T.C. Lei

Microstructural characterization of laser-clad TiCp-reinforced Ni–Cr–B–Si–C composite coatings on steel

Abstract A continuous-wave CO 2 laser was used to clad a TiC particle-reinforced Ni–Cr–B–Si–C composite coating on to an AISI 1045 steel substrate. The microstructure of the clad was investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was found that partial dissolution of the TiC occurred on melting, and epitaxial growth of the remaining particles takes place by the precipitation of TiC upon cooling. This epitaxial TiC layer has an additional alloying effect by using elements such as chromium and silicon from the matrix. The pushing of TiC particles (TiC p ) by the solidification front causes them to be unequally spaced. Besides the TiC p , the microstructure of the coating is a mixture of feather-like colonies and a small amount of (γ-Ni+M 23 C 6 ) eutectic. The feather-like colonies consist of CrB laths distributed in a γ-Ni solid solution and grow radially from the TiC particles because of the large difference between the thermal properties of TiC and the Ni–Cr–B–Si–C matrix alloy.

Microstructure and mechanical properties of SiC whisker reinforced ZrO2 (2 mol% Y2O3) based composites

Microstructure, mechanical properties and toughening mechanisms of hot-pressed SiC whiskers (SiCw) reinforced ZrO2 (2 mol% Y2O3) based composites were investigated by using TEM, SEM and XRD techniques and mechanical tests. It was shown that the Vickers hardness, elastic modulus, fracture toughness and flexural strength of ZrO2(2Y) ceramics are obviously improved by the addition of SiCw. TEM observations showed that whiskers and the ZrO2 matrix are bonded tightly and there is no obvious second phase formed on the SiCw/ZrO2 interface. Both whisker toughening and transformation toughening are effective in ZrO2 (2mol% Y2O2) ceramics so that a good combination of high strength and high fracture toughness can be obtained.

Laser cladding of ZrO2-(Ni alloy) composite coating

The microstructure of laser-clad 60 vol.% ZrO2 (partially stabilized with 2 mol% Y2O3) plus 40 vol.% Ni alloy composite coating on steel 1045 was investigated by scanning electron microscopy, electron probe microanalysis, X-ray diffraction, energy-dispersive X-ray analysis and microhardness tests. The composite coating consists of a pure ZrO2 clad layer in the outer region and a bonding zone of Ni alloy adjacent to the substrate. The pure ceramic layer exhibits fine equiaxed ZrO2 grains in the outer zone and columnar ZrO2 dendrites in the inner zone, growing from the ceramic layer-bonding zone interface. This ceramic layer is composed of metastable t′-ZrO2 phase and a very small amount of m-ZrO2 phase and displays a microhardness of 1700 HV0.2. The high heating and cooling rate caused by laser cladding restrains the t → m phase transformation in the ZrO2 ceramic layer. Interdiffusion of alloy elements takes place in the bonding zone, in which the coexistence of ZrO2 particles, Ni-based solid solution and (Fe,Cr)23C6 particles in the interdendritic regions was found.

Dynamic recrystallization of ferrite in a low carbon steel during hot rolling in the (F+A) two-phase range

The dynamic restoration mechanisms of metals during hot working have been one of the main topics in the past two decades. In most cases, it has been found that dynamic recovery is the sole dynamic restoration mechanism in ferrite ([alpha]-iron) due to its high stacking fault energy. Dynamic recrystallization is found to occur only in high purity ferrite and in ferritic interstitial free steels. In the latter case, the nucleation of dynamic recrystallization takes place via the bulging of grain boundaries. However, all of the above work has been done on single-phase ferrite, and the dynamic restoration behavior of ferrite in coarse two-phase structures is still unclear. In a previous study on the hot deformation of low carbon steel in the ferrite-austenite (F+A) two-phase range, the authors found that the nucleation mechanism of dynamic recrystallization in the austenite phase was greatly influenced by the presence of F/A phase boundaries. In this paper, the dynamic restoration of ferrite in (F+A) two-phase structures and the role of F/A phase boundaries were investigated.

Time-dependent tetragonal to monoclinic transition in hot-pressed zirconia stabilized with 2 mol % yttria

The kinetics of isothermal transformation from tetragonal (T) to monoclinic (M) structure in zirconia stabilized with 2 mol% yttria was systematically studied using thermal expansion analysis, electron probe microanalysis, X-ray diffractometry and transmission electron microscopy. Results showed that tetragonal phase obtained from rapid cooling transforms isothermally to monoclinic phase during holding at 200–400‡C for 4 h. The time-temperature-transformation (TTT) curve has a characteristic C-shape, with 300‡C as the “nose” temperature. A small amount of M phase can be detected when the holding temperature is not in the preceding temperature range. The isothermal transformation kinetics can be expressed in terms of the Johnson-Mehl-Avrami equation in which the nucleation and growth of new phase are both functions of time with the activation energy determined to be 28.67 KJ mol−1. The metastability of T phase is associated with cooling rate, stabilizer content, as well as grain size.In situ observation of M phase propagation induced by the electron beam reveals the T → M transition to be diffusion controlled.

Microstructure, mechanical properties and thermal shock behaviour of Al2O3+ZrO3+SiCw composites

The strengthening and toughening effects and mechanisms of combining both whisker reinforcing and zirconia phase transformation toughening in the Al2O3 matrix were systematically investigated in this paper. TEM observations showed that most of the SiCwAl2O3 and SiCwZrO2(2Y) interfaces are bonded tightly and there are no distinct second phases or intermediate layers formed at the interfaces. The addition of SiCw or ZrO2(2Y) particles evidently improve the mechanical properties of Al2O3 ceramics and the more obvious strengthening and toughening effects can be obtained by adding both SiCw and ZrO2(2Y) particles simultaneously to the Al2O3 matrix. The main toughening mechanisms in the Al2O3 + ZrO2 + SiCw composites are whisker bridging and pull-out, crack deflection as well as dynamic t-m phase transformation toughening and microcrack toughening. The toughening effects of both SiCw and ZrO2(2Y) particles are shown to be additive, but the addition of ZrO2(2Y) particles decreases the strengthening effect of SiCw. The addition of SiCw can obviously improve the thermal shock resistance of Al2O3 and Al2O3 + ZrO2(2Y) ceramics and the thermal shock resistance of Al2O3 + SiCw composites can be further improved by adding a few of ZrO2(2Y) particles (10vol%).

Microstructure of laser-clad SiC-(Ni alloy) composite coating

The laser cladding technique was used to produce Ni alloy coatings with different SiC particle (SiCp) contents on steel 1045. The complete dissolution of SiCp took place during laser melting and led to a microstructural evolution of the coatings associated with the SiCp content. M7X3 or M23X6-type carboborides and Ni-base solid solution are found as the main microstructural constituents of the clad layers, and the volume fraction of the carboborides increases with increasing SiCp content. When the SiCp content reaches 30 vol.%, a silicide of Ni3Si2 phase and a few spherical graphite precipitates are observed, and the γ matrix shows a modulated structure. The addition of SiCp can significantly enhance the microhardness and wear resistance of the coatings.

Deformation and restoration behaviour of ferrite-austenite two-phase structures

Abstract Pure iron and four low carbon steels with different carbon contents were compressed in a ferrite-austenite two-phase range at strain rates of 0.002, 0.02, 0.2 and 2 s−1. The experimental results show that dynamic recovery occurs in the ferrite (F) phase, and dynamic recrystallization of austenite (A) nucleates through the grain boundary bulging mechanism where there were A-A boundaries before deformation and proceeds through the formation and subsequent movement of new A-A boundaries at F-A phase boundaries in the case where no A-A boundaries existed before deformation. The shape of the true stress-true strain (σ-e) curves is affected by both the volume fractions and the morphologies of the constituents. The peak stress of the σ-e curve of two-phase structures responds to the strain rate in the same way as that of single-phase structures. The peak stress and the stable state stress on σ-e curves of two-phase structures are higher than those calculated from the linear law of mixtures owing to the existence of F-A boundaries.

Interface structure and mechanical properties of Al2O3-20vol%SiCw ceramic matrix composite

Abstract Al2O3–20vol%SiCw composites with as-received and acid-leached whiskers were fabricated by a hot-pressing technique. The microstructure, mechanical properties, fracture behavior and toughening mechanisms of the composites were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), X-ray photo spectroscopy (XPS), energy dispersive analysis of X-ray (EDAX) techniques and three-point bending tests. The results show that whisker surface acid-leached treatment obviously reduces the oxygen content of SiC whisker and makes the whisker surface smooth, resulting in composites with higher fracture toughness.

Mechanical properties of Al2O3 and Al2O3 + ZrO2 ceramics reinforced by SiC whiskers

The effect of the SiC whisker content on the mechanical properties of Al2O3 and Al2O3 + 20 vol% ZrO2 (2 mol% Y2O3) ceramic composites has been investigated. It is shown that the strength and fracture toughness of the composites are increased by the addition of 0–30 vol% SiC whiskers with only one exception that 30 vol% SiC whisker leads to a decrease in the flexure strength. The addition of 20 vol% ZrO2 (2 mol% Y2O3) significantly improves the mechanical properties of the Al2O3 + SiC whisker (SiCw) composites and the t-m phase transformation of ZrO2 is enhanced by the residual stresses caused by the thermal incompatibility between the SiCw and the matrix. The toughening effect of both SiC whiskers and the t-m phase transformation of ZrO2 (2 mol% Y2O3) is shown to be additive, but the addition of ZrO2 decreases the strengthening effect of the SiC whiskers.