Mathematical modelling of peritectic transformation in binary systems

A simple diffusional analysis of peritectic transformat-ion based on the linearized concentration gradient approximation and a rigorous numerical model of the peri-tectic transformation as well as the solid state homo-genization process following liquid depletion has been presented.The overall and interface mass balance equations are utilized to calculate the rate of movement of the interfaces in the finite geometry. The predictions of the present models, show a better agreement with the experi-mentally determined kinetic data from the Cd-Ag and Pd-Bi systems as compared to those by the earlier proposed 'models based on quasi-static interface or time-invariant or Laplacian concentration profiles. However, the computed kinetics differ from the observed rates of transformation at a later stage (-50% transformation), perhaps, due to the deviation from the idealized cell configuration consi-dered in the calculations

Initiation sites for discontinuous precipitation in some Cu-base alloys

A systematic effort has been made to investigate the suitability of various interfaces, natural as well as artificial, to initiate discontinuous precipitation. The interfaces studied in the present investigation include sample surface (external), and grain and interphase boundaries. It has been demonstrated that in addition to grain boundaries, non-conventional initiation sites like coherent faces of a twin or eutectic/eutectoid boundaries under favourable conditions may also nucleate discontinuous precipitation. In general, the ability of an interface to undergo thermally activated migration appears to be the most important criterion for the initiation of discontinuous precipitation

Structure of nanocomposites of Al-Fe alloys prepared by mechanical alloying and rapid solidification processing

Structures of Al-based nanocomposites of Al-Fe alloys prepared by mechanical alloying (MA) and subsequent annealing are compared with those obtained by rapid solidification processing (RSP). MA produced only supersaturated solid solution of Fe in Al up to 10 at.% Fe, while for higher Fe content up to 20 at.% the nonequilibrium intermetallic Al5Fe2 appeared. Subsequent annealing at 673 K resulted in more Al5Fe2 formation with very little coarsening. The equilibrium intermetallics, Al3Fe (Al13Fe4), was not observed even at this temperature. In contrast, ribbons of similar composition produced by RSP formed fine cellular or dendritic structure with nanosized dispersoids of possibly a nano-quasicrystalline phase and amorphous phase along with α-Al depending on the Fe content in the alloys. This difference in the product structure can be attributed to the difference in alloying mechanisms in MA and RSP

Adaptive strategies of smallholder farming systems to changing climate conditions in the vicinity of Kogyae Strict Nature Reserve within the Forest-Savanna Transitional Zone of Ghana

The strong climate linkages to farming systems render them and dependent communities vulnerable to climate change and variability. Knowledge of existing responses to climate change is important for the formulation of policies and adaptive strategies for resilience. The paper examines a fifty-year climatic record from 1961-2011, community perceptions, experiences and knowledge for evidence of climate change and impacts. Climate change-driven farmer adaptive responses were critically examined relative to farming practices; and crop climatic requirements for growth, development, maturity and harvesting. A mixed methodological approach was adopted to address issues of climate change, exposures, sensitivities and adaptive responses. The results showed that the area has experienced a steady rise in temperature, reduced rainfall amounts of 3.0mm per annum, reducing events of rainfall excesses and increasing deficits, narrowing of rainfall period and a shift of the double to a single rainfall maxima regime. Farmers have observed these patterns of changes and experienced the impacts. Consequently, evidence-driven adaptive responses in the transformation of farming practices, timing of cultivation and choice of crops have been developed by farmers. It is recommended that further adaptive strategies be planned to improve farmers’ adaptive capacities and reduce sensitivity of crops to climate perturbations

Mechanism and kinetics of type II discontinuous coarsening in a Zn-4 at% Ag alloy

Discontinuous coarsening (DC) may succeed discontinuous precipitation (DP) either at the same (DCI) or another temperature (DCII). The present study concerns mechanism and kinetics of DCII in a Zn-4 at% Ag alloy in the range 353-513 K following DP at 393 K for 60 h. DCII colonies prefer to initiate either from one or both sides of the interfaces between the former DP colonies. A suitable comparison of the kinetic data reveals that interlamellar spacing (λ) and steady-state growth velocity (v) values in DCII are significantly different than those in DP. On the other hand, the kinetics of DCIvis-a-vis DCII in terms of λ and v are comparable to each other, though the calculated values of the driving forces between them differ marginally. A detailed kinetic analysis of DCII through the Livingston-Cahn model leads to an underestimation of the activation energy (Q b) of grain boundary chemical diffusion of Ag in Zn-Ag (=30.7 kJ mol-1), whereas the same obtained from the modified Petermann-Hornbogen model (=61.0 kJ mol-1) compares well with that for DP/DCI (reported elsewhere by us), and grain boundary self diffusion of Zn. Considering thatQ b in DCII is nearly 50% of the activation energy for volume/matrix diffusion of Ag in Zn, it appears that DCII in the present alloy is a boundary diffusion controlled process

Prediction of Solubility of Carbonitrides in High Strength Low Alloy Steel

High Strength Low Alloy Steels are of particular interest for large submarines, offshore structures, line-pipe and other structural applications that require a combination of strength and toughness

On amorphization and nanocomposite formation in Al-Ni-Ti system by mechanical alloying

Amorphous structure generated by mechanical alloying (MA) is often used as a precursor for generating nanocomposites through controlled devitrification. The amorphous forming composition range of ternary Al-Ni-Ti system was calculated using the extended Miedema's semi-empirical model. Eleven compositions of this system showing a wide range of negative enthalpy of mixing (-ΔH mix) and amorphization (-ΔH amor) of the constituent elements were selected for synthesis by MA. The Al88Ni6Ti6 alloy with relatively small negative ΔH mix (-0.4 kJ/mol) and ΔHamor (-14.8 kJ/mol) became completely amorphous after 120 h of milling, which is possibly the first report of complete amorphization of an Al-based rare earth element free Al-TM-TM system (TM = transition metal) by MA. The alloys of other compositions selected had much more negative ΔHmix and Hamor; but they yielded either nanocomposites of partial amorphous and crystalline structure or no amorphous phase at all in the as-milled condition, evidencing a high degree of stability of the intermetallic phases under the MA environment. Hence, the negative ΔH mix and ΔH amor are not so reliable for predicting the amorphization in the present system by MA

Codeposition of nanocrystalline aluminides on a copper substrate

The present study concerns codeposition of nanocrystalline aluminide particles (NbAl3 and Cu9Al4) along with electrodeposition of Cu on a Cu substrate. It is shown that the success of codeposition primarily depends on the selection of an appropriate electrolyte. Following codeposition under an optimum deposition condition, the microstructure, phase identity and composition of the deposit layer have been studied. In addition, microhardness and electrical resistivity of the deposit have been determined. A suitable correlation of the microstructure and composition of the deposit with its properties suggests that codeposition of NbAl3 is more effective in enhancing the hardness. However, codeposition beyond a limit adversely affects the electrical conductivity. The optimum conditions for codeposition to enhance hardness without adversely affecting conductivity have been determined. Finally, it is predicted codeposition could be a suitable technique for developing a surface composite microstructure with uniform distribution of nanocrystalline aluminide particles

Structure of nanocomposites of Al-Fe alloys prepared by mechanical alloying and rapid solidification processing

Abstract. Structures of Al-based nanocomposites of Al-Fe alloys prepared by mechanical alloying (MA) and subsequent annealing are compared with those obtained by rapid solidification processing (RSP). MA produced only supersaturated solid solution of Fe in Al up to 10 at.% Fe, while for higher Fe content up to 20 at.% the nonequilibrium intermetallic Al 5 Fe 2 appeared. Subsequent annealing at 673 K resulted in more Al 5 Fe 2 formation with very little coarsening. The equilibrium intermetallics, Al 3 Fe (Al 13 Fe 4 ), was not observed even at this temperature. In contrast, ribbons of similar composition produced by RSP formed fine cellular or dendritic structure with nanosized dispersoids of possibly a nano-quasicrystalline phase and amorphous phase along with α-Al depending on the Fe content in the alloys. This difference in the product structure can be attributed to the difference in alloying mechanisms in MA and RSP

Kinetics of the eutectoid transformation in the Cu-In system

The present study concerns a detailed investigation of the kinetics of the eutectoid transformation in the Cu-In system based on both the isothermal growth rate of the eutectoid colony (monitored by microstructural change) and enthalpy changes during non-isothermal heating (determined by differential scanning calorimetry) of solution-treated and quenched samples. The maximum growth distance of the eutectoid cells and the equilibrium interlamellar spacing have been determined by optical and scanning electron microscopy in the temperature range 600-825 K. The reaction front velocity was observed to increase with the isothermal ageing temperature in the temperature range studied. A detailed analysis of the isothermal growth kinetics through the models available in the literature has yielded an activation energy of 125-127 kJ mol for the operating diffusion process, which is comparable with that for discontinuous precipitation in Cu-In or for grain boundary tracer diffusion of 115 In in Cu, but significantly lower than that for volume diffusion of In in the β Cu-In alloy. A subsequent differential scanning calorimetric study has indicated a similar activation energy of 133 kJ mol −1 for the concerned eutectoid transformation. It is thus concluded that the eutectoid transformation in the Cu-In system is a boundary-diffusion-controlled process. C 1999 Kluwer Academic Publisher