Chapter 11 Martensitic Strengthening

Systems that Show Martensitic Transformations

Free energy versus temperature for austenite (parent phase) and martensite. Free Energy vs. Temperature for Austenite and Martensite

(a) Lenticular martensite in an Fe–30% Ni alloy. (Courtesy of J. R. C. Guimarães.)‏ (b) Lenticular (thermoelastic)‏ martensite in Cu–Al–Ni alloy. (Courtesy of R. J. Salzbrenner.) ‏ Morphologies of Martensite

Lath martensite. (Reprinted with permission from C. A. Apple, R. N. Caron, and G. Krauss, Met. Trans., 5 (1974)‏ 593.)‏ Lath Martensite

Fracture toughness vs. yield strength for twinned and dislocated martensites in a medium-carbon (0.3% C) steel. (Courtesy of G. Thomas.)‏ Twinned and Dislocated Martensites

Acicular martensite in stainless steel forming at intersection of slip bands.(Courtesy of G. A. Stone.)‏ Acicular Martensite

(a) Transmission electron micrograph showing a group of twins inside martensite transformed at –140 ◦ C and 2 GPa. (b) Dark-field image of twins on (112)b plane; (c)‏ Stereographic analysis for habit (in FCC) and twin (in BCC) planes. (From S. N. Chang and M. A. Meyers, Acta Met., 36 (1988) 1085.)‏ Twins Inside Martensite

Martensite lenses (M)‏ being traversed by twins, which produce self accommodation. (Courtesy of A. R. Romig.)‏ Twins within Martensite Lenses

0.6% proof stress (one-half of tensile stress)‏ vs. (carbon concentration) 0.5 for Fe–Ni–C lath martensite at various temperatures. The slopes are shown as fractions of the shear modulus, G. (Adapted with permission from M. J. Roberts and W. J. Owen, J. Iron Steel Inst., 206 (1968) 37.)‏ Strength of Martensite

Effect of prior austenite grain size on the yield stress of three commercial martensitic steels. (Adapted with permission from R. A. Grange, Trans. ASM, 59 (1966) 26.)‏ Strength of Martensite

Distortion produced by martensite lens on a fiducial mark on surface of specimen. Distortion Produced by Martensite Lens

Change in M s temperature as a function of loading condition. (Adapted with permission from J. R. Patel and M. Cohen, Acta Met., 1 (1953)‏ 531.)‏ Martensite Start Temperature as Function of Loading Condition

Tensile curves for Fe–Ni–C alloy above Ms, showing martensite forming in elastic range (stress assisted). (Courtesy of J. R. C. Guimarães.)‏ Temperature dependence of the yield strength of Fe–31% Ni–0.1% C., predeformed by shock loading. (Adapted with permission from J. R. C. Guimarães, J. C. Gomes, and M. A. Meyers, Supp. Trans. Japan Inst. of Metals, 17 (1976) 41.)‏ Mechanical Effects

Volume fraction transformed (right-hand side), f, and stress (left-hand side) as a function of plastic strain for an austenitic (metastable) steel deformed at –50 ◦ C; experimental and idealized stress–strain curves for austenite, martensite, and mixture are shown. (After R. G. Stringfellow, D. M. Parks, and G. B. Olson, Acta Met., 40 (1992) 1703.)‏ Strain-induced Martensite

Microcracks generated by martensite. (a) Fe–8% Cr–1% C (225 martensite sectioned parallel to habit plane). (Courtesy of J. S. Bowles, University of South Wales.)‏ (b) Carburized steel. (Reprinted with permission from C. A. Apple and G. Krauss, Met. Trans., 4(1973) 1195.)‏ Microcracks in Martensite

(a) Pseudoelastic stress–strain curve for a single- crystal Cu–Al–Ni, alloy at 24 ◦ C (72 ◦ C above M s ). (b) Dependence on temperature of stress–strain characteristics along the characteristic transformation temperatures. Strain rate: 2.5 × 10 −3 min −1. (Reprinted with permission from C. Rodriguez and L. C. Brown, in Shape Memory Effects, (New York: Plenum Press, 1975), p. 29.)‏ Shape Memory Effect

Schematic representation of pseudoelastic (or superelastic) effect. (a) Initial specimen with length L 0. (b, c, d)‏ Formation of martensite and growth by glissile motion of interfaces under increasing compressive loading. (e) Unloadingof specimen with decreasing martensite. (f) Final unloaded configuration with length L 0. (g) Corresponding stress– strain curve with different stages indicated. Pseudoelastic Effect

Sequence showing how growth of one martensite variant and shrinkage of others results in strain ε L. (Courtesy of R. Vandermeer.)‏ Strain Memory Effect

Schematic representation of strain-memory effect in compression, tension, and bending. Variant A favors a decrease in dimension in the direction of its length, whereas variant B favors an increase in dimension. Strain-Memory Effect in Compression

Schematic representation of strain-memory effect. (a) Initial specimen with length L 0. (b, c, d) Formation of martensite and growth by glissile motion of interfaces under increasing compressive stresses. (e) Unloading of specimen. (f) Heating of specimen with reverse transformation. (g) Corresponding stress–strain curve with different stages indicated. Strain-Memory Effect

(a) Lenticular tetragonal zirconia precipitates in cubic zirconia (PSZ). (b) Equiaxial ZrO 2 particles (bright) dispersed in alumina (ZTA). (Courtesy of A. H. Heuer.)‏ Martensitic Transformation in Ceramics

Martensite in Cubic Zirconia Atomic-resolution transmission electron micrograph showing extremity of tetragonal lens in cubic zirconia; notice the coherency of boundary. (Courtesy of A. H. Heuer).

ZrO 2 -rich portion of ZrO 2 –MgO phase diagram. Notice the three crystal structures of ZrO 2 : cubic, monoclinic, and tetragonal. Zirconia Phase Diagram

TEM of martensitic monoclinic lenses in ZrO 2 stabilized with 4 wt% Y 2 O 3 and rapidly solidified; the zigzag pattern of lenses is due to autocatalysis. (Courtesy of B. A. Bender and R. P. Ingel.)‏ Martensite in Zirconia

(a) Zirconia-toughened alumina (ZTA) traversed by a crack. Black regions represent monoclinic (transformed) zirconia, gray regions tetragonal (untransformed) zirconia. (b) Partially stabilized zirconia (PSZ). Lenticular precipitates transformed from tetragonal to monoclinic in the vicinity of a crack. Notice the brighter transformed precipitates. (Courtesy of A. H. Heuer.)‏ Zirconia-Toughened Alumina