Microstructure and mechanical properties of continuous Al₂O₃fibre reinforced Ni₄₅Al₄₅Cr_7.5Ta_2.5alloy (IP75) matrix composites

Abstract

Single crystalline Al 2 O 3 fibres (sapphire), coated with the NiAl alloy IP75 by physical vapour deposition (PVD), were assembled to fabricate composites by means of diffusion bonding. The microstructure and chemistry of both as-coated fibre and as-diffusion bonded composites were investigated by electron microscopy and microanalysis. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature, and the composite tensile strength was measured at 900 C and 1100 C. An amorphous layer with a thickness of about 400 nm formed between the fibre and the matrix during the PVD process and was maintained during diffusion bonding. A Laves phase precipitated along NiAl grain boundaries in the IP75 matrix. This caused a lower tensile strength of the IP75/Al 2 O 3 composite at high temperatures compared to as-cast monolithic IP75 and rendered the composite useless for structural applications.

Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

“…Properties and further possible applications of IP75 were discussed in several more recent investigations [1047][1048][1049][1050][1051][1052]. NiAl-Ta-Cr IP75 coatings were produced by thermal spraying applying the HVOF (high-velocity oxygen fuel) technique [1047], and fiber composites consisting of single-crystal a-Al 2 O 3 fibers that were PVD-coated with IP75 and hotpressed in vacuum were tested with respect to their mechanical properties [1048,1049]. However, a thick amorphous layer formed at the interfaces where debonding occurred making this kind of composite material unsuitable for high-temperature applications [1048].…”

“…NiAl-Ta-Cr IP75 coatings were produced by thermal spraying applying the HVOF (high-velocity oxygen fuel) technique [1047], and fiber composites consisting of single-crystal a-Al 2 O 3 fibers that were PVD-coated with IP75 and hotpressed in vacuum were tested with respect to their mechanical properties [1048,1049]. However, a thick amorphous layer formed at the interfaces where debonding occurred making this kind of composite material unsuitable for high-temperature applications [1048]. A Cr-free variant containing additions of Nb and consisting of NiAl grains with (Nb,Ta)(Ni,Al) 2 Laves phase covering the grain boundaries was patented in 2015 for gas turbine applications [1053], but no further results about the materials' properties were published.…”

Tensile Strength of Ni45Al45Cr7.5Ta2.5‐Coated Sapphire Fibers

“…8) show the compressive axial thermal stress, σ zz , that develops in a sapphire fiber deposited with IP75 with and without a BN interlayer along its c ‐axis at different temperatures. Sputtered IP75 coatings develop a columnar structure 42 which considerably differs from the cast IP75 microstructure 43 . Hence, lower thermal residual stresses may develop due to the less than dense microstructure.…”

“…Si may be introduced in the IP75 during the casting process 50 and forms SiO with α‐Al 2 O 3 already at T >925°C 51 . Al 2 SiO 5 is identified by X‐ray diffraction in the cast IP75 alloy, 43 and hence Si is present in the alloy. Cr, which is an alloying element of IP75, has been found to react with α‐Al 2 O 3 at 1250°C 52 (compared with 1300°C used here).…”

Improvement of compressive strength and ductility in NiAl–Cr(Nb)/Dy alloy by rapid solidification and HIP treatment

“…On the other hand, the alloys processed by powder metallurgy with finer microstructure show improved room temperature mechanical properties but seriously lowered yield stresses at high temperatures [7]. It is well known that the mechanical properties of these alloys sensitively depend on the grain size of NiAl matrix and the morphology and distribution of strengthening Laves phase, which can be controlled by adjusting processing parameters, microalloying and heat treatment [17,18]. And moreover, recent studies [19,20] have shown that the rapid solidification and subsequent hot isostatic pressing (HIP) treatment can well improve the mechanical properties of the NiAl based alloy.…”