Investigation of precipitation behavior and related hardening in AA 7055 aluminum alloy

The precipitation behavior and related hardening in AA 7055 aluminum alloy aged at 120 and 160 °C was investigated in detail. GPI zones were the dominant phase in the alloy upon ageing at 120 °C for 60 min. The metastable η′ phase begins to precipitate in the alloy after being aged at 120 °C for 60 min, and turns to be the main phase after ageing for 300 min. When the alloy was aged at 160 °C, the precipitation was significantly promoted. The results also revealed that the transformation of small GPI zone to η′ phase is the dominant mechanism for η′ formation. Formation and growth of GPI zones and η′ phases led to the increase of the yield strength, while formation and coarsening of η resulted in the decrease of the strength. η′ is responsible for the peak hardening of this alloy.     

Age hardening and precipitation in a cast magnesium-rare-earth alloy

The precipitation sequence responsible for the age-hardening behaviour of a cast Mg-1.3 wt% rare-earth alloy has been investigated by analytical electron microscopy. Very fine intermediate precipitates formed at an early stage of ageing. Plate-shaped Mg₃MM precipitates (MM = misch metal) and hexagonal prism-shaped Mg₁₂MM precipitates were primarily responsible for age hardening. Precipitate morphologies, crystal structures and crystallographic orientation relationship were determined for the various types of precipitates that formed during ageing at different temperatures.     

Phase transformation in Cu-Cr-Zr-Mg alloy

After solid solution treatment, the effects of aging processes on the microstructure and electrical conductivity of Cu-Cr-Zr-Mg lead frame alloy were investigated in order to determine the phase transformation and the time-temperature-transformation (TTT) behavior. The results show that aged at 470 °C the fine precipitation of an ordered CrCu₂(Zr,Mg) phase is found in copper matrix as well as fine Cr and Cu₄Zr, aged at 550 °C the phase CrCu₂(Zr,Mg) is decomposed into Cu₄Zr and Cr phase, aged at 600 °C the precipitates is bcc Cr only. The phase transformation kinetics equation was deduced from the Avrami empirical formula based on the linear relationship between the electrical conductivity and the volume fraction of the phase transformation.     

Kinetics of precipitation hardening phase in aluminium alloy AA 6110

Abstract

The solid solution treated aluminium alloy AA 6110 was investigated using a differential scanning calorimetry (DSC) with different heating rates. Kinetic parameters, e.g. the activation energy and the Avrami exponent, were calculated. The peak temperatures of the hardening phase β″ from each heating rate were collected to calculate the activation energy of the aluminium alloy AA 6110 using various mathematical models: the Kissinger, Ozawa and Boswell models. It was found that the activation energies from each model were within the range of 106–114 kJ mol^?1. These activation energies were found to be lesser than those for the bulk diffusion of aluminium, magnesium and silicon atoms in aluminium matrix. The average Avrami exponent, n of 2·36 was calculated using the developed Matusita model and corresponded to a one-dimensional linear growth of fresh nuclei of needle-like β″ in this aluminium alloy AA 6110.     

Kinetics of precipitation hardening in SiC whisker-reinforced 6061 aluminium alloy

Ageing behaviour at 180 °C of 6061 aluminium alloy-SiC_w composites, drawn from bars obtained in various extruded ratios, and 6061 aluminium alloy used as matrix, have been compared. These materials were dissolved in a salt bath at 529 and 557 °C for 2 h, quenched in ice-water, and aged at 180 °C in an oil bath for increasing periods. Ageing kinetics were studied with Brinell hardness measurements and differential scanning calorimetry (DSC). Various samples of the composite, deriving from bars with Φ20, Φ35 and Φ50 mm in diameter, and 6061 aluminium alloy, show the same ageing mechanism; however, the ageing rates results increased for composites. While 6061 aluminium alloy shows its maximum hardness value after about 4–5 h at 180 °C, the 6061-SiC_w composites reach theirs in 2–3 h. Moreover, for composites hardness abruptly decreases after 3 h, while aluminium alloy keeps its maximum value for an ageing time as long as 6 h. Thermal analysis allows us to put together a definite DSC trace for every microstructural state. The highest hardness values are obtained as a result of the formation of a Guinier Preston (GP) needle-shaped zones, which progressively become more thermally stable with protracted isothermal treatment at 180 °C. The different ageing process rates observed for composites and for the 6061 alloy are correlated with the sizes of the reinforcements. Dimensional analysis of whiskers has been performed by light scattering and scanning electron microscopy. Ordinarily the longer the average length of the whiskers in the samples, the faster the ageing process. Higher temperatures are required for composite solutions than for 6061 alloy. On the other hand, 6061-SiC_w samples solutionized at higher temperature and then quenched sometimes show microcrack formation in the materials.     

Laser transformation hardening of low alloy hypoeutectoid steel

Abstract

The principles of laser transformation hardening were investigated using a low alloy special steel having a microstructure of pearlite and proeutectoid ferrite. Temperature fields and phase transformations were modelled. Particular attention was paid to increases of the Ac₁ and Ac₃ transformation temperatures owing to the rapid thermal cycles produced by laser heating. Dissolution of proeutectoid ferrite is shown to control the formation of a homogeneous hardened case. Experimental data are in good agreement with the predictions of the model. A diagram was constructed which describes the case geometry and microstructure in terms of the process variables and is an aid to optimising practical processing parameters. The models are flexible and may be used for laser transformation hardening of other ferrous alloys having inhomogeneous microstructures.

MST/1606     

Activation energies of the amorphous-crystalline transformation in the Fe40Ni40P14B6 metallic alloy

The amorphous — crystalline phase change of the metallic glass Fe₄₀Ni₄₀P₁₄B₆ has been studied by examining the change in the electric resistivity, using isochronal and isothermal heating. The isochronal measurements indicate that the crystallization occurs in two stages. One stage appears as a change with pre-annealing in the height of the resistivity-temperature curve at 600 K. This stage characterizes an induction period for recrystallization activated by (1.46+0.03) eV. The other stage, occurring at temperature above 600 K and manifesting itself as a remarkable drop in the resistivity indicates the onset of crystallization. From the isothermal studies of this stage, the energy activating the crystallization is found to be (3.58+0.03) eV. The minimum energy activating the vacancy formation in the crystallized matrix has been calculated to be (1.96+0.03) eV. These results are compared with the corresponding data obtained for the amorphous and crystalline metallic glasses Fe₄₀Ni₄₀B₂₀ and Fe₃₂Ni₃₆Cr₁₄P₁₂B₆.     

Phase transformation and magnetic properties of annealed Fe52.2Co46V1.8 alloy

In this work, FeCoV alloys were annealed in vacuum at various temperatures for different time, with and without applying an external magnetic field, respectively. The effect of the annealing on their magnetic properties has been investigated in regard with microstructural characterizations. X-ray diffraction analysis shows that the alloy is characterized of α-Fe bcc structure. It has been found that an order-disorder phase transformation took place locally in the surface around 993 K. In addition, α-γ phase transformation occurs at the same temperature at which the Curie point is observed in the TG curve. Coercivity is decreased with increasing annealing temperatures, and changed from about 3.2 kA/m to 78 A/m when the annealing temperatures are increased to 1173 K. Microstructural observations show that the decrease of coercivity after annealing is contributed mainly by grain growth. The coercivity of the alloy is further decreased by 32 A/m after magnetic field annealing at 1033 K. Hysteresis loops of the alloys after field annealing at 1033 K featured more rectangular shape with smaller coercivity, compared with those after vacuum annealing.     

Phase transformation behavior of zinc metastannates obtained by aqueous precipitation at different temperatures

Phase transformation studies in ZnO–SnO₂ system from zinc metastannate (ZnSnO₃) to zinc orthostannate (Zn₂SnO₄) with annealing temperature are reported. Non-centrosymmetric oxides show unique symmetry dependent and spontaneous polarization properties, which are technologically important. ZnSnO₃ particles were synthesized by a simple aqueous synthesis at low temperatures designed with the assistance of potential–pH diagrams. ZnSnO₃ particles synthesized at 4 °C are more porous losing the ilmenite structure upon annealing at 200 °C, while the other samples prepared at higher temperatures (25–65 °C) becomes amorphous at 300 °C. The phase transformation into the inverse spinel orthostannate phase occurs around 750 °C in all the samples.     

Phase transformation behavior of porous NiTi alloy fabricated by powder metallurgical method

Nickel titanium shape memory alloys (NiTi-SMAs) were successfully produced from elemental Ni/Ti powders by powder metallurgical method and then subjected to age treatment. Microstructure was examined by SEM and XRD and phase transformation temperatures were measured by dilatometric method. The phase transformation temperatures increased with both duration and temperature of the age treatment. The porous product exhibited desirable shape memory effect.     

Cu-Cr-Zr-Y合金时效析出行为研究

研究了时效参数和变形量对 Cu 0. 39Cr 0.11 Zr 0. 41Y 合金性能的影响。结果表明合金经950℃×1h固溶后,在480℃时效可获得较高的导电率和硬度;时效前对合金施以冷变形可以加速时效初期第二相的析出,并显著提高其电导率和硬度,60%变形合金 480℃×2h时效处理后,导电率和显微硬度分别为83.32% IACS 和 161Hv,而固溶后直接时效仅为70.58%IACS和112Hv。微量稀土元素 Y的加入,使Cu 0. 39Cr 0. 11Zr 0. 41Y 合金的显微硬度比 Cu 0.41Cr 0.10Zr合金提高8~10Hv,导电率略有降低。     

Phase transformation behavior in a multipurpose dental casting gold alloy during continuous heating

Phase transformation in a multipurpose dental casting gold alloy during continuous heating was studied by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. The behavior can be explained by the following reaction sequences in the nodule: ₁(FCC) + ₂ (L1₂) ₁(fcc) + ₂(L1₂) + (L1₀), where fcc is face centred cubic. A discontinuous precipitation with very fine nodules contributed to the hardening and the growth produced the softening. This multipurpose gold alloy is characterized by the introduction of a PtZn ordered phase with L1₀ structure instead of a CuAu I phase.     

Structural modification and phase transformation kinetics: crystallization of amorphous Fe40Ni40P14B6 eutectic alloy

The effect of processing histories (fluxing and pre-annealing) on the amorphous structure and the crystallization kinetics of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy prepared by melt spinning has been studied by differential scanning calorimetry, X-ray diffraction, transmission and scanning electron microscopy. For isothermal crystallization, an incubation time exists, and for isochronal crystallization, an abnormally sharp crystallization peak (with the transformed fraction corresponding to the transformation-rate maximum f _p as less than 0.632) occurs. Subjected to fluxing and pre-annealing, the incubation time (in isothermal crystallization) decreases, whereas the initial crystallization temperature (in isochronal crystallization) declines as well as the less sharp crystallization peak and f _p approaches 0.632. A kinetic model considering transient nucleation is proposed and analyzed, which could describe well the singular crystallization behavior of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy. A recipe based on the kinetic model is also proposed to obtain the kinetic parameters from experiment data. Via kinetic analysis and amorphous structural characterization, it is considered that pre-annealing and fluxing promote relaxation of the system close to the meta-stable equilibrium state; the atomic structure becomes more similar to the correspondingly crystallized phase, thus declining the amorphous stability and alleviating the transient effect on nucleation.     

Interconnection between the solidification and precipitation hardening processes of an AlSiCu alloy

Horizontal directional solidification and precipitation hardening experiments were performed with the Al5.5Si3Cu alloy. Solidification thermal and microstructural parameters such as growth and cooling rates (V_L and T_R, respectively) and the secondary dendrite arm spacing (λ₂) were determined experimentally. As-solidified samples were selected and submitted to the T6-heat treatment (T6-HT). The T6-HT was carried out considering two solution times: 8 and 5?h at 490?±?2°C, followed by quenching in warm water (60?±?2°C), aging for 5?h at 155?±?2°C and air-cooling. Higher HV values were observed for the solution time of 5 h. As highlights of this work, the influence of the processing parameters (V_L, T_R and λ₂) on the distribution, morphology and size of the eutectic Si particles has been investigated.     

Massive transformation in an Fe–Cu alloy

A typical microstructure characteristic of massive transformation such as “boundary crossing” and “irregular and jagged boundaries” was observed in an Fe–Cu alloy after water quenching and after cooling at a rate of 0.1 °C s^-1, which is far slower than that specified previously for the massive reaction. On the basis of the microstructure evolution and differential thermal analysis (DTA), it is shown that the massive transformation from γ to α occurs during isothermal holding at 810 °C but not at 830 °C. During isothermal holding at 810 °C, the composition of the supercooled γ phase is shifted into the metastable α one-phase region with the continuous precipitation of the Cu-rich ε phase. These experimental results imply that the supercooling into the one-phase region is a sufficient condition for the massive reaction to operate as long as the supercooled temperature is high enough to allow the thermally activated atomic jumps across the phase boundary. This revised version was published online in November 2006 with corrections to the Cover Date.     

A study of precipitation hardening of commercial Al-9 wt% Si alloy

Structural and morphological investigations of the processes occurring in Al-9 wt% Si silumin during isothermal ageing at high temperatures were performed using hardness measurements and electron microscopic observations. It was established that the alloy achieved maximum hardness after being aged for 2.7×10³ sec at a temperature of about 463 K. During this time, formation of clusters of supersaturating atoms, probably of spherical symmetry, was observed, and which, as the ageing time lengthened, assumed the form of needles (GP zones). It is suggested that GP zones contain foreign atoms (impurities, e.g. Fe, Mn, Na) which may tend to increase the stability of these zones. During continued ageing, in the period between 1.1×10⁴ and 2.2×10⁴ sec, excess silicon precipitates appeared. These precipitates increased in size progressively as the ageing time was increased, whereas the GP zones, after reaching a certain magnitude, remained unchanged. The change GP zones -phase rod-like precipitates occurs after ageing for approximately 8.6×10⁴ sec. Differences between the course of ageing in Al-9 wt% Si silumin and in (after supersaturation) single-phase alloys from a pseudobinary Al-Mg₂Si system, are discussed.