Internal friction in a Ni–Ti-based glassy-crystal alloy
Introduction
According to a unique combination of their functional properties bulk glassy alloys, and especially glassy/crystal dual phase materials, have attracted significant attention of the materials scientists since the end of the past century. Following the success in early transition metal based bulk glassy samples production [1], [2], among late transition metals a large number of bulk glassy samples was initially produced in Cu-based system alloys [3], [4], [5], [6], [7]. Later bulk glass formation was also achieved in Ni-based system alloys [8], [9], [10]. In general, bulk metallic glasses suffer from their limited room-temperature ductility, which drastically restricts their application areas. Ductilization of bulk gassy alloys is therefore challenging from both the standpoint of fundamental understanding of underlying mechanisms of plasticity in amorphous materials and from the standpoint of prospective applications.
The formation of dual-phase structures is one of the possible ways of improving plasticity of the samples. By alternation of the alloy composition and preparation conditions the glassy/crystal phase composite samples were further developed [11], [12]. These alloys exhibited better mechanical properties than those having a homogeneous glassy structure. The dual-phase crystal-glassy materials containing crystalline [13], [14] and quasi-crystalline [15] inclusions represent a class of high-strength materials with improved room-temperature plasticity. The glassy dual-phase can be produced in the supercooled liquid state [16], upon casting [17] or by devitrification of a glassy phase upon heating [18].
The connection between the plasticity and phase transitions in alloys opens an area for future research enabling new structural design towards good combinations of strength and ductility [19]. Deformation-induced martensitic transformations in which the austenite phase tends to transform into martensite during plastic deformation can enhance significantly the ductility and the fracture toughness of crystalline alloys [20]. This leads to the redistribution of stresses giving rise to a composite effect, which is associated with the observed high uniform elongation. The phenomenological modeling of the transformation induced plasticity (TRIP) was done related to the volume change due to the martensitic phase transformation [21]. The initial model was extended to triaxial stresses developed later [22], [23] to include the orientation effect as well as the plastic accommodation of martensitic transformation shear. An energy criterion has been developed [24] to derive the overall behavior of a ductile system undergoing martensitic transformation assuming that the martensitic domains are randomly oriented in the matrix. The ductilization takes place simultaneously with a hardening effect which is created by the transformation of retained austenite to martensite. The strain or stress induced martensitic transformations have been verified to be very effective for the ductility enhancement in a large variety of alloys. In a recent work the authors examined the microstructure changes and the deformation behavior of the as-cast Ni40Cu10Ti33Zr17 bulk glassy composite which exhibits a deformation-induced martensitic transformation which gave rise to enhanced ductility [25].
In the present work the attention is paid to early deformation stages. Dynamic mechanical behavior of the Ni40Cu10Ti33Zr17 alloy was studied in different regimes of deformation at relatively small deformations in pseudo-elastic mode. Mechanical spectroscopy technique is very sensitive to different structural transformations in alloys [26]. Martensitic transformation observed in situ leads to a huge dissipation of mechanical energy of vibration. In case of thermoelastic martensitic transformation in different crystals, including NiTi-based alloys, the theory of energy losses is well developed (e.g., [27], [28]). Internal energy dissipation (damping) due to TRIP effect is less studied case.
Section snippets
Experimental methods
An ingot of the Ni40Cu10Ti33Zr17 alloy (alloy composition is given in nominal atomic percents) was prepared by arc-melting mixtures of Ni, Cu, Ti and Zr each exceeding 99.9 mass% purity in an argon atmosphere. From this ingot, the bulk rod samples of 1 and 2 mm in diameter were prepared by Ar gas injection casting into a copper mould. The microstructure of the samples was examined by X-ray diffractometry (XRD) with monochromatic Cu Kα radiation by scanning (SEM) equipped with energy dispersive
Structural studies
The structure of the samples was studied by X-ray diffraction and SEM. As shown in Fig. 1a the structure of the Ni40Cu10Ti33Zr17 contains a glassy phase (broad peak from about 36 to 45 degrees) in addition to the cP2 (Pearson symbol) cubic phase, or the B2 phase according to the Strukturbericht symbols, having a lattice parameter a equal to 0.308 nm. SEM image of the polished cross section obtained in backscattered electrons shows alternations of darker and lighter areas (Fig. 2a). The EDX
Conclusions
Amplitude dependent internal friction in the Ni40Cu10Ti33Zr17 alloy samples was studied together with the structural changes monitored by XRD. It is found, that the internal friction values at initial loading are always higher than the subsequent ones. When the subsequent step of loading to a larger deformation takes place, the observed internal friction level in the amplitude range of the previous step is lower than that in the internal friction curve measured at the previous step. These facts
Acknowledgement
This study is supported by RFBR (Project 13-03-91330-ННИОа) and World Premier International Research Center Initiative (WPI), MEXT, Japan.
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