Jianian Gui

EBSD and TEM study of self-accommodating martensites in Cu75.7Al15.4Mn8.9 shape memory alloy

Abstract Morphologies and orientation relationships of martensites in Cu75.7Al15.4Mn8.9 shape memory alloy were studied for the first time by means of electron backscatter diffraction (EBSD) technique, transmission electron microscopy (TEM) and phenomenological calculation. The martensite was identified to be an 18R1 type monoclinic one with its β angle very close to 90°. Each plate group of martensite variants consists of four variants A, B, C and D showing characteristic spear- and fork-like morphologies. Variants A and C (and equivalently B and D) are twin-related with respect to the ( 1 2 8) 18 R plane of the 18R1 martensite, which is transformed from, and nearly parallel to (with an error of only 0.60°), a (0 1 1) P plane of the parent phase. Variant-pair A–D (and equivalently B–C) is twin related with respect nearly to the (2 0 20) 18 R plane, which is transformed from a ( 1 0 0) P parent plane. However, the discrepancy between these two planes, (2 0 20) 18 R and ( 1 0 0) P , is 2.01°, larger than that (0.60°) between ( 1 2 8) 18 R and (0 1 1) P . Therefore, this twin boundary prefers to consist microscopically of several low index planes, forming a zigzag morphology, as observed by TEM. Phenomenological calculation of the cubic D03 parent phase to monoclinic M18R1 martensite transformation revealed that although the magnitude of the shape strain produced by the martensitic transformation is calculated to be rather large (m=0.2005), the average shape deformation of each self-accommodating plate group is rather small. And the total average shape deformation for six plate groups becomes nearly zero, except for a small volume contraction. The angles between the basal plane of the 18R1 martensite and the twin planes of different variant-pairs in a plate group, determined from EBSD and TEM experiments, are consistent with phenomenological calculations.

On some discrepancies in the literature about the formation of icosahedral quasi-crystal in Al-Cu-Fe alloys

To clarify some discrepancies in the literature about the formation of icosahedral quasi-crystal (IQC) in Al-Cu-Fe alloys, microstructures, and constituent phases of Al62.5Cu25Fe12.5 and Al65Cu20Fe15 alloys were studied, Each dendritic arm of the primarily solidified lambda -Al13Fe4 phase is a single crystal that possesses no definite orientation relationship with the IQC, formed by peritectic reaction (L + lambda + beta --> IQC) or a solid-state reaction (Cu-rich phases + lambda + beta --> IQC), This fact disproves an assumption that h-phase is an approximant of the IQC, Two types of cubic phase, beta -phase with CsCl structure containing more Fe and tau (3) phase, which is a superstructure and contains less Fe, were observed depending on the composition and thermal history of the samples.

Effect of ageing in the two-phase region in a Cu–Zn–Al shape memory alloy

Abstract A plateau in the biased displacement versus temperature curves was observed for Cu–Zn–Al based shape memory alloys (SMAs) which were aged at the two-phase region for a long time. This corresponds to the plateau in the reversely transformed quantities of martensite versus temperature curves, which has been confirmed by the in-situ X-ray diffraction study. These plateaus are caused by the stabilized martensite which is aged at the two-phase region and undergoes structural changes.

In situ transmission electron microscopy observations of precipitation and a new orientation relationship between γ-Mg17Al12 and magnesium-based matrix in an Mg–Al–Zn–Sn alloy

An in situ observation of the precipitation of γ-Mg17Al12 phase in a die-cast Mg–Al–Zn–Sn alloy was performed using a transmission electron microscope equipped with a heating stage maintained at 403 K for 100 min. The addition of a small amount of Sn to the AZ91 system accelerates the development of the γ-Mg17Al12 phase formed during continuous precipitation. A new orientation relationship between the γ-Mg17Al12 precipitate and α-Mg matrix was identified as .

Intra-layer ordering and inter-layer disordering of the Li2MnO3 phase in Li1.07Mn1.93O4–δ cathode materials: electron diffraction investigation and DIFFaX simulation of X-ray diffraction patterns

A previous transmission electron microscopy (TEM) analysis revealed the existence of monoclinic Li2MnO3 in the lithium-rich and oxygen-deficient Li1.07Mn1.93O4−δ powder. Interestingly, the monoclinic phase exhibits different nanoscale lamellar variants involving a rotation of the stacking direction by 120 or 240° along the pseudo-threefold axis, i.e. the [103]M//[111]C (M and C denote the monoclinic and cubic phases, respectively) zone axis. Here, a theoretical X-ray diffraction (XRD) study of Li2MnO3 employing the DIFFaX program is presented. It is found that, with the occurrence of different stacking configurations, the characteristic superstructure reflections with 2θ between 20 and 35° (Cu Kα) in the XRD pattern become more and more broadened with the increasing degree of stacking disorder, indicating that XRD may fall short in detecting the presence of the monoclinic Li2MnO3 phase. Moreover, selective peak asymmetry appears when the stacking sequence becomes extremely disordered. Further selected-area electron diffraction and theoretical neutron diffraction investigation may clarify the similar ambiguity concerning the crystal phases of other structurally related compound cathode materials for lithium-ion batteries (e.g. LiNi1/2Mn1/2O2, LiNi1/3Co1/3Mn1/3O2).

The role of the Φ phase in the solidification process of Al–Cu–Fe icosahedral quasicrystal

Abstract Several Al–Cu–Fe alloys with compositions of Al 48–60 Cu 33–50 Fe 0–10 was prepared and the phase constituents of these alloys quenched from various temperatures were identified by using X-ray diffraction (XRD), differential thermal analysis (DTA), SEM with dispersive X-ray spectrometer (EDXS) attached to, and transmission electron microscopy (TEM) (including HRTEM). The present investigation confirmed again the existence of the Φ phase with a chemical composition of Al 10 Cu 10 Fe, having two variants. The high temperature variant (designated as Φ 1 phase) is stable when temperature is higher than nearly 600 °C and has a structure of τ 3 phase, which is a three-time modulation structure along a 〈111〉 B2 direction. The low temperature variant (designated as Φ 2 phase) is stable when temperature is lower than nearly 500 °C and has a 10-time modulation structure along a 〈011〉 B2 direction. We show the preferred polythermal projection and reaction scheme. Compared with the suggestion proposed by Gayle et al. [Metall. Trans., A 23A (1992) 2409.], Faudot [Ann. Chim. Fr. 18 (1993) 445.], and Zhang and Lueck [J. Alloys Compd. 343 (2002) 53.], the main amendment is to divide the previous β region into β+Φ two regions. The important reactions of the Φ phase involving the icosahedral quasicrystal (IQC) are (1) quasiperitectic at U 8 : L+β→Φ+IQC; (2) quasiperitectic at U 5 : L+IQC→Φ+ω; (3) peritectic (between U 8 and U 5 ): L+IQC→Φ.

Orientation relationships of martensite variants determined by electron backscatter diffraction

Orientation relationships between the parent phase and 2H type martensite, and between different variants in a plate group of martensite in the CuAlNi shape memory alloy were determined by electron backscatter diffraction. The result reveals clearly that the basal planes of different correspondence variants of the 2H martensite originate from different 110P planes of the parent phase, and the [010]2H axis of each correspondence variant originates from one of the <001>P axes of the parent phase. Our result reveals also the typical relationships of four habit-variants A, B, C and D in a martensite plate group, and that the habit-variants A and C (and equivalenty B and D) are twin-related by an 1212H mirror plane, and the habit-variants A and D (and B and C) are twin-related by an 1012H mirror plane. Usually, variants A and C (and B and D) form spear morphology and variants A and D (and B and C) form fork morphology.

Transmission electron microscopy analysis of the crystallography of precipitates in Mg-Sn alloys aged at high temperatures

The crystallographic orientation relationships (ORs) of precipitated β-Mg2Sn particles in Mg–9.76 wt% Sn alloy aged at 573 K for 5 h, corresponding to its peak hardness, were investigated by advanced transmission electron microscopy (TEM). OR-3 of (110)β//(0001)α and [\overline 111]β//[1\overline 210]α and OR-4 of (110)β//(0001)α and [001]β//[2\overline 1\overline 10]α are the key ORs of β-Mg2Sn particles in the alloy. The proportions of β-Mg2Sn particles exhibiting OR-3 and OR-4 were determined as 75.1 and 24.3%, respectively. Crystallographic factors determined the predominance of OR-3 in the precipitated β-Mg2Sn particles. This mechanism was analyzed by a three-dimensional invariant line model constructed using a transformation matrix in reciprocal space. Models of the interface of precipitated β-Mg2Sn and the α-Mg matrix were constructed via high-resolution TEM and atomic resolution high-angle annular dark-field scanning TEM.

Transmission electron microscopy investigations of the microstructures in rapidly solidified Mg-Sn ribbons

Ribbons of an Mg-9.76 wt.%Sn alloy have been fabricated using rapid solidification technology. X-ray diffraction and transmission electron microscopy confirm that the rapidly solidified ribbons consist of α-Mg, β-Mg2Sn and β″-Mg3Sn phases. The α-Mg grains are refined in the ribbons. The predominant fraction of the β-Mg2Sn phase distributes at the grain boundaries of the α-Mg grains. The minor fraction of the β-Mg2Sn phase reveals a spherical morphology with a typical grain size of 170 nm. The orientation relationship between the β″-Mg3Sn particles and α -Mg matrix is identified in the ribbons and an atomic structure model of the β″-Mg3Sn phase is proposed.

Theoretical derivation of the crystallographic parameters of polytypes of long-period stacking ordered structures with the period of 13 and 14 in hexagonal close-packed system

Based on crystallographic theory, there are 63 kinds of polytypes of 13H long-period stacking order (LPSO) structure, 126 kinds of polytypes of 14H LPSO structure, 120 kinds of polytypes of 39R LPSO structure, and 223 kinds of polytypes of 42R LPSO structure in a hexagonal close-packed (HCP) system, and their stacking sequences and space groups have been derived in detail. The result provides a theoretical explanation for the various polytypes of the LPSO structure.