Tzeng-Feng Liu

Phase Transformation in an Fe-10.1Al-28.6Mn-0.46C Alloy

The microstructure of an (α + γ) duplex Fe-10.1Al-28.6Mn-0.46C alloy has been investigated by means of optical microscopy and transmission electron microscopy (TEM). In the as-quenched condition, extremely fine D03 particles could be observed within the ferrite phase. During the early stage of isothermal aging at 550 °C, the D03 particles grew rapidly, especially the D03 particles in the vicinity of the α/γ grain boundary. After prolonged aging at 550 °C, coarse K’-phase (Fe, Mn)3AlC precipitates began to appear at the regions contiguous to the D03 particles, and —Mn precipitates occurred on the α/γ and α/α grain boundaries. Subsequently, the grain boundary β-Mn precipitates grew into the adjacent austenite grains accompanied by a γ→ α + β-Mn transition. When the alloy was aged at 650 °C for short times, coarse. K-phase precipitates were formed on the α/γ grain boundary. With increasing the aging time, the α/γ grain boundary migrated into the adjacent austenite grain, owing to the heterogeneous precipitation of the Mn-enrichedK phase on the grain boundary. However, the α/γ grain boundary migrated into the adjacent ferrite grain, even though coarse K-phase precipitates were also formed on the α/γ grain boundary in the specimen aged at 750 °C.

PHASE-TRANSFORMATIONS IN AN FE-7.8AL-29.5MN-1.5SI-1.05C ALLOY

Phase transformations in an Fe-7.8Al-29.5Mn-l.5Si-1.05C alloy have been investigated by means of optical microscopy and transmission electron microscopy. In the as-quenched condition, a high density of fine (Fe,Mn)3AlC carbides could be observed within the austenite matrix. When the as-quenched alloy was aged at temperatures ranging from 550 °C to 825 °C, aγ → coarse (Fe,Mn)3AlC carbide + DO3 reaction occurred by a cellular precipitation on theγ/γ grain boundaries and twin boundaries. Both of the observations are quite different from those observed by other workers in Fe-Al-Mn-C alloys. In their studies, it was found that the as-quenched microstructure was austenite phase(γ), and (Fe,Mn)3AlC carbides could only be observed within the austenite matrix in the aged alloys. In addition, aγ →α (ferrite) + coarse (Fe,Mn)3AlC carbide reaction or aγ →α + coarse (Fe,Mn)3AlC carbide +β-Mn reaction was found to occur on theγ/γ grain boundary in the aged Fe-Al-Mn-C alloys.

Orientation relationships among M23C6, M6C, and austenite in an Fe-Mn-Al-Mo-C alloy

Both M23C6 and Mi6C carbides were observed to precipitate within the austenite phase in an Fe-24.6 pct Mn-6.6 pct Al-3.1 pct Mo-1.0 pct alloy after being quenched from 1200 °C and aged at 700 °C. By means of transmission electron microscopy and diffraction techniques, the orientation relationships among M23C6, M6C, and the austenite phase were determined as follows: {fx567-1} The present result of the orientation relationship between M6C and the austenite phase is in disagreement with that reported by Maziasz[14] for M6C in an austenitic stainless steel.

Phase transformations in an Fe–8Al–30Mn–1.5Si–1.5C alloy

The phase transformations in an Fe‐8Al‐30Mn‐1.5Si‐1.5C alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, the microstructure of the alloy was austenite phase containing fine (Fe,Mn) 3AlC carbides. The fine (Fe,Mn) 3AlC carbides were formed during quenching by a spinodal decomposition. When the as-quenched alloy was aged at temperatures ranging from 550 to 1000C, the phase transformation sequence as the aging temperature increased was found to be (Fe,Mn)3AlC carbide C D03 ! .Fe; Mn/3AlC carbideC B2 ! .Fe; Mn/3AlC carbideC ! . This transformation has never before been observed in the Fe‐Al‐Mn‐C and Fe‐Al‐Mn‐Si‐C alloys.

L-J PHASE IN A CU2.2MN0.8AL ALLOY

A new type of precipitate (designated L-J phase) with two variants was observed within the (DO3 + L21) matrix in a Cu2.2Mn0.8Al alloy. Transmission electron microscopy examinations indicated that the L-J phase has an orthorhombic structure with lattice parametersa = 0.413 nm,b = 0.254 nm andc = 0.728 nm. The orientation relationship between the L-J phase and the matrix is (100)L-J//(011)m, (010)L-J//(111)m and (001)L-J//(211)m. The rotation axis and rotation angle between two variants of the L-J phase are [021] and 90 deg. The L-J phase has never been observed in various Cu-Al, Cu-Mn, and Cu-Al-Mn alloy systems before.

Phase transition in an Fe-23.2Al-4.1Ni alloy

The phase transition in an Fe-23.2 at. pct Al-4.1 at. pct Ni alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, the microstructure of the alloy is a mixture of (A2 + B2) phases. When the as-quenched alloy is aged at temperatures ranging from 500 °C to 1050 °C, the phase transition sequence is found to be (A2 + B2∗) → (B2 + B2∗) → B2 → A2, where B2∗ is also a B2-type phase. It is worthwhile to note that the coexistence of two kinds of ordered B2 phase has not previously been observed by other workers in the Fe-Al-Ni ternary alloy system.

Grain boundary precipitation behaviors in an Fe-9.8Al-28.6Mn-0.8Si-1.0C alloy

The phase tram&ormations in Fe-Al-Mu-C alloys have been extensively studied by many workers( 1 15). Based on their studies, it {is seen that when au alloy with a chemical composition in the range of Fe-@ 11) wt pet Al(28-35) wt pet M&(0.81.3) wt pet C was solution heat-treated and then quenched, the microstructure was single austenite (y) phase. When the as-quenched alloy was aged at temperatures ranging from 550°C to 750°C for short times, fine (Fe,Mn),AlC carbides (K’-phase) having au L’ 1 &pe structure were observed to appear within the austenite matrix, but not on the austenite grain boundaries. However, when the aging time was increased within this temperature range, a y _ a (ferrite) + tc transition (12), or a y cL + p-Mn transition (1 l13), or a y II + K + p-Mn transition (14) occurred on the austenite grain boundaries. The K phase is also an (Fe,Mn),AlC carbide, which was formed on the grain boundary as a coarse particle( 1214). Recently, the present workers performed transmission electron microscopy observations on the microstntctures of the Fe-9.8Al-28.6Mn-O.8Si-l.OC ahoy after being solution heat-treated, quenched and then aged at 6OO”C(16). In the as-quenched condition, the microstructure of the alloy was single austenite phase. After being aged at 600°C for short times, fine (Fe,Mn)&lC carbides were observed within the austenite matrix and no gram boundary precipitates could be detected. This is similar to that found by other workers in the Fe-AlMn-C alloys(6-12). However, after prolonged aging at 6OO”C, a y DO, + K transition occurred on the grain boundaries. This transition has never before been observed by other workers in the Fe-Al-Mn-C and Fe-AlMn-Si-C alloys( 1-I 5,17-22). Extending the previous study, it is interesting to study successively the microstructures of this alloy after being aged at above 600 o C.

PHASE TRANSFORMATION IN AN FE-9.0AL-29.5MN-1.2SI ALLOY

The microstructure of an (α + γ) duplex Fe-9.0Al-29.5Mn-l.2Si alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, extremely fine D03 particles were formed within the ferrite matrix by a continuous ordering transition during quenching. After being aged at 550 °C, the extremely fine D03 particles existing in the as-quenched specimen grew preferentially along (100) directions. With increasing the aging time at 550 °C, a (Si, Mn)-rich phase (designated as “L phase”) began to appear at the regions contiguous to the D03 particles. The L phase has never been observed in various Fe-Al-Mn, Fe-Al-Si, Fe-Mn-Si, and Mn-Al-Si alloy systems before. When the as-quenched specimen was aged at temperatures ranging from 550 °C to 950 °C, the phase transformation sequence occurring within the (α + D03) region as the aging temperature increases was found to be (α + D03 + L phase) → (α + D03 + A13 β-Mn)→ (B2 + D03 + A13 β-Mn)→ (B2 + A13β-Mn)→ (α + A13 β-Mn)→ (α +γ)→α.

The as-quenched microstructures of Fe-9Al-30Mn-1.2C-xSi alloys

The microstructures and the mechanical properties of the austenitic Fe-Al-Mn-C alloys have been studied by many workers. Recently, the authors have made transmission electron microscopy observations on the phase transformations of an Fe-8 wt pct Al-29 wt pct Mn-0.9 wt pct C alloy containing 1.5 wt pct Si[22]. In the previous study, they have shown that the as-quenched microstructure of the alloy was a mixture of austenite and ferrite phases, and extremely fine DO{sub 3} particles were formed within the ferrite matrix. This result is quite different from that observed by other workers in the Fe-Al-Mn-C and Fe-Al-Mn-C-Si alloys. Extending this work, it is interesting to study successively the effects of silicon content on the as-quenched microstructures of the Fe-Al-Mn-C alloy. Therefore, the purpose of this work is an attempt to study the as-quenched microstructures of the Fe-9Al-30Mn-1.2C alloy with 0, 0.5, 1.0, 1.5 and 2.5 silicon contents, respectively.

Phase transition in a Cu–14.1Al–9.0Ni alloy

Abstract The phase transition of the Cu–14.1 wt.% Al–9.0 wt.% Ni alloy has been examined by transmission electron microscopy and energy-dispersive X-ray analysis. In the as-quenched condition, the microstructure of the alloy was D03 phase that contained extremely fine L-J precipitates. During the early stage of isothermal aging at 500 °C, a high density of the extremely fine B2 particles was observed within the extremely thin lamellar γ1′ martensite, which was formed by a D03→γ1′ martensitic transformation during quenching. With increasing aging time at 500 °C, the isothermal phase transition sequence was found to be D03→D03+B2→D03+B2+α→B2+α+γ2. This transition significantly differs from that observed by other researchers. Besides, both Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W) orientation relationships between the B2 particle and the α phase could be detected in the aged alloy.