Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown -Mo, Mo₃Si, and Mo₅SiB₂ phases. In addition, particles of SiO₂ formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while -Al₂O₃ formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of -Mo and Mo₃Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by -Mo, crack deflection and branching. Flexural strength is related to volume fraction of the -Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.     

Effects of Nb and C additions on the crystallization behavior, microstructure and magnetic properties of B-rich nanocrystalline Nd–Fe–B ribbons

The effects of Nb and C additions on the crystallization behavior, microstructure and magnetic properties of B-rich Nd_9.4Fe_79.6−xNb_xB_11−yC_y (x = 0, 2, and 4; y = 0, 0.5, and 1.5) alloy ribbons have been investigated. The results show that Nb and C additions change the crystallization behavior of Nd_9.4Fe_79.6B₁₁, avoid the formation of metastable Nd₂Fe₂₃B₃ phase, leading to the simultaneously precipitation of α-Fe and Nd₂Fe₁₄B phases. The results also show that Nb and C additions suppress the formation and growth of the soft α-Fe phases, leading to the presence of a large amount of Nd₂Fe₁₄B phases. Nb and C additions also refine the structure, and thus increase the exchange coupling interaction between the soft and hard phases. Excellent magnetic properties of B_r = 0.85 T, _iH_c = 1106 kA/m, and (BH)_max = 117 kJ/m³ have been achieved in Nd_9.4Fe_75.6Nb₄B_10.5C_0.5 alloy ribbons.     

Sn effect on microstructure and mechanical properties of ultrafine eutectic Ti–Fe–Sn alloys

Microstructural investigations on ultrafine eutectic (Ti₆₅Fe₃₅)_100−xSn_x alloys with x = 0, 1 and 3 at.% reveal that additional Sn is effective to control formation of the micron-scale dendrites and to decrease the length-scale of lamellar spacing with enhancing macroscopic plasticity at room temperature compression. Hence, it is possible to understand the influence of the microstructural change on the plasticity of the ultrafine eutectic Ti–Fe–Sn alloys.     

Structures and superparamagnetic properties of overaged Fe–Al–Mn–C alloys

Microstructures and superparamagnetic properties in aged-hardened Fe–9%Al–30%Mn– (x)C,Si alloys, resulting from overaging at a temperature of 823 K for 48 h to 313 days, have been investigated by transmission electron microscopy (TEM), X-ray diffraction patterns, and vibrating sample magnetometry (VSM). The results reveal that the precipitate κ-phase [(Fe,Mn)₃AlC] decomposition in this alloy, overaged at 823 K for one week, resulted from two separate mechanisms: (1) wetting of the antiphase boundary segment (APBs) of D0₃ [(Fe/Mn)₃Al] domains by the B2 [(Fe/Mn)Al] phase; and (2) precipitation of the B2 [(Fe/Mn)Al] phase within the domain. A superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for ≈120–313 days. The super-soft magnetic property was mainly attributable to the ferromagnetic spinel-ordered (B2 [(Fe/Mn)Al]+D0₃ [(Fe/Mn)₃Al]) phases and ordered B2 with monoclinic ′Mn structures.     

An anelastic spectroscopy, differential scanning calorimetry and X-ray diffraction study of the crystallization process of Mg–Ni–Fe alloys

The effects of heating-induced crystallization on the structural and mechanical properties of Mg–Ni–Fe amorphous ribbons were studied by anelastic spectroscopy, differential scanning calorimetry (DSC) and X-ray diffraction. DSC results show that the crystallization occurs through several non-reversible steps, which correspond to significant changes in the Young's modulus and concomitant irreversible elastic energy loss peaks. Moreover, an anelastic peak is found at 215 K, which for the first time indicates the presence of some dynamical process related to the simultaneous presence of different phases. The formation of a metastable Mg₆Ni phase is detected, which transforms into Mg and Mg₂Ni stable phases. A quantitative analysis of the different phases present at the different steps was also carried out.     

The role of Fe and Ti additions in the microstructure of Nb–18Si–5Sn silicide-based alloys

The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)_ss, (Nb,Ti)₃Sn, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si, FeNb₄Si, and Fe₂Nb₃ and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)₃Si, (Nb,Ti)₃Sn and the Ti rich oxide. In NV8 the formation of Nb₅Si₃ was destabilised, the stability of Nb₃Si was enhanced and the eutectic between Nb₅Si₃ and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb₄Si and Fe₂Nb₃ phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)₃Sn, β(Ti,Nb)_ss, (Ti,Nb)₃Si and (Ti,Nb)₅Si₃ phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)_ss + (Ti,Nb)₅Si₃ → (Ti,Nb)₃Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb₃Sn, Ti₃Sn, and Nb₃Si, Ti₃Si, and Ti and Nb base 5-3 silicides are discussed.     

Concepts derived from phase diagram studies for the strengthening of Fe–Al-based alloys

Fe–Al-based alloys, i.e. alloys which contain either disordered A2 -(Fe,Al), B2-ordered FeAl or D0₃-ordered Fe₃Al as majority phase, have a considerable potential for developing materials for structural applications, but insufficient strength and creep resistance have been identified as obstacles for the use of Fe–Al-based alloys at high temperatures. At the ‘Discussion Meeting on the Development of Innovative Iron Aluminium Alloys’ held at the Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf on March, 9th 2004 a couple of presentations were made with emphasis on improving these properties at high temperatures. In the current article those strengthening mechanisms which are provided by the phase diagram—solid-solution hardening, strengthening by precipitates, or ordering—are reviewed. Besides results obtained for the binary Fe–Al system special emphasis is put forward to those ternary systems for which results have been presented at the ‘Discussion Meeting’.     

Levels of thermal stability in some glassy alloys of the Ge–Sb–Se system by differential scanning calorimetry

The thermal stability and crystallization of alloys in the Ge–Sb–Se system were studied by differential scanning calorimetry (DSC). A comparison of various simple quantitative methods to assess the level of stability of the glassy materials in the above-mentioned system is presented. All of these methods are based on characteristic temperatures, obtained by heating of the samples in non-isothermal regime, such as the glass transition temperature, T_g, the temperature at which crystallization begins, T_in, the temperature corresponding to the maximum crystallization rate, T_p, or the melting temperature, T_m. In this work, a parameter K_r(T) is added to the stability criteria. The thermal stability of some ternary compounds of Ge_xSb_0.23−ySe_0.77−x+y type has been evaluated experimentally and correlated with the activation energies of crystallization by this kinetic criterion and compared with those evaluated by other criteria.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

Dependence of the brittle-to-ductile transition temperature (BDTT) on the Al content of Fe–Al alloys

The brittle-to-ductile transition temperature (BDTT) of binary Fe–Al alloys with between 9.6 and 45 at.% Al was investigated in the as-cast state by four-point bending tests. An increase of the BDTT was observed with increasing aluminium content between 9.6 and 19.8 at.%. Up to 41.3 at.%, the BDTT did not change significantly. A sharp increase of the BDTT occurred between 41.3 and 45 at.% Al. Transgranular cleavage was observed at a composition of 25 at.% Al, mixed-mode fracture between 39.6 and 41.3 at.% Al and intergranular fracture at 45 at.% Al. The results indicate that the increase in BDTT is correlated with the transition from mixed-mode to intergranular fracture.     

The effect of thermal ageing on microstructure and mechanical properties of Al–Si–Fe/Mg alloys

In this paper the microstructure and ageing behavior of Al–Si–Fe/Mg alloys produced through sand-casting route is presented. 4.0–6.0 wt% Mg was added to Al–Si–Fe alloy. Standard mechanical properties test samples were prepared from the sand cast 25 mm diameter by 45 mm rods. Thermal ageing was done for 6 h at 200 °C. The ageing characteristics of these alloys were evaluated using tensile properties, hardness values, impact energy and microstructure as criteria. The thermal aged samples exhibited higher yield strength, tensile strength and hardness values as the weight percent of magnesium increased up to 5 wt% in the Al–Si–Fe/Mg alloys as compared to as-cast samples. The optimum values were obtained at 5 wt%Mg. Lower percent elongation, reduction in area and impact energy values were obtained for age-hardened Al–Si–Fe/Mg alloy samples as compared to as-cast samples. The increases in hardness values and strength during ageing are attributed to the formation of coherent and uniform precipitation in the metal lattice. It was found that the age-hardened showed acceleration in ageing compared to the as-cast alloy. However, the 5 wt%Mg addition to the alloy showed more acceleration to thermal ageing treatment. These results show that better mechanical properties are achievable by subjecting the as-cast Al–Si–Fe/Mg alloys to thermal ageing treatment.     

The influence of residual stress and crystallite size on the magnetic properties of electrodeposited nanocrystalline Pd-Co alloys

Nanocrystalline Pd-Co alloys were obtained by electrodeposition from an ammoniacal chloride bath. The influence of the crystallite size and the residual stress on the magnetic properties of the alloys was investigated. The residual stress increased as the applied current density was increased. It was associated to the high nucleation rate during electrodeposition and correlated to the lattice strain, estimated from the XRD patterns. Also from the XRD patterns the average crystallite size and the lattice constant were determined by Scherrer's and Rietveld's methods, respectively. Both parameters were directly influenced by the applied current density. Magnetic properties such as coercivity, remanence, saturation magnetization and squareness showed strong dependence on the residual stress and crystallite size. Coercivity higher than 1 kOe was achieved when a high current density was applied. High coercivity was attributed to the presence of residual stress and to the small crystallite size of deposits.     

Thermal property characterization of a Ti–4 wt.%Nb–4 wt.%Zr alloy using drop and differential scanning calorimetry

The thermal properties of Ti–4 wt.%Nb–4 wt.%Zr alloy, namely the enthalpy increment and heat capacity have been characterized as a function of temperature using drop and differential scanning calorimetry, respectively. The measured data clearly attested to the presence of a phase change from α (hcp) to β (bcc) phase at about 1100 ± 5 K. In fact, the alloy exhibited a transformation domain in the temperature interval 1100–1170 K. The enthalpy associated with the α → β phase change is estimated to be about 73 (±5%) J g⁻¹. The jump in the specific heat at the transformation temperature is 1714 (±7%) J kg⁻¹ K⁻¹. The drop and differential scanning calorimetry results are consolidated to obtain the first experimental data on the thermodynamic quantities of this alloy.     

Thermodynamic activity measurements in the B2 phases of the Fe–Al and Ni–Al systems

The vaporisation of Fe–Al and Ni–Al alloys has been investigated in the temperature range 1140–1600 K and 1178 to 1574 K, respectively, by Knudsen effusion mass spectrometry (KEMS). Eleven different Fe–Al and also eleven Ni–Al compositions have been investigated in the composition ranges 30–51 at.% Al and 38–53 at.% Al, respectively. The Fe–Al samples have been investigated mostly in the B2 region of the phase diagram. The partial pressures and thermodynamic activities were evaluated directly from the measured ion intensities formed from the equilibrium vapour over the alloy and the pure element. From the temperature dependence of the activities the partial and integral molar enthalpies and entropies of mixing have been obtained. These are the most accurate data obtained by mass spectrometry on Fe–Al and Ni–Al systems so far. Nearly temperature independent integral enthalpies and entropies of mixing over the wide temperature range investigated were found, with the mixing entropies being large and negative.     

Interstitial effects of B addition on the metamagnetic transition and magnetocaloric effect in tetragonal Cu2Sb-type Mn1.95Cu0.05Sb alloys

Mn_1.95Cu_0.05SbB_x (x = 0, 0.06 and 0.1) alloys had been prepared and B interstitial effects on metamagnetic transition were studied. The metamagnetic transition temperature was reduced and thermal hysteresis was widened by higher B concentration. The saturation magnetization and the magnetic entropy change were increased by moderate amount of B addition. However, too high B composition led into the sluggish metamagnetic transition. By relating with crystallographic structure, our results further indicated that the electron density of the atoms at MnⅡ position plays critical role on influencing the metamagnetic transition in tetragonal Cu₂Sb-type Mn_1.95Cu_0.05Sb alloys.     

Effect of Ce addition on the microstructure and damping properties of Cu–Al–Mn shape memory alloys

The effect of rare earth element Ce addition on the microstructure, martensitic transformation, mechanical properties and damping behavior of the Cu–Al–Mn shape memory alloys (SMAs) had been investigated. It is shown that the Ce addition makes the grain refinement and affects the martensitic transformation temperature. The tensile strength and the ductility of the Cu–Al–Mn alloys can be enhanced by the Ce addition. Damping capacity tan δ of the martensite for the Cu–Al–Mn–Ce alloys is strain amplitude dependent. The Ce addition has obvious effects on the damping properties of the martensite. With the increase of the Ce content, the damping capacity increases initially and then decreases.     

Influence of Mn and Cu addition on the hyperfine parameters of amorphous and nanocrystalline FeCoNbB alloys

The effects of Cu and Mn addition on the hyperfine field of FeCoNbB HITPERM alloys are discussed from Mössbauer spectrometry. Amorphous and nanocrystalline samples at different stages of the nanocrystallization were studied. The effect of Cu addition correlates with the observed refinement of the microstructure. Mn mainly partitions to the matrix, decreasing the average hyperfine field of the amorphous matrix, although some Mn remains in the nanocrystalline grains, presumably, in a concentration below the maximum solubility of Mn in -Fe.     

Influence of composition and annealing conditions on the site-occupation in the σ-phase of Fe–Cr and Fe–V systems

Neutron diffraction technique was used to study the site-occupation in the σ-phase in the Fe–Cr and Fe–V systems. It was found that all five sites A, B, C, D and E are “mixed”, i.e. occupied by both elements. The occupation is neither random – the degree of randomness increases with Fe content – nor regular, i.e. sites A and D are predominantly occupied by Fe atoms while B, C and E sites are occupied preferentially by Cr or V atoms. For all five sites the increase of Cr(V) concentration results in an irregular decrease of the number of Fe atoms on each site, the rate of decrease being the smallest for the sites A and D. In the Fe–Cr system the population of A with Fe atoms, N_A, is similar to that of D, N_D, while for the system Fe–V N_D > N_A and the difference increases with V content. The lowest Fe population in both investigated systems has B, but N_B(V) < N_B(Cr). N_C = N_E for Fe–V, while N_C > N_E for Fe–Cr. The influence of sample preparation conditions (plastic deformation prior to phase transformation, annealing time, t_a, and temperature of annealing, T_a) were also tested on a σ-Fe_53.8Cr_46.2 sample. In general the influence of these conditions on the site population is small. The largest effect had the plastic deformation, and the smallest one the annealing time in case of non-deformed samples. The most insensitive sites were revealed to be D and E, and the most sensitive ones were B and A.     

Influence of the history of a melt on the electrical resistivity of cadmium–antimony liquid alloys

Miller, Paces and Komarek (MPK) [Trans. Metall Soc. AIME 230 (1964) 1557] observed an influence of the history of a melt on the electrical resistivity of several cadmium–antimony alloys. In this work we complete the experimental work of MPK with new accurate experiments in order to verify the existence of the phenomenon and to precise by defining the conditions where it appears. We used three important improvements on the experimental design of MPK. First we used quartz cells instead of pyrex cells used by MPK so allowing the heating of the melt well above the temperature of 530 °C attained by MPK. Secondly our experimental design allows to mix mechanically the liquid alloy in order to achieve a macroscopic homogeneity of the melt. Finally we measure simultaneously the thermopower of the Cd₆₀–Sb₄₀ liquid alloy. The time evolution of our experiment is fully described with our conclusions.