Browsing by Author "D.K. Misra"

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Amorphous to icosahedral phase transformation in rapidly quenched Al-Cu-V and Al-Cu-Ti alloys
(2003) D.K. Misra; R.S. Tiwari; O.N. Srivastava
A comparative investigation of stabilization of amorphous phase and its transformation behaviour in Al75Cu15V10, Al65Cu25V5Ti5 and Al 50Cu45Ti5 alloys has been carried out employing X-ray diffraction (XRD) and transmission electron microscopy (TEM) and supported by differential scanning calorimetry (DSC). The DSC curve corresponding to amorphous alloy Al75Cu15V10 shows two stages of crystallization with peak positions at 306 °C and 420 °C. XRD and TEM investigations reveal that the first stage of crystallization corresponds to nanosized crystal formation of Al2Cu phase, which is dispersed in the amorphous matrix. The second stage of crystallization corresponds to growth of icosahedral phase. Partial substitution of Ti in place of V in Al55Cu25V 5Ti5 alloy stabilizes amorphous phase, which after subsequent annealing at 400 °C for 2 h transforms to a crystalline phase. Amongst several compositions in the Al-Cu-(Ti,V) alloy system prepared during present investigation, amorphous-to-icosahedral phase transformation has only been observed in Al50Cu45Ti 5 alloy after isothermal annealing at 300 °C for 2 h. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.
Devitrification of rapidly quenched Al-Cu-Ti amorphous alloys
(Indian Academy of Sciences, 2003) D.K. Misra; R.S. Tiwari; O.N. Srivastava
X-ray diffraction, transmission electron microscopy and differential scanning calorimetry were carried out to study the transformation from amorphous to icosahedral/crystalline phases in the rapidly quenched Al 50Cu45Ti5 and Al45Cu 45Ti10 alloys. In the present investigation, we have studied the formation and stability of amorphous phase in Al50Cu 45Ti5 and Al45Cu45Ti10 rapidly quenched alloys. The DSC curve shows a broad complex type of exothermic overlapping peaks (288-550°C) for Al50Cu45Ti 5 and a well defined peak around 373°C for Al45Cu 45Ti10 alloy. In the case of Al50Cu 45Ti5 alloy amorphous to icosahedral phase transformation has been observed after annealing at 280°C for 73 h. Large dendritic growth of icosahedral phase along with α-Al phase has been found. Annealing of Al50Cu45Ti5 alloy at 400°C for 8 h results in formation of Al3Ti type phase. Al45Cu 45Ti10 amorphous alloy is more stable in comparison to Al50Cu45Ti5 alloy and after annealing at 400°C for 8 h it also transforms to Al3Ti type phase. However, this alloy does not show amorphous to icosahedral phase transformation.
Effect of Fe substitution on the stabilization of the icosahedral phase in Al-Cu-V alloys
(2001) D.K. Misra; R.S. Tiwari; O.N. Srivastava
The effect of Fe substitution on the formation and stabilization of icosahedral phase in Al-Cu-V alloys has been investigated by employing transmission electron microscope and X-ray diffraction. In the as quenched Al75-xCu15+xFe5V5 alloys with x=0, 5 and 10 it has been found that grain size of icosahedral phase increases from 156nm to 0.53μm with increasing x. All the three alloys exhibit the formation of α-Al and θ-Al2Cu phases along with the i-phase. The nanoquasicrystalline and α-Al phases in Al75Cu15Fe5V5 alloy transform to Al7Cu2Fe phase by anealing at 550°C for 4 hrs by peritectic type of reaction. A curious type of pseudo-twelve fold electron diffraction pattern devoid of any diffuse scattering has been observed in some local regions of Al65Cu25Fe5V5 alloy. The angles and details of twelve sectors are varying alternatively. A plausible explanation in terms of microtwin domains of θAl2Cu structure in [1̄10] orientation has been given to explain the characteristic features of the pseudotwelve fold pattern.
Formation of a new F.C.C. τ2 phase in rapidly quenched Al55Cu35V10 alloy
(2003) D.K. Misra; R.S. Tiwari; O.N. Srivastava
X-ray diffraction and transmission electron microscopy experiments were carried out to study the structure of rapidly solidified as-cast and annealed Al55Cu35V10 alloy. The as-cast Al55Cu35V10 alloy shows the presence of a new f.c.c. τ2 phase (a=0.58nm) along with a b.c.c. (a=0.89nm) phase which after subsequent annealing transforms into single f.c.c. phase (a=0.58nm). In this paper, it is also reported that these phases are crystalline approximants to an icosahedral phase on the basis of e/a (valence electron per atom) constant line.
Growth kinetics of icosahedral phase and formation of handed structure in Al50Cu45Ti5 alloy during solid state transformation
(2004) D.K. Misra; R.S. Tiwari; O.N. Srivastava
X-ray diffraction and transmission electron microscopy investigation were carried out to study the transformation from amorphous to icosahedral/ crystalline phases on the rapidly quenched Al45+xCu 45Ti10-x (for x = 0, 2, 4, 5 and 6) alloys. Various composition of Al-Cu-Ti alloys namely Al45+xCu45Ti 10-x (for x = 0, 2, 4, 5 and 6) have been synthesized to investigate the influence of electron concentration per atom (e/a) and atomic size on the stabilization of amorphous and icosahedral phases. All these alloys exhibit amorphous phase. However, only in case of Al50Cu45Ti 5 alloy, amorphous to icosahedral phase transformation has been observed. In the beginning, amorphous phase transforms to icosahedral and a-Al phases. A parabolic growth behaviour of icosahedral phase has been found at isothermal annealing temperatures 280°C, 310°C, 320°C, 330°C and 350°C. The activation energy associated with the growth of icosahedral phase has been found to be ≈ 110 ± 20 KJ/mole. A novel micorstructure has been found in the form of banded structures surrounding the icosahedral grains originating at the later stage of transformation. It has been suggested that the banded structure is originating due to cellular precipitation in solute rich a-Al region which surrounds the icosahedral phase. Stability of icosahedral phase in Al50Cu45Ti5 alloy has also been compared with the other alloys of Al-Cu-TM (where TM = transition metals) series of alloys. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.
Indentation characteristics of rapidly solidified Al-Cu-V And Al-Cu-Ti alloys
(Indian Institute of Metals, 2008) R.K. Mandal; A. Mandal; D.K. Misra; R.S. Tiwari; O.N. Srivastava
We report indentation characteristics of metallic glasses and nano-composites in Al-Cu-V as well as in Al-Cu-Ti systems. The melt spun ribbons of these alloys are employed to study the surface hardness characteristics of the specimen containing microstructural features at various length scales. The characterization of these materials has been done with the help of transmission electron microscope, scanning electron microscope and X-ray diffractometer. The surface hardness characteristics of melt spun ribbons with and without crystallization have been studied using a micro-hardness tester. We shall discuss the indentation behavior of ribbons in relation to their structures and microstructures.