Madangopal Krishnan

A novel B19' martensite in nickel titanium shape memory alloys

Abstract A new type of the B19′ (monoclinic) Ni–Ti martensite—one which is internally twinned by the (0 0 1)m compound twinning mode, has been found in thermally cycled Ni–Ti shape memory alloys. This is an unexpected and remarkable martensite type on account of the fact that the (0 0 1)m compound twinning mode does not qualify to occur as the fine structure in the B2–B19′ martensite transformation. On the basis of the good concurrence of the observed crystallographic parameters with those predicted by the phenomenological theory of martensite transformations, it has been determined that this martensite is a product of the R phase-B19′ martensite transformation. However, the (0 0 1)m compound twinning mode can qualify to occur as the lattice invariant shear (LIS) only when the rhombohedral angle of the R phase is less than the critical value of 86.2°. The preference of this twinning mode as the LIS to the [0 1 1]m Type II twinning mode, which is the normally observed substructure of the Ni–Ti martensites, has been rationalized to be due to the closer proximity of the orientation relationships to the lattice correspondence and the lower magnitudes of twinning shear, shape strain shear, twin interface energy and nucleation strain energy.

The effect of microstructure on the shape recovery of a Fe–Mn–Si–Cr–Ni stainless steel shape memory alloy

Abstract In this work the effect of varying the microstructure on the shape memory properties of a Fe-15Mn-7Si-9Cr-5Ni (wt.%) stainless steel shape memory alloy was evaluated using a simple bend test. The best shape recovery was obtained for a single-phase austenite microstructure and for a two-phase microstructure composed of an austenite matrix and Fe 5 Ni 3 Si 2 type intermetallic grain boundary phase. The maximum shape recovery was achieved at the reversion temperature of 600 °C and when the pre-stain was less than 2%.

Neutron diffraction evidence for kinetic arrest of first order magneto-structural phase transitions in some functional magnetic materials

Neutron diffraction measurements, performed in the presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first order phase transition from (i) the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5, (ii) the higher temperature ferromagnetic phase to the lower temperature antiferromagnetic phase in the half-doped charge ordered compound La0.5Ca0.5MnO3 and (iii) the formation of glass-like arrested states in both compounds. The cooling and heating under unequal fields protocol has been used to establish phase coexistence of metastable and equilibrium states, and also to demonstrate the devitrification of the arrested metastable states in the neutron diffraction patterns. We also explore the field–temperature dependent kinetic arrest line TK(H), through the transformation of the arrested phase to the equilibrium phase. This transformation has been observed isothermally in reducing H, as also on warming in constant H. TK is seen to increase as H increases in both cases, consistent with the low-T equilibrium phase having lower magnetization.

Applicability of Scheil–Gulliver solidification model in real alloy: a case study with Cu-9wt%Ni-6wt%Sn alloy

The present work explores the possibilities of the application of Scheil–Gulliver equation in modelling the solidification of a real alloy. For this study, Cu-9 wt%Ni-6 wt%Sn alloy was chosen which exhibits profuse micro-segregation during solidification, and hence easy to quantify experimentally. Also, this alloy is spinodally strengthened high strength copper alloy and has industrial importance. In this study, thermodynamic assessment using Scheil–Gulliver solidification model was carried out. Subsequently, the assessed result was compared with the experimentally obtained results from energy-dispersive X-ray spectroscopy analysis, and a good agreement was observed between these results. Therefore, it could be concluded that the solidification of this particular alloy system can be modelled using Scheil–Gulliver equation.

Is the observation of {112} pseudotwins in the B2 phase of Ni-Ti-Fe shape-memory alloys a case of misidentification?

We present transmission electron microscopy observations and a crystallographic analysis that show that the only significant report on the observation of {112} pseudotwins in B2 structures by Goo et al. (1985, Acta metall., 3, 1725) is a misidentification of the {114} true twins.

Calorimetric Studies of Dissolution Kinetics of Ni2(Cr,Mo) Phase in Ni-Cr-Mo Alloys Using Non-Isothermal Approach

The kinetics of dissolution of ordered phase with Pt2Mo structure has been studied in two nickel chromium alloys – one without molybdenum and another with molybdenum - using differential scanning calorimetry. The activation energy for dissolution, determined using three nonisothermal approaches was found to be ~ 418 kJ /mole for both the alloys. This value agreed very well with the activation energy for coarsening of g″ precipitates in Ni-Cr-Mo matrix and is close to activation energy for mobility of chromium and molybdenum in complex nickel alloy matrix.

Microstructural Irreversibilities under Thermal Cycling in Ni-Ti-Fe Shape Memory Alloys

The thermal cycling (quenching in liquid nitrogen and reverting back to room temperature: austenite martensite reversible transformation) response of Ni-Ti-Fe shape memory alloys has been investigated. It was clearly noted that residual deformation, estimated in terms of noticeable differences in austenite grain size, depend on the relative clustering of fine grains. During repeated thermal cycling, the residual deformation, in-grain misorientation developments and retained martensite content scaled together: bringing out a clear picture of microstructural irreversibility.

Evolution of Texture and Microstructure during Cold Rolling and Annealing of Fe-14Mn-6Si-9Cr-5Ni Shape Memory Alloy

Evolution Texture and microstructure has been investigated in a Fe-14Mn-6Si-9Cr-5Ni shape memory alloy during cold rolling and annealing. The starting solution-annealed material show a nearly random texture with microstructure composed of equi-axed austenite grains with some e martensite plates inside. Cold rolling induces a strong alloy type texture with Brass {011}<211> and Goss {011}<100> as major components. Annealing of the cold deformed material produces a nearly random texture. The microstructural investigation reveals that with increasing cold deformation the amount of stress induced e and a’ martensite volume fraction increases. The electron back scattered diffraction (EBSD) phase mapping shows that reversion of the e martensite starts only after commencement of recrystallization.

The martensitic microstructure of 5M and NM martensites in off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys

In recent years, extensive studies have been performed on many aspects of the NiMnGa alloys, such as crystal structure, phase transformation, magnetic field induced strain, magnetic properties, mechanical behaviour and the effects of magnetic fields on martensitic transformation. Depending upon the composition, the most important martensites that exist in off-stoichiometric Ni2MnGa alloys are the modulated 5M and 7M, and, the non-modulated tetragonal NM ones. In this study, a systematic investigation was carried out on the 5M and NM martensite in well characterised Ni50Mn28.9Ga21.1 and Ni53Mn25Ga22 alloys, respectively using transmission electron microscopy (TEM) and X-ray diffractometry for determining crystal structure, substructure, intervariant interfaces and self-accommodating microstructure. It was established that both NM and 5M are internally twinned martensites with tetragonal and monoclinic crystal structures, respectively, Figures 1–4. The crystallographic features of the martensitic microstructures as observed by TEM are found to be in agreement with those predicted by computations based on the phenomenological theory of martensite crystallography.

The Application of Ni – Ti Shape Memory Alloys as Fasteners

Abstract This article covers the various materials aspects of the shape memory alloys that go into the design of fastening devices. A brief overview of the shape memory phenomenon and the martensite transformation during free and constrained recovery has been given to clarify the role of the martensitic transformation in the success of this application. Finally, the design of an Ni-Ti-Fe fastening sleeve has been discussed.