Martensitic transformation,crystal structure and shape memory effect in Ni55−xMn25Ga20Cox alloys

ABSTRACT

The effect of cobalt addition instead of nickel on the crystal structure, martensitic transformation behaviour and shape memory effect were investigated. Within the analysed range of chemical composition, a single non-modulated martensite was detected at ambient temperature. Cobalt addition modified the lattice parameters and, thus, affected the tetragonality of the martensite unit cell. The hysteresis of martensitic transformation was differently affected by the type of heat treatment applied. For furnace cooled samples, the hysteresis decreased from 50°C to 30°C; in the case of water quenched samples, the hysteresis sharply increased up to 60°C. The shape memory effect, measured as the recoverable strain upon annealing after compression tests, reached a fully recoverable deformation at 10?at.-% of cobalt.

This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.     

Modification of the Ti<Subscript>40</Subscript>Cu<Subscript>36</Subscript>Zr<Subscript>10</Subscript>Pd<Subscript>14</Subscript> BMG Crystallization Mechanism with Heating Rates 10-140 K/min

The article presents investigations of Ti₄₀Cu₃₆Zr₁₀Pd₁₄ bulk metallic glass crystallization process heated with the rates of 10, 60, 100 and 140 K/min. High heating rates experiments were performed in a new type of differential scanning calorimeter equipped with a fast responding thermal sensor. Phase composition and microstructure were studied with x-ray diffraction and transmission electron microscopy. The observed crystallization proceeded in two separate steps. Applied high rates of heating/cooling resulted in the crystallization of only one CuTi phase, replacing typical multi-phase crystallization. The microstructure after crystallization was polycrystalline with some amount of amorphous phase retained. Kinetic parameters were determined with the use of the Kissinger and Friedman iso-conversional analysis and Matusita–Sakka iso-kinetic model. The kinetic analysis supplies results concerning autocatalytically activated mechanism of primary crystallization with decreasing activation energy and small density of quenched-in nuclei, in good agreement with previous structural investigations. The mechanism of secondary crystallization required dense nuclei site, increasing activation energy and large nucleation frequency. The amorphous phase of Ti₄₀Cu₃₆Zr₁₀Pd₁₄ BMG revealed high thermal stability against crystallization. Application of high heating rates in DSC experiments might be useful for the determination of mechanism and kinetic parameters in investigations of metallic glasses crystallization, giving reasonable results.     

Microstructural and Phase Composition Differences Across the Interfaces in Al/Ti/Al Explosively Welded Clads

The microstructure and phase composition of Al/Ti/Al interfaces with respect to their localization were investigated. An aluminum-flyer plate exhibited finer grains located close to the upper interface than those present within the aluminum-base plate. The same tendency, but with a higher number of twins, was observed for titanium. Good quality bonding with a wavy shape and four intermetallic phases, namely, TiAl₃, TiAl, TiAl₂, and Ti₃Al, was only obtained at the interface closer to the explosive material. The other interface was planar with three intermetallic compounds, excluding the metastable TiAl₂ phase. As a result of a 100-hour annealing at 903 K (630 °C), an Al/TiAl₃/Ti/TiAl₃/Al sandwich was manufactured, formed with single crystalline Al layers. A substantial difference between the intermetallic layer thicknesses was measured, with 235.3 and 167.4 µm obtained for the layers corresponding to the upper and lower interfaces, respectively. An examination by transmission electron microscopy of a thin foil taken from the interface area after a 1-hour annealing at 825 K (552 °C) showed a mixture of randomly located TiAl₃ grains within the aluminum. Finally, the hardness results were correlated with the microstructural changes across the samples.

     

Formation of a quasicrystalline phase in Al–Mn base alloys cast at intermediate cooling rates

Al-rich 94Al–6Mn and 94Al–4Mn–2Fe alloys were suction-cast to evaluate the feasibility of obtaining bulk quasicrystal-strengthened Al-alloys at intermediate cooling rates alloyed with non-toxic, easily accessible and affordable additions. The influence of different cooling rates on the potential formation of a quasicrystalline phase was examined by means of scanning and transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Increased cooling rates in the thinnest castings entailed a change in sample phase composition. The highest cooling rates turned out to be insufficient to form an icosahedral quasicrystalline phase (I-phase) in the binary alloy. Instead, an orthorhombic approximant phase occurred (L-phase). The addition of Fe to the 94Al–6Mn binary alloy enhanced the formation of a quasicrystalline phase. At intermediate cooling rates of 10²–10³ K/s, various metastable phases were formed, including decagonal and icosahedral quasicrystals and their approximants. Rods (1 mm in diameter) composed of I-phase particles embedded in Al matrix exhibited a hardness of 1.5 GPa, much higher than the 1.1 GPa of 94Al–6Mn.     

Modeling of Reactive Diffusion: Mechanism and Kinetics of the Intermetallics Growth in Ag/Ag Interconnections

The phenomenological model describing the growth of intermetallic phases in multi-component systems is presented. Full time-dynamics approach is applied without the often-used simplifications such as flux constancy. General form of the species flux is considered, which consists of chemical potential gradient as a driving force for diffusion with additional drift term. Stefan-type (moving) boundary conditions are applied. In the present form, the model assumes local equilibrium at each interface and that the process of growth of intermediate phases is controlled by diffusion of reagents through the layers and/or chemical reactions at the boundaries. The model is solved in its full generality. Numerical method for the solution of the problem has been developed. Specially selected change of dependent variables transforms the moving boundary problem into an equivalent fixed boundary problem. Such problem has been treated using the method of lines which converts partial differential equations into a system of ordinary differential equations, which is subsequently solved numerically. The obtained solution was tested and compared with analytic ones available in special cases, showing satisfactory agreement. The growth of intermetallic phases in Ag/Sn/Ag system has been modeled and compared with experimental results.     

On Constituents of the Periodic Layered Microstructure Developed in the Solid-State Reaction Between Zinc and Co2Si

Metallurgical and Materials Transactions A - The composition and crystal structure of the constituents of the periodic layered microstructure developed during the solid-state reaction between Co2Si...     

Emf Measurements in the Liquid Au-Cu-Sn Lead-free Solder Alloys

An emf method was employed to determine tin activities in the liquid Au-Cu-Sn alloys using solid electrolyte galvanic cells. The whole concentration range of the Gibbs triangle was covered, and emf readings were taken for both heating and cooling mode within temperature interval: 900-1360 K. Emf versus T relations were approximated by line equations, and its coefficients were listed in tables. Tin activity versus composition curves were constructed, displaying negative deviation from ideal behavior. Activity data were compared to the respective values calculated from COST 531 database using binary formalism, and relatively good agreement was observed.     

Specific Heat Capacities of Some Ternary Aluminides

Two different methods were employed to prepare samples of the ternary intermetallic phase: AlNi₂Ti, and of Al_0.4Fe_0.5Ni_0.1, Al_0.4Fe_0.3Ni_0.3, Al_0.4Fe_0.1Ni_0.5 solid solutions, respectively. Alloys were then subjected to heat treatment for homogenization. Phase identification was made by XRD phase analysis and by the SEM BSE technique, and chemical composition was determined using the EDS method. Heat capacity measurements were made within a temperature range from 100 to 600 K using the DSC technique. Results were listed in tables and then fitted to a Kelley equation; an example of C _p vs. T curve is given.