M. Iqbal

Hardness and microstructural studies of electron beam welded joints of Zircaloy-4 and stainless steel

Abstract Electron beam welded joints between Zircaloy-4 and stainless steel 304L are investigated due to their importance in the nuclear industry. The molten and heat affected zones (HAZs) are found to be free of defects. Diffusion of Fe, Cr and Ni is observed in Zircaloy-4 near the molten zone and of Zr and Sn in the stainless steel. A rod-shaped intermetallic compound Zr(Cr,Fe) 2 and eutectic phases ZrCr 2 –liquid (Zr,Fe) and Zr 2 Fe–Zr 2 Ni are present in the molten zone. The hardness of the molten zone, containing Zr(Cr,Fe) 2, is much higher than the rest of the molten zone and the HAZs.

Effect of aging on the hardness and impact properties of Hastelloy C-276

Nickel base alloys constitute an important class of materials, which are used under demanding conditions of high corrosion resistance and high temperature strength. A wide range of these alloys exists due to the ability of nickel matrix to accommodate large amounts of elements, such as chromium, iron, copper, molybdenum, etc., while retaining essentially a single phase face-centered-cubic structure [1]. Ni-Cr-Mo-W alloys, named as Hastelloys, are known for their good mechanical and physical properties and resistance to a wide variety of corrosive environments. The alloys have found widespread use in chemical, aerospace and nuclear industry because of these properties. Hastelloy C-276 has been previously studied due to its many applications. Effect of aging treatment on the microstructure and corrosion properties of this alloy has been investigated by a number of researchers [2–5]. Precipitation of intermetallic phases ( μ and P) and carbides has been reported in most of these studies. Theμ phase is known to be hard and brittle [5]. On the other hand this alloy has been considered to be non-hardenable by conventional aging treatment [6]. Present study has been carried out to investigate the effect of aging treatment on the hardness and impact energy of the material, as no detailed results are available on these aspects. The results would be correlated with the precipitates produced due to heat treatment. Hastelloy C-276 was received in the form of a sheet having thickness 3 mm. The composition of the alloy is given in Table I. Samples were solution treated at 1200◦C for 1 h and water quenched. Aging was carried out at two temperatures 650 ◦C and 850◦C for time varying up to 240 h. Specimens were etched in a solution of 10–12%. HCl in ethanol and then they were examined in scanning electron microscope (SEM). Elemental analysis was done using X-ray energy dispersive microanalysis system (EDS) attached with SEM. Precipitates were extracted for analysis using carbon replica technique. Samples were prepared for impact testing with size 3× 10× 55 mm3 and having notch of 45◦ angle. These samples were tested with 50 J hammer. Hardness of the alloy was measured in three conditions, i.e. as-received, solution treated and aged by Rockwell hardness tester (HRB). Hardness was found to remain constant for the samples aged at 650 ◦C. Its value is the same as that of the solution treated samples. It suggests that no hardening phases are produced at 650◦C for aging time up to 240 h. According to the previously reported [2] time-temperature-precipitation diagram one of the hardening phases, i.e., μphase is not formed at 650◦C up to 500 h of aging. However M 6C carbides may be produced after about 10 h of aging. In the present case no precipitates have been observed by SEM examination in the samples aged at 650 ◦C for aging period up to 240 h. It may be that precipitation of carbides may take much longer than the aging time period in the present case. Room temperature absorbed energies of the samples were attempted by impact testing after various aging treatments. At 650 ◦C, no sample could be broken showing again that hardening was not produced by aging at this temperature. Hardness of the samples aged at 850 ◦C increases with aging time up to 96 h and then remains constant. These results are plotted in Fig. 1. These measurements suggest that aging at 850 ◦C hardens the alloy. This observation is contrary to the previously quoted statement

Diffusion bonding of stainless steel to Zircaloy-4 in the presence of a Ta intermediate layer

Diffusion bonding of stainless steel to Zircaloy-4 in the presence of Ta as interlayer has been investigated using diffusion couples heat treated at 1150 °C under vacuum for 3 h. Different types of regions have been observed in stainless steel, Zircaloy-4 and Ta foil. A Cr rich layer formed in stainless steel is found to act as a diffusion barrier. A region observed with the Ta foil in Zircaloy-4 is rich in Cr, Ta, Fe and Zr while two zones are formed in Zircaoly-4 containing Zr, Ta, Cr, Fe and Ni. The zone rich in Zr gives black contrast and the other zone gives grey contrast and these are identified as eutectic phases of the type TaCr2–Liq.-β-Zr and Ta(Cr,Fe)2–Liq.-β-Zr.

Alloying of immiscible Ge with Al by ball milling

Abstract Mechanical alloying (MA) is a versatile technique to produce nanocrystalline and amorphous alloys. Its special advantage is to form alloys of immiscible elements. High energy planetary ball milling has been used to produce Al–2 wt.% Ge alloy. Powders of Al (1–125 μm) and Ge (0.5–2 μm) were milled together with a powder to ball weight ratio of 1:20 up to 400 h. The size and shape of the elemental and alloy powder particles were examined in a scanning electron microscope (SEM) while their microanalysis was performed by energy dispersive system (EDS) attached with SEM. It has been observed that 300 h of milling produces homogeneous alloy. X-ray diffraction (XRD) patterns confirm complete alloying of Ge with Al. As the size of the atoms of Al and Ge are different, elastic strain energy because of misfit has been calculated to be about 2.6 kcal/mol. It has been concluded that among the factors contributing towards the enthalpy, stress exerted by dislocations on solute atoms is the major driving force for alloying.

Precipitation hardening in Inconel* 625

Abstract The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760°C for different intervals of time ranging from 1 to 335 h, has been measured. Peak hardening is found to occur much earlier at 760°C than at 700°C. Also the peak hardness is higher at 700°C than at 760°C. The results have been discussed in terms of precipitation. Scanning electron microscopy revealed the presence of precipitates in specimens aged at 760°C for a longer time. Electron probe microanalysis results show these precipitates to be rich in Ni, Nb, and Mo indicating that these are γ″ precipitates of Ni3 (Nb, Mo) type. Transmission electron microscopy confirmed that these are γ″ precipitates. It also suggests that nucleation takes place heterogeneously on dislocations and stacking faults. Longer aging causes somewhat uniform nucleation but still preferentially on the secondary defects. At 700°C γ′ precipitates have been observed in addition to γ″ precipitates. The orientation relationship between the precipitates and the matrix has also been determined.

Role of solute segregation on microstructure and mechanical properties of zircaloy-4

The Microstructure of two different lots of Zircaloy-4 was investigated by using a scanning electron microscope (SEM) attached to an energy dispersive system (EDS). The alloy containing Ni higher than the ASTM specification shows parallel plate structure with segregation of Ni, Fe, and Cr at the triple points of β-grains, and it failed during forging due to crack initiation. The second alloy having Ni content within the ASTM specification has basketweave morphology with minute segregation of Fe and Cr and was forged successfully. The absorbed energy is higher for the successfully forged alloy than for the alloy that failed during forging. It was also observed that the fracture started from the segregated area rich in Ni, Fe, and Cr, and propagated along the plate. The alloy with parallel plate structure is harder than the alloy with basketweave structure.

Crystallization behaviour of amorphous (Fe0.99Mo0.01)78Si9B13 alloy

An amorphous Fe-Mo-Si-B alloy has been investigated to establish the crystallization behaviour as a function of temperature. Techniques of x-ray diffraction and Mossbauer spectroscopy are applied to identify and characterize various phases produced as a result of heat treatment. Stable phases Fe2B and alpha-Fe(Mo, Si) and metastable phase Fe3B have been identified at 500degreesC by both techniques. The metastable phase transforms to stable alpha-Fe(Mo, Si) solid solution and Fe2B after 600degreesC. The distribution of Si and Mo atoms in the solution is found to be of short-range order and it changes with temperature.