Morphological and calorimetric studies on the

Amorphous Al₅₀Zr₅₀ alloy powders have been prepared by rod-milling technique using mechanical alloying (MA) method. The amorphization and crystallization processes of the alloyed powders were followed by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The results have shown that the formation of amorphous Al₅₀Zr₅₀ alloy powders occurs through three stages, agglomeration, disintegration, and homogenization. At the disintegration stage, the alloyed powders contain many fine layers of Al and Zr. An amorphous phase has been formed at about 880 K as a result of heating these layered particles in a thermal analyzer. The crystalline-to-amorphous transformation at this stage of milling is attributed to a thermally assisted solid-state amorphizing reaction. The present study corroborates the similarity of the amorphization process through the MA with the solid-state interdiffusion reaction in multilayered thin films. The amorphization temperature, T_a, and the activation energy of amorphization, E_a, are 675 and 156 kJ/mol, respectively. In addition, the enthalpy change of amorphization, ΔH_a, was evaluated to be -3.5 kJ/mol. On the other hand, the crystallization temperature, T_x, and enthalpy change of crystallization, ΔH_x, were 1000 K and −68 kJ/mol, respectively.     

Amorphization reaction of Ni-Ta powders during mechanical alloying

This study examined the amorphization behavior of Ni _x Ta_100−x alloy powders synthesized by mechanically alloying (MA) mixtures of pure crystalline Ni and Ta powders with a SPEX high energy ball mill. According to the results, after 20 hours of milling, the mechanically alloyed powders were amorphous for the composition range between Ni₁₀Ta₉₀ and Ni₈₀Ta₂₀. A supersaturated nickel solid solution formed for Ni₉₀Ta₁₀, as well. X-ray diffraction analysis reveals two different types of amorphization reactions. Through an intermediate solid solution and by direct formation of amorphous phase. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, the crystallization temperature of amorphous Ni-Ta powders increased with increasing Ta content. In addition, the activation energy of amorphous Ni-Ta powders reached a maximum near the eutectic composition.     

Preparation and thermal stability of Zr-Ti-Al-Ni-Cu amorphous powders by mechanical alloying technique

This study examined the amorphization feasibility of Zr_70−x−y Ti _x Al _y Ni₁₀Cu₂₀ alloy powders by the mechanical alloying (MA) technique. According to the results, after 5 to 7 hours of milling, the mechanically alloyed powders were amorphous basically in the ranges of 0 to 12.5 at. pct Ti and 2.5 to 17.5 at. pct Al. These ranges are larger than those of bulk amorphous alloys prepared by a squeeze mold casting technique. Most of the amorphous mechanically alloyed powders exhibited a wide supercooled liquid region of more than 60 K before crystallization. The glass-transition and crystallization temperatures of mechanically alloyed samples were different from those prepared by squeeze casting. It is suspected that different thermal properties arise from the introduction of impurities during the MA process. The amorphization behavior of Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ was examined in detail. The X-ray diffraction and extended X-ray absorption fine structure (EXAFS) results show the fully amorphous powders formed after 5 hours of milling. A kinetically modified thermodynamic phase transformation process was observed for the glass-transition behavior in the Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ amorphous powder.     

Mechanical alloying of nb-al powders

The effect of mechanical alloying (MA) on solid solubility extension, nanostructure formation, amorphization, intermetallic compound formation, and the occurrence of a face-centered cubic (fcc) phase in the Nb-Al system has been studied. Solid solubility extension was observed in both the terminal compositions and intermetallic compounds: 15 pct Nb in Al and 60 pct Al in Nb, well beyond the equilibrium and even rapid solidification levels (2.4 pct Nb and 25 pct Al, respectively) and increased homogeneity range for the NbAl₃ phase. Nanostructured grains formed in all compositions. In the central part of the phase diagram, amorphization occurred predominantly. Only NbAl₃, the most stable intermetallic, formed during MA; in most cases, a subsequent anneal was required. On long milling time, an fcc phase, probably a nitride, formed as a result of contamination from the ambient atmosphere.     

Ball-milling-induced crystallization and ball-milling effect on thermal crystallization kinetics in an amorphous FeMoSiB alloy

Microstructure evolution in a melt-spun amorphous Fe_77.2Mo_0.8Si₉B₁₃ alloy subjected to high-energy ball milling was investigated by means of X-ray diffraction (XRD), a transmission electron microscope (TEM), and a differential scanning calorimeter (DSC). It was found that during ball milling, crystallization occurs in the amorphous ribbon sample with precipitation of an α-Fe solid solution, and the amorphous sample crystallizes completely into a single α-Fe nanostructure (rather than α-Fe and borides as in the usual thermal crystallization products) when the milling time exceeds 135 hours. The volume fraction of material crystallized was found to be approximately proportional to the milling time. The fully crystallized sample with a single α-Fe nanophase exhibits an intrinsic thermal stability against phase separation upon annealing at high temperatures. The ball-milling effect on the subsequent thermal crystallization of the amorphous phase in an as-milled sample was studied by comparison of the crystallization products and kinetic parameters between the as-quenched amorphous sample and the as-milled partially crystallized samples. The crystallization temperatures and activation energies for the crystallization processes of the residual amorphous phase were considerably decreased due to ball milling, indicating that ball milling has a significant effect on the depression of thermal stability of the residual amorphous phase.     

Amorphization of Al50(Fe2B)30Nb20 Mixture by Mechanical Alloying

An amorphous Al₅₀(Fe₂B)₃₀Nb₂₀ powder mixture was prepared by mechanical alloying in a high-energy planetary ball-mill under argon atmosphere. Morphologic, microstructural, and structural changes during the milling process were followed by scanning electron microscopy and X-ray diffraction. Rietveld analysis of X-ray diffraction patterns was used to follow the solid-state amorphization transformation during the milling process of the prepared powder. The reaction between elemental Al, Fe₂B, and Nb powders leads to the formation of the Al(Fe,B) and Al(Fe,Nb,B) solid solutions after 4 and 6 hours of milling, respectively. An amorphous structure is achieved after 20 of milling. These amorphous powders are crystallized on further milling time (36 hours). The observation by scanning electron microscope shows a phenomenon of fracturing followed by compaction of the powder particles.     

Effect of milling temperature on mechanical alloying in the immiscible Cu-Ta system

Elemental powder blends with atomic composition of Cu_100−x Ta _x (x=10, 30, 50, 70, and 90) were ball milled in a SPEX mill at several temperatures (room temperature (RT), liquid nitrogen temperature (LN₂T), −80 °C, and 95 °C) to examine the effect of milling temperature on the extent of alloying and microstructural refinement. For the Cu-rich powders (10<x<50), high-energy ball milling to steady state at all temperatures produced a mixture of nanocrystalline Cu and Ta with no observable extension of mutual solid solubility. Compared with milling at RT, cryomilling (LN₂T) caused further refinement of Cu crystallites, while the same steady-state grain size was reached for Ta crystallites. On the Ta-rich side (50<x<90), ball milling at all temperatures led to refined Cu and Ta grain sizes. Partial amorphization seemed to be present, which apparently increased in extent with increasing contamination from the milling media upon extended milling. Very similar results were obtained for milling at RT and LN₂T. It was concluded that high-energy ball milling at LN₂T did not drastically enhance the amorphization reaction between Cu and Ta nor extend their mutual solubility. The limited power of cryomilling to alloy immiscible elements such as Cu-Ta is explained as a consequence of the inability to fully suppress, during energetic collisions, the atomic mobility responsible for phase separation even when the milling is conducted at the nominal LN₂T. The temperature dependence of milling-induced microstructural refinement and alloying is analyzed in terms of the dynamics of the generation and annihilation of the nonequilibrium vacancies in an externally driven system. It is predicted that externally forced mixing as well as diffusion assisted by high-energy ball milling can be merely weakly temperature dependent between RT and LN₂T. As a result, the extension of solubility by using cryomilling is feasible only in limited systems, and this process cannot be expected to alloy all immiscible elements.     

Amorphization of Ti1−x Mn x

Three amorphous Ti_1−x Mn _x alloy powders, withx = 0.4, 0.5, and 0.6, were prepared by mechanical alloying (MA) of the elemental powders in a high-energy ball mill. The amorphous powders were characterized by X-ray diffraction (XRD) and high-resolution transmission elec- tron microscopy (HRTEM). The crystallization temperatures for these alloys detected by dif- ferential scanning calorimetry (DSC) varied from 769 to 830 K. The calculated enthalpies of mixing in these amorphous phases are relatively small compared with those for other Ti-base binary alloys. The criteria for solid-state amorphization reaction are examined. It is suggested that the kinetics of nucleation and growth favors the formation of the amorphous phases and the supply of atoms for nucleation and growth is predominantly through the defective regions induced by MA. Formerly Graduate Student, National Tsing Hua University     

Solid-gas reactions driven by mechanical alloying of niobium and tantalum in nitrogen

Solid-gas reactions of niobium and tantalum with molecular nitrogen driven by mechanical alloying (MA) have been investigated by X-ray diffraction, transmission electron microscopy, and differential thermal analysis. It was found that the phase transition followed a sequence of Nb₂N → Nb₃N₄ → NbN when Nb was milled with N₂. However, Ta₂N and an amorphous phase were formed when Ta was milled with N₂. The chemosorption of nitrogen onto the clean metal surfaces created by ball milling is believed to be the fundamental process governing solid-gas reactions, and the defects generated during MA can promote the diffusion of adsorbed nitrogen, and consequently the formation of metal nitrides. The difference in phase transition between the two systems is discussed.     

Bulk titanium-rich alloys containing nanoscale disordered regions

Nanoscale disordered regions were obtained in titanium-rich alloys via annealing of a metastable crystalline phase. This metastable bcc solid solution (β) was formed in the alloy Ti₆₅Cr₁₃Cu₁₆Mn₄Fe₂ following arc melting and cooling or simply by air cooling a solutionized ingot. Since only air cooling was required to retain the β phase, large ingots can be produced. During subsequent annealing, partial amorphization occurred in conjunction with a phase transformation from β to β′. The premise for this amorphization is the reduction of the free energy difference between an amorphous phase and a metastable crystalline phase with proper alloy additions. Comparisons are made with amorphized powders at the same composition produced by mechanical milling. This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory.     

Fabrication and characterizations of new glassy Co<Subscript>71</Subscript>Ti<Subscript>24</Subscript>B<Subscript>5</Subscript> alloy powders and subsequent hot pressing into a fully dense bulk glass

A single glassy phase of Co₇₁Ti₂₄B₅ alloy has been synthesized by high-energy ball milling the elemental powders at room temperature, using the mechanical alloying method. The synthetic glassy powder obtained after 130 ks of ball milling exhibits good soft magnetic properties with a polarization magnetization and coercivity values of 1.01 T and 2.86 kA/m, respectively. This ternary glassy alloy in which its glass transition temperature (T _g ) lies at a rather high temperature (805 K) crystallizes at 868 K through a single sharp exothermic peak with an enthalpy change of crystallization (ΔH _x ) of −3.28 kJ/mol. The supercooled liquid region before crystallization, ΔT _x of the synthesized glassy powders shows a large value (63 K) for a ternary system. The reduced glass transition temperature (ratio between T _g and liquidus temperatures, T _l (T _g /T _l )) was found to be 0.55. The end product of the glassy powder (130 ks) was compacted in an argon gas atmosphere at 835 K with a pressure of 780 MPa, using the hot-pressing technique. The consolidated sample is fully dense (∼99.5 pct) and maintains its chemically homogeneous glassy structure. The measured polarization magnetization and coercivity values of as-consolidated powders are measured and found to be 0.96 T and 2.92 kA/m, respectively. The Vickers microhardness of the bulk glassy Co₇₁Ti₂₄B₅ sample is measured and found to be in the range between 7.32 and 7.46 GPa.     

A study on the microstructural evolution of Al-25 at. pct V-12.5 at. pct M (M = Cu,Ni, Mn) powders by planetary ball milling

The alloying behavior of Al-25 at. pct V-12.5 at. pct M (M = Cu, Ni, Mn) by planetary ball milling of elemental powders hours as been investigated in this study. In Al₃V binary system, an amorphous phase was produced after 6 hours and the amorphous phase was mechanically crystallized after 20 hours. The large difference in the diffusivities between Al and V atoms in Al matrix results in the formation of the amorphous phase when the homogeneous distribution of all the elements in a powder was achieved at 6 hours. According to thermal analyses, the amorphous phase in the binary Al₃V was crystallized at 350 °C. The addition of ternary elements (Cu, Ni, Mn) increased the activation energy for the crystallization to D0₂₂ phase by interfering with the diffusion process. Therefore, ternary element addition improved the thermal stability of the amorphous structures. The amorphous phase in the 12.5 at. pct Ni added Al₃V was crystallized to D0₂₂ phase at 540 °C. The mechanical crystallization of the amorphous phase in the ternary element-added Al-V system either occurred later or was not observed during ball milling up to 100 hours. It is thought that the amorphous intermetallic compacts could be produced more easily in ternary element-added alloys by using an advanced consolidation method.     

Oxidation of Ni — Ta alloys. III. Mechanism of oxidation

The results of an x-ray diffraction and metallographic study of the kinetics of scale formation during the oxidation of Ni(Ta), Ni₃Ta, and NiTa in air at 600–1000°C are analyzed. The free energies, equilibrium oxygen pressures, and mass balances of the oxidation reactions were calculated, and conditions for the formation of NiO·Ta₂O₅ and NiO on the alloy determined. It is shown that the oxidation process is controlled primarily by the diffusion of oxygen and counter-diffusion of Ni⁺² in the scale, and involves oxidation, reduction, and synthesis reactions. A multilayer scale is formed, consisting of an outer layer containing only oxides (NiO, NiO·Ta₂O₅, Ta₂O₅) and an inner one which additionally contains nickel. The protective ability of the outer scale depends upon the concentrations of NiO and NiO·Ta₂O₅ in it. Preferential oxidation of tantalum is responsible for the appearance of a subscale consisting of Ni(Ta) + Ta₂O₅ on the intermetallic Ni₃Ta, and Ni₃Ta + Ni(Ta) + Ta₂O₅ on NiTa. Differences in molar volumes of phases result in the formation of pores and cracks at interphase boundaries, particularly in the inner scale on Ni₃Ta. A change in the oxidation mechanism occurs at T ≥ 850°C as a result of the p → n transition in Ta₂O₅, which leads to retarded oxygen diffusion and the appearance of Ta⁺⁵ diffusion in the intermetallic. This, as well as the diffusion of Ni⁺², promotes the healing of macrodefects in the scale. However, it also results in enrichment of the outer scale in pentoxide, which decreases its protective ability. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 69–78, May–June, 2000.     

Compaction and characterization of mechanically alloyed nanocrystalline titanium aluminides

Blended elemental (BE) Ti-24 at. pct Al-11 at. pct Nb (Ti-24-11) and Ti-55 at. pct Al (Ti-55) powders and prealloyed (PA) Ti-24-11 powders were mechanically alloyed in a SPEX mill or an attritor. After SPEX milling for 10 hours, the BE Ti-24-11 powder contained the B2/bcc phase, while the BE Ti-55 powder showed the presence of an amorphous phase. The PA Ti-24-11 powder containing the B2 phase showed a decrease of crystal size on milling. These powders were consolidated by hot isostatic pressing (“hipping”), Ceracon process, and dynamic methods. On compaction, the B2/bcc phase in the Ti-24-11 sample transformed to a mixture of the B2 and orthorhombic (“O”) phases, while the amorphous phase in the Ti-55 powder crystallized to a mixture of the γ-TiAl and α ₂-Ti₃Al phases. The finest grain size in compacted material was obtained in the dynamically consolidated powder, and the grain size in the hot isostatic pressed (“hipped”) powder became larger with the increasing hipping temperature.     

Comparative Study of Mechanical Alloying Induced Nanocrystallization and Amorphization in Ni-Nb and Ni-Zr Systems

The nanocrystallization and amorphization processes in Ni₆₀Nb₄₀ and Ni₆₀Zr₄₀ binary alloys during mechanical alloying (MA) were studied in detail. The mechanical alloying behavior of these alloy systems was compared with respect to the rate of refinement of grain size, ultimate grain size, and rate of amorphization reaction. For both compositions, MA leads to the refinement of grain size and enhancement of internal strain, followed by the amorphization reaction. The higher melting temperature metal Nb exhibits smaller grain size and greater internal strain, although the Ni and Nb grain size approach a similar value of ~15 nm after 20 hours of milling time. The refinement of grain size and enhancement of internal strain was observed to occur with a slower rate during MA of Ni₆₀Zr₄₀ alloy compared to Ni₆₀Nb₄₀ alloy. In all cases, an ultrafine layered structure with a typical thickness of 30 nm, containing nanoscale size grains with a typical size of 15 nm and a high density of dislocations, develops prior to the amorphization reaction. This observation suggests that numerous high-speed diffusion paths such as grain boundaries and dislocations are necessary to allow a high diffusion rate at low temperature and therefore permits the amorphization reaction to take place kinetically. The Ni-Zr system is a better glass former in MA than Ni-Nb system; i.e., the start time of amorphization reaction for Ni₆₀Zr₄₀ was about half that for Ni₆₀Nb₄₀. These results were discussed in terms of physical and chemical characteristics of the constituent elements of the alloy systems. Furthermore, the thermodynamically stable phase in each system was predicted using a semi-empirical Miedema model, and the results were compared with the structure formed in MA of Ni-Zr and Ni-Nb powder mixture.     

Milling maps and amorphization during mechanical alloying

The effect of various milling parameters such as, milling intensity, ball: powder weight ratio and number of balls on the glass forming ability of an elemental blend of composition Ti₅₀Ni₅₀ has been studied by mechanical alloying. In order to understand the results, all the milling parameters have been converted into two energy parameters, namely, impact energy of the ball and the total energy of milling. In a milling map of these two parameters, the conditions for amorphous phase formation have been isolated. A similar exercise has been carried out for Ti₅₀Cu₅₀ as a function of milling time at two milling intensities. The results indicate that a minimum impact energy of the ball and a minimum total energy are essential for amorphization by mechanical alloying.     

Amorphization in Zr3Ai irradiated with 1-mev e- and Kr+

In situ electron microscopy is used to study irradiation-induced amorphization in Zr₃Al from 10 to 295 K by 1-MeV electrons and at 295 K with 1-MeV Kr⁺. The onset of amorphization is observed when the long-range order parameter decreases substantially with both electron and Kr⁺. Diffuse streaks are observed in the diffraction pattern prior to amorphization. This is attributed to a softening of the shear elastic constant,C′ = (C ₁₁ −C ₁₂)/2, due to static displacement of atoms. This observation is consistent with a large softening of shear modulus reported in Zr₃Al irradiated with 1-MeV Kr⁺, for which a shear elastic instability has been identified prior to the onset of amorphization. In order to complete amorphization with electrons, a large dose, ≳26 dpa, is required at 10 K, while with Kr₊, amorphization is completed by a dose of only 0.8 dpa. After the same dose of 26 dpa with electrons at 57 and 295 K, only partial amorphization and rhombohedral distortion are observed. Defect aggregation is also observed during irradiation at all three temperatures. The anomalously large dose required for complete amorphization with electrons is ascribed to point defect migration, as evidenced by the formation of defect aggregates. The partial amorphization at higher temperatures is explained by two factors, faster defect migration and rhombohedral distortion, which both provide alternative paths to amorphization for reducing the accumulated strain. Formerly with the Materials Science Division, Argonne National Laboratory and the Department of Materials Science and Engineering, Northwestern University. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, Illinois, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Alloying evolution and stability of Al65Cu20Ti15 during process of amorphisation by high energy ball milling

Abstract

Mechanical alloying of Al₆₅Cu₂₀Ti₁₅ powder blend has been carried out by high energy vibrating ball mill. The process of amorphisation in the mechanically alloyed Al₆₅Cu₂₀Ti₁₅ powder and the stability of the amorphous phase during ball milling were investigated. Almost completely amorphous powder was achieved after 25 h ball milling. Examination of the microstructural constituents using X-ray diffraction and transmission electron microscopy shows that the amorphisation process was controlled by the transformation of both Al based solid solution and intermetallic compounds (Al₂Cu, Cu₉Al₄ and AlCu₂Ti). However, that prolonging the ball milling time to 30 h led to the appearance of Cu₉Al₄, the Al₆₅Cu₂₀Ti₁₅ composite comprising nanocrystalline and amorphous phases could be stable after 50 h ball milling.