D.Z. Yang

Damage of aluminum borate whisker reinforced 6061 aluminum composite under impact of hypervelocity projectiles

Abstract An aluminum borate whisker reinforced 6061 aluminum composite (AlBOw/6061Al) semi-infinite target was impacted by hypervelocity projectiles with velocity of 2.1 km s−1. The results showed that a columned crater with hemispherical bottom was formed. The crater formed on the AlBOw/6061Al composite is much smaller than that on the 6061 aluminum alloy target. Spallation occurred along the edge of the crater on the composite target, and some whiskers were pulled out in the vicinity of the crater, severely compressed deformation of the composite material occurred and the whiskers were obviously smashed into smaller particles. Due to the acute damage caused by hypervelocity projectile impact, the micro-hardness is lower in the areas near crater than that far away from the crater.

Macro- and microdamage behaviors of the 30CrMnSiA steel impacted by hypervelocity projectiles

Abstract Macro- and microdamage behaviors of a 30CrMnSiA steel under impacts of GCr15 steel projectiles have been studied. The results show that, in the impact velocities of 2.5∼7 km s−1, hemispherical craters are often formed in semi-infinite targets, and with decreasing the target thickness or increasing the projectile velocity, the form of craters changes to conical one, and obvious spall phenomenon occurs on the back face of targets. Depth and diameter of craters increase with the projectile velocity, and can be calculated by the formulas: Pc=0.47 v02/3 and Dc=0.85 v02/3. Microstructure analyses show that, there are three kinds of microdamages: microcracks, microvoids and adiabatic shear bands, in the region around the crater. The size of the microdamage zone is much larger than that of the macrodamage one for the medium thick targets, inferring that when studying the damage behaviors of materials under impacts of hypervelocity projectiles, attention should also be paid to microdamages besides macrodamages. Adiabatic transformed shear bands always form networks and shows white contrast in scanning electron microscope (SEM) micrographs. While, when these transformed shear bands are studied by a SEM attachment in transmission electron microscope (TEM), it is found that obvious melt has occurred in the head of bands. A lot of microcracks have been formed in the tail and many deformation shear bands are observed in junctures of transformed bands and the matrix. TEM results show that, dislocation cells are the predominant deformation structure in the region just near the crater, and with increasing the distance from the crater, the density of dislocation decreases, and dislocation tangling is the main feature. Deformation twins are also found in the region just near the crater, while only micro-twins in individual laths of martensite are observed in the region a little farther from the crater.

TEM observation of the alpha(2)/O interface in a Ti3Al-Nb alloy

A study of the microstructures of a Ti-24Al-14Nb-3V-0.5Mo (at%) alloy was carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), anc high-resolution electron microscopy (HREM). The results indicate that the O-phase is directly derived from the alpha(2)-phase and is distributed all over the primary alpha(2)-grains with fine plate-like form. Transformation of the alpha(2) to the O-phase is explained by a diffusional growth mechanism, and the interface between the alpha(2) and O-phases is completely coherent

Micro-damage of Ti–6Al–4V alloy under hypervelocity projectile impact

Abstract The micro-damage of Ti–6Al–4V alloy under high velocity projectile impact was studied. The results show that adiabatic shear bands are very easy to form in the target material and primarily distribute in the area adjacent to the crater. The voids and micro-cracks are formed in the adiabatic shear bands during the impact process. The formation of adiabatic shear bands is discussed.

Deformation and fracture behavior of a RSP Al–Li alloy

Abstract The deformation and fracture behaviors in a rapid solidification processed (RSP) Al–Li alloy have been studied by tensile tests, transmission electron microscope (TEM) and scanning electron microscope (SEM) observations. The results show that the interaction mechanism between dislocations and δ′ precipitates is shearing in underaged, peak aged and some overaged conditions. In the early stage of deformation (δ 5%) of the alloy, δ′ precipitates are cut into pieces by dislocations from different directions, suggesting that cross slip is predominant. In severely overaged conditions, the deformation is mainly concentrated on the precipitate-free zones (PFZs), and only smaller δ′ precipitates near PFZs are sheared. The fine grain size (2–3 μm) produced by rapid solidification cannot change the inherent intergranular fracture mode of Al–Li alloys. TEM observations show that the nominal intergranular fracture in the alloy is actually caused by the cracking of PFZs but not by the fracture of grain boundary. Therefore, it is suggested that in alloys with PFZs in their microstructure, PFZ is the main reason that results in the intergranular fracture, and the width of PFZ is not an important factor in controlling the fracture mode. Tensile tests show that the peak strength of the alloy does not correspond to the transition stage of dislocations’ shearing to bypassing of δ′ precipitates. The high volume fraction of PFZ and the small spacing of δ′ precipitates are thought to be reasons for such a phenomenon.

Jerky flow behavior in a rapid solidification processed Al-Li alloy

Abstract The specific jerky flow behavior in a rapid solidification processed (RSP) Al–Li alloy has been studied systematically. The results show that stress–strain curves of the as-quenched RSP Al–Li alloy tensioned with a moderate strain rate (6.9×10 −4 s −1 ) show a typical plateau step feature with obvious physical yielding; the width of each plateau (except for the yield plateau) increases linearly with increasing total strain, while the height of the steps is almost the same for one specimen. Small and regular serrations usually begin to appear beyond the third plateau of transverse specimens, and the size of serrations in each plateau is equal, while that on the latter plateau is larger than that on the former. The specific jerky flow behavior of the RSP Al–Li alloy is suggested to be formed by initiation and propagation of a local deformation band during the tensile deformation process, which is caused by the combined effects of static and dynamic strain aging of Li and Mg atoms. The yield plateau is thought to be related to the static strain aging that occurred in the time period between aging treatment and tensile tests. Other plateaus are suggested to be caused by the first stage of dynamic strain aging (DSA I), while small and regular serrations on plateaus are attributed to the second stage of dynamic strain aging (DSA II). High Li content, high volume fraction of δ ′ phase and very fine grains in the RSP Al–Li alloy are major causes leading to the specific jerky flow behavior. The plateau step characteristic in stress–strain curves of longitudinal specimens is not so typical as that of transverse specimens. The influence of artificial aging and tensile strain rates on the jerky flow behavior is also discussed.

Effect of quenching rate on microstructures of a NiAl alloy

Abstract A study of the microstructures of a NiAl base alloy in different conditions was carried out using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that water quenching from the 1100°C solution temperature cannot restrain the precipitation of Ni 3 Al phase at grain boundaries, while liquid nitrogen quenching can restrain the precipitation of Ni 3 Al phase effectively. Martensite laths in specimens quenched in liquid nitrogen are twins with the twinning plane of [111] and the twinning axis of [211]. On aging of specimens quenched in liquid nitrogen, Ni 3 Al precipitates distribute homogeneously among NiAl martensite or at boundaries of martensite laths and the growth rate of precipitates is higher than that in water-quenched specimens.

Impact fracture of rapid solidification processed Al-Li alloys

Abstract The deformation-fracture behavior in impact test of rapid solidification processed (RSP) Al-Li alloys has been studied systematically. The results show that there are both intrinsic and the extrinsic toughening effects in RSP Al-Li alloys. The intrinsic toughening effect is controlled by the micro-mechanism of fracture in the alloys, while the extrinsic toughening is produced by cracking of the prior powder boundaries (PPBs). The level of extrinsic toughening effect is determined by the relative strength of grains, grain boundaries, and PPBs. Large secondary cracks across the transverse section of impact specimens form easily along relatively weak PPBs in the as-quenched condition, leading to a marked extrinsic toughening effect. However, in the peak aged and overaged conditions, cracks propagate mainly along grain boundaries, and the extrinsic toughening effect is inhibited.

High temperature deformation behavior and processing map of hot isostatically pressed Ti-47. 5Al-2Cr-2Nb-0. 2W-0. 2B alloy using gas atomization powders

The hot compressive deformation behavior of hot isostatically pressed Ti-47. 5Al-2Cr-2Nb-0. 2W-0. 2B alloy using gas atomization powders was systematically investigated and the processing map was obtained in the temperature range of 1323–1473 K and strain rate range of 0.001–0.5 s−1. The calculated activation energy in the above variational ranges of temperature and strain rate possesses a low activation energy value of approximately 365.6 kj/mol based on the constitutive relationship models developed with the Arrhenius-type constitutive model respectively considering the strain rate and deformation temperature. The hot working flow behavior during the deformation process was analyzed combined with the microstructural evolution. Meanwhile, the processing maps during the deformation process were established based on the dynamic material model and Prasad instability criterion under different deformation conditions. Finally, the optimal hot processing window of this alloy corresponding to the wide temperature range of 1353– 1453 K and the low strain rate of 0.001–0.1 s−1 was obtained.

Fractal characteristic of the microstructure in Alnico8

Alnico alloys are of technological importance owing to their excellent magnetic stability and good comprehensive properties. Considerable interest has arisen concerning the chemical compositions of the Fe-Co and Ni-Al rich phases which evolve from the spinodal decomposition of a bcc single phase solution at high temperatures [1–5]. However, emphasis has been seldom placed on kinetics of microstructural evolution at high temperatures. Recently, we have observed the splitting of Fe-Co rich particles in Alnico8 thermomagnetically treated [6]. On the other hand, some investigations [7– 9] have shown that microstructures in some materials have been found to be of fractal characteristics. However, different structures often exhibit different fractal natures, which implies that they are formed usually by different kinetic mechanisms. To this end, in this paper, our attention is focused on development of fractal characteristics in the microstructure formed during thermomagnetic treatment, which is expected to enhance understanding of the mechanisms of the microstructural evolution in Alnico8. The chemical composition of the Alnico8 studied is as follows (at mass percentage): 33.2Co, 16.3Ni, 5.3Al, 3.6Ti, 3.0Cu, 1.5Si, 1.0Nb, bal. Fe. The bulk Alnico8 was solution treated for 50 min at 1250 ◦C and then vacuum quenched. Thin laminar samples were cut from the quenched Alnico8 and subsequently subjected to a magnetic heat treatment with a field of 270 kA m−1 perpendicular to the section plane at 800 ◦C for various times such as 1.5, 5 and 10 min, and finally vacuum quenched. Transmission electron microscope (TEM) observations were made using a JEOL 200CX microscope with an accelerating voltage of 200 kV. Small angle X-ray scattering (SAXS) performed in transmission geometry with Cu Kα radiation on a Rigaku diffractometer was employed to evaluate the spatial fractal characteristics in size and morphology of Fe-Co rich particles. Fig. 1 shows TEM micrographs of the samples thermomagnetically treated at 800 ◦C for various times. TEM images were taken with electron beams along the [001] direction. The dark and isolated areas in all TEM images corresponded to rod-like Fe-Co rich particles, the long axis of which was parallel to the [001] direction [6]. The splitting of large Fe-Co rich particles was observed in the specimens with thermomagnetic treatment for 1.5 and 5 min, respectively, as the arrows in Fig. 1a and b indicate. In Fig. 1a small particles originated from the splitting of larger ones [6]. To compare Fig. 1a with b, it is easily found that the split particles coarsened. As a result of splitting, the resultant microstructure was fine and homogenous as shown in Fig. 1c. For a fractal object, N (R) is proportional to Rdf , where N (R) represents the number of particles within a radius R around a given particle, and df is the fractal dimension [10]. Based on this, direct measurements from TEM images have been taken on Fe-Co rich particles for all samples in two-dimensional Euclidian space, which covered wide ranges of R. Logarithmic plots of N (R) versus R for all samples measured are shown in Fig. 2. Each set of data are well described by a power-law behavior, as expected for a fractal. The linear least-squares fits to the data sets give estimates of fractal dimensions for 1.5, 5 and 10 min, which are 1.941, 1.945 and 1.784, respectively. In addition, SAXS experiments were also performed on all samples with different treatments to obtain directly three-dimensional information about the fractal behavior of Fe-Co rich particles. It has been however pointed out that since a fractal object has the spatial self-similarity, the scattering intensity obeys a power-law decay: I ∼ q−p [11]. If p 3, then p= df− 2d, where d is the Euclidian space dimension 3, which is the case of a compact object with smooth or rough surface depending on the value of p. Fig. 3 shows the curves of lgI versus lgq for the samples thermomagnetically treated for various times. On the curves, there exist obvious linear regions between lgI and lgq for all samples, which also indicates clearly that Fe-Co rich particles with different sizes are self-similar and of fractal characteristic in statistical sense. The exponents p for 1.5, 5 and 10 min are 2.566, 2.687 and 1.917, respectively, which are all less than 3 and thus equivalent exactly to the fractal dimensions. The fractal dimensions 2.566 and 2.687 obtained from the SAXS data for 1.5 and 5 min are in good agreement with the theoretical dimension 2.5, which has been figured out by Meakin on the basis of the diffusion-limited aggregation (DLA) model in threedimensional space [12]. In addition, for 10 min, a fractal dimension 1.917 derived from the SAXS data is also basically in accord with the fractal dimension 1.8 calculated from the cluster-cluster aggregation (CCA) growth model reported in Reference [13]. This gives a hint that the evolution of Fe-Co rich particles for