Double twinning in Ni–Mn–Ga–Co

Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5 at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110 °C respectively up to 160 °C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism.     

Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys

Sluggish diffusion kinetics is an important contributor to the outstanding properties of high-entropy alloys. However, the diffusion kinetics in high-entropy alloys has never been probed directly. Here, the diffusion couple method was used to measure the diffusion parameters of Co, Cr, Fe, Mn and Ni in ideal-solution-like Co–Cr–Fe–Mn–Ni alloys. These parameters were compared with those in various conventional face-centered cubic metals. The results show that the diffusion coefficients in the Co–Cr–Fe–Mn–Ni alloys are indeed lower than those in the reference metals. Correspondingly, the activation energies in the high-entropy alloys are higher than those in the reference metals. Moreover, the trend of the normalized activation energy is positively related to the number of composing elements in the matrix. A quasi-chemical model is proposed to analyze the fluctuation of lattice potential energy in different matrices and to explain the observed trend in activation energies. Greater fluctuation of lattice potential energy produces more significant atomic traps and blocks, leading to higher activation energies, and thus accounts for the sluggish diffusion in high-entropy alloys.     

Fatigue in Ti–6Al–4V–B alloys

The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening.     

GP-zones in Al–Zn–Mg alloys and their role in artificial aging

The structure of GP-zones in an industrial, 7xxx-series Al–Zn–Mg alloy has been investigated by transmission electron microscopy methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)-zones are formed over a wide temperature range, from room temperature to 140–150°C, independently of quenching temperature. The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450°C, by aging at temperatures above 70°C. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated <110> domains. The structural relation to the η′-precipitate is discussed.     

Bulk ferromagnetic glasses in the Fe–Ni–P–B System

The ferromagnetic metallic glass Fe₄₀Ni₄₀P₁₄B₆, available only as 30–50 μm thick ribbons, has been extensively studied over the last three decades. We used a flux-melting and water-quenching technique to prepare bulk glassy Fe₄₀Ni₄₀P₁₄B₆ alloys in the form of 2-mm diameter spheres and 1-mm diameter rods. The Curie temperature for the bulk glasses is higher than the average value of Curie temperatures reported for the rapidly quenched ribbons. The glass-transition temperature and the crystallization temperature of the bulk glasses are lower and higher, respectively, than the average values reported for rapidly quenched ribbons, making the supercooled-liquid region as wide as 42 K. The bulk glasses crystallize by a homogeneous nucleation followed by a growth at a constant rate. The nucleation rate in the bulk glasses is four orders of magnitude lower than in the rapidly quenched ribbons, suggesting that the previous thickness limitation was due to impurities in the melt (heterogeneous nucleation).     

Secondary precipitation in Al–Zn–Mg–(Ag) alloys

Secondary ageing of age-hardenable aluminium alloys occurs at temperatures below the solvus of GP zones after a preliminary ageing at a higher temperature. The phenomenon has technological interest, as it may be included in heat treatments giving a substantial benefit on the mechanical properties. In the present work, positron annihilation lifetime spectroscopy (PALS) is applied in combination with Vickers hardness measurements for an investigation on secondary ageing of Al–4wt.%Zn–3wt.%Mg–xAg, where x=0, 0.1, 0.2, 0.3, 0.5 wt.%. Ageing regimes have been characterised by the substantially different evolutions that are observed. The results shed light on the interplay between the formation of coherent solute aggregates (clusters or GP zones) and the precipitation of semi-coherent or incoherent precipitates, which are in competition to control the hardening effects. PALS data show that secondary ageing in the ternary Al–Zn–Mg alloys produces coherent aggregates even in the presence of a well-developed stage of semi-coherent or incoherent precipitation that is obtained if the alloys are first aged to peak hardness. In the presence of Ag, on the contrary, the effects of coherent aggregation during secondary ageing are observed only if the preliminary ageing is interrupted well before reaching peak hardness.     

Phase equilibria in the system Al–Co–Si

The Al–Co–Si system was studied for three isothermal sections at 600 °C (equilibria with Si), 800 °C (alloys up to 50 at.% Co) and 900 °C (alloys with more than 50 at.% Co). A total number of seven ternary compounds were characterized in the ternary system and the homogeneity ranges of the various ternary solid solutions of binary Co–Al and Co–Si compounds were studied. X-ray powder diffraction and optical microscopy was used for initial sample characterization and electron probe microanalysis of the annealed samples was used to determine the phase compositions within the ternary system. Lattice parameters have been determined for all ternary compounds and the change of lattice parameters with the composition is given for the solid solution phases.     

Defect-induced Au precipitation in Fe–Au and Fe–Au–B–N alloys studied by in situ small-angle neutron scattering

Nanoscale Au precipitation in high-purity Fe–Au and Fe–Au–B–N alloys has been studied by in situ small-angle neutron scattering during isothermal aging at 550 °C and complementary ex situ transmission electron microscopy. The high temperature precipitation behavior in samples having received different degrees of cold deformation has been studied to explore the potential self-healing of deformation-induced defects by Au precipitation. It is found that dislocations induced by prior plastic deformation strongly facilitate the formation of Au precipitates, as no significant precipitation is observed for undeformed samples. Defect-induced Au precipitates are formed both at dislocations and along grain boundaries where the defect density is high. The fact that the Au atoms only precipitate on deformation-induced defects demonstrates that solute gold atoms act as efficient self-healing agents in the ferrous matrix. The addition of B and N is found to retard the Au precipitation.     

Phase equilibria in Ti–Ni–Pt ternary system

Phase equilibria in Ti–Ni–Pt ternary system have been experimentally determined through diffusion triple technique combined with alloy samples approach. Assisted with electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques, isothermal sections at 1073 and 1173 K of this system were constructed and existence of ternary phase Ti₂(Ni,Pt)₃ was confirmed. In addition, binary compounds Ti₃Pt₅ and TiPt_3– were found to be stable at 1073 and 1173 K, and remarkable ternary solubility in some binary compounds was detected, e.g., solubility of Pt in TiNi can be up to about 36% (molar fraction) at 1073 K and 40% (molar fraction) at 1173 K. Furthermore, a ternary invariant transition reaction TiNi₃+Ti₃Pt₅→Ti₂(Ni,Pt)₃+TiPt₃₊ at a temperature between 1073 and 1173 K was deduced.     

Rhenium distribution in the matrix and near the particle–matrix interface in a model Ni–Al–Ta–Re superalloy

The distribution of Ni, Al, Ta, and Re in a model Ni-base superalloy is investigated in the as-heat treated alloy by energy dispersive X-ray analysis and three-dimensional atom probe. Re was found to partition mainly to the γ phase in which it forms clusters with sizes of about 1 nm and mutual average distances of 20 nm.     

Studying recovery processes in a strain-hardened Al–Mg–Mn–Fe–Si alloy

The material of a shell structure subjected to 20-year use under ambient conditions has been studied. The structure and mechanical characteristics of a strain-hardened AMg6 alloy, as well as the effect of subsequent holdings of this alloy for 10–3000 h at temperatures of 50, 70, 80, 100, 130, 150, 180, and 220°C, on changes in its dislocation structure and mechanical characteristics have been investigated. It has been shown that, in the structures under study, the AMg6 alloy has a cellular structure with a high density of dislocations and the ultimate strength σ_u = 445.5 ± 2.5 MPa, the proof stress σ_0.2 = 326.5 ± 3.5 MPa, and the relative elongation δ = 11.7 ± 0.5%. Polygonization in the alloy occurs at a temperature of 220°C and the initial stage of the recovery process corresponds to a temperature range of 50–100°С in which the softening process can be divided into two stages, i.e., stage (1) of active softening due to the interaction of point defects with each other and stage (2) of the stabilization of the characteristics of the alloy.     

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.     

Laves phase fields in Cr–Zr–Nb and Cr–Zr–Hf alloy systems

The phase fields of the Laves phase in the Cr–Zr–Nb and Cr–Zr–Hf alloy systems were investigated at 1573 K. The Laves phase formed in the Cr–Zr–Nb alloy system had a broad off-stoichiometric range in the center of ZrCr₂–NbCr₂ pseudo-binary line, while the Laves phase formed in the Cr–Zr–Hf alloy system had a uniform and limited off-stoichiometric range along ZrCr₂–HfCr₂ pseudo-binary line. The results are discussed, in terms of geometric concept of atomic sizes of the constituent atoms.     

Role of vacancy–solute complex in the initial rapid age hardening in an Al–Cu–Mg alloy

We have investigated the origin of the initial rapid hardening of an Al–1.3 at.% Mg–1.7 at.% Cu alloy by coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopy. Quenched-in vacancies are bound to Mg atoms rather than Cu atoms initially and the vacancy–Mg complexes easily migrate to vacancy sinks at 150°C. Vacancy–Mg–Cu complexes form during the initial 1 min aging at vacancy sinks, meanwhile vacancy density decreases rapidly. These results support that the dislocation–solute interaction is the origin of the initial rapid hardening.     

Bulk metallic glass formation in the Mg–Cu–Zn–Y system

The element Cu in the bulk glass-forming alloy Mg₆₅Cu₂₅Y₁₀ was substituted with the element Zn to form a Mg₆₅Cu₂₀Zn₅Y₁₀ alloy, which caused a significant improvement of the glass-forming ability of Mg₆₅Cu₂₅Y₁₀ alloy. For the Mg₆₅Cu₂₀Zn₅Y₁₀ alloy, fully glassy rod with a 6-mm diameter can be obtained by copper mold casting.     

Control of crystallinity in sputtered Cr–Ti–C films

The influence of Ti content on crystallinity and bonding of Cr–Ti–C thin films deposited by magnetron sputtering have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy. Our results show that binary Cr–C films without Ti exhibit an amorphous structure with two non-crystalline components; amorphous CrC_x and amorphous C (a-C). The addition of 10–20 at.% Ti leads to the crystallization of the amorphous CrC_x and the formation of a metastable cubic (Cr_1?xTi_x)Cy phase. The observation was explained based on the tendency of the 3d transition metals to form crystalline carbide films. The mechanical properties of the films determined by nanoindentation and microindentation were found to be strongly dependent on the film composition in terms of hardness, elasticity modulus, hardness/elasticity ratio and crack development.     

β Phase decomposition in Ti–5Al–5Mo–5V–3Cr

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ～500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ～90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ～1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties.     

Analysis of phases in a Cu–Cr–Zr alloy

Phases of a Cu–0.31%Cr–0.21%Zr alloy were analyzed by scanning electronic microscope and energy dispersive X-ray spectroscopy (EDXS) and transmission electronic microscope (TEM). The EDXS results showed that there are three types of phases in the alloy, Cu-matrix, chromium-rich and zirconium-rich phases; coarse phases mainly consist of zirconium-rich phase. TEM result showed that fine chromium distributed in matrix and Cu₅₁Zr₁₄ phase was found in matrix.     

Microstructure and microsegregation in Al-rich Al–Cu–Mg alloys

Microstructure and microsegregation in two directionally solidified Al alloys, Al–3.9Cu–0.9Mg and Al–15Cu–1Mg (in wt%), were investigated for cooling rates between 0.78 and 0.039 K/s. Transverse and longitudinal sections were examined to exhibit dendritic microstructures. Fractions of solids formed were determined using quantitative image analysis and solute redistribution in the primary phase was determined using area scans. The model employed to calculate microsegregation is based on the Scheil model but including solid-state diffusion, dendrite arm coarsening and undercooling of the dendrite tip and the formation of eutectic. The model-calculated results were found to be in good agreement with the experimentally determined concentration distributions in the primary α phase and the amounts of phases formed. It was found that the dendrite morphology was best described by a cylindrical arm geometry and that the accuracy of the phase diagram could have a significant influence on the microsegregation predictions. For the alloy with low copper content, two types of embedded droplets were observed.