Effects of Annealing and Pressure on Devitrification and Mechanical Properties of Amorphous Al<Subscript>87</Subscript>Ni<Subscript>7</Subscript>Gd<Subscript>6</Subscript>

Annealing studies at different temperatures, as well as those conducted with 940 MPa hydrostatic pressure, were conducted on amorphous ribbons of Al₈₇Ni₇Gd₆. The studies were performed to investigate the evolution of structure under different conditions and to particularly examine the effects of superimposed hydrostatic pressure during annealing. This amorphous alloy devitrifies at low temperatures via the precipitation of nano-crystalline α-Al particles. The effects of these various exposures on the amount of devitrification have been quantified using a variety of analytical techniques (i.e., X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM)). In addition, the effects of devitrification on the mechanical properties have been quantified using microhardness indentation and uniaxial tension tests. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Shape memory effect in a rapidly quenched Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy

The structural and thermomechanical properties of rapidly quenched layered amorphous–crystalline Ti₅₀Ni₂₅Cu₂₅ composite materials with various ratios of amorphous and crystalline phases are studied. These layered composite materials are shown to exhibit the two-way shape memory effect accompanied by bending deformation without additional thermomechanical treatment. The ratio of amorphous and crystalline phases is found to affect the reversible change in the shape of the composite material.     

Oxidation Behavior of a Pd<Subscript>43</Subscript>Cu<Subscript>27</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> Bulk Metallic Glass and Foam in Dry Air

The oxidation behavior of both Pd₄₃Cu₂₇Ni₁₀P₂₀ bulk metallic glass (Pd4-BMG) and its amorphous foam containing 45 pct porosity (Pd4-AF) was investigated over the temperature range of 343 K (70 °C) to 623 K (350 °C) in dry air. The results showed that virtually no oxidation occurred in the Pd4-BMG at T < 523 K (250 °C), revealing the alloy’s favorable oxidation resistance in this temperature range. In addition, the oxidation kinetics at T ≥ 523 K (250 °C) followed a parabolic-rate law, and the parabolic-rate constants (k _p values) generally increased with temperature. It was found that the oxidation k _p values of the Pd4-AF are slightly lower than those of the Pd4-BMG, indicating that the porous structure contributes to improving the overall oxidation resistance. The scale formed on the alloys was composed exclusively of CuO at T ≥ 548 K (275 °C), whose thickness gradually increased with increasing temperature. In addition, the amorphous structure remained unchanged at T ≤ 548 K (275 °C), while a triplex-phase structure developed after the oxidation at higher temperatures, consisting of Pd₂Ni₂P, Cu₃P, and Pd₃P.     

Infrared Brazing of Ti<Subscript>50</Subscript>Ni<Subscript>50</Subscript> Shape Memory Alloy and Inconel 600 Alloy with Two Ag-Cu-Ti Active Braze Alloys

Infrared brazing of Ti₅₀Ni₅₀ SMA and Inconel 600 alloy using Cusil-ABA and Ticusil filler metals has been investigated. The joints were dominated by Ag-Cu eutectic with proeutectic Cu in the Cusil-ABA brazed joint and with proeutectic Ag in the Ticusil one. A continuous curved belt composed of a Ni₃Ti layer and a (Cu _x Ni_1?x )₂Ti layer formed in the brazed Ti₅₀Ni₅₀/Ticusil/Inconel 600 joint. On the Ti₅₀Ni₅₀ SMA side, an intermetallic layer of (Cu _x Ni_1?x )₂Ti formed in all joints, with x values around 0.81 and 0.47. Layers of (Cu _x Ni_1?x )₂Ti, Ni₃Ti, and mixed Ni₃Ti and Ni₂Cr intermetallics were observed next to the Inconel 600 substrate in the brazed Ti₅₀Ni₅₀/Cusil-ABA/Inconel 600 joint. The maximum shear strengths of the joints using the Cusil-ABA filler metal and the Ticusil filler metal were 324 and 300 MPa, respectively. In the Cusil-ABA brazed joint, cracks with cleavage-dominated fracture propagated along the (Cu _x Ni_1?x )₂Ti interfacial layer next to the Ti₅₀Ni₅₀ SMA substrate. In the Ticusil brazed joint, ductile dimple fracture occurred in the Ag-rich matrix near the Inconel 600 alloy substrate. The absence of a detrimental Ti-Fe-(Cu) layer on the Inconel 600 substrate side can effectively improve the shear strength of the joint.     

Multistage Devitrification Behavior of Mg<Subscript>65</Subscript>Cu<Subscript>25</Subscript>Tb<Subscript>10</Subscript> Bulk Metallic Glass

The devitrification of Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) has been studied by time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) simultaneously. By analyzing the interference peaks on SAXS patterns and the Bragg peaks on WAXS patterns, it is found that devitrification initiates by activation of quenched-in short-range orders. Crystallization proceeds in three stages. During stage I, icosahedral clusters are formed that transforms to a quasi-crystalline 1/1 approximant during stage II, accompanied by the formation of cubic TbMg₃. In stage III, the 1/1 approximant transforms to a 2/1 approximant. The orthorhombic CuMg₂ phase is formed at a higher temperature when the quasi-crystalline phase starts to decompose. Pair distribution functions were evaluated to demonstrate these structural evolutions in real space. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

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.     

Microstructural Evolution and Mechanical Properties of Nanointermetallic Phase Dispersed Al<Subscript>65</Subscript>Cu<Subscript>20</Subscript>Ti<Subscript>15</Subscript> Amorphous Matrix Composite Synthesized by Mechanical Alloying and Hot Isostatic Pressing

The structure and mechanical properties of nanocrystalline intermetallic phase dispersed amorphous matrix composite prepared by hot isostatic pressing (HIP) of mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous powder in the temperature range 573 K to 873 K (300 °C to 600 °C) with 1.2 GPa pressure were studied. Phase identification by X-ray diffraction (XRD) and microstructural investigation by transmission electron microscopy confirmed that sintering in this temperature range led to partial crystallization of the amorphous powder. The microstructures of the consolidated composites were found to have nanocrystalline intermetallic precipitates of Al₅CuTi₂, Al₃Ti, AlCu, Al₂Cu, and Al₄Cu₉ dispersed in amorphous matrix. An optimum combination of density (3.73 Mg/m³), hardness (8.96 GPa), compressive strength (1650 MPa), shear strength (850 MPa), and Young’s modulus (182 GPa) were obtained in the composite hot isostatically pressed (“hipped”) at 773 K (500 °C). Furthermore, these results were compared with those from earlier studies based on conventional sintering (CCS), high pressure sintering (HPS), and pulse plasma sintering (PPS). HIP appears to be the most preferred process for achieving an optimum combination of density and mechanical properties in amorphous-nanocrystalline intermetallic composites at temperatures ≤773 K (500 °C), while HPS is most suited for bulk amorphous alloys. Both density and volume fraction of intermetallic dispersoids were found to influence the mechanical properties of the composites.     

Fabrication of Porous Ti-rich Ti<Subscript>51</Subscript>Ni<Subscript>49</Subscript> by Evaporating NaCl Space Holder

Net-shaped porous Ti-rich Ti₅₁Ni₄₉ alloy with well-controlled porosity, pore size, and pore shape are fabricated by pressing-and-sintering compacts containing fine Ti and Ni powders and coarse NaCl powders. After sintering at 1323 K (1050 °C) for 30 minutes in a high vacuum, the NaCl space holder is removed by evaporation, and the remaining Ti and Ni powders are sintered with about 2.3 vol pct liquid phase. The sintered Ti₅₁Ni₄₉ compacts have porosities of 26, 64, 70, 78, and 85 pct, and no distortion is observed. DSC tests show that the M _S temperature and ΔH are about 347 K (74 °C) and 28 J/g, respectively, and that they are almost independent of the porosity and close to those of wrought Ti-rich TiNi alloys. These porous Ti₅₁Ni₄₉ compacts exhibit a homogeneous microstructure, and the compressive properties and porosity are close to those of human bones.     

Improvement in the Shape Memory Response of Ti<Subscript>50.5</Subscript>Ni<Subscript>24.5</Subscript>Pd<Subscript>25</Subscript> High-Temperature Shape Memory Alloy with Scandium Microalloying

A Ti_50.5Ni_24.5Pd₂₅ high-temperature shape memory alloy (HTSMA) is microalloyed with 0.5 at. pct scandium (Sc) to enhance its shape-memory characteristics, in particular, dimensional stability under repeated thermomechanical cycles. For both Ti_50.5Ni_24.5Pd₂₅ and the Sc-alloyed material, differential scanning calorimetry is conducted for multiple cycles to characterize cyclic stability of the transformation temperatures. The microstructure is evaluated using electron microscopy, X-ray diffractometry, and wavelength dispersive spectroscopy. Isobaric thermal cycling experiments are used to determine transformation temperatures, dimensional stability, and work output as a function of stress. The Sc-doped alloy displays more stable shape memory response with smaller irrecoverable strain and narrower thermal hysteresis than the baseline ternary alloy. This improvement in performance is attributed to the solid solution hardening effect of Sc.     

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.     

Corrosion Behavior of Fe<Subscript>41</Subscript>Co<Subscript>7</Subscript>Cr<Subscript>15</Subscript>Mo<Subscript>14</Subscript>C<Subscript>15</Subscript>B<Subscript>6</Subscript>Y<Subscript>2</Subscript> Bulk Metallic Glass in Sulfuric Acid Solutions

An Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk metallic glass with a diameter of 5 mm was prepared with the copper-mold-casting method. The corrosion resistance of this amorphous steel in sulfuric-acid solutions was determined by electrochemical measurements. The passive film formed on the surface of the alloy after immersion in the 0.5-mol/l H₂SO₄ solution for 1 week was analyzed by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements show that the corrosion resistance of the amorphous steel in the 1 mol/l-H₂SO₄ solution is superior to a stainless steel (SUS 321), and is almost the same as Ti6Al4V, which shows that the amorphous steel has an excellent corrosion resistance in sulfuric-acid solutions. As the concentration of the sulfuric-acid solutions increases from 0.5 mol/l to 4 mol/l, the corrosion resistance of the amorphous steel decreases. The XPS result reveals that a bilayer structure of protective film formed on the surface of the amorphous steel in a H₂SO₄ solution. The compositions of the inner part of the film are MoO₂, Cr₂O₃, CoO, and FeO, and those of the outer film are MoO₃, Cr(OH)₃, Co(OH)₂, and Fe(OH)₃.     

Influence of Ni Content on the Microstructure and Dry Sliding Wear Properties of Cr<Subscript>13</Subscript>Ni<Subscript>5</Subscript>Si<Subscript>2</Subscript>/<Emphasis Type="Italic">γ</Emphasis> Intermetallic Alloys

The effect of Ni content on microstructure, hardness, and wear resistance was studied for the Cr₁₃Ni₅Si₂-base intermetallic alloys toughened by Ni-base solid solution (γ). Volume fraction and microhardness of the Cr₁₃Ni₅Si₂ primary dendrite as well as the average hardness of the Cr₁₃Ni₅Si₂/γ alloy decrease with the increasing Ni content. The Cr₁₃Ni₅Si₂/γ alloys have excellent wear resistance under dry sliding wear test conditions, which increases under high contact load wear conditions and decreases under low contact load wear test conditions with the increasing Ni content. The high wear resistance is due to the combination of high toughness of γ and high hardness of Cr₁₃Ni₅Si₂ and formation of a transferred cover layer on the worn surface during wear process. The wear rate of the Cr₁₃Ni₅Si₂/γ alloy is governed by the slow process of microspalling or pullout of the cracked Cr₁₃Ni₅Si₂ primary dendrites. The Cr₁₃Ni₅Si₂/γ alloys have extremely low load sensitivity of wear and the load-sensitivity coefficient of wear decreases drastically as the Ni content increases.     

Titanium-Based Metallic Glass Composites with Good Plasticity

Three types of Ti-based alloys (an amorphous material, an amorphous composite with intermetallic crystals, and an intermetallic compound) of the compositions Ti_41.5Zr_2.5Hf₅Cu_42.5−x Ni_7.5+x Si₁ (x = 0, 5, and 15) were fabricated to study the effect of composition on glass formability and microstructure, and the dependence of mechanical properties on microstructure were investigated at room temperature. The results show that the amorphous composite has an excellent combination of both ultrahigh strength (2245 MPa) and large plastic strain (9 pct), which is a significant improvement compared to both the fully amorphous and intermetallic structures. In addition, it is also found that the crystal phases in the amorphous matrix can obstruct the shearing-off of the shear bands by inducing them to interact, deflect, and branch, resulting good plasticity in the amorphous composite. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synchrotron Characterization of Transformation Sequences in TiNi-Based Shape Memory Alloys during Thermal Cycling

In-situ synchrotron radiation has been used to provide direct analysis of the transformation sequences in TiNi-based shape memory alloys during thermal cycling. The high resolution, narrow peak width Debye–Scherrer diffraction spectra enabled positive identification and quantification of the phase transformation sequences, which is not possible through normal laboratory studies. The results facilitate a clearer understanding of the development and influence of intermediate phases such as R or B19 on sequential martensitic transformations. Ti_50.2Ni_49.8 transformed predominately via a single-step B2 ↔ B19′ transformation, although evidence of the R phase was found during cooling in every cycle. The martensitic start temperature was depressed by ~0.6 °C per cycle, while the R-phase start temperature was found to be unaffected. Ti₅₀Ni₄₁Cu₉ transformed through a two-step B2 ↔ B19 ↔ B19′ sequence, with the B2 → B19 transformation reaching completion prior to the formation of any B19′. The transformation temperatures of Ti₅₀Ni₄₁Cu₉ were found to be insensitive to thermal cycling, remaining constant over the studied cycle range.     

Oxidation Behavior of CuZr-Based Glassy Alloys at 400 °C to 500 °C in Dry Air

The oxidation behavior of the Cu_47.5Zr_47.5Al₅ (Cu3) and Cu₄₇Ti₃₄Zr₁₁Ni₈ (Cu4) bulk metallic glasses (BMGs) was studied over the temperature range of 400 °C to 500 °C in dry air. The oxidation kinetics of both alloys generally followed a multistage parabolic-rate law, and the steady-state parabolic-rate constants (k _p values) fluctuated with temperature for the Cu3 BMG, but increased with increasing temperature for the Cu4 BMG. The scales formed on the BMGs were strongly dependent on the temperature and alloy composition, and were composed primarily of tetragonal-ZrO₂ (t-ZrO₂) and minor amounts of Al₂O₃, Cu₂O, and CuO at 400 °C for the Cu3 BMG, while the monoclinic-ZrO₂ (m-ZrO₂) phase is present at T ≥ 425 °C, and the Cu₂O phase is absent at 500 °C. Conversely, the scales formed on the Cu4 BMG consisted exclusively of CuO at 400 °C, while minor amounts of t-ZrO₂, TiO₂, and ZrTiO₄ formed at 425 °C to 450 °C, and TiO was also detected at higher temperatures. It was found that both amorphous Cu3 and Cu4 substrates transformed into different crystalline phases, and were strongly dependent on temperature and duration of time. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred during the TMS Annual Meeting February 25–March 1, 2007, in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

<Emphasis Type="Italic">In-Situ</Emphasis> High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition in a Ni<Subscript>2</Subscript>MnGa Ferromagnetic Shape-Memory Crystal

The full information on the changes in many crystallographic aspects, including the structural and microstructural characterizations, during the phase transformation is essential for understanding the phase transition and “memory” behavior in the ferromagnetic shape-memory alloys. In the present article, the defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a uniaxial pressure of 50 MPa applied along the [110] crystallographic direction were studied by the in-situ high-energy X-ray diffuse-scattering experiments. The analysis of the characteristics of diffuse-scattering patterns around different sharp Bragg spots suggests that the influences of some defect clusters on the pressure-induced phase-transition sequences of Ni₂MnGa are significant. Our experiments show that an intermediate phase is produced during the premartensitic transition in the Ni₂MnGa single crystal, which is favorable for the nucleation of a martensitic phase. The compression stress along the [110] direction of the Heusler phase can promote the premartensitic and martensitic transition of the Ni₂MnGa single crystal. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Study of the Ni<Subscript>41.3</Subscript>Ti<Subscript>38.7</Subscript>Nb<Subscript>20</Subscript> wide transformation hysteresis shape-memory alloy

The shape-memory characteristics in the Ni_41.3Ti_38.7Nb₂₀ alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M _s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ _s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M _s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni_41.3Ti_38.7Nb₂₀ alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.     

Structural Transitions and Magnetic Properties of Ni<Subscript>50</Subscript>Mn<Subscript>36.7</Subscript>In<Subscript>13.3</Subscript> Particles with Amorphous-Like Phase

The high-energy ball-milling method was used for fabricating Ni₅₀Mn_36.7In_13.3 fine-sized particles. The as-melt polycrystalline Ni₅₀Mn_36.7In_13.3 alloy exhibits a 14 M modulated martensite structure at room temperature (RT). The atomic pair distribution function analysis together with the differential scanning calorimetry technique proved that the 14 M modulated martensite transformed to a metastable amorphous-like structure after ball milling for 8 hours. Annealing of the ball-milled particles with the amorphous-like phase first led to the crystallization to form a B2 structure at 523 K (250 °C), and then an ordered Heusler L2₁ structure (with a small tetragonal distortion) at 684 K (411 °C). The annealed particles undergo different structural transitions during cooling, tailored by the atomic arrangements of the high-temperature phase. Low-field thermomagnetization measurements show that the ball-milled particles with the amorphous-like structure or the atomically disordered crystalline structure exhibit a magnetic transition from the paramagnetic-like to the spin-glass state with decreasing temperature, whereas the crystalline particles with the ordered Heusler L2₁ structure present a ferromagnetic behavior with the Curie temperature T _c ≈ 310 K (37 °C).