Weiping Hu

Origin of grain boundary motion during diffusion bonding by hot pressing

Abstract The microstructural changes during diffusion bonding of Al 2 O 3 fibres with boron-doped Ni 3 Al plates by hot pressing were investigated. Particular attention was paid to the conversion of the bonding surfaces to an interior grain boundary and its migration during the hot-pressing treatment. The microstructure changes were found to depend on the grain size of the matrix material prior to hot pressing. Fine-grained matrix material led to a fast migration of the bonded interface in order to establish force equilibrium at grain boundary junctions at the prior surfaces. In coarse-grained material, the bonded interface moved after much larger hot pressing time by strain-induced boundary migration due to the accumulated plastic-strain during prolonged hot-pressing treatment. The fibres were found to strongly impede migration of the bonded interface, which can interfere with the perfection of the bonding process.

Thermal residual stress analysis in continuous Al2O3 fiber reinforced NiAl composites

Abstract Thermal residual stresses (TRSs) in continuous Al2O3 fiber reinforced NiAl composites with and without a BN interlayer were studied by using the finite element method (FEM). The FEM model includes the effect of neighboring fibers of the composite. A minimum sample thickness and a minimum size of the surrounding composite have to be exceeded in order to obtain the correct TRS distribution in simulation. It is shown that the TRS, caused by different thermal expansion of fiber and matrix in the NiAl composite, is sufficiently large to cause fiber damage owing to twinning on different rhombohedral planes in the sapphire fiber. The introduction of a BN interlayer or a higher fiber volume fraction (40–60%) essentially reduces the TRS level in the NiAl composite and lessens the fiber damage. The residual stress in the NiAl composites was experimentally measured by nanoindentation. A good agreement between measurements and finite element analysis was achieved.

Interfacial microstructure and reaction of BN-coated single crystal Al2O3 fiber reinforced NiAl matrix composites

Analytical electron microscopy was carried out on BN-coated 125 μm single crystal Al2O3 fiber reinforced NiAl matrix composites fabricated by diffusion bonding. NiAl was observed to react with BN to form a layer of AlN with a thickness of about 100 nm at the NiAl/BN interface during fabrication of the composite. The BN layer, deposited on Al2O3 fibers using chemical vapor deposition, exhibited a turbostratic structure with {002} planes of BN showing highly preferred orientation. Perfect bonding was obtained at the BN/Al2O3 interface with the BN {002} basal planes locally parallel to the surface of Al2O3.

Finite-element analysis of the hot-pressing consolidation of continuous Al2O3 fibers-reinforced NiAl composites

Abstract The finite-element method was used to model the consolidation process of continuous Al2O3 fiber-reinforced NiAl composites. Actual creep behavior of NiAl was accounted for including the instantaneous strain and primary creep behavior. Primary creep played a very important role in the consolidation process at high temperatures and high pressures. Various processing parameters were investigated to evaluate the effects of temperature and hot-pressing stress on the consolidation behavior. It was found that the effect of pressure on consolidation was more significant at high temperatures due to its strong influence on the creep rate during power-law creep. The effects of fiber volume fraction and the arrangement of the matrix-coated fibers were also analysed. The simulations recommend an optimal fiber volume fraction in the range of 30%–50 %. Fully dense composites were fabricated according to the predicted processing conditions, and good agreement between simulation and experiment was achieved.

High-temperature creep behavior and hot-pressing consolidation of NiAl

Abstract The steady-state creep behavior of NiAl was studied by constant strain-rate tests in the strain-rate regime 1.5 × 10−4 ≤ e˙ ≤ 1.0 × 10−2 s−1. Primary creep of NiAl was investigated at different stresses (10 MPa–40 MPa) in the temperature range from 1200 to 1400 °C. The steady-state creep rate was fitted to a temperature-compensated power-law expression e˙ = B(σ/μ)n exp(–Q/RT), from which a stress exponent n ≈ 3.5 and an activation energy Q ≈ 398 kJ/mol were derived, respectively. The obtained activation parameters indicate that dislocation creep dominates the creep behavior of NiAl at temperatures above 1200 °C up to 1400 °C. The transient creep behavior at 40 MPa could be fitted to a power-law equation ∊ = ∊0 + btm with b ∞ σ3.5 and 0.52 < m < 0.68. The creep data of NiAl obtained in this study were utilized for modeling and simulation of the consolidation process of NiAl composites during hot pressing by finite-element analysis.

Diffusion of Ni into Al2O3-fibres during hot pressing of Al2O3Ni3Al long fibre composites

Abstract Intermetallic matrix composite specimens of Ni 3 Al-matrix reinforced with continuous Al 2 O 3 fibres (FP-fibres) were investigated by means of TEM, SEM, EDS and EPMA with regard to chemical stability during diffusion bonding. The specimens were hot pressed at 1280°C and 20 MPa for 2 h in vacuum and air. The experimental results revealed a noticeable diffusion of Ni into the fibres during hot pressing. The diffusion coefficients of Ni in the fibres were estimated from the analytical results of EPMA-analysis. The estimated values for T = 1280°C were D v Ni = (5 ± 2) × 10 −14 cm 2 ·s −1 for volume diffusion and D gb Ni = (6 ± 2) × 10 −9 cm 2 ·s −1 for grain boundary diffusion. Based on these results it is proposed that Ni-diffusion in the fibres involves both volume and grain boundary diffusion. In addition, oxygen diffusion in the fibres during hot pressing in air is conceived to proceed by diffusion along grain boundaries as well as through the bulk. The influence of Ni- and oxygen-diffusion in the fibres on the formation and growth of Ni-reaction products during hot pressing in air is discussed.

Microstructure of a eutectic NiAl—Mo alloy directionally solidified using an industrial scale and a laboratory scale Bridgman furnace

Abstract The eutectic NiAl-9Mo (at.%) alloy was directionally solidified using an industrial scale Bridgman furnace and using a liquid metal cooling Bridgman furnace to produce in-situ composites of aligned Mo fibers in an NiAl matrix. In the first part of the experiment, an investment casting shell mold system, produced for NiAl alloys, was used and investigated for the NiAl-9Mo alloy in the industrial scale furnace. Due to the lower temperature gradient of ∼2.8 K mm−1, the microstructure of the samples was dendritic. In the second part of the experiment, samples with well aligned Mo fibers were produced in the laboratory Bridgman furnace using a temperature gradient of ∼11 K mm−1 and growth rates of 0.33–0.66 mm min−1.