Deformation of two C36 laves phases by microhardness indentation at room temperature

Microhardness indentation and electron microscopy were used to study the room-temperature deformation behavior of two Laves phases, ZrFe₂ and MgNi₂, in two-phase alloys. Evidence of extensive plastic deformation was found around the indentations. Activation of basal and nonbasal slip systems forms a distinctive cell structure in both Laves phases. Slip on planes nearly normal to the basal plane occurs in the lamellar form of ZrFe₂. Burgers vectors of the dislocations and resulting shear displacements are studied using high-resolution transmission electron microscopy (TEM). Curved or zigzag nonbasal slip surfaces in atomic scale were observed. Each slip plane can be considered to be composed of a series of small facets of high atomic density. A common Burgers vector and displacement vector component of 1/4 [0001] was found on many different slip planes. The “zonal glide” concept for slip on prismatic planes is discussed.     

The γ → ε-martensitic transformation and its reversion in the FeMnSiCrNi shape-memory alloy

The hcp martensitic transformation and its reversion in an Fe-16.86Mn-4.50Si-10.30Cr-5.29Ni alloy have been studied. The fine structure and morphologies of ε-martensite were systematically investigated using transmission electron microscopy. It is found that, in the overlapped region of the stacking faults, the nucleus of ε-martensite may form by shear and grow along the {111}γ plane, which is not the stacking fault plane. The nucleus may grow into a small, thin plate and may act as the basal structure unit of ε-martensite. When the stress increases, the thin plates continue to grow into an ε-martensite ribbon along this preferred orientation and, during the subsequent recovery annealing, the reverse transformation of these basal structure units occurs. As a result, the ε-martensite plates shrink in both length and thickness.     

Crystallography of Martensite in TiAu Shape Memory Alloy

The twin structure, habit plane orientation, and morphology of B19 martensite in TiAu, which is a candidate shape memory alloy (SMA) for high-temperature and biomedical applications, were investigated by conventional transmission electron microscopy. Almost all internal twins were {111} type I twins as lattice-invariant deformation (LID). The <211> type II twin was scarcely observed in TiAu, unlike in TiPd and TiPt SMAs. The habit plane roughly corresponded to the twinning plane (K ₁ plane) of the <211> type II twin because of the superb lattice parameter ratio of TiAu. As a result, an energy-minimizing microstructure referred to as “twins within twins” appears as the major microstructure. The selection rules for the twinning of LID are also discussed considering the results of extensive studies on LID in SMAs.     

锻造方式对Ti-6Al-4V合金组织及取向分布的影响

在快锻液压机上对Ti-6Al-4V合金进行了锻造变形,采用扫描电镜、背散射电子衍射技术以及X射线衍射技术研究了不同锻造方式下合金组织及晶粒取向的变化规律.在单向镦拔和换向镦拔两种不同锻造方式下,难变形区、小变形区及大变形区中α相及β相的分布差别不大,组织均匀性基本一致,两种变形方式下锻坯不同区域的应变稍有差别.进一步对不同变形区域形变织构的定量分析可知:在应变较小的边缘区域,变形主要以{0001}基面滑移为主,形成基面织构;在应变较大的内部区域,织构明显转向{1120}、{1010}等柱面织构;在应力集中的位置,会产生{1122}、{1011}等锥面织构.两种锻造方式均能提高Ti-6Al-4V合金中形变织构的均匀性,而且换向镦拔优于单向镦拔.      

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.     

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.     

Deformation modes in NbAl3

The DO₂₂ lattice of the NbAl₃ intermetallic compound shows very limited ductility at room temperature. In this study the slip and twinning systems that are active during the deformation process were investigated. Evaluation of the possible deformation modes was performed and contrast analysis in the transmission electron microscope revealed both expected and unexpected deformation modes. Two types of dislocations were found in the deformed structure, namely thea 〈110〉 superdislocation on the {112} plane and loops of unidentified dislocations on the {010} plane. No evidence of 〈201〉 superdislocations was found, probably due to the fact that this type of dislocation is expected to move in groups of four. Twins of the {112} type were found to play an important role in the deformation process since they supply a component of shear perpendicular to the (001) plane.     

Intermetallic phases formed during DC-casting

The Al-Fe and Al-Fe-Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al-Fe-Si phases were determined by EDS-analysis of extracted crystals.     

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.     

Transformation behavior of nearly stoichiometric Ni-Mn alloys

The transformation behavior of Ni-Mn alloys in the vicinity of the stoichiometric composition has been studied using transmission electron microscopy, X-ray diffraction, optical microscopy, and electrical resistivity measurements. The transformation behavior was found to be markedly different in Mn-rich alloys and Ni-rich alloys. In Mn-rich alloys a martensitic transformation between L2₀ (B2) and L1₀ structures takes place, which possesses many features common to alloys exhibiting a thermoelastic martensitic transformation. On the other hand, in Ni-rich alloys an order-disorder transformation between A2 and L1₀ structures occurs. The martensitic transformation features {111} transformation twins as the transformation substructure while the ordering reaction involves {101} order twins. In the Mn-rich alloys, the martensitic phase, if either slowly cooled or annealed at intermediate temperatures, becomes “tempered”, resulting in a noncrystallographic, essentially featureless microstructure apart from the presence of occasional {111} twins. Formerly with University of Illinois.     

Effect of interfacial reaction on bending strength of Al18B4O33 whisker-reinforced aluminum composites

The present study deals with the mechanism of the interfacial reaction and the microstructure of the interface in aluminum borate whisker-reinforced pure aluminum matrix composites prepared by a squeeze casting process. By means of X-ray diffraction analysis (XRDA), differential thermal analysis (DTA), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM), it has been found that no interfacial reaction occurred under the as-cast condition, whereas aluminum reacted with the whiskers when the composite was reheated at a temperature higher than 726 °C. The reaction produced γ-Al₂O₃. Based on HRTEM observation, it is considered that the whiskers bond to the aluminum matrix directly after squeeze casting. The observation also shows that there is a specific orientation relationship between the reaction product and whisker, {002} Al₁₈B₄O₃₃ ‖ {220} γ-Al₂O₃, <200> Al₁₈B₄O₃₃ ‖ <111> γ-Al₂O₃, which leads to a coherent interface with a mismatch of less than 1 pct. The interfacial bonding state changed after heating under different conditions. Propagation of cracks in each interfacial state was observed by SEM, and the effects of the interfacial reaction on the bending strength were studied with the microstructure of the interface.     

Hexagonal dislocation networks in titanium

Hexagonal dislocation networks which occurred in as-annealed commercial Ti-A 50 rod and following partial extrusion under hydrostatic pressure at room temperature were studied using transmission electron microscopy. For the as-annealed condition networks were observed on the prism, basal and {2•1•1x} planes, while for the extrusion networks were only observed on the basal plane. The various stages in the development of the networks are presented and mechanisms by which they form are proposed. The self energies of the dislocation hexagons constituting the various networks are calculated using the equations proposed by de Wit and Ruff. They increase in the order: a) networks on the basal plane in the as-annealed condition, b) networks on the prism plane in the asannealed condition and c) networks on the basal plane in the partially extruded condition. This paper is based on Doctoral Thesis presented in October 1973 by S. P. AGRAWAL to the Department of Metallurgical Engineering and Materials Science at the University of Kentucky.     

The role of twinning in the cyclic stress-strain behavior of directionally solidified γ/γ′-δ

Under cyclic straining of the γ/γ’-δ directionally solidified alloy a special type of hysteresis loop with an inflection in the compressive loading region was obtained. Observations gave direct evidence that the inflection was due to a process in which the 5 twins formed during tension untwinned under the subsequent compression. Thin foil electron microscopy established the 5 twins to be two specific variants of the four {211} variants, the selectivity being imposed by the crystallographic relation of the 5 phase with the γ/γ’ matrix. Characteristic distribution of (010) 5 faults was also noted in the foils and they were found associated with the {211} untwinning events. The compressive {011} 5 twinning mode reported in literature was not observed; the forward plastic strain in tension was reversed by {211} untwinning mode at a compressive stress level much lower than that required for the onset of the {011} twins.     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in hydrogen at 22 °C

The hydrogen-induced fracture behavior of notched single crystals of the PWA 1480E nickel-based superalloy was studied. Notched single crystals with seven different crystal orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in an hydrogen atmosphere at 34 MPa. The notch tensile strength degradation in hydrogen was orientation dependent. The specimen with the [100] orientation had the greatest strength degradation, while the crystal with the [111] orientation had the least. A stereoscopic technique combined with the use of planar γ′ morphologies was applied to identify cleavage plane orientations. All specimens failed predominately by {100}-type cleavage within about 0.5 mm of the notch and {111}-type cleavage toward the center of the crystal. Cleavage on {111}-type planes in the center of the crystals was not related to testing in hydrogen. Microcracking along the {100} γ/γ′ interfaces was observed in the area near the fractured surface. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies indicated that {100}-type cleavage within the notch region controlled the hydrogen-induced cleavage fracture in notched single crystals. Formerly graduate Student, Auburn University     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in helium at 22 °C

The fracture behavior of single crystals of the PWA 1480E nickel-base superalloy was studied using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Notched single crystals with seven different crystal growth orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in a helium atmosphere at 34 MPa. Gamma prime particles were orderly and closely aligned with the cube edges along the [100], [010], and [001] directions of theγ matrix. The cuboid morphology of theγ’ precipitate was not influenced by the crystal growth orientation. The specimen with the [110] orientation was the strongest, while the crystal with the [100] orientation was the weakest. A stereoscopic technique, combined with the use of planary’ morphologies, was applied to identify the cleavage plane orientation. All specimens failed predominately by {lll}-type cleavage which originated from combined slip on various {111} planes. In most cases, deformation was found to occur inhomogeneously in intense slip bands lying on {111} planes and aligned parallel to the different slip directions. Both SEM and TEM studies indicated that {lll}-type slip was the controlling factor during cleavage fracture of single crystals of the PWA 1480E nickel-base superalloy. Formerly Graduate Student, Auburn University     

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.     

Deformation structure in a Ti-24Al-11Nb alloy

A Ti-24Al-11Nb alloy has been heat-treated so as to obtain a microstructure of coarse α₂ particles (D0₁₉ structure based on Ti₃Al) in a matrix of the ordered β_o phase (B2 structure based on Ti₂AlNb). Dislocation structures generated by tensile strains of ∼2 pct at room temperature have been analyzed by transmission electron microscopy The β_o phase is shown to deform inhomogeneously on {110}, {112}, and {123} planes by α/〈211〉 slip. The slipband structure is complex, consisting of segments of heavily pinned edge dislocations with periodic cross slip of screw components on to secondary slip planes. Incompatibility stresses at α₂/β_o interfaces can generate fine α[100] slip as well. The α₂ phase deforms independently by α dislocation slip. Slipbands in the β_o phase can shear the α₂ phase by activatingc +a/2 slip on and slip planes, as well asa slip on higher order pyramidal planes, where the parallelism of the specific slip system is permitted by the Burgers relationship between the two phases.     

The morphology and substructure of {225}F martensite in an Fe−Mn−Cr−C alloy

A transmission electron microscopy and diffraction study of martensite plates in an Fe-3 pct Mn-3 pct Cr-1 pct C alloy was carried out with particular attention to details of the martensite substructure. A corresponding optical metallographic study of plate morphology was made. A variability in martensite substructure was observed from plate to plate, although a (252)_F ^* plate was generally associated with (112)_B transformation twins and {111}_F stacking faults. The particular (111)_F fault variant gave rise to a wedge-shaped plate morphology. Planar {101}_B inhomogeneities were frequently observed in the martensite, and most of these appear to be derived from austenite stacking faults. In general, more than one type of inhomogeneity was observed in a single martensite plate and the “typical” plate substructure was rather difficult to characterize, although the habit plane was found invariably to be (252)_F.