Isothermal transformations in an Fe-9 pct Ni alloy
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Owen[1] first reported massive transformations in iron substitutional alloys. Since that time, it has become apparent that there are two massive transformations in iron alloys: equiaxed ferrite[2] (␣p) and massive ferrite (␣q).[2,3] The nomenclature in brackets, here and later is attributed to Araki.[4] Wilson also contends[5–8] that the massive transformation in Cu-9.3 pct Al may occur partly if not completely by shear to resemble massive martensite in the polished and etched microstructure. In References 9 and 10, transformations occurring in an Fe-9 pct Ni alloy at various cooling rates were reported. On cooling at 0.13 K/s (equivalent to furnace cooling), composition invariant massive ferrite formed between 575 ⬚C and 556 ⬚C. The transformation range means that the massive ferrite forms in the (␣ ⫹ ␥) phase region, below the A3 of 715 ⬚C[11,12] and T0[13,14] of 623 ⬚C ⫾ 8 ⬚C.[9,10,15] This result confirms directly Massalski’s observation[16] that the massive transformation occurs in an Fe-9.12 pct Ni alloy in the two-phase region beyond the retrograde ␣/(␣ ⫹ ␥) phase boundary on cooling at 7 K/s. A typical optical micrograph of massive ferrite and transmission electron microscopy (TEM) micrograph are shown in figures 1[3] and 2,[17] respectively. Note the irregular grain boundaries and high dislocation density. In contrast, a typical optical micrograph of equiaxed ferrite and TEM micrograph are shown in Figure 3.[18] The TEM micrograph illustrates the low dislocation density and the equiaxed microstructure similar to well-annealed metals. To return to the Fe-9.14 pct Ni alloy on cooling at 0.13 K/s, below 556 ⬚C, Widmansta¨tten ferrite grew from the grain boundary E.A. WILSON, Emeritus Fellow, is with the School of Engineering, Sheffield Hallam University, Sheffield S1 1WB, United Kingdom, S.H. CHONG, formerly Postgraduate Student, School of Engineering, Sheffield Hallam University, is with Wieland Metals (Singapore) Ltd., 19, Joo-Koon Crescent, Singapore 629 017. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
ferrite exhibiting tent relief,[9,19–22] on prepolished surfaces. Subsequent microanalysis by TEM showed an enhancement of Ni at the grain boundaries of the massive ferrite in the furnace-cooled Fe-9.14 pct Ni alloy.[9,10] This indicates that Ni must partition in the interface during transformation. The observation of martensite-austenite (MA) constituent in furnace-cooled Fe-9.14 pct Ni[9,10] also indicates that C partitions in the interface, although this point is debated later under further discussion. Current thinking is that there are six transformations in pure iron; equiaxed ferrite (␣p) at 870 ⬚C ⫾ 10 ⬚C, massive ferrite (␣q) 800 ⬚C ⫾ 10 ⬚C, Widmansta¨tten ferrite (␣W) 760 ⬚C ⫾ 10 ⬚C, bainitic ferrite 725 ⬚C ⫾ 20 ⬚C (␣0B), lath (massive)
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