magnetization curve under three different measurement protocols (i.e. isothermal, loop, and isofield measurement protocol) is found to be nearly same indicating a reversible MCE in the present alloy system. Our work provides a path to design new magnetic shape memory Heusler alloys for magnetic refrigeration and also suggest that any of the above measurement protocol can be used for the calculation of ΔSiso for materials satisfying geometrical compatibility condition.

Kinetic Constraints of δ-Ferrite to the Formation of Kappa (κ) Carbide in a Fe-4Mn-9Al-0.3C Wt Pct Low-Density Steel

Aluminum is the key ingredient in low-density steels. It leads to the formation of δ-ferrite in a large fraction. In order to study the influence, two different experimental approaches were devised. In a first of its kind, the sample was arc melted in an argon atmosphere. Later, the same alloy composition was ball milled and sintered at 1000 °C by avoiding liquid to δ-ferrite in the system. In the current study, the comparative analysis led by DICTRA simulation indicates that kappa (κ) carbide has difficulty forming in the present alloy system when δ-ferrite is already available, particularly during solidification casting.

Negative Strain Rate Sensitivity in Two-Phase Nanocrystalline CoCrFeMnNi High-Entropy Alloy with Broader Grain Size Distribution Studied by Nanoindentation

Two-phase microstructure was realized in CoCrFeMnNi system using a combination of ball milling and spark plasma sintering at 1273 K. Microstructure consisted of finer average grain size with broader grain size distribution, complex solid solution phases (a major face-centered cubic phase and a minor body-centered cubic phase) with very close lattice constants, dislocations and nano twins. A high hardness of 6.3 GPa and a negative strain rate sensitivity (with no associated serrated flow) of − 0.0206 were observed. Various interactions between dislocations and grain boundaries/interphase boundaries/twin boundaries, prevailing atomic level complex chemistry in the lattices, interfaces and dislocation cores might be the reasons for the observed flow characteristics. Dislocation–solute atom interactions observed in conventional dilute alloys that have resulted in negative strain rate sensitivity and dynamic strain aging may not be operating in the present alloy system under the given set of test conditions.

Quantum criticality in the (Cr98.4Al1.6)100-yMoy alloy system

Indications of quantum critical behaviour (QCB), driven by electron-hole pair breaking effects through Mo doping of Cr98.4Al1.6 are reported. Previous studies on the electrical resistivity (ρ), Seebeck coefficient (S), Sommerfeld electronic specific heat coefficient (γ) and magnetic susceptibility (χ) as a function of temperature on the (Cr98.4Al1.6)100-yMoy alloy system point towards the existence of a quantum critical point (QCP) at yc ≈ 4.5. In the present study the Hall coefficient RH measurements and an analysis of the previous results are used to substantiate the presence of a QCP in the alloy system. The charge carrier density at 2 K, (qRH(2K))−1, of the alloy systems increases continuously with y from the antiferromagnetic (AFM) phase through yc ≈ 4.5 and into the paramagnetic (P) phase. Scaling relationships between the magnetic contribution to the electrical resistivity, Δρ2K, the magnetic contribution to the Sommerfeld electronic specific heat coefficient, Δγ, and (qRH(2K))−1, are furthermore used to support the presence of a QCP in the alloy system. dS/dT in the temperature limit T→ 2 K depicts an anomalous behaviour in the form of an upturn on decreasing y through the QCP, yc ≈ 4.5. The critical exponents in the vicinity of the QCP were found to be β = 0.5 ± 0.1 for the Δρ2K/ρ2K(y) curve and γ = 0.4 ± 0.1 for the TN(y) curve. The nature of the QCP in the present alloy system is classified as an incommensurate (I) spin-density-wave (SDW) type, driven mainly by electron-hole pair breaking effects.

Microstructure and Phase Transformation in Ni50Mn40Sn10 Shape Memory Alloy

In the present work, a ternary Heusler alloy of composition Ni50Mn40Sn10 with a high martensitic transformation temperature has been prepared by arc-melting technique. The crystal structure of the obtained alloy is investigated by X-ray diffraction. The microstructure of the specimen sintered at 950°C for 72 h followed by ice-water quenching shows needle-like martensitic structure. Energy dispersive spectroscopy is utilized for compositional study. The differential scanning calorimetry result shows that the alloy undergoes a martensitic transformation at 190°C upon cooling. The M–H curve suggests paramagnetic nature of the martensite phase at room temperature. The present alloy system has a great potential to be used for high-temperature shape memory applications. Experimental data of this study may be useful for developing Ga-free high-temperature shape memory alloys.

Study of magneto-structural phase transitions and magnetocaloric effects in Co-based Heusler alloys synthesized via mechanical milling

In the present study, the effect of complete replacement of Cr by Fe dramatically enhances the structural, magnetic and magnetocaloric effect (MCE) of Co2(Cr1−xFex)Al (x = 0 and 1) Heusler alloys was studied. The present Heusler alloy systems were successfully prepared by mechanical alloying via high energy ball milling technique. Evolution of the B2 type disordered phase along with A2 type structure has been observed through X-ray diffraction. The substitution of Fe would lead to the development of remarkable enhancement of MCE observed near room temperature. The magnetic entropy (ΔSM) values of 10.77 J/kg-K and 17.46 J/kg-K, and refrigerant capacity (RC) 136.31 mJ/cm3 and 30.30 mJ/cm3 respectively are observed under an applied field of 20 kOe. The increment in magnetic entropy and refrigeration capacity with increasing Fe was associated with variation in lattice defects, and the presence of small amounts of magnetic/nonmagnetic secondary phases. Therefore, the increment in magnetic properties of these alloys was attributed to the enhancement of d-d exchange interaction due to the possible occupancy of vacant sites by Fe atoms, which indicates the development of relative anisotropy.

Effects of rare-earth element, Y, additions on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

The effects of rare-earth element, Y, additions on the microstructures and mechanical properties of CoCrFeNi alloy have been investigated. The new series of CoCrFeNiYx (x = 0, 0.05, 0.1, 0.2, and 0.3) high entropy alloys were synthesized by vacuum arc-melting method. Microstructure characterizations were performed by the means of X-ray diffraction, scanning electron microscope, and transmission electron microscope. It was found that alloying Y element could lead to the formation of a simple hexagonal structure phase (CaCu5 type). And another Ni3Y-type hexagonal structure phase was observed in the alloy with higher Y contents (0.3 at%). The phase evolution of the present alloy system was evaluated using the previous criteria (ΔHmix−δ,Ω−δ,Δχ, VEC, and Λ). The results of the nanoindentation measurements on different phases indicated that the hexagonal structure phases (~ 10.5 GPa) had a higher nanohardness than the face-centered cubic phase (~ 3 GPa). Furthermore, the maximum shear stress of the incipient plasticity was calculated to be 3.2 GPa for the face-centered cubic phase and 5.2 GPa for the hexagonal structure phase. The dislocation nucleation under the indenter in different phases was also discussed. The Vickers hardness and yield strength increased with increasing the Y content, while the fracture strength and plastic strain decreased. The strengthening mechanisms of the present alloys included solid-solution strengthening and the second phase strengthening. Fracture surface observations suggested a ductile fracture in the face-centered cubic phase and a cleavage fracture in the hexagonal structure phase for the alloys containing Y element. The results of this investigation can provide a guide for the design of new high entropy alloy system with excellent properties.

Kinetics and thermal stability of the Ni62Nb38–x Ta x (x=5, 10, 15, 20 and 25) bulk metallic glasses

We studied thermal stability and its relationship to the glass-forming ability (GFA) of the Ni62Nb38–xTax (x=5, 10, 15, 20, 25) bulk metallic glasses (BMG) from a kinetic point of view. By fitting the heating-rate dependence of glass transition temperature (Tgonset) and crystallization temperatures (Txonset and Txpeak) of the Ni62Nb38–xTax BMG using the Vogel-Fulcher-Tammann (VFT) equation, we obtained the ideal glass transition and crystallization temperatures (Tg0 and Tx0) and the fragility parameter (m), and also constructed continuous heating transition (CHT) diagrams for crystallization of the BMG. The CHT diagrams of the BMG indicate enhanced thermal stability by Ta addition; the Tg0 as well as the Tx0 also illustrates this improved stability limit. The compositional dependence of m, which agrees well with that of the reduced glass-transition temperature, indicates a strong correlation between liquid fragility and glass-forming ability in the present alloy system. These results provide new evidence for understanding thermal stability, liquid fragility, and GFA in BMG.

Relationship between crystallographic compatibility and thermal hysteresis in Ni-rich NiTiHf and NiTiZr high temperature shape memory alloys

The relationship between the crystallographic compatibility of austenite and martensite phases and the transformation thermal hysteresis (ΔT) of Ni-rich Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys undergoing B2–B19′ martensitic transformation was studied as a function of microstructure, via differential scanning calorimetry, transmission electron microscopy, and X-ray diffraction. An experimental linear relationship of ΔT vs λ2 (the second eigenvalue of the transformation stretch matrix) was observed for these NiTi(Hf/Zr) alloys, but with a shallower slope as compared to the universal behavior followed by alloys showing B2–B19 martensitic phase transformation. Several ternary NiTiCu and binary NiTi alloys undergoing the B2–B19′ transformation were also found to deviate from the universal behavior attributed to alloys that undergo a B2–B19 transformation, and instead, follow the trend revealed for the present alloy systems. Aged NiTi(Hf/Zr) samples, which consist of very fine nano-precipitates, followed the newly established ΔT vs λ2 linear relationship, due to only minor changes in the microstructure. In contrast, samples with large precipitates, exhibited a large deviation from this relationship due to much more drastic changes in microstructure. Finally, despite the poor crystallographic compatibility of the austenite and martensite lattices observed in the present alloys, rationalized by large deviation of λ2 values from 1, relatively low ΔT values were measured. This behavior is actually consistent with the newly established relationship for ΔT vs λ2 for B2–B19′ transforming alloys, which is much less sensitive to compatibility (shallower slope). It is concluded that such a difference in the ΔT vs λ2 slope must be a consequence of the crystallography of monoclinic martensite formation in NiTi-based alloys as long as other factors such as plasticity or major constraints to the martensitic transformation do not intervene.

Superior Mechanical Properties of AlCoCrFeNiTi x High-Entropy Alloys upon Dynamic Loading

High-entropy alloys with composition of AlCoCrFeNiTi x (x: molar ratio; x = 0, 0.2, 0.4) under quasi-static and dynamic compression exhibit excellent mechanical properties. A positive strain-rate sensitivity of yield strength and the strong work-hardening behavior during plastic flows dominate upon dynamic loading in the present alloy system. The constitutive relationships are extracted to model flow behaviors by employing the Johnson-Cook constitutive model. Upon dynamic loading, the ultimate strength and fracture strain of AlCoCrFeNiTi x alloys are superior to most of bulk metallic glasses and in situ metallic glass matrix composites.

Ductility behaviour of cubic titanium niobium nitride ternary alloy: a first-principles study

The ductility and hardness behaviour of NbxTi1−xN (x = 0, .25, .5, .75 and 1) ternary alloy has been studied. Bulk modulus, Young’s modulus, shear modulus, Poisson’s ratio and anisotropy energy have been calculated. Analysis of G/B and Cauchy’s pressure shows that TiN is brittle in nature in good agreement with other theoretical results. Systematic addition of Nb with TiN shows that Nb.75Ti.25N is ductile. The charge density plot shows weak directional contours that enclose Ti and N due to the ductile behaviour of the alloy, namely Nb.75Ti.25N. The estimated hardness of Nb.75Ti.25N is 19.78 GPa, which is 70 % of hardness value of TiN (29.4 GPa) and thus addition of niobium enhances the ductility of TiN. The Debye temperature of the present alloy system is also reported.

Influence of Ti addition on the microstructure and mechanical properties of a 5% Cr–Mo–V steel

A systematic study to determine the effect of Ti addition on the microstructure and mechanical properties of a 5% Cr–Mo–V tool steel has been made in terms of optical and electron microscopy, as well as X-ray diffraction, thermal analysis, and small angle neutron scattering. With the characterization of the as-quenched structure, the microstructural variation as a function of aging time was investigated. The Ti addition effectively refines the prior austenite grain structure as expected. More importantly, it can change the precipitation behavior of secondary carbides formed upon aging. As this change is attributed to the resisted Cr diffusion into cementite, the resultant precipitation of M23C6 carbides was restricted. Hence this leads to a significant improvement of the hardness to toughness balance, which is coupled with grain refinement. This variation in precipitation behavior was considered to have a thermodynamic aspect, especially the driving force for nucleation of equilibrium carbides involved in the present alloy system.