Kinetic study of non-isothermal crystallization in Al80Fe10Ti5Ni5 metallic glass

This study investigated the crystallization behavior of a kinetically metastable Al₈₀Fe₁₀Ti₅Ni₅ amorphous phase. The Al₈₀Fe₁₀Ti₅Ni₅ amorphous phase was synthesized via the mechanical alloying of elemental powders of Al, Fe, Ti, and Ni. The microstructures and crystallization kinetics of the as-milled and annealed powders were characterized using X-ray diffraction, transition electron microscopy, and non-isothermal differential thermal analysis techniques. The results demonstrated that an Al₈₀Fe₁₀Ti₅Ni₅ amorphous phase was obtained after 40 h of ball milling. The produced amorphous phase exhibited one-stage crystallization on heating, i.e., the amorphous phase transforms into nanocrystalline Al₁₃(Fe,Ni)₄ (40 nm) and Al₃Ti (10 nm) intermetallic phases. The activation energy for the crystallization of the alloy evaluated from the Kissinger equation was approximately 538±5 kJ/mol using the peak temperature of the exothermic reaction. The Avrami exponent or reaction order n indicates that the nucleation rate decreases with time and the crystallization is governed by a three-dimensional diffusion-controlled growth. These results provide new opportunities for structure control through innovative alloy design and processing techniques.     

Vacuum Brazing TC4 Titanium Alloy to 304 Stainless Steel with Cu-Ti-Ni-Zr-V Amorphous Alloy Foil

Dissimilar metal vacuum brazing between TC4 titanium alloy and 304 stainless steel was conducted with newly designed Cu-Ti-Ni-Zr-V amorphous alloy foils as filler metals. Solid joints were obtained due to excellent compatibility between the filler metal and stainless steel substrate. Partial dissolution of stainless steel substrate occurred during brazing. The shear strength of the joint brazed with Cu_43.75Ti_37.5Ni_6.25Zr_6.25V_6.25 foil was 105 MPa and that with Cu_37.5Ti₂₅Ni_12.5Zr_12.5V_12.5 was 116 MPa. All the joints fractured through the gray layer in the brazed seam, revealing brittle fracture features. Cr₄Ti, Cu_0.8FeTi, Fe₈TiZr₃ and Al₂NiTi₃C compounds were found in the fractured joint brazed with Cu_43.75Ti_37.5Ni_6.25Zr_6.25V_6.25 foil, and Fe₂Ti, TiCu, Fe₈TiZr₃ and NiTi_0.8Zr_0.3 compounds were detected in the joint brazed with Cu_37.5Ti₂₅Ni_12.5Zr_12.5V_12.5 foil. The existence of Cr-Ti, Fe-Ti, Cu-Fe-Ti, and Fe-Ti-V intermetallic compounds in the brazed seam caused fracture of the resultant joints.     

Structure and phase transformations in TiNiFe ternary alloys subjected to plastic deformation by high-pressure torsion and subsequent heat treatment

The results of a complex study of ternary TiNiFe alloys with a low-temperature shape-memory effect subjected to megaplastic deformation by high-pressure torsion (HPT) with subsequent heat treatment are presented. Investigations have been performed using X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. It has been established that, at moderate degrees of reduction, the plastic deformation in the Ti₅₀Ni₄₉Fe₁ alloy induces a B2 ? B19′ thermoelastic martensitic transformation and the formation of a developed banded dislocation and twin structure in the B19′ martensite; in the Ti₅₀Ni₄₇Fe₃ alloy, a mainly analogous dislocation substructure is formed, but in the B2 austenite. The megaplastic deformation by HPT at room temperature leads to the amorphization of the Ti50Ni49Fe1 alloy and to the high-angle nanofragmentation of the Ti50Ni47Fe3 alloy. Specific features of the evolution of the structure and martensitic transformations in the TiNiFe ternary alloys after plastic deformation and heat treatment have been established. It has been found that the heat treatment of both alloys after HPT at temperatures of 553–773 K results in the formation of a nanocrystalline or mixed nano-submicro-crystalline structure.     

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

The common Ti₄₄Ni₄₇Nb₉ and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti₄₄Ni₄₇Nb₉ alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti₅₀Ni₄₀Cu₁₀ alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.     

Oxidation Behavior of Ti50Ni40Cu10 Shape-Memory Alloy in 700–1,000 °C Air

The isothermal-oxidation behavior of Ti₅₀Ni₄₀Cu₁₀ shape memory alloy (SMA) in 700–1,000 °C air was investigated by TGA, XRD, SEM and EPMA. Experimental results indicate that a multi-layered oxide scale formed, consisting of an outermost Cu₂O(Ni,Ti) layer, a layer of the mixture of TiO₂, TiNiO₃ and irregular small pores, a layer of the mixture of Ni(Ti,Cu), TiO₂ and irregular large pores, a Ti(Ni,Cu)₃ layer and an innermost Ti₃₀Ni_43–47Cu_27–23 layer. The apparent activation energy for the oxidation reaction of Ti₅₀Ni₄₀Cu₁₀ SMA is determined to be 180 kJ/mol, and the oxidation rate follows a parabolic law. A schematic oxidation mechanism of Ti₅₀Ni₄₀Cu₁₀ SMA is proposed to explain the observed results.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Influence of Ti additions on martensitic transformation and magnetic properties of cast Ni51Fe22−xGa27Tix shape memory alloys

The effect of Ti addition on the microstructure, martensitic transformation, magnetic and mechanical properties of polycrystalline Ni₅₁Fe_22?x Ga₂₇Ti _x (x=0, 2 and 4) ferromagnetic shape memory alloy was investigated by scanning electron microscope, differential scanning calorimetry and X-ray diffraction. The results showed that the martensitic transformation temperature increases monotonously with the increase of fraction of Ti substitution for Fe. The increase in the martensite transformation temperatures should be related to the change of the electron concentration after the addition of Ti to Ni₅₁Fe_22?x Ga₂₇Ti _x alloys. According to the results of X-ray diffraction and magnetic properties, Ti has significant effect the structure of Ni₅₁Fe_22-x Ga₂₇Ti _x . Adding of 4 at% Ti altered the structure of the matrix from five-layered tetragonal martensite of Ni₅₁Fe₂₂Ga₂₇ and Ni₅₁Fe₂₀Ga₂₇Ti₂ alloys to non-modulated tetragonal martensite. Magnetic properties proved that the alloy transits from ferromagnetic, five-layered tetragonal martensite, to paramagnetic, non-modulated martensite structure, with increasing Ti content to 4 at.%. Saturation magnetization, remnant magnetization and coercivity of the alloy were significantly influenced by Ti additions. Hardness values of Ni₅₁Fe₂₂Ga₂₇ increased by the addition of Ti.     

Kinetics of crystallization process for bulk glass forming Zr-base alloys

The crystallization kinetics of Zr₆₅Ni₁₀Cu_17.5Al_7.5 (alloy I) and Zr_52.5Ni_14.6Cu_17.9Al₁₀Ti₅ (alloy II) are investigated. Two-stage crystallization takes place during continuous heating of the glassy alloy I, resulting in the transformation of the glass to the metastable α-Zr-solid solution and Zr-base quasicrystals with an activation energy of 309 KJ/mol in the first-stage and the formation of Zr₂Cu compound and the stable α-Zr-solid solution with an activation energy of 227 KJ/mol in the second-stage. For alloy II, one-stage crystallization with an activation energy of 333 KJ/mol occurs during continuous heating of the glass, resulting in the formation of Zr₃Al and α-(Zr,Ti)-solid solution. Based on the DSC data and calculations, both the alloys go through with three stages of crystallization mechanism during isothermal annealing, i.e. (1) surface nucleation and growth, (2) three-dimensional nucleation and growth, and (3) crystal growth. The TEM observation on Zr_52.5Ni_14.6Cu_17.9Al₁₀Ti₅ alloy is in good agreement with the calculations.     

Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_17.5Al_7.5Ni₁₀ and Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.     

Effect of post-deformation annealing on the R-phase transformation temperatures in NiTi shape memory alloys

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni₄Ti₃ precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni₄Ti₃ precipitates.     

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys (SMAs) were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti₅₀Ni₄₀Cu₁₀ alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti₅₀Ni₄₀Cu₁₀ alloy shows a lower shape recovery rate than the porous Ti₅₀Ni₅₀ alloy, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy behaves reversely. In addition, the damping capacity (or internal friction, IF) of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti₅₀Ni₃₀Cu₂₀ alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti₅₀Ni₅₀ alloy.     

Effects of annealing at 800 and 1000 °C on phase precipitates and hardness of Al7Cr20FexNi73−x alloys

The effects of Fe content on the microstructure, phase constituents and microhardness of the as-cast, 800 °C- or 1000 °C-annealed Al₇Cr₂₀Fe_xNi_73?x (x=13?66) alloys were investigated. Not all these alloys are composed of the single FCC phase. The BCC and B2 phases are found. It is confirmed that the BCC phase in the Al₇Cr₂₀Fe₆₆Ni₇ alloy is transformed from the FCC phase at about 900 °C during cooling. While in the 800 °C-annealed Al₇Cr₂₀Fe₆₀Ni₁₃ alloy, the FCC phase is stable and the hardness decreases. After annealing at 1000 °C, for the precipitation of the B2 particles, the Al content in the FCC phase decreases, which results in decreasing of the alloy hardness. Moreover, after annealing at 800 °C, a small amount of Al-rich B2 particles precipitate at the phase boundary and some nanocrystal BCC phase precipitates in the FCC matrix, which increases the hardness of the Al₇Cr₂₀Fe_xNi_73?x (x=41?49) alloys. These results will help to the composition design and processing design of the Al?Cr?Fe?Ni based high-entropy alloys.     

层叠Ni/Ti热扩散形成金属间化合物的规律

选择Ni和Ti粉末及其机械合金化粉末制备Ni/Ti扩散偶,利用扫描电镜和X射线衍射等手段研究了Ni/Ti扩散偶在固相热处理作用下金属间化合物的形成及生长规律.随着热处理温度的提高,Ni3Ti,Ti2Ni和NiTi金属间化合物的数量增加明显;随热处理保温时间的增加,NiTi金属间化合物呈抛物线规律生长,而对Ni3Ti和Ti2Ni的生长影响不大.结果表明,金属间化合物在形成过程中,Ni3Ti和Ti2Ni优先形成,达到一定厚度后,NiTi金属间化合物开始形成并快速增长.     

Influences of additional alloying elements (V,Ni, Cu,Sn, B) on structure and mechanical properties of high-strength hypereutectic Ti–Fe–Co bulk alloys

High-strength nonequilibrium hypereutectic bulk alloys were obtained recently in the Ti–Fe and Ti–Fe–Co systems by arc-melting. Following these results, the influences of the additional alloying elements (V, Ni, Cu, Sn, B) on high strength hypereutectic Ti–Fe–Co bulk alloys are studied and analyzed in the present work. The structure of the hypereutectic quaternary Ti₆₇Fe₁₄Co₁₄Sn₅, Ti₆₇Fe₁₄Co₁₄V₅, Ti₇₀Fe₁₇Co₇Cu₆, Ti₇₀Fe₁₇Co₇Ni₆, and Ti_69.4Fe_14.8Co_14.8B₁ alloys obtained in the form of arc-melted ingots of about 20–30 mm diameter and 10–15 mm height was studied by X-ray diffractometry and scanning electron microscopy. The mechanical properties were tested by an Instron-type machine. Ti₆₇Fe₁₄Co₁₄Sn₅ alloy exhibits a high ultimate compressive strength of 1830 MPa and a large plastic strain of 24% which exceeds the ductility values obtained for Ti–Fe and Ti–Fe–Co alloys. The addition of Sn causes formation of a relatively rough eutectic structure which is preferable for the high strength hypereutectic alloys. Rough primary dendrites and eutectic rods of the cP2 intermetallic phase act as efficient barriers for shear strain and cracks propagation while fine eutectic rods of submicron size are quite effortlessly cut by deformation bands and cracks.     

Glass-forming ability and stability of ternary Ni-early transition metal (Ti/Zr/Hf) alloys

Four Ni-bearing Ti, Zr and Hf ternary alloys of nominal composition Zr_41.5Ti_41.5Ni₁₇, Zr₂₅Ti₂₅Ni₅₀, Zr_41.5Hf_41.5Ni₁₇ and Ti_41.5Hf_41.5Ni₁₇ were rapidly solidified in order to produce ribbons. The Zr–Ti–Ni and Ti–Hf–Ni alloys become amorphous, whereas the Zr–Hf–Ni alloy shows precipitation of a cubic phase. The devitrification of all three alloys was followed and the relative tendency to form nanoquasicrystals and cF96 phases analysed. The relative glass-forming ability of the alloys can be explained by taking into account their atomic size difference. Addition of Ni often leads to quasicrystallisation or quasicrystal-related phases. This can be explained by the atomic radius and heat of mixing of the constituent elements. The phases precipitated at the initial stages of crystallisation indicate the possible presence of Frank–Kasper polyhedral structure in the amorphous alloys. Structural analysis reveals that the Laves and the anti-Laves phases have the same polyhedral structural unit, which is similar to the structural characteristics of glass.     

Microstructure properties and microhardness of rapidly solidified Al<Subscript>64</Subscript>Cu<Subscript>20</Subscript>Fe<Subscript>12</Subscript>Si<Subscript>4</Subscript> quasicrystal alloy

This paper presents differences in the microstructure and microhardness properties of conventional casting (ingot) and rapidly solidified Al₆₄Cu₂₀Fe₁₂Si₄ quasicrystal (QC) alloys. The phases present in the Al₆₄Cu₂₀Fe₁₂Si₄ ingot alloy were determined to be icosahedral quasicrystalline (IQC) Ψ-Al₆₅Cu₂₀Fe₁₅, cubic β-AlFe, tetragonal θ-Al₂Cu, and monoclinic λ-A1₃Fe₄ phases, whereas only IQC Ψ-Al₆₅Cu₂₀Fe₁₅ and cubic β-AlFe phases were identified in the rapidly solidified alloy. The microhardness value of the melt spun alloy was measured to be approximately 790 kg/mm². Microhardness increases with increasing solidification rates.     

Microstructure and Tensile Behavior of Al8Co17Cr17Cu8Fe17Ni33 (at.%) High-Entropy Alloy

Microstructure evolution and tensile behavior of the high-entropy alloy Al₈Co₁₇Cr₁₇Cu₈Fe₁₇Ni₃₃ (at.%) are investigated at room temperature and at 500°C in the as-cast state and under different heat-treatment conditions. Detailed microstructural characterizations are carried out using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The equilibrium phase evolution as a function of temperature was calculated using the Thermo-Calc software (Thermo-Calc Software, Stockholm, Sweden) integrated with TTNi-7 database. The observed majority phase is a face-centered cubic solid solution for all tested specimens. Tensile ductility at room temperature and at elevated temperature is enhanced by heat treatment at 1150°C. An embrittlement phenomenon has been observed after a heat treatment at 700°C resulting in significant degradation in tensile properties.     

Microstructure and mechanical properties of duplex tiAl alloys containing Mn

A study has been made of the effect of Mn on the structure and compressive mechanical properties of duplex TiAl alloys. In order to clarify the separate effect of Mn and Al contents, the effect of Ti/Al ratio was also studied by analyzing the behavior of alloys with different Ti/Al ratios at the fixed Mn content. Addition of 3 at.% Mn to the binary Ti₅₃Al₄₇ alloy decreases the tetragonality and the unit cell volume of γ structure. It also promotes the formation of twin-related structure, the refinement of interlamellar spacing and grain size, and the increase in the volume fraction of lamellar grains. Increase in Ti/Al ratio at the fixed Mn content results in the further decrease in the tetragonality and unit cell volume and the refinement of grain size, while it decreases the volume fraction of lamellar grains. The modification of microstructure directly influences the compressive properties and deformation mode of duplex TiAl alloys. It has been found that Mn addition and increase in Ti/Al ratio enhance the plasticity of duplex alloys. Generation of mobile dislocations at the twin intersections has been found to occur in Mn containing alloys. Such dislocation generation at the twin intersections as well as the promotion of deformation twins in Mn containing alloys are all beneficial for improving the ductility of duplex TiAl alloys.     

热处理对机械合金化Ni/Ti冷喷涂层金属间化合物形成的影响

选择了三种球磨时间制备的Ni/Ti机械合金化粉末,通过冷喷涂制备了不同结构的Ni/Ti涂层.涂层组织结构采用扫描电镜(SEM)和X射线衍射(XRD)进行了表征分析.试验发现,随着粉末球磨时间的增加,热处理后的冷喷涂合金转变为金属间化合物的温度下降,涂层的组成相由Ni3Ti,B2-NiTi和Ti2Ni逐渐变成Ni3Ti和Ti2Ni;随着热处理温度的增加,涂层组织中不同成分的金属间化合物的相对量会发生一定改变.结果表明,热处理过程中形成的B2-NiTi金属间化合物在冷却时表现出较高的稳定性.     

Microstructure and superelastic properties of free forged Ti–Ni shape-memory alloy

Elemental titanium (Ti) and nickel (Ni) powders were consolidated by spark plasma sintering (SPS) to fabricate Ti–51%Ni (mole fraction) shape-memory alloys (SMAs). The objective of this study is to enhance the superelasticity of SPS produced Ti–Ni alloy using free forging as a secondary process. Products from two processes (with and without free forging) were compared in terms of microstructure, transformation temperature and superelasticity. The results showed that, free forging effectively improved the tensile and shape-memory properties. Ductility increased from 6.8% to 9.2% after forging. The maximum strain during superelasticity increased from 5% to 7.5% and the strain recovery rate increased from 72% to 92%. The microstructure of produced Ti–51%Ni SMA consists of the cubic austenite (B2) matrix, monoclinic martensite (B19′), secondary phases (Ti₃Ni₄, Ti₂Ni and TiNi₃) and oxides (Ti₄Ni₂O and Ti₃O₅). There was a shift towards higher temperatures in the martensitic transformation of free forged specimen (aged at 500 °C) due to the decrease in Ni content of B2 matrix. This is related to the presence of Ti₃Ni₄ precipitates, which were observed using transmission electron microscope (TEM). In conclusion, free forging could improve superelasticity and mechanical properties of Ti–51%Ni SMA.