Phase transformations in nanocrystalline alloys synthesized by mechanical alloying

The effect of nanometer grain size and extensive grain boundary regions in nanocrystalline alloy systems was investigated for the chemical order-disorder, structural, precipitation, and spinodal phase transformations. The kinetic paths for approach to the chemically ordered phase from the disordered phase in FeCo-Mo alloys were observed to be the same at different temperatures due to grain boundaries acting as short-circuited diffusion paths for atom movements. The structure of Fe₃Ge was bcc for small crystallite size and the equilibrium fcc phase developed only after a critical grain size was attained. This was understood as a manifestation of the Gibbs Thomson effect. The precipitation phase transformation in Fe-Mo alloys proceeded by a rapid movement and clustering of the Mo atoms to the grain boundaries that was correlated to the size of the nano grains, and subsequent formation of the Mo rich lambda phase directly in the grain boundary regions. The composition fluctuation domains for spinodal decomposition in nanophase Fe-Cr alloys were observed to be linearly correlated to the growth of grains.     

Effect of molybdenum on grain growth of W-Mo-Ni-Fe heavy alloys

The grain growth phenomena of tungsten heavy alloys with various concentrations of Mo were investigated. The early formation of a eutectic liquid phase with the alloying of Mo to W-Ni-Fe facilitated growth and spheroidization of tungsten grains in the initial stage of isothermal hold. However, when the concentration of molybdenum was so high as to leave behind non-dissolved Mo grains, coalescence of W grains around Mo grains resulted in the formation of larger W-Mo grains having Mo-rich cores. These larger grains later grew at the expense of the smaller grains and grain growth was statistically very fast. During grain growth in the later stage of isothermal hold, W had a higher potential than Mo to precipitate from the liquid phase onto the grains. This competitive behavior led to the gradual accumulation of Mo atoms in the liquid phase, which not only retarded the growth of grains, but also caused precipitation of intermetallic phases in the interfaces between the solid grains and liquid phase during cooling.     

The formation, structure and properties of nanocrystalline Ni-Mo-B alloys

The structure, structure evolution and microhardness of nanocrystalline Ni-Mo-B alloys were studied by X-ray diffraction, differential scanning calorimetry, transmission and high resolution electron microscopy and microhardness measurements. The nanocrystalline structure was produced by controlled crystallization of amorphous alloys. The annealed samples consist of the FCC nanocrystals with the amorphous regions between them. The grain size of the nanocrystals is about 20 nm and depends on the chemical composition of the alloy. The chemical composition of the amorphous phase between the nanocrystals changes at the annealing. A slight grain growth was observed when the annealing time increases. The diffusion of Mo and B from FCC to the amorphous phase occurs at the annealing. It results in the lattice parameter change. The microhardness of the alloys increases during the annealing. The microhardness values are the same in all alloys before the nanocrystalline structure decomposition. The microhardness is inconsistent with the Petch-Hall equation. The microhardness of the alloys is determined by the microhardness of the amorphous phase bands located between the nanocrystalline grains.     

A model of discontinuous precipitation based on the balance and maximum production of the entropy

A model of discontinuous precipitation in supercooled binary polycrystalline alloys at reduced temperatures, taking place as a result of the diffusion-induced grain boundary migration, is constructed with allowance of grain boundary diffusion. The proposed approach allows independent determination of the main parameters, including the interlamellar distance, the maximum velocity of the phase transformation front, and the concentration jump at this boundary. This is achieved by using a set of equations for the (i) mass transfer in the moving interphase boundary, (ii) balance of the entropy fluxes at the phase transformation front, and (iii) maximum rate of the free energy release. The model uses a minimum of thermodynamic information on the two-phase system: the curvature of the Gibbs potential surface in the decomposing phase and the free energy of the interface between the new phases. Theoretical results are compared to the available experimental data.     

Fabrication of nanocrystalline alloys Cu–Cr–Mo super satured solid solution by mechanical alloying

This work discusses the extension of solid solubility of Cr and Mo in Cu processed by mechanical alloying. Three alloys processed, Cu–5Cr–5Mo, Cu–10Cr–10Mo and Cu–15Cr–15Mo (weight%) using a SPEX mill. Gibbs free energy of mixing values 10, 15 and 20 kJ mol⁻¹ were calculated for these three alloys respectively by using the Miedema's model. The crystallite size decreases and dislocation density increases when the milling time increases, so Gibbs free energy storage in powders increases by the presence of crystalline defects. The energy produced by crystallite boundaries and strain dislocations were estimated and compared with Gibbs free energy of mixing values. The energy storage values by the presence of crystalline defects were higher than Gibbs free energy of mixing at 120 h for Cu–5Cr–5Mo, 130 h for Cu–10Cr–10Mo and 150 h for Cu–15Cr–15Mo. During milling, crystalline defects are produced that increases the Gibbs free energy storage and thus the Gibbs free energy curves are moved upwards and hence the solubility limit changes. Therefore, the three alloys form solid solutions after these milling time, which are supported with the XRD results.     

Microstructure and mechanical properties of ultra-fine grained MoNbTaTiV refractory high-entropy alloy fabricated by spark plasma sintering

The MoNbTaTiV refractory high-entropy alloy(RHEA) with ultra-fine grains and homogeneous microstructure was successfully fabricated by mechanical alloying(MA) and spark plasma sintering(SPS).The microstructural evolutions,mechanical properties and strengthening mechanisms of the alloys were systematically investigated.The nanocrystalline mechanically alloyed powders with simple bodycentered cubic(BCC) phase were obtained after 40 h MA process.Afterward,the powders were sintered using SPS in the temperature range from 1500 ℃ to 1700 ℃.The bulk alloys were consisted of submicron scale BCC matrix and face-centered cubic(FCC) precipitation phases.The bulk alloy sintered at 1600℃ had an average grain size of 0.58 μm and an FCC precipitation phase of 0.18 μm,exhibiting outstanding micro-hardness of 542 HV,compressive yield strength of 2208 MPa,fracture strength of 3238 MPa and acceptable plastic strain of 24.9% at room temperature.The enhanced mechanical properties of the MoNbTaTiV RHEA fabricated by MA and SPS were mainly attributed to the grain boundary strengthening and the interstitial solid solution strengthening.It is expectable that the MA and SPS processes are the promising methods to synthesize ultra-fine grains and homogenous microstructural RHEA with excellent mechanical properties.     

Study on martensitic transformation of mechanically alloyed nanocrystalline Fe–Ni

Phase transformation of Fe–Ni powders with different nickel content during mechanical alloying was studied, as well as reverse transformation of mechanically alloyed nanocrystalline Fe–Ni upon heating. Results show that nickel content plays an important role in the phase transformation tendency during mechanical alloying. When heated at 300 °C, neither grain size nor phase changes in Fe–30 wt.% Ni milled for 80 h, indicating the nanometer-sized martensite is very stable below 300 °C. When the temperature increases to 350 °C, concurrently with grain growth reverse transformation takes place. The reverse transformation temperature of mechanically alloyed nanocrystalline Fe–Ni is higher than that of bulk alloys.     

Phase transformation and precipitation hardening behavior of Cr and Fe in BS40CrFeSn alloy

Phase transformation and precipitation hardening behavior of the water atomized copper alloy powder was studied by aging treatment, to develop high strength Cu–40Zn–X (X: Cr, Fe, Sn) alloys by powder metallurgy process. Super-saturated solid solution elements of Cr and Fe are formed in the brass matrix, and single β phase was retained in the raw powder after water atomization. Solid solubility of Cr and Fe decreased with increase of aging temperature, and phase transformation evolved from single β phase to α + β duplex phase structure after aged at the elevated temperature of 773 K and over. It was clarified that Cr showed higher precipitation potential than Fe in the brass matrix. The hardness depended strongly upon solid solubility of Cr and Fe, and upon phase transformation.     

Preparation and Characterization of Multilayered Al/Fe-Mo-Si-B Alloy with Nanostructure

The bulk multilayered Al/Fe-Mo-Si-B alloy with nanostructure was prepared by annealing the alternate layers consisting of metal Al and amorphous (Fe0.99,Mo0.01)78 Si9B13 alloy ribbons for 30 min at 873 K under pressure of 3~5.5 GPa. The structures and grain sizes of the Fe-MoSi-B nanocrystalline alloy were measured and analyzed. It was found that the pressure could restrain the growth of the grains and influence the formation of phases. The dependence of grain sizes for α-Fe(Mo,Si) and Fe2B on pressure was given. The morphologies of Al/Fe-Mo-SiB nanocrystalline alloy intedeces were observed by SEM. Two intedecial phases formed at various pressures were established by TEM and EDAX, and an unknown Fe-rich one with nanostructure was also observed. The dependence of the intedecial phases on pressure and its formation and growth mechanism were discussed     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Formation of nanocrystalline phases during thermal decomposition of amorphous Ni-P alloys by isothermal annealing

The microstructure and the average grain size were investigated by x-ray diffraction and transmission electron microscopy for nanocrystalline (n) Ni-P alloys with 18, 19, and 22 at.% P. A detailed study of the nanocrystalline states obtained along different heat treatment routes has been performed: (1) a-->ni by isothermal annealing of the melt-quenched amorphous (a) Ni-P alloys; (2) ni-->nii by isothermal annealing of the nanocrystalline ni state; (3) ni-->nii by linear heating of the ni state. The heats evolved during the structural transformations were determined by differential scanning calorimetry. From these studies, a scheme of the structural transformations and their energetics was constructed, which also includes previous results on phases obtained by linear heating of the as-quenched amorphous state of the same alloys. Grain boundary energies also have been estimated. In some cases it was necessary to assume a variation of the specific grain boundary energy during the phase transformation to understand the enthalpy and microstructure changes during the different heat treatments.     

Microstructural evolution and strengthening mechanisms in Ti–Nb–Zr–Ta,Ti–Mo–Zr–Fe and Ti–15Mo biocompatible alloys

The microstructural evolution and the strengthening mechanisms in the two quaternary alloys, TNZT (Ti–34Nb–9Zr–8Ta) and TMZF (Ti–13Mo–7Zr–3Fe), and one binary alloy, Ti–15Mo, have been investigated. In the homogenized condition both the quaternary alloys exhibited a microstructure consisting primarily of a β Ti matrix with grain boundary α precipitates and a low volume fraction of primary α precipitates while the binary alloy showed single-phase microstructure with large β grains. On ageing the homogenized alloys at 600 °C for 4 h, all the alloys exhibited the precipitation of refined scale secondary α precipitates distributed homogeneously in the β matrix. However, after ageing while the hardness of TMZF marginally increased, that of the TNZT and Ti–15Mo alloys decreased. Furthermore, the modulus of TNZT decreased while other two alloys showed opposite trends. TEM studies indicate that there is initially a B2 ordering in TNZT that is destroyed after ageing causing a reduction in both hardness and modulus of this alloy. Also in Ti–15Mo, dissolution of ω precipitates on ageing causes the hardness to reduce, while the precipitation of secondary α causes an increase in the modulus. Using these examples, the important influence of thermal processing on the property–microstructure relationships in orthopaedic alloys for implant applications will be highlighted.     

第三组元(C、B)添加对Fe83Ga17合金相结构和磁致伸缩性能的影响

为了研究非平衡凝固条件下(Fe83Ga17)100-xMx(x=0、0.5、1、1.5;M=C、B)合金的相组成及其磁致伸缩性能,采用吹铸的方式制备了(Fe83Ga17)100-xMx,(x=0、0.5、1、1.5;M=C、B)合金,结果表明,合金保持了A2(bcc-Fe(Ga))相结构.C、B元素对合金微观组织产生了很大影响,C完全固溶于bcc-Fe中,B于晶界处大量富集,晶粒形状呈区域定向排列.添加C、B均对Fe83Ga17合金的磁致伸缩性能产生了抑制作用.添加C增大了弹性模量,当x=1时磁致伸缩值最大;添加B对合金组织产生了巨大影响,随着B含量的增加形成了明显的铸造织构,并且生成了富含Fe2B的相,x=1.5时磁致伸缩值最大.     

Comparison of microstructural evolution in Ti-Mo-Zr-Fe and Ti-15Mo biocompatible alloys

The microstructural evolution and attendant strengthening mechanisms in two biocompatible alloy systems, the binary Ti-15Mo and the quaternary Ti-13Mo-7Zr-3Fe (TMZF), have been compared and contrasted in this paper. In the homogenized condition, while the Ti-15Mo alloy exhibited a single phase microstructure consisting of large β grains, the TMZF alloy exhibited a microstructure consisting primarily of a β matrix with grain boundary α precipitates and a low volume fraction of intra-granular α precipitates. On ageing the homogenized alloys at 600 ^∘C for 4 h, both alloys exhibited the precipitation of refined scale secondary α precipitates homogeneously in the β matrix. However, while the hardness of the TMZF alloy marginally increased, that of the Ti-15Mo alloy decreased substantially as a result of the ageing treatment. In order to understand this difference in the mechanical properties after ageing, TEM studies have been carried out on both alloys in the homogenized and homogenized plus aged conditions. The results indicate that the ω precipitates dissolve on ageing in case of the Ti-15Mo alloy, consequently leading to a substantial decrease in the hardness. In contrast, the ω precipitates do not dissolve on ageing in the TMZF alloy and the precipitation of the fine scale secondary α leads to increased hardness.     

Effects of sintering aids on the densification of Mo–Si–B alloys

The effects of seven sintering aids (0.5?at.% Ni, Co, Fe, Cr, Zr, Nb, and Pd) on the densification of Mo–Si–B alloys of six different compositions (Mo, Mo–0.2Si, Mo–0.2Si–0.02B, Mo–2.5Si–2.5B, Mo–7Si–5B, and Mo–8.9Si–7.7B?at.%) are systematically investigated. It was found that Ni, Co, and Fe are effective in enhancing densification of Mo–Si–B alloys, and Ni is the most effective sintering aid. This study supports a previously proposed hypothesis that activated sintering results from enhanced mass transport in the sintering-aid-induced quasi-liquid intergranular films (a type of grain boundary complexion). The relative effectiveness of these sintering aids can be rationalized by analyzing several key thermodynamic parameters that control the stability of premelting-like grain boundary complexions. Future studies are needed to develop interfacial thermodynamic models and methods for computing “grain boundary complexion (phase) diagrams” for multicomponent systems, which can be a useful component for the “Materials Genome” project that will enable better predictions of the activated sintering and other materials phenomena.     

Characterisation of interfaces in nanocrystalline palladium

Structures of grain boundaries and triple line junctions in nanocrystalline materials are of interest owing to large fractions of atoms in nanocrystalline materials being at these interfacial positions. Grain boundary and triple line junction structures in nanocrystalline palladium have been studied using high-resolution transmission electron microscopy (HRTEM). The main micro structural features observed include the varying atomic structures of grain boundaries and the presence of disordered regions at triple line junctions. Also, there is variation in lattice parameters in different nanocrystalline grains. Geometric phase analysis is used to quantify atomic displacements within nanocrystalline grains. Displacement fields thus detected indicate links to the interface structures.     

Microstructural Evolution of Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-0.7Nd Titanium Alloy with Carbon Additions

The effect of carbon addition on microstructural evolution was studied in a near-α titanium alloy(Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-0.7Nd). It was found that flake and ribbon titanium carbides with a NaCl crystal structure formed in the as-cast alloys with carbon additions of over 0.17 wt pct. Flake carbide particles are the product of eutectic transformation and precipitate from the high-temperature β phase. The ribbon carbide particles are primary phases formed prior to the nucleation of any metallic phases. The as-cast alloys with carbide precipitation after heat-treatment atβt-30℃ followed by water quenching showed the spheroidization of α lamellae and partial dissolution of carbide particles. After annealing at βt 15℃, carbide particles are mostly distributed at the grain boundary and spheroidized through mixed grain boundary plus bulk diffusions.     

Heterostructured crystallization mechanism and its effect on enlarging the processing window of Fe-based nanocrystalline alloys

The harsh melt-spinning and annealing processes of high saturation magnetization nanocrystalline softmagnetic alloys are the biggest obstacles for their industrialization. Here, we proposed a novel strategy to enlarge the processing window by annealing the partially crystallized precursor ribbons via a heterostructured crystallization process. The heterostructured evolution of Fe_84.75Si₂B₉P_3C0.5Cu_0.75(at.%)alloy ribbons with different spinning rate were studied in detail, to demonstrate the gradient nucleation and grain refinement mechanisms. The nanocrystalline alloys made with industrially acceptable spinning rate of 25-30 m/s and normal annealing process exhibit excellent magnetic properties and fine nanostructure. The small quenched-in crystals/clusters in the free surface of the low spinning rate ribbons will not grow to coarse grains, because of the competitive grain growth and shielding effect of metalloid elements rich interlayer with a high stability. Avoiding the precipitation of quenched-in coarse grains in precursor ribbons is thus a new criterion for the composition and process design, which is more convenient than the former one with respect to the homogenous crystallization mechanism, and enable us to produce high performance nanocrystalline soft-magnetic alloys. This strategy is also suitable for improving the compositional adjustability, impurity tolerance, and enlarging the window of melt temperature,which is an important reference for the future development of composition and process.     

High‐Strength Nanotwinned Al Alloys with 9R Phase

Light‐weight aluminum (Al) alloys have widespread applications. However, most Al alloys have inherently low mechanical strength. Nanotwins can induce high strength and ductility in metallic materials. Yet, introducing high‐density growth twins into Al remains difficult due to its ultrahigh stacking‐fault energy. In this study, it is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned (nt) columnar grains with high‐density 9R phase in Al(Fe) solid solutions. The nt Al–Fe alloy coatings reach a maximum hardness of ≈5.5 GPa, one of the strongest binary Al alloys ever created. In situ uniaxial compressions show that the nt Al–Fe alloys populated with 9R phase have flow stress exceeding 1.5 GPa, comparable to high‐strength steels. Molecular dynamics simulations reveal that high strength and hardening ability of Al–Fe alloys arise mainly from the high‐density 9R phase and nanoscale grain sizes.     

Structure and Magnetic Properties of Fe_(76.5)Si_(13.5)B_9Cu_1 Alloy with Nanoscale Grain Size

The structure and magnetic properties of Fe76.5Si13.5B9Cu1 alloys with a nanocrystalline (NC) bcc Fe(Si) phase trom about 23 to 46 nm in diameter, which were first formed into amorphous ribbons and then annealed at various temperatures between 703 and 773 K, have been investigated. At annealing temperatures from 703 to 748 K, the single NC bcc(Si) phase is obtained in the crystallized alloys. The grain size and the Si-content in the NC bcc Fe(Si) phase for the alloys annealed at different temperatures are presented. The soft magnetic properties and the saturation magnetostriction for the alloys with the NC bcc Fe(Si) phase are also measured. The results show that, the saturation magnetizotion and the permeability are improved for the alloys with only the NC bcc Fe(Si) phase and become better with decreasing of the NC bcc phase size, and the saturation magnetostriction declines for the alloys with increasing Si-content in the NC bcc Fe(Si) phase.