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.     

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.     

Orientation Relationship Between Magnetic Domains and Twins in Ni<Subscript>52</Subscript>Fe<Subscript>17</Subscript>Ga<Subscript>27</Subscript>Co<Subscript>4</Subscript> Magnetic Shape Memory Alloy

The orientation relationship between magnetic domain and twins in the directional solidified Ni₅₂Fe₁₇Ga₂₇Co₄ magnetic shape memory alloy was analyzed by electron backscatter diffraction and magnetic force microscopy. The twin interface plane was determined to be \( \{ \bar{1}10\} \) plates. The magnetic domains walls with a misorientation about 5 deg belong to low angle boundaries. According to the orientation relationship between twins and magnetic domains, the intersection angle on the observed surface can be estimated.     

Preparation and Properties of Ti<Subscript>50</Subscript>Cu<Subscript>28</Subscript>Ni<Subscript>15</Subscript>Sn<Subscript>7</Subscript> Bulk Metallic Glass by Vacuum Hot Pressing

Amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized by a mechanical alloying (MA) technique. Differential scanning calorimetry (DSC) results showed that, after 7 hours of exposure to the milling process, amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders exhibit a wide supercooled liquid region of 61 K. Consolidation of amorphous powders were performed at a temperature slightly higher than the glass transition temperature under a pressure of ∼1.2 GPa, and bulk metallic glass (BMG) discs can be prepared successfully. However, we noticed partial crystallization during the hot pressing process and were not able to achieve full densification of BMG. The Vickers microhardness of Ti₅₀Cu₂₈Ni₁₅Sn₇ BMG was 634 kg/mm², and the trace of the indentation revealed that pre-existing particle boundaries or interfaces between nanocrystals and amorphous matrix may serve as the crack initiation sites. Thus, typical brittle failure of Ti₅₀Cu₂₈Ni₁₅Sn₇ BMG was observed and resulted in relatively low fracture stress compared to that estimated by the microhardness. 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 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.     

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).     

Infrared Brazing Fe<Subscript>3</Subscript>Al Using Ag-Based Filler Metals

The microstructural evolution and bonding shear strength of infrared brazed Fe₃Al using Ag and BAg-8 (72Ag-28Cu in wt pct) braze alloys have been studied. The Ag-rich phase alloyed with Al dominates the entire Ag brazed joints, and the shear strength is independent of the brazing time. The BAg-8 brazed joint contains Ag-Cu eutectic for all brazing conditions, and its shear strength increases slightly with increasing brazing time. The highest shear strength of 181 MPa is acquired from the joint infrared brazed at 1073 K (800 °C) for 600 seconds. A thin layer of Fe₃Al is identified at the interface between the brazed zone and the substrate for both braze alloys. An Al depletion zone in the Fe₃Al substrate next to the interfacial Fe₃Al is identified as the α-Fe phase. The dissolution of Al from the Fe₃Al substrate into the molten braze causes the formation of α-Fe in the Fe₃Al substrate.     

Phase analysis of multicomponent alloys using an Ni<Subscript>50</Subscript>Co<Subscript>25</Subscript>Mo<Subscript>25</Subscript> alloy as an example

method for phase analysis of three-component alloys is proposed. It is based on a pair interaction model and an experimental determination of the sign of pair chemical interaction energy and includes an electron-microscopic investigation of microstructures above and below the ordering–separation phase transition temperature for each diffusion couple. This method is used to study an Ni₅₀Co₂₅Mo₂₅ alloy. The phases that precipitate in this alloy over the entire heating temperature range, including the liquid state, are detected.     

Mechanical behaviour of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> based alloys and composites

The demand for materials to be used in the components operating above 1100°C in advanced aero-engines drives the development of the silicide-based intermetallic alloys and composites, including the titanium silicides. The mechanical behaviour of Ti₅Si₃ and its composites has been reviewed with emphasis on the microstructure-property relationships. It is found that the grain size is a critical parameter, and smaller grain sizes are desirable for reducing the magnitude of internal residual stress caused by the crystallographic anisotropy in coefficients of thermal expansion. The reduction in grain size leads to significant improvement in hardness, room temperature flexural strength and fracture toughness. On the other hand, the high temperature strength observed at slow strain rates and creep resistance are higher in the samples with the coarser grain sizes. Further improvements in the strength, fracture toughness and high temperature creep resistance are possible, either through the development of multiphase alloys, or by the use of ceramic reinforcements in composites.     

Li-Ion Battery with LiFePO<Subscript>4</Subscript> Cathode and Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> Anode for Stationary Energy Storage

Li-ion batteries based on commercially available LiFePO₄ cathode and Li₄Ti₅O₁₂ anode were investigated for potential stationary energy storage applications. The full cell that operated at flat 1.85 V demonstrated stable cycling up to 200 cycles followed by a rapid fade. A Li-ion full cell with Ketjen black modified LiFePO₄ cathode and an unmodified Li₄Ti₅O₁₂ anode exhibited negligible fade after more than 1200 cycles with a capacity of ~130 mAh/g at C/2. The improved stability, along with its cost-effectiveness, environmental benignity, and safety, make the LiFePO₄/Li₄Ti₅O₁₂ combination Li-ion battery a promising option for storing renewable energy.     

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.     

Mechanically induced solid-state devitrifications of Zr<Subscript>70</Subscript>Pd<Subscript>20</Subscript>Ni<Subscript>10</Subscript> glassy alloy powders

A single glassy phase of Zr₇₀Pd₂₀Ni₁₀ alloy powder was synthesized by mechanical alloying the elemental powders for 48 hours, using a high-energy ball-milling technique. The obtained glassy phase transformed into a metastable big-cube phase upon increasing the ball-milling time (100 hours). After 150 hours of milling, a complete glass-metastable-phase transformation was achieved, and the end product was nanocrystalline big-cube powder, which has a lattice constant of 1.23 nm. As the ball-milling time was further increased the big-cube phase could no longer withstand the mechanical deformation that was generated by the milling media and transformed into a new metastable phase of nanocrystalline fcc Zr₇₀Pd₂₀Ni₁₀. The lattice constant of this metastable phase was calculated to be 0.455 nm. The reported metastable phases here are new and have never been, so far as we know, reported for the ternary Zr-Pd-Ni system, or its binary-phase relations.