Microstructural evolution and phase transformation in the liquid-solid Al/Ni diffusion couple

The liquid-solid Al/Ni diffusion couple was successfully fabricated by annealing at 1373?K for 48?h followed by water-quenching. Cross-sectional scanning and transmission electron microscopic analyses show that the multilayered diffusion zones comprise the following sequence of layers: γ-Ni(Al) | γ′-Ni₃Al | β′-NiAl | Ni-rich β-NiAl | β-NiAl. The Ni-rich β-NiAl upon quenching undergoes a martensitic transformation from β (B2) to β′ (L1₀). The β′ martensite is found to be internally twinned on the {111}<112>system. The volume changes and strains due to martensitic phase transformation, the precipitation of γ′-Ni₃Al from γ-Ni(Al) and lattice mismatch between Ni-rich β-NiAl and β-NiAl in the Al/Ni diffusion couple are quantitatively determined. The cuboidal γ′ phase coherently precipitates cube-on-cube in γ-Ni(Al). Composition fluctuations existing in the supersaturated solid solution γ-Ni(Al), provide sufficient driving force for the precipitation and facilitate nucleation and growth of the γ′ phase under isothermal annealing.     

Effect of ausforming temperature and strain on the bainitic transformation kinetics of a low carbon boron steel

The effect of ausforming temperature and strain on the bainitic transformation kinetics was investigated in a low carbon boron steel. A new mechanism, which is based on the competition between the increase in nucleation rate and the decrease in average volume of bainite sheaf after deformation, is proposed. The increase in nucleation rate is due to the decrease in boron concentration at the grain boundaries after small deformation and the formation of sub-grain boundaries at the grain interior after large deformation. The decrease in average volume of bainite sheaf is ascribed to the frequent impingement of bainite sub-units after deformation. The increase in nucleation rate after deformation results in the decrease in incubation time, which is confirmed from the experiment. The increase in nucleation rate overcomes the decrease in average volume of bainite sheaf, resulting in the increase in transformation velocity and volume fraction after small deformation. On the contrary, the decrease in the average volume of bainite sheaf overcomes the increase in nucleation rate after large deformation, leading to the decrease in transformation velocity and volume fraction of bainite.     

Five-parameter crystallographic characteristics of the interfaces formed during ferrite to austenite transformation in a duplex stainless steel

The crystallography of interfaces in a duplex stainless steel having an equiaxed microstructure produced through the ferrite to austenite diffusive phase transformation has been studied. The five-parameter interface character distribution revealed a high anisotropy in habit planes for the austenite–ferrite and austenite–austenite interfaces for different lattice misorientations. The austenite and ferrite habit planes largely terminated on (1 1 1) and (1 1 0) planes, respectively, for the austenite–ferrite interfaces associated with Kurdjumov–Sachs (K–S) and Nishiyama–Wasserman (N–W) orientation relationships. This was mostly attributed to the crystallographic preference associated with the phase transformation. For the austenite–ferrite interfaces with orientation relationships which are neither K–S nor N–W, both austenite and ferrite habit planes had (1 1 1) orientations. Σ3 twin boundaries comprised the majority of austenite–austenite interfaces, mostly showing a pure twist character and terminating on (1 1 1) planes due to the minimum energy configuration. The second highest populated austenite–austenite boundary was Σ9, which tended to have grain boundary planes in the tilt zone due to the geometrical constraints. Furthermore, the intervariant crystallographic plane distribution associated with the K–S orientation relationship displayed a general tendency for the austenite habit planes to terminate with the (1 1 1) orientation, mainly due to the crystallographic preference associated with the phase transformation.     

Effect of prior austenite grain size on the formation of carbide-free bainite in low-alloy steel

ABSTRACT

Inconsistencies exist in literature regarding the effect of prior austenite grain size (PAGS) on the extent and kinetics of bainite transformation. Attempts have been made in the present work to address these issues in a low-alloy carbide-free bainitic steel using dilatometry, over a range of PAGS. The bainite transformation kinetics in the above-mentioned conditions have been analysed quantitatively using established kinetic model to extract information related to the transformation mechanisms in such conditions. Greater obstruction from grain boundaries in fine-grained austenite restricts sheaves of bainite to develop completely and thereby reduces the volume fraction of bainite in comparison with coarse-grained austenite. Initial nucleation rate of bainite transformation increases with decreasing PAGS due to an increase in the nucleation site density. However, the maximum nucleation rate decreases consistently with decreasing PAGS due to gradual reduction in the autocatalytic factor.     

Ce emission in hexagonal and orthorhombic phases of CaSO4

CaSO₄ exists in several phases. The most common phase of CaSO₄ is orthorhombic and reported Ce³⁺ emission corresponds to this phase. However, significant change in the emission of Ce³⁺ is observed when CaSO₄ crystallizes in hexagonal phases. The emission is observed at 354 nm as compared to the spilt band at 305 and 326 nm for the orthorhombic phase. The preparation procedure and photoluminescence spectra for orthorhombic and hexagonal phases are described in this paper.     

Effect of high pressure on metastable and stable intermediate hcp phases formed in the Ag-17at%Ge alloy

Abstract

A metastable hexagonal close-packed (hcp) phase obtained by rapid quenching from the melt has been compressed to 5.7 GPa and annealed up to 1023 K. The axial ratios (c/a) of the hcp structure at the initial state, the stable state annealed under high pressure (5.7 GPa, 673 K) and the quenched state from high pressure and high temperature condition are 1.630, 1.635 and 1.628, respectively.

The volume reduction of the hcp structure by application of high pressure gives rise to increase the c/a ratio, which corresponds to an apparent reduction in the number of valence electrons per atom (e/a) in the Hume-Rothery alloys.     

Martensitic transformation and magnetic properties of ferromagnetic shape memory Fe66Pd30Rh4 alloys

This study mainly shows that in the Fe₆₆Pd₃₀Rh₄ (at%) alloys, the L1₀ phase plays an important role in magnetostriction due to the interplay of L1₀ martensitic twins with magnetic domains. The L1₀ martensitic twin structure exhibits a strong magnetocrystalline anisotropy energy constant (K_u=1.27-2.84×10⁶ (ergs/cm³)) along the tetragonal c axis direction. In addition, the L1₀ tetragonal martensitic twin structure shows both a perfect shape memory and a reversible shape memory effect; therefore, it is expected to be applicable in magneto-mechanical applications (such as microactuators or springs). However, in this study, we discover that solution treatment (ST) and aging heat treatments of Fe₆₆Pd₃₀Rh₄ ferromagnetic shape memory alloys influence the behavior of the martensitic transition, which is associated with the change in magnetic properties. The process of a thermoelastic L1₀+L1_m twin phase decomposition→non-thermoelastic L1₀+L1_m+α_bct structure in Fe₆₆Pd₃₀Rh₄ alloys during solution treatment and aging at 400-550 °C for various times is studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The relation of phase separation morphology to the magnetic property change is examined with a superconducting quantum interference device (SQUID) magnetometer, and magnetostriction measurement is performed with a strain gage method and magnetostrictive meter setup. The results indicate that the process of martensitic transformation during aging leads to an increase in coercivity and a decrease in magnetostriction, simultaneously.     

Phase transformation and quantum confinement effect in CdSe/Se multilayer thin films prepared by physical vapour deposition

CdSe/Se multilayer (ML) thin films with different thickness ratios of Se and CdSe sublayers were prepared by using a thermal evaporation method. Prepared samples were annealed at temperature 300 K. From X-ray diffraction (XRD) studies, samples prepared at room temperature showed a (100) plane of CdSe with wurtzite structure, whereas the annealed samples confirmed the cubic structure. Stress created in ML systems was calculated from XRD data and found that it increases with decreasing particle size. The energy band gap value of a CdSe/Se ML thin film is shifted to a value higher than that of the bulk CdSe (1.74 eV) semiconductor. This is due to decrease in the crystallite size smaller than the Bohr exciton diameter of CdSe (11.2 nm). Crystallite sizes (≈5 nm) were calculated from UV–VIS data with the predictions of an effective mass approximation model. The photoluminescence peak of the ML samples is split into two bands having nearest values due to the emissions from spin–orbit split-up of the excited energy state.     

Investigation of the effect of pressure on thermodynamic properties and thermoelastic phase transformation of CuAlNi alloys: A molecular dynamics study

Thermoelastic phase transformations and thermodynamic properties of CuAlNi alloys at 0, 1, 2 and 3 GPa pressures were investigated by using MD simulation in this study. The interactions between atoms were modelled by Sutton-Chen type of embedded atom method (SCEAM) that is based on many-body interaction. It was observed that thermoelastic phase transformation in the ternary alloy system occurred at the end of thermal process. Radial distribution function (RDF) was used in order to analysis the structures obtained from MD simulation using the simulation techniques’ thermodynamic parameters. The transformation temperatures, enthalpy and entropy of the ternary alloy system have been observed to be changing with the applied pressure. In addition, it was found that the elastic energy has been decreased about 22% by applied pressure whereas Gibbs free energy has been increased about 60% by applied pressure. The values of the thermodynamical parameters obtained in this study were observed to be in close agreement with the experimental study.     

Phase transformation and nanograting structure on TiO2 rutile single crystal induced by infrared femtosecond laser

In this paper, TitaiJum dioxide （TiO2） rutile single crystal was irradiated by infrared femtosecond laser pulses with repetition rate of 250 kHz. For a P-polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was formed . The periodicity is much less than the laser wavelength. The direction of nanograting alignment depends on the polarization laser beam. Micro-Raman spectra show that the intensity of Eg Raman vibrating mode of rutile phase increases and that of Alg Raman vibrating mode decreases apparently within the ablation crater. With the increase of irradiation time and laser average power, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO2 single crystal is partly transformed into anatase phase.[第一段]     

Fast relaxation of the metastable helium state 2S0 in collisions with molecules and collisional lasing on the He (2P1-2S0) transition

In the present study, the laser absorption method was used to measure the rates of quenching of the metastable state He(2¹S₀), the lower laser level in the self-terminating helium laser, with H₂O, NH₃, N₂O, and CO₂ molecules. For the above molecules, the quenching rate constants were found to equal (1.2 ± 0.3)10^− 9, (0.8 ± 0.2)10^− 9, (1.9 ± 0.2)10^− 9 and (2.2 ± 0.4)10^− 9 cm³ s^− 1. Under excitation with long (up to 750 ns) open discharge generated electron beam pulses, lasing on the transition He (2¹P₁⁰-2¹S₀) was examined. In the mixtures He-H₂O and He-NH₃, lasing durations almost equal to the pump-pulse duration were obtained. In the mixtures of He with CO₂ and N₂O, no lasing prolonged in comparison with pure helium was found. The data obtained were explained considering two quenching mechanisms for the state He(2¹S₀): in collisions with molecules and in collisions with plasma electrons having low energies due to fast relaxation of the vibrational states of H₂O and NH₃ molecules.     

Effect of dose on irradiation-induced loop density and Burgers vector in ion-irradiated ferritic/martensitic steel HT9

Samples of F/M steel HT9 were irradiated to 20?dpa at 420°C, 440°C and 470°C in a transmission electron microscope with 1?MeV Kr ions so that the microstructure evolution could be followed in situ and characterised as a function of dose. Dynamic observations of irradiation-induced defect formation and evolution were made at the different temperatures. Irradiation-induced loops were characterised in terms of their Burgers vector, size and density as a function of dose and similar observations and trends were found at the three temperatures: (i) both a/2 <111> and a <100> loops are observed; (ii) in the early stage of irradiation, the density of irradiation-induced loops increases with dose (0–4?dpa) and then decreases at higher doses (above 4?dpa), (iii) the dislocation line density shows an inverse trend to the loop density with increasing dose: in the early stages of irradiation, the pre-existing dislocation lines are lost by climb to the surfaces while at higher doses (above 4?dpa), the build-up of new dislocation networks is observed along with the loss of the radiation-induced dislocation loops to dislocation networks; (iv) at higher doses, the decrease of number of loops affects more the a/2 <111> loop population; the possible loss mechanisms of the a/2 <111> loops are discussed. Also, the ratio of a <100> to a/2 <111> loops is found to be similar to cases of bulk irradiation of the same alloy using 5?MeV Fe²⁺ ions to similar doses of 20?dpa at similar temperatures.     

AlN在AlN/VN纳米多层膜中的相转变及其对薄膜力学性能的影响

采用反应磁控溅射制备了AlN/VN纳米多层膜.研究了多层膜调制周期对AlN生长结构的影响以 及纳米多层膜的力学性能.结果表明：小周期多层膜中的AlN以亚稳的立方相(c-AlN)存在并 与VN形成共格外延生长的超晶格.薄膜产生硬度和弹性模量升高的超硬效应.大调制周期下， AlN从立方结构转变为稳定的六方相（h-AlN），并使多层膜形成纳米晶的“砖墙”型结构. 讨论认为VN的模板作用有利于c-AlN的生长，但不能显著提高其临界厚度. 关键词： 薄膜的力学性能 外延生长 亚稳相     

Effects of high-pressure heat treatment on the solid-state phase transformation and microstructures of Cu61.13Zn33.94Al4.93 alloys

The phase transformation activation energy of the Cu61.13Zn33.94Al4.93 alloys, which were treated at 4 GPa and 700℃ for 15 minutes, was calculated by means of differential scanning calorimetry curves obtained at various heating and cooling rates. Then, the effects of high-pressure heat treatments on the solid-state phase transformation and the microstructures of Cu61.13Zn33.94Al4.93 alloys were investigated. The results show that high-pressure heat treatments can refine the grains and can change the preferred orientation from (111) to (200) of α phase. Compared with the as-cast alloy, the sample with high-pressure heat treatment has finer grains, lower β′→β and β→β' transformation temperature and activation energy. Furthermore, we found that high cooling rate favours the formation of fine needle-like α phase in the range of 5-20℃/min.     

A study on the formation of crystalline phases during solidification and crystallisation in the bulk metallic glass of Zr53Cu21Al10Ni8Ti8 composition

The present paper considers the formation of crystalline phases during solidification and crystallisation of the Zr₅₃Cu₂₁Al₁₀Ni₈Ti₈ alloy. Solidification was carried out by a copper mould casting technique, which yielded a partially crystalline microstructure comprising a ‘big cube phase’ in a dendritic morphology and a bct Zr₂Ni phase. Detailed high-resolution microscopy was carried out to determine possible mechanisms for the formation of the crystalline phases. Based on microstructural examinations, it was established that the dendrites grew by the attachment of atomistic ledges. The bct Zr₂Ni phase, formed during solidification and crystallisation, showed various types of faults depending on the crystallite size, and its crystallography was examined in detail. It has been shown that the presence of these faults could be explained by anti-site occupancy in the bct lattice of the Zr₂Ni phase.     

Ni75Cr25-xAlx合金中L12相和D022相形核孕育期的微观相场模拟

基于微观相场动力学模型,运用原子图像、平均长程序参数和平均成分偏离序参数,研究了Al浓度对Ni₇₅Cr_25-xAl_x合金中L1₂相和D0₂₂相形核孕育期的影响以及孕育期与沉淀相析出顺序之间的关系.结果表明,L1₂相和D0₂₂相的形核孕育期不仅与Al浓度有关,而且与两相析出的先后顺序密切相关.当Al浓度小于7.5%时,先析出相为D0₂₂相,随着Al浓度的增大,D0₂₂相形核孕育期延长,后析出的L1₂相的形核孕育期也延长,L1₂相的原子簇?聚?速度加快;当Al浓度大于7.5%时,先析出相为L1₂相,随着Al浓度的增大,L1₂相的形核孕育期缩短,后析出的D0₂₂相形核孕育期也缩短.当Al浓度为7.5%时,L1₂相和D0₂₂相几乎同时析出,两者的孕育期没有明显的差别. 关键词： 形核孕育期 序参数 沉淀 微观相场模拟     

The effects of interstitial oxygen on superconducting electronic phases in strontium and oxygen co-doped La1.937Sr0.063CuO4＋δ

Strontium and oxygen co-doped La1.937Sr0.063CuO4＋δ superconductor with Tc≈ 40K, which is obtained by oxidizing strontium-doped starting ceramic sample La1.937Sr0.063CuO4 in NaC10 solution, is annealed under different conditions to allow interstitial oxygen to redistribute. The evolution of the intrinsic superconducting property with the oxygen redistribution is studied in detail by magnetic measurements in various fields. It is found that there occurs the electronic phase separation from the single superconducting phase with Tc ≈ 40 K into two coexisting superconducting states with values of Tc： 15 and 40K or of 15 and 35 K in this system, depending on annealing condition. Our results indicate that the 15, 35 and 40 K superconducting phases associated with the excess oxygen redistribution are all thermodynamically meta-stable intrinsic states in this Sr/O co-doped cuprate.     

Influence of Ni/Mn ratio on magnetostructural transformation and magnetocaloric effect in Ni_(48-x)Co_2Mn_(38+x)Sn_(12)(x = 0, 1.0, 1.5, 2.0,and 2.5) ferromagnetic shape memory alloys

An investigation on the magnetostructural transformation and magnetocaloric properties of Ni_(48-x)Co_2Mn_(38+x)Sn_(12)(x = 0, 1.0, 1.5, 2.0, and 2.5) ferromagnetic shape memory alloys is carried out. With the partial replacement of Ni by Mn in the Ni_(48)Co_2Mn_(38)Sn_(12) alloy, the electron concentration decreases. As a result, the martensitic transformation temperature is decreased into the temperature window between the Curie-temperatures of austenite and martensite. Thus, the samples with x = 1.5 and 2.0 exhibit the magnetostructural transformation between the weak-magnetization martensite and ferromagnetic austenite at room temperature. The structural transformation can be induced not only by the temperature,but also by the magnetic field. Accompanied by the magnetic-field-induced magnetostructural transformation, a considerable magnetocaloric effect is observed. With the increase of x, the maximum entropy change decreases, but the effective magnetic cooling capacity increases.