Phase Equilibria and Phase Transition of the Ni–Fe–Ga Ferromagnetic Shape Memory Alloy System

Phase equilibria and martensitic and magnetic transitions of the β (B2 and L2₁) phase in the Ni–Fe–Ga system were investigated. The b phase was found to be in equilibrium with the γ (A1 structure) or γ′ (L1₂ structure) phase. The Curie temperature, T _c , equilibrium temperature, T _o 5 (Ms + Af)/2, martensitic transition starting temperature, M _s , and reverse transition finishing temperature, Af , of the β single–phase alloys were sensitive to the Fe and Ga compositions. The Fe substitution for Ni decreased and increased the T _o and T _c , respectively. The Ga substitution for Ni or Fe decreased both the T _o and T _c . The entropy change accompanying the reverse martensitic transition showed compositional dependence due to the magnetic contribution. The saturation magnetization I _s of the Ni–Fe–Ga system showed a strong dependence on the magnetic valence Z _M . The Is values of the Ni–Fe–Ga alloys annealed at 1023 K showed the same Z _m dependence as other ferromagnetic shape memory alloy (FSMA) systems. This article is based on a presentation made in the symposium entitled "Phase Transformations in Magnetic Materials," which occurred during the TMS Annual Meeting, March 12-16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS–MPMD and ASMI–MSCTS Phase Transformations Committee.     

Thermal Properties,Microstructure and Microhardness of Cu–Al–Co Shape Memory Alloy System

In this study, the effects of Co content on the crystal structure, transformation temperatures and microstructure of Cu–Al–Co shape memory alloy system were investigated. It was found that Cu–Al–Co alloy system has also 18R type martensitic structure, which is commonly observed in copper-based systems. The transformation temperatures were found to be higher than 250 °C and they do not show a linear increase or decrease with Co content. The microstructural examination revealed the presence of martensite phase and precipitates. It was realized that the size of precipitate increases with increasing Co content. It can be stated that the hardness increased with increasing Co content. All these results point out that increasing Co content increases the strength of Cu–Al–Co shape memory alloy system.     

Calculation of Thermodynamic Properties of Cu–Al–(Ag,Au) Shape Memory Alloy Systems

The results of calculation of thermodynamic properties for ternary Cu–Al–Ag and Cu–Al–Au shape memory alloy systems at temperature of 1,100 K, obtained using general solution model, are presented in this paper. The activities, activity coefficients, partial and integral molar quantities were determined for selected cross-sections in explored systems.     

Phase Transformation Behaviors and Effects of Terbium in Polycrystalline Ni-Mn-Ga Magnetic Shape Memory Alloys

The phase transformation behaviors of two kinds of magnetic shape memory alloys NisoMn25 x Ga25-x and Ni50 Mn29Ga21-x Tbx were studied. When the composition of Ni in these alloys was constant, increasing Mn and reducing Ga contents make martensitic transformation temperatures rise obviously. Simultaneously, thermal hysteresis of phase transformation reduce but Curie temperature una|ters. When terbium was added, phase transformation temperature went up further and Curie temperature kept constant. The alloys still show strong ferromagnetism and properties of thermoelastic martensite phase transformation.     

Reverse Shape Memory Effect Related to α → γ Transformation in a Fe-Mn-Al-Ni Shape Memory Alloy

In this study, we investigated the shape memory behavior and phase transformations of solution-treated Fe_43.61Mn_34.74Al_13.38Ni_8.27 alloy between room temperature and 1173 K (900 °C). This alloy exhibits the reverse shape memory effect resulting from the phase transformation of α (bcc) → γ (fcc) between 673 K and 1073 K (400 °C and 800 °C) in addition to the shape memory effect resulting from the martensitic reverse transformation of γ′ (fcc) → α (bcc) below 673 K (400 °C). There is a high density of hairpin-shaped dislocations in the α phase undergoing the martensitic reverse transformation of γ′ → α. The lath γ phase, which preferentially nucleates and grows in the reversed α phase, has the same crystal orientation with the reverse-transformed γ′ martensite. However, the vermiculate γ phase, which is precipitated in the α phase between lath γ phase, has different crystal orientations. The lath γ phase is beneficial to attaining better reverse shape memory effect than the vermiculate γ phase.     

Strain-induced Precipitation Behavior ofδPhase in Inconel 718 Alloy

To study the precipitation dynamics ofδphase in Inconel 718alloy,two-stage interrupted compression method was used in the region of cold deformation temperatures and the temperatures range from 875to 975℃.The precipitation-time-temperature(PTT)curve ofδphase was obtained by analyzing the softening kinetics curves.For verifying the type of the precipitates and confirming the validity of the test,the transmission electron microscopy(TEM),scanning transmission electron microscopy(STEM)and energy dispersion spectrum(EDS)were employed.Experimental results indicated that the PTT curve forδprecipitation exhibited a typicalCshape and the nose points of start and finish precipitation were about 5sat 920℃and 2 815sat 940℃,respectively.In addition,the nucleation ofδwas heterogeneous.The nucleation sites varied with temperatures,including dislocation,grain boundary and stacking fault withinγ″phase.Andδparticles grew quickly at higher temperature with lower density.Moreover,the driving force of nucleation was mainly including chemical free energy,interfacial energy and dislocation distorted energy.And the dislocation distorted energy could decide the density of nucleation in the strain-induced process.     

Effect of Annealing Temperature and Time on Martensitic Transformation Temperatures and Mechanical Properties of the Ti–50.7 at % Ni Shape Memory Alloy

Russian Journal of Non-Ferrous Metals - The influence of temperature and time of recrystallization annealing on the characteristic temperatures of martensitic transformation and mechanical...     

Influence of Aging Treatment on Precipitation Behavior of η Phase in Ni-Co-Cr Alloy

The influence of aging treatment on precipitation behavior of -η phase in a new Ni-Co-Cr fastener superal-loy, AEREX350, was investigated by means of optical microscopy, scanning electron microscopy (SEM) and trans-mission electron microscopy (TEM). The precipitation behavior of η phase with two kinds of morphologies in this material was found to be sensitive to the temperature and time of aging treatment. Time-temperature-transformation curves of these two η phase were achieved. Furthermore, diffraction patterns revealed the semi-coherent orientation relationship between η phase and γ matrix. This approach may be used to optimize the microstructure of the alloy for excellent mechanical properties.     

Low-Cyclic Fatigue Behavior of Peak-Aged Mg–Nd-Based Alloy

Metallurgical and Materials Transactions A - In low cycle fatigue, Mg–Nd based alloys exhibit initial cyclic hardening and then softening. The cyclic hardening and softening behaviour of a...     

CuZnAl Shape Memory Alloy Refined with Composite Rare Earths La Ce

TheCu-basedshapememoryalloys(SMA)havelowc0standrichresourcesandmayhavewidepracticalapplications,0wingtotheirbettershapememoryeffect(SME)thanNi-TiSMA.However,someseri0usproblemssuchasinter-granularfractureduetolargegrainsizeandsta-bilizati0nofmartensitemusthes0lvedbeforetheyareputinuse,Thewell-knownmethodforreducinggrainsizeistheadditionoflesssolub1e.l....t.[l'2j.Ithasbeenreportedthattherareearthelementshavedramaticeffectsinreducinggrainsizeandimprovingthemechanicalproper-tiessuchasbetterf…     

Influence of Aging Treatment on Precipitation Behavior of η Phase in a Ni-Co-Cr Alloy

 The influence of aging treatment on precipitation behavior of η phase in a new Ni-Co-Cr fastener superalloy, AEREX350, was investigated by means of optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation behavior of η phase with two kinds of morphologies in this material was found to be sensitive to the temperature and time of aging treatment. Time temperature transformation curves of these two η phase were both achieved. Furthermore, diffraction patterns revealed the semi-coherent orientation relationship between η phase and γ matrix. This approach may be used to optimize the microstructure of the alloy for excellent mechanical properties.     

The Wear Behavior of In-Situ Al–AlN Metal Matrix Composites

Al–AlN composites are synthesized using NH₄Cl + CaO powder as a nitrogenation precursor in the melt of pure aluminum. In-situ formation of AlN to varying volume fraction is attempted using different proportion of NH₄Cl + CaO precursor into the aluminum melt held at 700 °C. Mechanical properties of synthesized metal matrix composites are evaluated for different volume fraction and distribution of AlN particles in aluminum matrix. Agglomeration of AlN is noticed with increasing precursor addition and synthesis time into the aluminum matrix. Due to heterogeneous distribution of AlN particles in aluminum matrix, marginal changes in hardness are observed. Pin on disc, dry sliding wear of metal matrix composites is carried to study wear behavior of synthesized composites. Composite with good dispersion of AlN particulates has shown higher hardness and wear resistance. Present paper discusses wear behavior of composites with different weight fraction of AlN tested under load and sliding distance as wear parameters. The shearing mechanism of agglomerate due to friction and its correlation with the wear loss is also highlighted in the present paper.     

Combined Effect of Calcium and Silicon on the Phase Composition and Structure of Al–10%Mg Alloy

Phase transformations in the Al–Ca–Mg–Si system in the region of aluminum–magnesium alloys are investigated using the Thermo-Calc program. The liquidus projection of the quaternary system is constructed with a Mg content of 10% and it is shown that phases Al4Ca, Mg₂Si, and Al₂CaSi₂ can crystallize (in addition to the aluminum solid solution (Al)) depending on the calcium and silicon concentrations. The crystallization character of quaternary alloys is investigated with the help of a polythermal cross section calculated at concentrations of 10% Mg and 84% Al. Based on the analysis of phase transformations occurring in alloys of this section, the presence of the Al–Al₂CaSi₂–Mg₂Si quasi-ternary section in the Al–Ca–Mg–Si system was assumed. Three experimental alloys were considered from a quantitative analysis of the phase composition, notably, Al–10% Ca–10% Mg–2% Si, Al–4% Ca–10% Mg–2% Si, and Al–3% Ca–10% Mg–1% Si. Metallographic investigations and electron-probe microanalysis were performed using a TESCAN Vega 3 scanning electron microscope. Critical temperatures are determined using a DSC Setaram Setsys Evolution differential calorimeter. The experimental results agree well with the calculated data; in particular, a peak at t ~ 450°C is revealed for all alloys in curves of the nonequilibrium solidus and invariant eutectic reaction L → (Al) + Al₄Ca + Mg₂Si + Al₃Mg₂. It is established that the structure of the Al–3% Ca–10% Mg–1% Si alloy is closest to the eutectic alloy. It is no worse that the AMg10 alloy in regards to density and corrosion resistance and even surpasses it in hardness, which allows us to consider this alloy as the basis for the development of a new cast material: “natural composites.”     

Electrochemical Behavior of AlB12–AlN Composites in Natural Environments

Powder Metallurgy and Metal Ceramics - Electrochemical processes occurring on ceramic AlB12–AlN composites induced by polarization in a 3% NaCl aqueous solution at 37°C are examined for...