Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 °C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni_24.7Ti_50.3Pd_25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 °C and ∼8.5 °C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 °C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82 GPa and hardness of ∼4.7 GPa in martensite phase.     

Influence of interface microstructure on the mechanical properties of titanium/17-4 PH stainless steel solid state diffusion bonded joints

In the present study, titanium was diffusion bonded to a type 17-4 precipitation hardening stainless steel in vacuum at different temperatures and times. Bonded samples were characterized using light microscopy, scanning electron microscopy (SEM) and X-ray diffraction technique (XRD). The inter-diffusion of the chemical species across the diffusion interface was evaluated by electron probe microanalysis (EPMA). Up to 850 °C for 60 min, FeTi phase was formed at the diffusion interface; however, α-Fe + λ, χ, Fe₂Ti and FeTi phases and their phase mixtures were formed above 850 °C for 60 min and at 900 °C for all bonding times. The maximum tensile strength of ∼342.4 MPa and shear strength of ∼260.3 MPa along with 12.8% elongation were obtained for the diffusion couple processed at 950 °C. The thicknesses of different reaction products at the bond interface play an important role in determining the mechanical properties of the joints. The residual stress of the bonded joints increases with the increases in bonding temperatures and times.     

Effect of heat treatment on microstructure,hardness and rollability of V55Ti30Ni15 alloy membranes

The effect of heat treatment on the microstructure, hardness and rollability of V₅₅Ti₃₀Ni₁₅ alloy membranes has been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine NiTi particles precipitate from the supersaturated V-matrix solid solution at temperatures above 600 °C, increase in quantity until 800 °C, then dissolve back into the V-matrix with a further increase in temperature up to 950 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 950 °C. In the present study, a comparison has been made between the rollability of the as-cast and the heat treated state selected for deformation at different rolling temperatures. The percent reduction in thickness of the heat-treated alloy (800 °C/18 h) has been found to be up to 30% higher than that of the as-cast alloy, even at room temperature (cold rolling).     

Microstructural development of diffusion-brazed austenitic stainless steel to magnesium alloy using a nickel interlayer

The differences in physical and metallurgical properties of stainless steels and magnesium alloys make them difficult to join using conventional fusion welding processes. Therefore, the diffusion brazing of 316L steel to magnesium alloy (AZ31) was performed using a double stage bonding process. To join these dissimilar alloys, the solid-state diffusion bonding of 316L steel to a Ni interlayer was carried out at 900 °C followed by diffusion brazing to AZ31 at 510 °C. Metallographic and compositional analyses show that a metallurgical bond was achieved with a shear strength of 54 MPa. However, during the diffusion brazing stage B₂ intermetallic compounds form within the joint and these intermetallics are pushed ahead of the solid/liquid interface during isothermal solidification of the joint. These intermetallics had a detrimental effect on joint strengths when the joint was held at the diffusion brazing temperature for longer than 20 min.     

The ambient and high temperature deformation behavior of Al–Si–Cu–Mg alloy with minor Ti,Zr, Ni additions

The principal aim of the present work was to investigate the effects of minor additions of nickel and zirconium on the strength of cast aluminum alloy 354 at ambient and high temperatures. Tensile properties of the as-cast and heat-treated alloys were determined at room temperature and at high temperatures (190 °C, 250 °C, 350 °C). The results show that Zr reacts only with Ti, Si and Al. From the quality index charts constructed for these alloys, the quality index attains minimum and maximum values of 259 MPa and 459 MPa, in the as-cast and solution-treated conditions; also, maximum and minimum values of yield strength are observed at 345 MPa and 80 MPa, respectively, within the series of aging treatments applied. A decrease in tensile properties of ∼10% with the addition of 0.4 wt.% nickel is attributed to a nickel–copper reaction. The reduction in mechanical properties due to addition of different elements is attributed principally to the increase in the percentage of intermetallic phase particles formed during solidification; such particles act as stress concentrators, decreasing the alloy ductility. Tensile test results at ambient temperatures show a slight increase (∼10%) in alloys with Zr and Zr/Ni additions, particularly at aging temperatures above 240 °C. Additions of Zr and Zr + Ni increase the high temperature tensile properties, in particular for the alloy containing 0.2 wt.% Zr + 0.2 wt.% Ni, which exhibits an increase of more than 30% in the tensile properties at 300 °C compared with the base 354 alloy.     

Microstructure stability and mechanical properties of an age-hardenable Ni–Mo–Cr alloy subjected to long-term exposure to elevated temperature

This paper characterizes the microstructure and mechanical properties of a nickel-based superalloy with a nominal composition of Ni–25Mo–8Cr (wt.%) after long-term exposures to elevated temperatures. The alloy is strengthened by long-range-ordered precipitates of an oI6 metastable phase with the Ni₂(Mo,Cr) stoichiometry. The alloy was annealed at 650 °C for 1000, 2000 and 4000 h, after it had been plastically deformed in order to accelerate diffusion processes occurring at elevated temperature and consequently to ease the formation of stable phases. The microstructure was characterized using TEM, SEM and X-ray phase analyses; mechanical properties were measured in tensile tests.It has been determined that the alloy loses its phase stability upon plastic deformation and subsequent long-term annealing at 650 °C. The microstructure, composed initially of a dispersed Ni₂(Mo,Cr) strengthening phase in a Ni-based solid solution, decomposes during annealing into a mixture of Ni₃Mo- and Ni₄Mo-type phases, Mo-lean Ni-based solid solution and a complex intermetallic P phase. The dominant new phase is a plate-shaped Ni₃Mo-type phase while the P phase appears as singular small precipitates. The Ni₃Mo phase is formed mainly in regions of highly localized deformation, e.g., in shear bands, and only occasionally nucleates in regions where the deformation was relatively uniform (dislocations or twins in one system). Regions adjacent to the plates of the Ni₃Mo phase are recrystallized and free from an Ni₂(Mo,Cr) strengthening phase. Changes in microstructure of the deformed alloy during long time annealing at 650 °C result in the decrease in the yield strength as well as tensile elongation at both room temperature and 650 °C. A significant decrease in elongation at 650 °C occurs only in specimens tested in air but not those tested in vacuum.     

The effects of Ta on the stress rupture properties and microstructural stability of a novel Ni-base superalloy for land-based high temperature applications

A novel polycrystalline Ni-base superalloy was developed for land-based high temperature applications, such as isothermal forging dies and industrial gas turbines. The alloy possessed surprisingly high stress rupture life of 52 h at 1100 °C/118 MPa which is comparable to the first generation single crystal (SC) superalloy and exhibited good microstructural stability. The effects of Ta addition on the phase change, stress rupture properties and microstructural stability of the alloy were investigated. The results indicated that Ta is a γ′-former and promotes the formation of eutectic γ′. The alloys with ∼7 vol.% eutectic γ′ possess higher stress rupture life at 1100 °C/118 MPa than the alloys with higher ∼20 vol.% eutectic. However, ∼20 vol.% excessive eutectic phases will enhance the stress rupture life at intermediate temperature of 760 °C for 686 MPa but weaken high temperature stress rupture properties. The (Al + Ta) content over 14.4 at.% led to the formation of large amounts of eutectic γ′ and exceeded the solubility of W and Mo in the residue liquid pool, which then promoted the precipitation of primary α-(W,Mo) and M₆C phases. Tantalum was also found as a strong MC carbides forming element. The order of ability to form monocarbide decreased from NbC to TaC to TiC. 6Al–0Ta (wt.%) alloys possessed good microstructural stability with no harmful topologically close-packed (TCP) phases being observed after thermal exposure at 850 °C/3000 h, 900 °C/1000 h. Only trace amounts of secondary plate-like M₆C carbides appeared in Ta-free and 5Al–4Ta (wt.%) alloys at 1100 °C/100–500 h. However, excessive Ta addition will destabilize the alloy and large amounts of secondary plate-like M₆C carbides precipitated after thermal exposure at 1100 °C. The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) results showed the existence of the plate-like M₆C carbides.     

Effects of annealing treatment on microstructure and hardness of bulk AlCrFeNiMo0.2 eutectic high-entropy alloy

Bulk AlCrFeNiMo_0.2 (in molar ratio) alloy ingot was prepared by vacuum induction melting and casting methods. The effects of annealing temperature variations (550–1050 °C) on the crystal structure, microstructure, and hardness were investigated. The hardness of the as-cast alloy was HV467, and the alloy exhibited a typical eutectic cell microstructure consisting of FeCrMo-rich solid solution (BCC structure) and NiAl-rich intermetallic compound (B2 structure). After annealing at 750–950 °C, a Mo-rich σ phase precipitated from the FeCrMo-rich solid solution, and apparent annealing hardening appeared. The hardness increased from HV479 to HV542. The hardening of this alloy was attributed to the transformation of the BCC phase to the σ phase. The σ phase changed back to the BCC phase at 1050 °C and the hardness decreased to HV478 which was nearly equal to that of the as-cast alloy. Overall, the AlCrFeNiMo_0.2 alloy exhibited excellent annealing softening resistance.     

Thermal oxidation of Ti6Al4V alloy: Microstructural and electrochemical characterization

Thermal oxidation (TO) of Ti6Al4V alloy was performed at 500, 650 and 800 °C for 8, 16, 24 and 48 h in air. The morphological features, structural characteristics, microhardness and corrosion resistance in Ringer's solution of TO Ti6Al4V alloy were evaluated and compared with those of the untreated one. The surface morphological features reveal that the oxide film formed on Ti6Al4V alloy is adherent to the substrate at 500 and 650 °C irrespective of the oxidation time whereas it spalls off when the alloy is oxidized at 800 °C for more than 8 h. X-ray diffraction (XRD) measurement reveals the presence of Ti(O) and α-Ti phases on alloy oxidized at 500 and 650 °C, with Ti(O) as the dominant phase at 650 °C whereas the alloy oxidized at 800 °C exhibits only the rutile phase. Almost a threefold increase in hardness is observed for the alloy oxidized at 650 °C for 48 h when compared to that of the untreated one. Thermally oxidized Ti6Al4V alloy offers excellent corrosion resistance in Ringer's solution when compared to that of the untreated alloy.     

Tyranno ZMI fiber/TiSi2–Si matrix composites for high-temperature structural applications

A Tyranno ZMI fiber/TiSi₂–Si matrix composite was fabricated via melt infiltration (MI) of a Si–16at%Ti alloy at 1375 °C under vacuum. The Si–Ti alloy was used as an infiltrant to conduct MI processing below 1400 °C and inhibit the strength degradation of the amorphous SiC fibers. The alloy matrix formed was dense and comprised primarily of TiSi₂–Si eutectic structures. The TiSi₂–Si matrix composite melt-infiltrated at 1375 °C showed a pseudo-plastic tensile stress–strain behavior followed by final fracture at ∼290 MPa and ∼0.9% strain. When the MI temperature was increased to 1450 °C, however, substantial reduction in the stiffness and ultimate strength occurred under tensile loading. Microstructural observations revealed that these degradations were attributed to the damages that occurred on the reinforcing fibers and pyrolytic carbon interfaces during the MI process. The present experimental results clearly demonstrated the effectiveness of the low-temperature MI process in strengthening Tyranno ZMI fiber composites and reducing the processing cost.     

Development of a new magnesium alloy ZW21

A new magnesium alloy named ZW21 has been developed through orthogonal experiment method and the effects of heat treatment on the alloy’s tensile properties have also been investigated. The results indicate that the alloy only has one Mg–Zn–Y(Nd) phase of Mg₃Zn₃(Y, Nd)₂ (named W phase) and has higher mechanical properties, lower cost and lighter weight compared with the other congeneric alloys. Its microstructure is composed of small equiaxed dendrites and interdendritic discontinuous net-like eutectic structures. The eutectic structures appear in divorced W phase laths in the thin regions between the dendrites and in regular W + α-Mg lamellar structures in the triangle regions. The eutectic structures, especially the W phase, with such distribution are harmful to the tensile properties and thus proper heat treatment can improve its properties through changing the W phase distribution. Solution treatment at 525 °C for 4 h (T₄ treatment) increases the elongation from 17.75% to 26.5%. Subsequent ageing treatment at 250 °C for 24 h (T₆ treatment) improves the ultimate tensile strength from 210 MPa to 243 MPa. The fracture modes of the as-cast, T₄- and T₆-treated alloys all obey the quasi-cleavage regime. The fracture of the as-cast alloy belongs to a mixed mode of intergranular and transgranular forms, but those of the T₄- and T₆-treated alloys follow the transgranular mode due to the relatively high bonding strength between the grains.     

Hot tensile behavior of an extruded Al–Zn–Mg–Cu alloy in the solid and in the semi-solid state

This is the first reported research into the tensile behavior of as-deformed Al–Zn–Mg–Cu alloy in the semi-solid state. Tensile tests of extruded 7075 aluminium alloy were carried out in the high temperature solid and semi-solid states. Based on the tensile results and microstructural examination, the tensile behavior can be divided into three stages according to the effect of liquid: one behaves in predominantly ductile character between 400 and about 520 °C (f_l ∼ 0.31%), one is governed by both of solid and liquid between 520 and 550 °C (f_l ∼ 2%), and almost completely dominated by liquid above ∼550 °C. A brittle temperature range (519–550 °C) is proposed, in which the as-deformed Al–Zn–Mg–Cu alloy exhibits large crack probability. An equation based on ultimate tensile stress and temperature is proposed.     

Investigation of high-strength and superplastic Mg–Y–Gd–Zn alloy

The Mg–7Y–4Gd–1Zn (wt.%) alloy was prepared by hot extrusion technology, and the microstructure, tensile properties and superplastic behavior have been investigated. The extruded alloy possesses high tensile strength both at room temperature and 250 °C, and especially the yield strength can remain above 300 MPa at 250 °C. The outstanding microstructure, i.e. bent 18R long period stacking ordered (LPSO) strips and dynamic recrystallization (DRX) Mg grains containing fine lamellae with 14H LPSO or stacking fault structures, is responsible for the excellent mechanical properties, and it is considered that the integrated performance can be further improved by controlling the size of LPSO phase. The alloy shows the maximum elongation of 700% at 470 °C and 1.7 × 10⁻⁴ s⁻¹. The predominant superplastic mechanism is considered to be grain boundary sliding assisted by lattice diffusion. The fracture of superplastic deformation is related to the microstructure evolution, i.e. the disappearance of LPSO phase and the formation of cubic phase. Both high temperature and stress contribute to the phase transformation.     

Effect of bonding time on microstructure and mechanical properties of transient liquid phase bonded magnesium AZ31 alloy

The transient liquid phase (TLP) bonding of a magnesium alloy AZ31 was undertaken using pure nickel interlayers. The formation of a eutectic between the magnesium and nickel was obtained by bonding at 515 °C and the microstructural developments across the joint region were examined as a function of hold time from 5 to 120 min. Reaction zones were identified within the joint and the changes of width of the reaction zones were examined with respect to changes in the joint shear strength and hardness. The results showed that as the bonding time was increased to 60 min, the width of the eutectic zone was completely removed and the joint solidified isothermally. Under these conditions a maximum hardness value of 179 VHN was recorded and the highest joint shear strength of 36 MPa was obtained. However, when the bonding time was increased to 120 min, the shear strength of the interface decreased and this was attributed to the formation and segregation of brittle Mg–Ni intermetallics within the joint.     

Microstructure and mechanical properties of extruded Mg96Zn1Y3 alloy

Microstructure and mechanical properties of an extruded Mg₉₆Zn₁Y₃ alloy have been investigated in the current study. Microstructure of as extruded alloy consists of α-Mg, long period stacking ordered (LPSO) phase and parallelepiped Y-riched phases, which exhibits a bimodal distribution of fine recrystallized and coarse elongated grains. Fine recrystallized grains with an average grain size of 1.5 μm are mainly pinned by bulk LPSO-14H phase, inside which both LPSO-18R phase and LPSO-14H phase coexist. The extruded Mg₉₆Zn₁Y₃ alloy exhibits a good combination of yield strength and elongation below 200 °C, which is mainly attributed to LPSO phases, stronger texture and finer DRX grains. The main deformation is controlled by load transfer mechanism below 250 °C, and deformation is controlled by non-basal dislocation movement between 300 °C and 325 °C. A good plasticity is attained at 350 °C with an elongation of 140%. The flow stress behavior at 350 °C is mainly controlled by non-basal slip accompanying with dynamic recrystallization and growth of grain.     

Effect of aging on the tensile properties and microstructures of a near-alpha titanium alloy

The effect of aging temperature between 650 °C and 750 °C for different aging times on the tensile properties and microstructures of Ti60 alloy were studied. The results show that the strength of the alloy increases first and then decreases with the aging temperature increases from 650 °C to 750 °C. The reduction of area of the alloy is more sensitive to the aging time than elongation. With increasing aging temperature and time, the volume fracture and grain size of silicides and α₂ phase increase gradually. The silicides have the strengthen effect on the Ti60 alloy, but the effect weakens when the silicides grow up. The loss of ductility is mainly attributed to the precipitation of α₂ phase after aging treatment.     

Improvement of yield strength of LM24 alloy

In this study, Al₂O₃ particles were employed to improve the microstructure of LM24 and therefore, to increase the yield strength and tensile strength of this kind of alloy. In situ Al₂O₃ particles were obtained by direct reaction between oxygen and Al melt at 750–800 °C. Microstructure examination shows that the size of in situ formed Al₂O₃ particles was about 1–2 μm, and interestingly, with addition of in situ Al₂O₃ particles, the coarse primary Si phase was disappeared completely. More important, the yield strength and the tensile strength of Al₂O₃/LM24 are increased by 52 MPa, 16 MPa than that of LM24 alloy with 0.1% Sb addition. The value of 181 MPa and 315 MPa is for yield strength and tensile strength of Al₂O₃/LM24 respectively. Besides, the yield strength and tensile strength are 180 MPa and 314 MPa respectively for Al₂O₃/LM24 alloy after remelting and casting. This verifies that the improvement of mechanical properties of such kind of material possesses stability and reliability.     

Diffusion bonding of 410 stainless steel to copper using a nickel interlayer

In the present work, plates of stainless steel (grade 410) were joined to copper ones through a diffusion bonding process using a nickel interlayer at a temperature range of 800–950 °C. The bonding was performed through pressing the specimens under a 12-MPa compression load and a vacuum of 10^? 4 torr for 60 min. The results indicated the formation of distinct diffusion zones at both Cu/Ni and Ni/SS interfaces during the diffusion bonding process. The thickness of the reaction layer in both interfaces was increased by raising the processing temperature. The phase constitutions and their related microstructure at the Cu/Ni and Ni/SS diffusion bonding interfaces were studied using optical microscopy, scanning electron microscopy, X-ray diffraction and elemental analyses through energy dispersive spectrometry. The resulted penetration profiles were examined using a calibrated electron probe micro-analyzer. The diffusion transition regions near the Cu/Ni and Ni/SS interfaces consist of a complete solid solution zone and of various phases based on (Fe, Ni), (Fe, Cr, Ni) and (Fe, Cr) chemical systems, respectively. The diffusion-bonded joint processed at 900 °C showed the maximum shear strength of about 145 MPa. The maximum hardness was obtained at the SS–Ni interface with a value of about 432 HV.     

Influence of thermal treatment on microstructure,mechanical and degradation properties of Zn–3Mg alloy as potential biodegradable implant material

In this study, the influence of homogenisation heat treatment effect on Zn–3Mg alloy proposed for biodegradable bone implants was investigated. The alloy was developed via casting process from high purity raw materials and homogenised at 360 °C for 15 h followed by water quenching. Results revealed that the microstructure of as cast alloy was composed of dendritic structure of Zn-rich phase distributed in segregated pattern within Mg₂Zn₁₁ eutectic phase. Exposure to the long duration heating of homogenisation apparently broke the dendrites and transformed them into connected precipitates within the alloy's matrix. Non-equilibrium thermal analysis revealed the formation of Mg₂Zn₁₁ eutectic phase which nucleated at 367 °C and solidified completely at 354 °C. The eutectic coherency point occurred at 368 °C and 424 s when 30% of solid has precipitated during solidification. Homogenisation resulted into lowering the alloy's tensile strength from 104 MPa to 88 MPa but improving elongation at fracture from 2.3% to 8.8%. The homogenised Zn–3Mg alloy showed improved corrosion resistance (corrosion rate = 0.13 mmpy) compared to the as-cast one (corrosion rate = 0.21 mmpy). The mechanical property and corrosion behaviour of the homogenised alloy seem suitable for biodegradable implant applications.     

Experimental investigation and thermodynamic calculation of the phase equilibria in the Cu–Mo–Ni ternary system

The phase equilibria at 900 °C, 1000 °C, 1100 °C, 1200 °C and 1300 °C in the Cu–Mo–Ni system were experimentally determined by means of optical microscopy (OM), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD) on the equilibrated alloys. The experimental results firstly found that the fcc-type miscibility gap exists at 900 °C, 1000 °C, 1100 °C and 1200 °C in the Cu–Mo–Ni system, and the solubility of Cu in the MoNi phase at 900 °C, 1000 °C, 1100 °C and 1200 °C are about 0.5 at.%, 1.5 at.%, 1.7 at.% and 4.0 at.%, respectively. The as-cast Cu₂₀Mo₂₀Ni₆₀ (at.%), Cu₂₀Mo₃₀Ni₅₀ (at.%), Cu₁₀Mo₆₀Ni₃₀ (at.%), Cu₇₀Mo₁₀Ni₆₀ (at.%), Cu₂₀Mo₆₀Ni₂₀ (at.%) and Cu₈₀Mo₁₀Ni₁₀ (at.%) alloys appear the separated macroscopic morphologies, which are caused by the liquid phase separation on cooling, while the as-cast Cu₁₀Mo₂₅Ni₆₅ (at.%), Cu₃₂Mo₅Ni₆₃ (at.%) and Cu_30.7Mo_6.3Ni₆₃ (at.%) alloys show the homogenous microscopic morphologies. On the basis of the experimental data investigated by the present and previous works, the phase equilibria in the Cu–Mo–Ni system were thermodynamically assessed by using CALPHAD (Calculation of Phase Diagrams) method, and a consistent set of the thermodynamic parameters leading to reasonable agreement between the calculated results and experimental data was obtained.