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.     

Thermally assisted and mechanically driven solid-state reactions for formation of amorphous AI33Ta67 alloy powders

The rod milling technique using the mechanical alloying (MA) process has been employed for preparing amorphous Al₃₃Ta₆₇ alloy starting from elemental Al and Ta powders. X-ray diffraction (XRD), differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are utilized to follow the progress of amorphization. The results show that during the first few kiloseconds of MA time, layered composite particles of Al and Ta are intermixed and form an amorphous phase upon heating to 685 K by DTA. This process is called thermally assisted solid-state amorphization (TASSA). During the early stage of milling, the number of layers of the composite particles increases. This leads to an increase in the heat formation of amorphous Al₃₃Ta₆₇ alloyvia the TASSA process, ΔH _TASSA ^a . After 360 ks (100 h) of the MA time, all Al atoms emigrate to Ta lattices to form a solid solution phase and the powder particles have no more layered structure. At this stage of milling, the value of ΔH _TASSA ^a becomes zero. This solid solution phase is not stable against the shear forces that are generated by the rods and transforms completely to an amorphous phase upon milling for 720 ks (200 hours). This phase transformation is attributed to the accumulation of several lattice imperfections, such as point and lattice defects, which raise the free energy from the more stable phase (solid solution) to a less stable phase (amorphous). After 1440 ks (400 hours) of MA time, a homogeneous amorphous phase is formed. The amorphization process in this case is attributed to a mechanical driven solid-state amorphization (MDSSA). The heat of formation of the amorphous phase formedvia the MDSSA process, ΔH _MDSSA ^a , has been calculated. Moreover, the crystallization characteristics indexed by the crystallization temperature, and the enthalphy of crystallization, of the amorphous phases formed by TASSA and MDSSA processes are investigated as a function of MA time. The role of amorphizationvia each process has been discussed. Formerly lecturer of Materials Science, Department of Mining and Petroleum Engineering, Faculty of Engineering, AI-Azhar University, Nasr City 11884, Cairo, Egypt     

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.     

Calorimetrically measured enthalpies for the reaction of Hf with H(D) 2 (g)

The enthalpy for the direct reaction of H₂ (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H₂ (g) + 1/(b - a)HfH_a(α) → 1/(b - a)HfH_b(δ) is -70 ± 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities α and δ are the coexisting phases anda andb are the corresponding (H/ Hf) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of ΔH°_f (HfH_1.5) = -107.5 kJ/mol and ΔH°_f (HfH_2.0) = -142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is -74.5 kJ/mol H and after the two-phase region, |ΔH_H| increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3. Formerly Graduate Student, Department of Chemistry, University of Vermont     

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     

Differential thermal analysis of hydrogen-induced amorphization in C15 laves phase GdFe2

Structural changes of the C15 Laves phase GdFeP₂ during differential thermal analysis in a hydrogen atmosphere have been investigated by X-ray diffraction and transmission electron microscopy. The DTA curves exhibit four exothermic peaks resulting from hydrogen absorption, hydrogen-induced amorphization (HIA), precipitation of GdH₂ and crystallization. As the hydrogen pressure increases, the onset temperatures of hydrogen absorption, HIA and crystallization become lower. Correspondingly, the activation energies for hydrogen absorption and for HIA decrease from 125 to 110 kJ/mol and from 130 to 95 kJ/mol, respectively, but that for the precipitation of GdH₂ is almost constant as 200 kJ/mol. These results indicate that as the hydrogen pressure increases, both the hydrogen absorption and amorphization occur easily.     

Calorimetrically Measured Enthalpies for

The enthalpy for the direct reaction of H₂ (g) with Zr has been measured by calorimetry at moderate, 323 K, and elevated, 928 K, temperatures over a large range of H contents. The elevated temperature enthalpies were determined for solution in the α phase, for the (α + β) phases, the β phase, the(β + γ) phases, and they phase. Simultaneously, the equilibrium pressures were measured. A combination of ΔG_H = 1/2RT In p _{H 2} , values and the enthalpies gives the corresponding entropies. At 323 K, where equilibrium pressures cannot be measured, the enthalpy for the reaction 1/2H₂ (g) + Zr/1.5 → ZrH_1.5 /1.5 was determined as -87 ± 1.5 kJ/mol H. The enthalpy for reaction of H₂ (g) and Zr (2.5 wt pct Nb) has been determined calorimetrically at 323 K and is found to decrease in exothermicity from —86 to —78 kJ/mol H in the range of (H/Zr) values from 0 to 1.5. Enthalpies of reaction were also measured at the same tem- perature for a two-phase alloy consisting of Zr-rich and Zr₂Ni phases with an overall stoichi- ometry of Zr_0.85Ni_0.15. Formerly Graduate Student, Formerly Graduate Student,     

Thermally enhanced and mechanically driven glass-formation reactions of multilayered Cu33Zr67 powders

A single phase of glassy Cu₃₃Zr₆₇ powders has been synthesized by postannealing the mechanically alloyed Cu₃₃Zr₆₇ multilayered composite powders at 685 K for 1.8 ks. The heat formation of the obtained glassy phase has been measured directly and found to be −2.59 kJ/mol. The glassy powders transform to a big-cube CuZr₂ metastable phase (E9 structure) upon heating to 731 K. This metastable phase does not withstand the temperature increase during the differential scanning calorimetry (DSC) analysis and, therefore, transforms completely into the most stable phase of tetragonal CuZr₂ (C16 structure) at 993 K. In contrast to the annealing process of the multilayered particles, the glassy phase that was obtained by milling the starting materials for a longer ball-milling time is crystallized into the tetragonal phase of CuZr₂ through a single step.     

Formation,Structure, and Crystallization Behavior of Cu-Based Bulk Glass-Forming Alloys

We studied Cu-Zr–based alloys having exceptionally high glass-forming ability (GFA) and investigated the influence of Ag and Al addition on their structure and crystallization behavior. Most of the bulk glassy alloys (BGAs) do not contain any crystals, while some samples studied by high-resolution transmission electron microscopy (HRTEM) were found to contain well-developed medium-range order zones and nanoparticles in a bulk form. The crystallization kinetics of Cu₅₅Zr₄₅, Cu₅₀Zr₅₀, Cu_55–x Zr₄₅Ag _x (x = 0, 10, 20), Cu₄₅Zr₄₅Al₅Ag₅, Cu₄₄Ag₁₅Zr₃₆Ti₅, and Cu₃₆Zr₄₈Al₈Ag₈ glassy alloys was analyzed. An influence of the cooling rate on the formation of glassy phase and thermal stability of the Cu-based glassy alloys on heating was also studied. The crystallization kinetics and phase composition of the ribbon-shape and bulk glassy samples of Cu₃₆Zr₄₈Al₈Ag₈ alloys were also analyzed. The results also indicate that the best glass-forming compositions are possibly located at slightly off-eutectic area, owing to the shift of the eutectic point due to the nonequilibrium processing conditions.     

Synthesis of new bulk metallic glassy Ti60Al15Cu10W10Ni5 alloy by hot-pressing the mechanically alloyed powders at the supercooled liquid region

The mechanical alloying method has been used to fabricate multicomponent Ti₆₀Al₁₅Cu₁₀W₁₀Ni₅ glassy alloy powders, using a low-energy ball-milling technique. The glassy powders that were obtained after 720 ks of milling have a sphere-like morphology with an average particle size of 0.38 μm in diameter. This new glassy alloy exhibits a glass transition temperature (T _g) at 733 K. It crystallizes at a crystallization temperature (T _x ) of 804 K through a single sharp exothermic peak, with an enthalpy change of crystallization (ΔH _x ) of −5.20 kJ/mol. The supercooled liquid region before crystallization ΔT _x of the obtained glassy powders shows a large value (71 K). The reduced glass transition temperature (ratio betweenT _g and liquidus temperatures,T ₁(T _G /T ₁) was found to be 0.46. The synthetic glassy powders were uniaxial hot-pressed into consolidated round objects with large dimensions (20 mm in diameter × 30 mm in height) in an argon gas atmosphere at several temperatures with a pressure of 936 MPa. The samples that were consolidated at the temperature range of 755 to 775 K (within the ΔT _x region) are fully dense (∼99.85 pct) and maintain the chemically homogeneous glassy structure. These hot-pressed glassy samples exhibit excellent mechanical properties for Ti-base metallic glasses. They have high Vickers microhardness values, in the range between 8.0 and 8.2 GPa. They also show high fracture strength (2.28 GPa) with an extraordinarily high Young’s modulus of 153 GPa. Neither yielding stress nor plastic strain could be detected for this glassy alloy, which shows an elastic strain of 1.39 pct.     

Designing Zr-Cu-Co-Al Bulk Metallic Glasses with Phase Separation Mediated Plasticity

New Zr-based bulk metallic glasses (BMGs) with improved plasticity were developed in the Zr-Cu-Co-Al system by a combination of Zr₄₅Cu₅₀Al₅ and Zr₅₅Co₂₅Al₂₀ BMGs with a certain concentration ratio. The compressive plasticity of the investigated alloys depends strongly on the concentration ratio of the two BMGs. Because of the positive enthalpy of mixing between Cu and Co (??H_Cu-Co?=?+9?kJ/mol), a strong repulsive interaction between Cu and Co is introduced, whereas an attractive interaction exists among the other constituent elements in the liquid state. When two BMGs are combined at a 1:1 concentration ratio, a maximum compressive plasticity of ~12?pct is achieved for the Zr₅₀Cu₂₅Co_12.5Al_12.5 BMG. The plasticity enhancement is attributed to atomic-scale chemical/structural fluctuations achieved through liquid-phase separation.     

Calorimetrically measured enthalpies for the reaction of H2(D2) (g) with Ti and Ti-Ni alloys at 323 K

Reaction enthalpies have been measured calorimetrically at 323 K for the reaction 1/2H₂ (g) + Ti (α, hcp) → TiH_1.5 and for the partial molar solution of H₂ in the δ phase. The magnitude of the enthalpy decreases from 68 at H/Ti = 0 to 65 kJ/mol H at H/Ti = 1.5. The enthalpy continues to slowly fall in magnitude with the increase of H content, and then, for (H/Ti) > 1.9, it falls more precipituously. ΔH _f ⁰ (TiH_1.5) = −98.4 kJ and ΔH _f ⁰ (TiH₂) = −130.3 kJ evaluated at 323 K. No differences in enthalpies were found between H and D. The results are discussed in terms of the existing solvus data for this system, which are important for the quantitative understanding of hydride-induced fracture. Enthalpies of reaction with H₂ have been determined for several Ti-Ni alloys which lie in the (Ti(α) + Ti₂Ni) two-phase field. The reaction with H₂ initially occurs with the Ti phase and then with the Ti₂Ni phase. The enthalpies are similar for the Ti phase as for pure Ti, indicating that this phase is relatively pure Ti. Reaction with the Ti₂Ni phase shows a plateau region with an enthalpy of reaction with 1/2H₂ of about −30 kJ/ mol H. Formerly Graduate Student, Department of Chemistry, University of Vermont.     

Calorimetrically measured enthalpies for the reaction of Hf with H(D)2 (g)

The enthalpy for the direct reaction of H₂ (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H₂ (g) + 1/(b - a)HfH_a(α) → 1/(b- a)HfH_b(δ) is -70 ± 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities α and δ are the coexisting phases anda andb are the corresponding (H/ Hf ) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of δH _f ^‡ (HfH_1.5) = -107.5 kJ/mol and δH _f ^‡ (HfH_2.0) = -142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is -74.5 kJ/mol H and after the two-phase region, |δH_H| increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3. Formerly Graduate Student, Department of Chemistry, University of Vermont     

Kinetics of phase evolution of Zn-Fe intermetallics

The intermetallic phases, Γ (Fe₃3Zn₁₀), Γ₁, (Fe₅Zn₂₁), δ (FeZn₇, and ζ (FeZn₁₃), are mechanically alloyed through ball milling of pure elemental Fe and Zn powders under a controlled atmosphere of argon gas. The state of the as-ball-milled materials was crystalline, except for the Γ phase, which was amorphous. Phase-evolution kinetics through differential scanning calorimeter (DSC) measurements of the as-ball-milled powders show three characteristic transition temperatures for the Γ₁, and ζ phases, two for the Γ phase, and only one for the δ phase. The activation energies for the evolution of the milled powders to their equilibrium crystalline phases are 170 ± 1, 251 ±2, 176± 1, and 737 ±4 kJ/mol for the Γ, Γ₁, δ, and ζ phases, respectively. These values show that the mechanisms for the metastable-to-stable phase transition in these intermetallics are different, whereas diffusion over short distances may be part of the entire process in all cases.     

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.     

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.     

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.     

Mg-Zn粉末机械合金化非晶转变

采用纯金属镁粉和锌粉为原料,通过机械合金化制取了Mg-Zn非晶合金粉末。借助扫描电镜(SEM)、透射电镜(TEM)、X-射线衍射仪、能谱分析仪等手段研究了Mg_(50)Zn_(50)粉末的机械合金化非晶转变过程。本实验证明了镁基合金在负值混合焓较小的条件下,可以通过机械驱动力的作用而发生MA非晶反应,且非晶反应能力很强,Mg_(50)Zn_(50)在50h内完成了非晶转变。非晶反应可能与颗粒的变形细化过程有关,原子的界面扩散在非晶反应过程中起重要作用。     

On the elastic mismatch in the order-disorder transformation and solid state amorphization of intermetallic compounds—II. Criteria for the solid-amorphous transformation in intermetallic compounds

Using simple thermodynamic considerations for the Gibbs-free energies of the crystalline and amorphous states a general condition for the solid-state amorphization of intermetallic compounds by irradiation or mechanical impact was derived. It was obtained that the amorphization is possible if the maximum elastic energy, ΔH_el^m, stored during the (chemical) order-disorder transition is larger than ΔH_top^a, the enthalpy of the topological change of amorphization. This condition can be rewritten into the form: the amorphization is not possible if the order-disorder temperature is less than the melting temperature. T_c < T_m, and if T_c >T_m—in contrast to Johnson's condition—the possibility of the amorphization depends on the relative magnitude of the elastic mismatch energy and the chemical energy of ordering. If their ratio is large enough the amorphization is possible. Detailed calculations, carried out for a large number of compounds, led to a satisfactory agreement with the experimental observations.     

Oxidation and Crystallization Behavior of Quinary Zr-Based Bulk Metallic Glasses

Non-isothermal and isothermal oxidation behavior of four Zr-based bulk metallic glasses (Zr₅₈Cu₂₂Co₄Ag₄Al₁₂, Zr₅₈Cu₂₂Co₂Ag₆Al₁₂, Zr₅₈Cu₂₂Fe₄Ag₄Al₁₂, and Zr₅₈Cu₂₂Fe₂Ag₆Al₁₂ (compositions are in at.%)) has been studied in oxygen environment. Non-isothermal oxidation has been performed at different heating rates up to 1,173 K to understand the effect of progressive crystallization on the oxidation behavior. In addition, crystallization behavior of the glassy alloys has been studied, and activation energies have been calculated in an inert and oxygen environment. Partial replacement of iron with silver and cobalt has a distinct effect on the oxidation and crystallization behavior of the alloys. Oxidation of the glassy alloys starts with the dissolution of oxygen in the amorphous matrix followed by rapid oxidation after crystallization.