Damage-failure transition in titanium alloy Ti-6Al-4V under dwell fatigue loads

The role of structural mechanisms responsible for the consequent staging of damage-failure transition as the combination and continuity of ductile and creep kinetics of the structure evolution and the modeling in dwell fatigue regime. Damage-failure transition is considered as critical phenomena, the structural-scaling transition, when the damage develops as specific phase with characteristic stages: nucleation of new phase and the phase growth kinetics. In the case of dwell fatigue, the nucleation stage is associated with slip localization, faceting, voids and microcrack initiation; the phase growth kinetics has the relation to specific non-linearity of the free energy release responsible for the staging of damage-failure transition. Statistically based phenomenological model of damage-failure transition specified the links of macroscopic material parameters with structural parameters responsible for the influence of microstructure on the structure sensitive mechanical properties. The developed conception of modeling of Ti alloys based on the duality of damage kinetics in dwell fatigue loads allowed us to propose the strategy of structural study to provide in perspective the links of structural parameters of /phases with phenomenological parameters responsible for different mechanisms of damage accumulation at LCF and stress hold regimes

STUDY OF PHASE TRANSFORMATIONS OF THE MICROSTRUCTURE OF ALLOYS BASED ON BINARY ALLOY OF STOICHIOMETRIC COMPOSITION Ni-Mn

Микроструктуру сплава номинального состава Ni50Mn50 изучали методами микроскопии видимого света (ОМ), сканирующей (СЭМ) и просвечивающей электронной микроскопии (ПЭМ). Анализ изображений выполнен в среде программного обеспечения SIAMS 800 и нейросети SIAMS AIM. Изучена микроструктура сплава в мартенситном состоянии и после естественного старения. Фазовый состав и параметры кристаллических решеток фаз устанавливали методом рентгеноструктурного фазового анализа (РФСА) и дифракции обратно рассеянных электронов (EBSD).The microstructure of the alloy with the nominal composition Ni50Mn50 was studied by visible light microscopy, scanning microscopy, and transmission electron microscopy. Image analysis was performed in the SIAMS 800 software environment and the SIAMS AIM neural network. The microstructure of the alloy in the martensitic state and after natural aging was studied. The phase composition and crystal lattice parameters of the phases were determined by X-ray diffraction phase analysis and electron backscatter diffraction.Работа выполнена в рамках государственного задания МИНОБРНАУКИ России (Шифр «Структура» Г.р. № 122021000033-2). Работа выполнена с использованием оборудования ЦКП «Испытательный центр нанотехнологий и перспективных материалов» ИФМ УрО РАН. При финансовой, технической и программной поддержке ООО «СИАМС»

Mechanical Behavior and Structural Characterization of a Cu-Al-Ni-Based Shape-Memory Alloy Subjected to Isothermal Uniaxial Megaplastic Compression

For the first time, uniaxial megaplastic compression was successfully applied to a polycrystalline shape-memory Cu-Al-Ni-based alloy. The samples before and after uniaxial megaplastic compression were examined by methods of X-ray diffraction, optical, electron transmission, and scanning microscopy. The temperature dependences of electrical resistance and the mechanical properties of the alloys under uniaxial tension were also measured. The mechanical behavior under uniaxial megaplastic compression in isothermal conditions in the range of 300–1073 K was studied using the Instron 8862 electric testing machine. The microstructure, phase composition, and martensitic transformations in the eutectoid alloy (Cu-14wt.%Al–4 wt.%Ni) were studied. The radical refinement of the grain structure of the initial hardened D03 austenite was found under controlled isothermal compression, due to dynamic recrystallization in the temperature range 673–1073 K and velocities of 0.5–5 mm/min. Compression at 873–1073 K was accompanied by simultaneous partial pro-eutectoid decomposition with the precipitation of the γ2 phase. Compression at temperatures of 673 and 773 K—that is, below the eutectoid decomposition temperature (840 K)—was accompanied by the precipitation of disperse γ2 and α phases, and ultradisperse B2’ particles. Cooling of the deformed alloy to room temperature after performing each regime of compression led to thermoelastic martensitic transformation, together with the precipitation of the β′ and γ′ phases. The formation of a fine-grained structure produced an unusual combination of strength and plasticity of the initially brittle alloy both under controlled uniaxial compression, and during subsequent tensile tests at room temperature

New Metastable Baro- and Deformation-Induced Phases in Ferromagnetic Shape Memory Ni₂MnGa-Based Alloys

Structural and phase transformations in the microstructure and new metastable baro- and deformation-induced phases of the Ni50Mn28.5Ga21.5 alloy, typical of the unique class of ferromagnetic shape memory Heusler alloys, have been systematically studied for the first time. Phase X-ray diffraction analysis, transmission and scanning electron microscopy, and temperature measurements of electrical resistivity and magnetic characteristics in strong magnetic fields were used. It was found that in the course of increasing the pressure from 3 to 12 GPa, the metastable long-period structure of martensite modulated according to the 10M-type experienced transformation into a final non-modulated 2M structure. It is proved that severe shear deformation by high pressure torsion (HPT) entails grainsize refinement to a nanocrystalline and partially amorphized state in the polycrystalline structure of the martensitic alloy. In this case, an HPT shear of five revolutions under pressure of 3 GPa provided total atomic disordering and a stepwise structural-phase transformation (SPT) according to the scheme 10M → 2M → B2 + A2, whereas under pressure of 5 GPa the SPT took place according to the scheme 10M → 2M → B2 → A1. It is shown that low-temperature annealing at a temperature of 573 K caused the amorphous phase to undergo devitrification, and annealing at 623–773 K entailed recrystallization with the restoration of the L21 superstructure in the final ultrafine-grained state. The size effect of suppression of the martensitic transformation in an austenitic alloy with a critical grain size of less than 100 nm at cooling to 120 K was determined. It was established that after annealing at 773 K, a narrow-hysteresis thermoelastic martensitic transformation was restored in a plastic ultrafine-grained alloy with the formation of 10M and 14M martensite at temperatures close to those characteristic of the cast prototype of the alloy

Influence of Heat Treatment and Deformation on the Structure, Phase Transformation, and Mechanical Behavior of Bulk TiNi-Based Alloys

We present a brief overview of the structural and phase transformations and mechanical properties of bulk binary TiNi shape memory alloys, which demonstrate attractive commercial potential. The main goal of this work was to create a favorable microstructure of bulk alloys using both traditional and new alternative methods of thermal and thermomechanical processing. It was found that the implementation of an ultrafine-grained structure by different methods determined an unusual combination of strength, ductility, reversible deformation, reactive resistance of these alloys to subsequent tensile or torsion tests at room temperature, and, as a consequence, the highly reversible effects of the shape memory and superelasticity. It is shown that the alloys Ti49.8Ni50.2 and Ti49.4Ni50.6 are incapable of aging, and, after being subjected to ECAP, were characterized by their high strength (σu up to 1200 MPa) and ductility (δ up to 60–70%). A combined treatment of multi-pass rolling and HT of the Ti49.5Ni50.5 and Ti49Ni51 alloys prone to aging have provided even greater strength (σu up to 1400–1500 MPa) with slightly lower ductility (25–30%). The microstructure, phase composition, and martensitic transformations in Ti-Ni alloys with varying Ni concentrations ranging from 50 to 51 wt.% were investigated by TEM, SEM, and X-ray methods. The mechanical behavior of the alloys was studied during tensile and torsion tests

Influence of Heat Treatment and Deformation on the Structure, Phase Transformation, and Mechanical Behavior of Bulk TiNi-Based Alloys

We present a brief overview of the structural and phase transformations and mechanical properties of bulk binary TiNi shape memory alloys, which demonstrate attractive commercial potential. The main goal of this work was to create a favorable microstructure of bulk alloys using both traditional and new alternative methods of thermal and thermomechanical processing. It was found that the implementation of an ultrafine-grained structure by different methods determined an unusual combination of strength, ductility, reversible deformation, reactive resistance of these alloys to subsequent tensile or torsion tests at room temperature, and, as a consequence, the highly reversible effects of the shape memory and superelasticity. It is shown that the alloys Ti49.8Ni50.2 and Ti49.4Ni50.6 are incapable of aging, and, after being subjected to ECAP, were characterized by their high strength (σu up to 1200 MPa) and ductility (δ up to 60–70%). A combined treatment of multi-pass rolling and HT of the Ti49.5Ni50.5 and Ti49Ni51 alloys prone to aging have provided even greater strength (σu up to 1400–1500 MPa) with slightly lower ductility (25–30%). The microstructure, phase composition, and martensitic transformations in Ti-Ni alloys with varying Ni concentrations ranging from 50 to 51 wt.% were investigated by TEM, SEM, and X-ray methods. The mechanical behavior of the alloys was studied during tensile and torsion tests