Volume 96, Issue 12

The crystallization kinetics of an amorphous magnesium-rich Mg 80 Cu 10 Y 10 alloy has been determined by isothermal and isochronal differential scanning calorimetry combined with ex-situ X-ray diffraction for phase analysis. Microstructure analysis has been performed by means of transmission electron microscopy. Complete crystallization of Mg 80 Cu 10 Y 10 has been found to take place in three steps. Independent of the type of annealing, during the first crystallization process at around 430 K particles of several phases are formed with an equiaxed morphology. Kinetic analysis applying a new analytical model demonstrated that Avrami nucleation and volume-diffusion-controlled growth are the operating nucleation and growth modes. The effect of yttrium on the transformation kinetics has been discussed.

The thermodynamic database for the Al –Fe– Mg– Mn– Si system is developed based on the constituent binary, ternary, and quarternary systems. The computed phase diagrams agree well with the experimental data. The obtained database is used to describe the solidification behavior of Al 356.1 (91.95Al–0.46Fe –0.3Mg–0.32Mn– 6.97Si, in wt.%) and Al 356.2 (92.77Al – 0.08Fe –0.35Mg–6.8Si, in wt.%) under equilibrium and Gulliver–Scheil non-equilibrium conditions. The reliability of the established thermodynamic database is also verified by the good agreement between calculation and experiment for both equilibrium and Gulliver– Scheil non-equilibrium solidifications. Microstructure and microsegregation of the directionally solidified Al 356.1 alloy are investigated with a growth rate of 0.04445 cm s –1 and a temperature gradient of 45 K cm –1 . Fractions of solids formed are measured by using quantitative image analysis of back-scattered electron, and solute redistribution in the primary (Al) is determined by means of an area scan approach with a total of 2400 electron probe microanalysis point counts. A micromodel, which includes solid-state diffusion, secondary dendrite arm coarsening, and dendrite tip and eutectic undercooling, is coupled with a multicomponent phase diagram calculation engine (PanEngine) to predict the microstructure and microsegregation of the solidified alloy. Quantitative agreement of the model prediction with the experiment is obtained for concentration distributions in the primary (Al), the types and amounts of phases formed, and the solidification path.

The precipitation behaviour of Pb-4 wt.% Sn alloys containing 0.001 –0.088 wt.% Ca has been mainly studied by resistivity measurements and transmission electron microscopy. The addition of Ca to the Pb–Sn binary alloy suppresses the discontinuous precipitation and accelerates the formation of continuous precipitates with a high coherency. Therefore, the addition of Ca induces a higher age hardening. TEM observations reveal that the continuous precipitates in the Ca-doped alloys contain Pb, Ca, and Sn, and have an L1 2 -type ordered structure. The discontinuous precipitation reaction in the Pb-4 wt.% Sn–Ca alloys is suppressed by the formation of continuous precipitates and, furthermore, the segregation of the solute elements at the grain boundaries.

The phase diagram of the Er – Zn system has been experimentally investigated over its whole composition range by using differential thermal analysis, X-ray diffraction, optical and scanning electron microscopy, and electron probe microanalysis. Seven intermetallic phases have been found and their crystal structure confirmed, namely ErZn, ErZn 2 , ErZn 3 , Er 13 Zn 58 , ErZn 5 , Er 2 Zn 17 , and ErZn 12 . Four eutectic reactions occur. A degenerate invariant effect has been found in the Zn-rich region. The results obtained in this investigation have been used, together with those of other previously studied rare earth – Zn systems, to predict the constitutional properties of the Tb –Zn and Ho– Zn systems, taking into account that the alloying behaviour of the different rare earths with the same element varies according to systematic patterns. The predicted phase diagrams of the Tb – Zn and Ho– Zn systems are compared with selected equilibria experimentally determined by analysing a few key compositions.

This work shows the nature of nucleation and growth of M 7 C 3 carbides in high-chromium white irons during eutectic solidification using scanning electron microscopy. Different chemical compositions of hypoeutectic high-chromium irons were used for the present study. Imaging was undertaken on a JEOL 6400 scanning electron microscope (SEM) on samples where solidification was interrupted by decantation techniques. Bar- and plate-like carbides, which seemed to nucleate on the surface of primary dendrites and at the solute-rich areas between secondary arms, were observed in these irons. The final carbide morphology in the alloys is discussed in terms of the eutectic nucleation and the preferred crystallographic growth direction of the carbide phase.

The manifestations of dynamic strain ageing in a commercial 2090 Al – Li alloy were investigated in a wide range of temperatures (–40 to 100 °C) and strain rates (4.16×10 –5 to 4.16×10 –3 s –1 ). Serrated yielding, as a macroscopic manifestation of dynamic strain ageing, occurs under all testing conditions. Negative strain-rate sensitivity of the flow stress is also observed. Temperature rise and strain rate reduction cause the increase of the yield strength and the work-hardening rate, as well as the decrease of elongation – the behavior being typical for the range in which C-type serrations appear. These phenomena are related to the dislocation substructure, in which features characteristic for solute atoms blocking of dislocations and Orowan bypassing are observed.

Surface fusions were performed by GTAW (Gas Tungsten Arc Welding) surfacing process on plate castings of Al– 7Si – Mg alloy with electric arc travel speeds from 200 to 800 mm/min in argon atmosphere. Their geometry and values of structural parameters λ were measured. Microhardness, frictional wear intensity and the fatigue strength of the alloy surface, refined through rapid solidification, have been investigated. Extensive calorimetric measurements of the net heat input for the gas tungsten arc welding process have been conducted. Arc and melting efficiencies were evaluated. A stepwise regression method was used to develop relationships between GTAW process parameters and energy transfer efficiency, as well as geometry, structure and hardness.

In order to develop a refiner of Mg– Al alloys, an Al – 1C (in wt.%) master alloy was synthesized using a casting method. The microstructure and grain-refining performance of the Al – 1C master alloy were investigated using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and a grain-refining test. The microstructure of the Al – 1C master alloy is composed of α-Al solid solution, Al 4 C 3 particles, and graphite phases. After grain refinement of AZ63B alloy by the Al – 1C master alloy, the mean grain size reached a limit when 2 wt.% Al – C master alloy was added at 800 °C and held for 20 min in the melt before casting. The minimum mean grain size is approximately 48 μm at the one-half radius of the ingot and is about 17% of that of the unrefined alloy. The Al – 1C master alloy results in better grain refinement than C 2 Cl 6 and MgCO 3 carbon-containing refiners.

A 1441 Al –Li –Cu–Mg– Zr alloy was subjected to retrogression treatment and the retrogressed states were immediately reaged to various tempers. Hardness measurement, tensile testing, and SEM fractography have been carried out to conventional peak-aged, retrogressed, retrogressed and reaged (RRA), and overaged tempers of the alloy. Retrogression treatment applied to peak-aged tempered alloys resulted in a decrease in hardness and tensile strength, an increase of ductility, and a serration phenomenon in the plastic region. These are primarily due to the dissolution of matrix strengthening δ′ precipitates into solid solution upon retrogression treatment. Retrogressed states have been reaged to various under-, peak-, and overaged tempers. Reaging the retrogressed state to peak-aged temper has resulted in attaining the original peak-aged strength and ductility value. SEM fractography study has been carried out and fracture surfaces exhibited predominantly intergranular fracture in presence of few dimples. TEM and DSC studies have been carried out to observe the microstructural changes upon RRA treatment and to explain the mechanical behaviour of the alloy.

The effect of microstructural parameters produced by α–β forging and heat treatment on the strength and ductility of Ti – 6Al – 4V, which is one of the typical α–β titanium alloys, has been studied. The results obtained in this study are as follows: (1) the aspect ratio of primary α (α p ) including grain boundary α decreases exponentially with the increase of the α–β forging ratio, (2) both strength and ductility have strong correlations with the aspect ratio of α p and increase linearly with the decrease of the aspect ratio of α p , (3) strength increases with the strengthening of the matrix structure by the heat treatment without any reduction in ductility, and (4) the α p of the lower aspect ratio restrains the growth of micro-voids, which results in improvement of both strength and ductility.

The interfacial reaction, change of intermetallic compound (IMC) morphology and shear strength of Sn–0.7 wt.% Cu solder/Ni joints were investigated during reflow at 255 °C for times of up to 20 min. Up to the reaction time of 1 min, a needle-type (Cu,Ni) 6 Sn 5 IMC with a hexagonal cross-section was formed. After reflow for 5 min, the (Ni,Cu) 3 Sn 4 IMC started to form under the (Cu,Ni) 6 Sn 5 IMC. After prolonged reflow time, the upper (Cu,Ni) 6 Sn 5 IMCs detached from the interface. Also, the morphology of the (Cu,Ni) 6 Sn 5 IMC changed from needle type to dodecagonal type with increasing reflow time. In the ball shear test, the shear strength did not change significantly as a function of the reflow time. The result of the shear test indicated that the shear strength of the Sn– 0.7Cu/Ni ball-grid array solder joint was not significantly related to the thickness and spalling phenomenon of the IMCs formed at the interface.

Powder-metallurgy processed high-speed steels containing up to 5 wt.% of Nb were fabricated by nitrogen gas atomization of elemental powders followed by hot isostatic pressing and hot working. Microstructural changes, particularly the carbide formation during the heat treatment and the resulting mechanical properties were investigated. After the heat treatment, the primary carbides were identified as MC and M 6 C-types. With Nb addition, the total volume of primary carbides increased from 16 to 23 vol.% and the volume fraction of MC-type carbide in the primary carbides also increased. The chemistry of the MC-type carbide changed from a V-rich to a V–Nb-rich carbide with increasing Nb content. Hardness significantly increased to a maximum of Rockwell C66 with 1 wt.% Nb and then slightly decreased with further Nb additions. Bend strength, on the other hand, gradually decreased with increasing Nb content. Fracture toughness is almost inversely proportional to the hardness and wear and softening resistance of the Nb alloyed steels are both superior to those of Nb-free steels.

In this study, an investigation was carried out in order to determine the effect of annealing temperature on the mechanical properties of H13 steel. Heat treatments were applied on samples at different temperatures in both, the as-received and as-quenched conditions. In order to indicate the effect of heat treatment temperature, hardness measurements and tensile tests were performed at all chosen temperatures. At relatively high temperatures the mechanical properties of the H13 steel have been diminished considerably.