{011} Twinning in Fe-Ni-C martensites

The substructures of two Fe-Ni-C alloys that form platelike martensite have been studied by transmission electron microscopy (TEM). Planar features with a {011 } habit are observed, in addition to the usual {112 } transformation twins and arrays of screw dislocations. The results of diffraction-contrast experiments are consistent with these {011 } defects being fine twins within which the carbon atoms occupy a different octahedral interstitial sublattice from the surrounding matrix. In any given martensitic plate, this twinning appears to occur preferentially on the {011 } plane containing the same {111 } direction as the operative {112 } transformation-twinning variant. The possibility that {011 } twinning occurs during the martensitic transformation and/or when virgin martensite is heated from subambient temperatures is discussed in relation to crystallographic observables such as the martensitic habit plane and axial ratios. The tetragonal symmetry of martensite dictates that the absolute value of the underlined index be fixed when obtaining equivalent crystallographic variants. Small contractions along the αa- andb-axes accompany the expansion along thec-axis. formerly with the Massachusetts Institute of Technology, formerly with the Massachusetts Institute of Technology     

The thermoelastic martensitic transformation inβ′ Ni-Al alloys: I. Crystallography and morphology

This investigation describes a study of the crystallographic features of the thermoelastic martensitic transformation inβ′ Ni-Al alloys. Experimental measurements of the habit plane, shape strain and orientation relationship have been made, and the results are found to be in good agreement with the predictions of the Bowles-Mackenzie phenomenological theory, assuming δ = 1.000, p₂', and {110}_β', and d₂ = <110> _β'. The martensite habit plane normals are close to {2, 14, 15}_β' and are typically clustered in self-accommodating groups of four crystallographically equivalent variants centered around {011}_β' poles. The experimental shape strain is found to be exactly an invariant plane strain with the displacement direction lying ∼92 deg from the habit plane normal.     

Isothermal martensite transformation in a 1.80 Wt Pct C steel

The linear products formed isothermally at 373 K in a 1.80 wt pct C steel (Ms = 346 K) were examined by means of transmission electron microscopy. They were first reported as “black line products” by Greninger and Troiano. The isothermal product was of a thin plate with about 0.5 μm width, and it contained {112}_b transformation twins and revealed a habit plane of {3 15 10}_f. The orientation relationship between austenite and product was close to the Nishiyama relationship. These crystallographic data were in good agreement with those calculated by the phenomenological theory of martensitic transformation. Consequently, the product was determined not to be lower bainite, but isothermal martensite. The black color of isothermal martensites resulted from the fact that they were easily etched by the precipitates of epsilon carbide formed during the isothermal holding.     

Electron microscope study on martensitic transformations in Fe- Pt alloys: General features of internal structure

Martensitic transformations of Fe-Pt alloys near the composition of Fe₃Pt with various degrees of order were examined systematically mainly by transmission electron microscopy. This alloy system exhibits two types of martensitic transformations, fcc-bct (bcc) and fcc-fct. The present experimental results indicate that each transformation is independent and compctitive. As the degree of order of austenite (L1₂ superlattice) increases, the dislocation density of bct (bcc) martensite is remarkably reduced and the shape of internal twins is changed, which is closely related to the thermoelastic behavior and the reversibility of the transformation.     

The thermoelastic martensitic transformation in β′ Ni-Al alloys: II. Electron microscopy

The substructure of martensite in a 63.4 at. pct Ni-Al alloy was studied by transmission electron microscopy with particular attention to the “mixed” martensite morphologies observed in Ni-Al alloys, {111} <11?2>_fct transformation twinning was the only mode of lattice-invariant shear found and the measured twin thicknesses lead to an inhomogeneous shear value,m ₂, in excellent agreement with that predicted from the crystallographic theory. The internal twins in the martensite plates forming characteristic mixed morphologies were found to be at definite angles to one another and these angles were carefully measured to be 96 deg and 125 deg for the “fork” type or “spear” type martensite morphologies. The corresponding angles as predicted from the Bowles-Mackenzie theory are 94.2 deg and 124.4 deg. The interface plane between the martensite variants that form the spear type morphology was found to be a {011}_β' type plane with the internal twins in each variant twin related to each other across this mirror plane.     

Structure and deformation behavior ofT 1 precipitate plates in an Al- 2Li- 1 Cu alloy

A high-resolution TEM study was performed in order to determine the structure, transformation mechanisms, and deformation behavior ofT ₁ precipitate plates in an Al-2Li-lCu alloy aged to peak strength. It is shown that a possible structure for theT ₁ plates may be an A₁BA₂C... stacking of close-packed planes with the A, planes mostly Al, the B and C planes containing a mixture of Cu and Al, and the A₂ planes mostly Li. Furthermore, the structural transformation necessary to achieve the A₁BA₂C ... stacking from the ABC ... matrix planes may be accomplished by the nu-cleation and propagation of a pair of Shockley partial dislocations on every third and fourth {111}_α matrix plane. After straining 3 pct, theT ₁ plates are sheared by dislocations, which cause a disruption in atomic order within the precipitates. Experimental evidence also indicates that the binding between Li and Cu in Al-Li-Cu alloys may be a major factor in determining microstructural evolution in this system, and comparison of diffraction data from hexagonal precipitates on {111}_α planes in a variety of Al alloys indicates that they may have similar structures. Formerly a Staff Scientist, ALCOA Laboratories.     

Thermoelastic martensite and shape memory effect in ductile Cu-Al-Mn alloys

Ductile shape memory (SM) alloys of the Cu-AI-Mn system have been developed by controlling the degree of order in the β phase. Additions of Mn to the binary Cu-Al alloy stabilize the β phase and widen the single-phase region to lower temperature and lower Al contents. It is shown that Cu-Al-Mn alloys with low Al contents have either the disordered A2 structure or the ordered L2₁ structure with a lower degree of order and that they exhibit excellent ductility. The disordered A2 phase martensitically transforms to the disordered Al phase with a high density of twins. The martensite phase formed from the ordered L2₁ phase has the 18R structure. The SM effect accompanies both the A2 → Al and L2₁ → 18R martensitic transformations. These alloys exhibit 15 pct strain to failure, 60 to 90 pct rolling reduction without cracking, and 80 to 90 pct recovery from bend test in the martensitic condition. Experimental results on the microstructure, crystal structure, mechanical properties, and shape memory behavior in the ductile Cu-AI-Mn alloys are presented and discussed.     

Crystallographic orientation relationships among η-Ni3Ti precipitate,reverted austenite,and martensitic matrix in Fe-10Cr-10Ni-2W maraging alloy

The precipitation and reversion behavior in Fe-10Cr-10Ni-2W maraging alloy during aging treatment were investigated. The fine rod-shaped η-Ni₃Ti phases were observed to be precipitated having two specific orientation relationships, termed as type I and type II orientation relationships, with martensitic matrix. The reverted austenite phases were also observed, in addition to η-Ni₃Ti precipitates which have two specific orientation relationships known as Kurdjumov-Sachs (K-S) orientation relationship and Nishiyama-Wassermann (N-W) orientation relationship, with the martensitic matrix during aging at a temperature above 550 ?C. By analyzing the observed electron diffraction patterns and computer-simulated electron diffraction patterns, unified orientation relationships among the martensitic matrix, η-Ni₃Ti precipitate, and reverted austenite phase were suggested. Two types of unified orientation relationships, named as K-S type and N-W type, were found to coexist as follows: $$K - S{\mathbf{ }}type:{\mathbf{ }}(100)_{\alpha '} ||(00 \cdot 1)_{\eta ^1 } ||(111)_\gamma ;{\mathbf{ }}[1\bar 11]_{\alpha '} ||[11 \cdot 0]_{\eta ^1 } ||[10\bar 1]_\gamma $$ $$N - W{\mathbf{ }}type:{\mathbf{ }}(100)_{\alpha '} ||(00 \cdot 1)_{\eta ^2 } ||(\bar 111)_\gamma ;{\mathbf{ }}[001]_{\alpha '} ||[2\bar 1 \cdot 0]_{\eta ^2 } ||[0\bar 11]_\gamma $$      

Competition among {110}, {112}, and {123} <111> slip modes in BCC metals

The conditions for the activation of various combinations of {ll0} <111>, {112} <1ll>, and {123} <111> slip modes have been examined by analyzing the yield loci. It is found that {ll0} slip will occur in preference to the other two modes if α > 1.155 and α₂ > 1.134, where α₁ and α₂ are respectively the critical resolved shear stress for slip on {112} and {123} systems relative to {ll0} slip. Slip occurs on {112} systems alone if α₁ < 0.866 and α₁ < 0.917α₂: and on {123} systems alone if α₂ < 0.945 and α₂ < 0.982α₁. The influence of {112} slip asymmetry and {112} <111> twinning on the choice of deformation modes has also been analyzed.     

The crystallography of ferrous martensites

The applicability of the phenomenological crystallographic theory to martensitic transformations in ferrous alloys is discussed, and it is concluded that only for the {3, 15, 10} _F and {252} _F transformations are experimental data sufficiently complete for detailed comparisons with predictions of the theory. Since crystallographic measurements have proved more difficult to obtain in the {252}F case a detailed assessment of electron microscopy observations and of reported measurements of the shape strain and orientation relationship is given for this transformation. The basic theory introduced, in the 1950's accounts satisfactorily for the {3, 15, 10} _F transformation but not for the {252} _F , case. Recent modifications of the theory based on multiple lattice invariant shears are also reviewed and are examined in the light of the crystallographic measurements. The use of experimental information to infer possible inhomogeneous deformation modes is emphasized.     

Deformation behavior in quenched and aged beta Ti-V alloys

The deformation characteristics of quenched and aged Ti-V alloys in the composition range 20 to 40 wt pct vanadium have been examined by optical metallography and transmission electron microscopy. A coarse lenticular deformation product similar in appearance to previously reported strain induced “martensites” was found to be associated with the occurrence of the omega phase. These features proved to be {112}〈111〉 twins. In the omega bearing alloys prolonged aging resulted in a transition of the deformation mode from twinning to slip at a point which corresponded either to the onset of embrittlement or alpha phase precipitation.     

Mechanical twinning in Ti50Ni47Fe3 and Ti49Ni51 alloys

Pseudo-twinning and mechanical twinning have been observed in a transmission electron microscopy study of Ti₅₀Ni₄₇Fe₃, and Ti₄₉Ni₅₁ alloys which have the B2(CsCl) structure. Observation of twinning in ordered alloys is rare and this is the first observation of twinning reported in a B2 structure. The twin planes are the {112} and {114} planes. For {112} pseudo-twins, the composition plane is not the twin plane and the pseudo-twin does not have the B2 structure. For {114} mechanical twins, the composition plane is the twin plane and the twin does have the B2 structure. It is shown that a shear on the {114} plane plus a shuffle of the atoms results in the ordered B2 structure in the twinned region.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

X-ray study of phase transformations in martensitic Ni-Al alloys

The formation of the Ni₅Al₃ and Ni₂Al phases in Ni-Al alloys with L1o ↔ B2 thermoelastic martensitic transformation has been studied by X-ray analysis. Ni₅Al₃ can form both the L1o and B2 structures, but the kinetics of L1o → Ni₅Al₃ and B2 → Ni₅Al₃ reactions are significantly different. A homogeneous mechanism for the former reaction and a mechanism of precipitation and growth for the latter are proposed. Ni₂Al forms from the B2 structure by the complex rearrangement of atoms. The initial stage of this reaction proceeds very rapidly and involves segregation of Ni atoms into Ni-rich zones leading to a Ni depletion in the surrounding regions. The nucleation of Ni₂Al retards the Ni₅Al₃ formation, so preaging in the B2 region affects the kinetics of the L1o → Ni₅Al₃ reaction on further aging in the L1o region. The microstructural mechanism for this effect is suggested.     

Deformation and microstructure in L10 type Ti-Al-V alloys

The addition of vanadium to polycrystalline, single-phase γ-TiAl significantly enhances stiffness from room temperature (RT) to 1173 K. The maximum yield stress in Ti-55A1-10V alloy is centered near 1073 K, in contrast with those of single-crystal Ti-54A1 at 873 and 1073 K, depending upon deformation orientation. The effect of strain rate on yield stress is present over the entire temperature range, but it is more prominent above 873 K, while the effect becomes significantly small between 673 and 873 K, similar to that observed in Ll₂ type Ni₃Al. The microstructure of deformed Ti-55A1-10V and Ti-49.5A1-10V alloys evolved by slip, twinning, and formation of stacking faults. Twinning is a major deformation mode in Ti-49.5-10V alloy and a minor mode in Ti-55A1-10V alloy. It appears that the amount of twins for a given deformation decreases with increasing Al content. The twin structures in both alloys were found to be true twins of {111}〈112]. No pseudotwinning was observed in the alloys deformed in the temperature range of RT to 1273 K. The stacking faults found in the deformed alloys had an extrinsic character. No intrinsic type of stacking faults were found in these alloys. In addition to the cross slip of superdislocations, the anomalous hardening up to 1073 K appears attributable to an ordinary superdislocation pinning structure, dislocation loops, and nonglissile “square shaped dislocations,” which are products of super-superdislocation reaction. These nonglissile or pinning structures are unwinded in the deformed Ti-55A1-10V alloy at 1273 K, consistent with the softening behavior at this temperature.     

The displacive cubic → tetragonal transformation in ZrO2 alloys

A displacive but nonmartensitic cubic (c) to tetragonal (t) phase transformation is ubiquitous in ZrO₂-Y₂O₃ alloys containing from ~4 to ~ 13 wt% Y₂O₃. The microstructures of the transformed phase are characterized by the presence of anti-phase domain boundaries (APB's), similar to those present in ordered alloys, and in fine-grained polycrystals, by {101} twins. The latter are interpreted as mechanical deformation twins, which relieve the strains arising from the (small) tetragonality of the product phase and the (small) molar volume change accompanying transformation.     

Transmission electron microscopy study of face-centered orthorhombic martensite in Ti-12.6 pct V alloy

The face-centered orthorhombic (fco) martensite newly found in a Ti-12.6 wt pct V alloy (11.9 at. pct V) has been investigated mainly by means of transmission electron microscopy and diffraction. The axial ratios of the fco martensite were greatly different from those of another orthorhombic martensite (distorted hexagonal) reported by Bagariatskiiet al. The lattice parameters of the bcc β matrix and the fco martensite were determined by X-ray diffraction to bea = 3.263A for the bcc β matrix,a = 3.561A,b = 4.386A,c = 4.467A for the fco martensite. The orientation relationship between the matrix and martensite lattices was found to be (0άcr11)β II (001)fco, [άcr111]β II [άcr110]fco by means of electron diffraction, and the habit plane was close to{l33}β. {1ll} twins and {ll0}, {l0l}, and {0ll} planar faults (possibly twins, also) were observed as internal inhomogeneities of the martensite plates.