In-situ study of the microstructure evolution during tension of a Mg-Y-Zn-Al alloy processed by rapidly solidified ribbon consolidation technique

Mg-Y-Zn-Al alloys processed by rapidly solidified ribbon consolidation(RSRC)technique exhibit an exceptional mechanical performance indicating promising application potential.This material has a bimodal microstructure consisting of fine recrystallized and coarse non-recrystallized grains with solute-rich stacking faults forming cluster arranged layers(CALs)and nanoplates(CANaPs),or complete long period stacking ordered(LPSO)phase.In order to reveal the deformation mechanisms,in-situ synchrotron X-ray diffraction line profile analysis was employed for a detailed study of the dislocation arrangement created during tension in Mg-0.9%Zn-2.05%Y-0.15%Al(at%)alloy.For uncovering the effect of the initial microstructure on the mechanical performance,additional samples were obtained by annealing of the as-consolidated specimen at 300 and 400℃ for 2 h.The heat treatment at 300℃ had no significant effect on the initial microstructure,its evolution during tension and,thus,the overall deformation behavior under tensile loading.On the other hand,annealing at 400℃ resulted in a significant increase of the recrystallized grains fraction and a decrease of the dislocation density,leading to only minor degradation of the mechanical strength.The maximum dislocation density at the failure of the samples corresponding to the plastic strain of 10-25% was estimated to be about 16-20×10^(14)m^(-2).The diffraction profile analysis indicated that most dislocations formed during tension were of non-basal and pyramidal types,what was also in agreement with the Schmid factor values revealed independently from orientation maps.It was also shown that the dislocation-induced Taylor hardening was much lower below the plastic strain of 3% than above this value,which was explained by a model of the interaction between prismatic dislocations and CANaPs/LPSO plates.

Corrosion behavior of Mg-8Li-3Zn-Al alloy in neutral 3.5%NaCl solution

The corrosion behavior of Mg-8Li-3Zn-Al alloy was investigated in neutral 3.5%NaCl aqueous solution by morphology observation and electrochemical tests.The weight loss method was to measure the corrosion rate.The electrochemical results indicated that the corrosion resistance of Mg-8Li-3Zn-Al alloy in 3.5%NaCl solution is poorer than that in distilled water.The Cl-anion leads to the initiation and development of the corrosion pits.And the corrosion products are mainly Mg(OH)_(2).

Creep Induced Phase Transformation in Extruded Zn-Al Alloy

Phase transformation and microstructural change of an extruded eutectoid Zn-Al alloy Zn76Al22Cu2 (wt pct) were investigated during creep testing by using SEM and X-ray diffraction techniques. Creep induced decomposition of a metastable η'T phase and a four phase transformation, α+ ε →T' +η, occurred during the creep testing. Also a microstructural change was observed from a lamellar structure into a spheroidized structure in the rupture part of the extruded alloy. It provided evidence of creep induced phase transformations which occurred in ageing process. The mechanism of creep rupture of the extruded Zn-Al alloy was also discussed.

ON THE STABILIZATION OF THERMOELASTIC MARTENSITE IN A Cu-Zn-Al ALLOY

The stabilization of thermoelastic martensite in a rapidly solidified Cu-Zn-A1 alloy is be- lieved to be the process of disordering in atomic configuration during which the structure of martensite gradually transforms into N9R(b/a=1/3^(1/2))from M18R.This is dependent upon the intrinsic decomposition tendency of the martensite.

Microstructures of Thermomechanically Treated Eutectoid Zn-Al Alloy

A continuously cast eutectoid Zn-Al alloy was extruded at about 250℃ and the complex microstructures of three phases (α. ε and η'E) were observed in the extruded eutectoid Zn-Al alloy.The fcc Al-rich α phase appeared as isolated particles with clear boundaries instead of the α phase occurring with diffuse boundary within the dendrites of the as-cast structure. A new unstable phase η'E was determined to have hexagonal close packed crystal structure. The decomposed β's, phase comprised the matrix. The new phase η'E decompoeed and a four phase transformation α+ε T'+η occurred afterwards during the isothermal holding. The extruded alloy tended to be stable after these two phase transformations. The early shrinkage and the following increase in dimension were related to the decomposition of the new phase η'E and the four phase transformation.

One-dimensional nano-structure of Cu-Zn-Al alloy

Cu-Zn-Al alloy of one dimensional nano-structure was prepared and thestructure of obtained nano-material was characterized by transmission electron microscope (TEM). Itwas shown that there are non-linear oscillations on the surface of Cu-Zn-Al alloy and theconsanguineous connection exists between non-linear oscillation and the growth process of onedimensional nano-structure. The diameter of one dimensional nano-structure is about 40 nm, and theratio of length to diameter is over 40. Finally, the growth mechanism of one dimensionalnano-structure was also studied.

Electroplating Zn-Al Alloy Technology

The method of controlling separating anode and separating power source was used to perform orthogonal optimization for the parameters in electroplating Zn Al alloy.The electroplating Zn-Al alloy technology was decided, in which the content of Al is about 12%-15% .

Tensile Stress Induced Phase Transformations in Zn-Al Alloy

Both furnace cooled and as-cast eutectoid Zn-Al alloys were investigated under external tensile stress at 100℃. It was observed that the external tensile stress caused decomposition of two metastable phases η'T and η'S which derived from both original state of the alloy, and a phase transformation, αf +ε→T' +η， in both furnace cooled and as-cast eutectoid Zn-Al alloys. Also spheroidized structure formed partially during tensile testing. Superplasticity of the alloy has been discussed correlating with the phase transformations and microstructural changes.

MORPHOLOGY AND STRUCTURE OF TRANSFORMATION PRODUCTS FORMED AT INTERMEDIATE TEMPERATURE IN A Cu-Zn-Al ALLOY

A TTT diagram for th ie precipitation formed at some intermediate temperatures through cooling from high lemperalure parenl phase in a Cu-27 27Zn-3.73Al allay is established by means of dilatormetric measurement.The morphology and structure of transformation products formed at some intermediate tem peratures isothermally through cooling from high temperature parent phase and up-quenching from DO_(3) parent phase are studied by metallographic.X-ray and electron microscopy analyses.Three regions in the two separate C curves are obtained according to different morphology of precipitate:rod-like a,plate-like bainite and a rods,and bainite plates.Prolonged aging makes bainite plate change gradually into a whose lattice parameters are no different from that of a formed equilibriumly from parent phase.The structure is almost orthorhombic long period structure for bainites formed from B_(2) and DO_(3) parent phase,but monoclinic for martensite from DO_(3).They correspond to the overlapping and separating of(1210)and(2010)diffraction peaks respectively,showing the lower degree of ordering in bainite.

A QUENCHING INDUCED PLANAR DEFECT OF MARTENSITE IN Cu-Zn-Al ALLOY

A new kind of planar defect named non-basal plane stacking faults has been studied by using trace analysis and lattice image techniques.It has been found that they are located in (125)_M plane of martensite lattice.By lattice image observation,the non-basal plane stack- ing faults are argued to be induced by two incomplete basal plane stacking faults and may be the results of rapid quenching.The defects are regarded as anomalous structures of martensite.

EFFECT OF MARTENSITE ORDERING ON SME IN A Cu-Zn-Al ALLOY

The correlation between the shape memory effect(SME)and the ordering degree in martensite formed through various heat-treatment processes,e.g.ice water quenching, step-quenching and aging etc.,has been studied in a Cu-26Zn-4Al alloy.The martensite or- dering degree is estimated by △d,the spacing difference of some pairs of diffracting planes with indices satisfying a relation of(h_1~2-h_2~2)/3=(k_2~2-k_1~2)/n(n=1 for 9R martensite,n=4 for 18R martensite).M 18R martensite is obtained from step-quenching,in which the value of △d increases with the holding duration of step-quenching,and the shape recovery rate η increases synchronouslly.9R martensite is obtained from direct water-quenching,the value of △d is quite large and SME is also good at just quenching state.But both △d and η decrease continuously with aging time at room temperature.This stabilization phe- nomenon of martensite is explained by the observation of TEM lattice fringe image,and it may be attributed to the clustering of quenched-in supersaturated vacancies at (001)_M close-packed plane in martensite and decreasing the ordering degree.A reduction in △d and η occuring in the specimens on step-quenching or aging at higher temperature, which may be related to the precipitation of the α-phase and the enrichment of solute atoms, decreases the ordering degree.

A Crystallography Study of Bainitic Transformation in a Cu-Zn-Al Alloy

The crystallographic nature of initial isothermal phase transformation in Cu-26Zn-4Al (wt-％) was investigated. The kinetic transforma- tion curve, morphology, crystal structure, substructure, orientation relationship and twin relationship of bainite plates have been studied by means of optical and transmission electron microscopy. The experimental results showed that the characteristics of initial 8→bainite transformation are not exactly consistent with that found in martensite transformation, for example, orientation relationship between matrix and bainite does not exist in martensite transformation.

Mobility of Various Intervariant Interfaces in 18R Martensite in a Cu-Zn-Al Alloy Part Ⅰ: In-situ Observations

The motion of intervariant intedeces under the action of applied stress in the internally faulted 18R martensite in a Cu-Zn-Al shape memory alloy has been studied. Transmission electron microscopy in situ observations show that the interfaces between 24 martensite variants have different reaction to applied stress. The A/C type and A/B type interfaces have good mobil-ity, the A/D type interface has bad mobiIity, and the different-group-intervariant interfaces are basically immobile.

KINETICS CHARACTERISTICS OF BAINITE FORMATION IN Cu-Zn-Al ALLOYS

TTT diagrams and kinetics curves of bainite formation through up quenching from martensitic slate and of dilatometry and cooling from high temperature parent phase in Cu-Zn-Al alloys are established by ments of dilatometry and metallographic inspection.Experimental results show that the kinetics characteristics of bainite formation obey the Austin-Rickett Equation with n=2.25 for up-quenched specimens and n=1.80 for specimens cooled from high temperature,and the activation energy of bainite formation is about.110 kJ/mol,corresponding to that ofthe diffusion of solute atoms.

Crossed Micro-bands in the Martensite Variant Pairs of a Thermomechanically Trained Cu-Zn-Al Alloy

The crossed micro-bands in the martensite variant pairs of a thermomechanically trained Cu25.66Zn-4.02Al (wt-%) alloy have been studied. It was found that the micro-bands pass through the interface from one martensite variant to another and their orientation inside different variants is distinct, which would remain in the parent phase after the reverse transformation and play an important role in the formation of preferential martensite. In fact, the crossed micro-bands are micro-twins for accommodating the stress fields generated during thermomechanically training

ORDER STATE AND STABILITY OF MARTENSITE IN AN AIR-QUENCHED Cu-Zn-Al ALLOY

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MICROSTRUCTURE ANALYSIS AND MECHANICAL PROPERTIES OF Zn-Al ALLOY ROD PRODUCED BY HEATED MOLD CONTINUOUS CASTING

The new technology of continuous casting by heated mold was used to produce directional solidification ZA alloy lines to eliminate the inter defects of these lines and increase their mechanical properties. The results are as follows: (1) The microstruc-ture of the ZA alloy lines is the parallel directional dendritic columnar crystal. Every dendritic crystal of eutectic alloy ZA5 was composed of many layer eutectic β and η phases. The micro structure of hypereutectic ZA alloys is primary dendritic crystal and interdendritic eutectic structure. The primary phase of ZA8 and ZA12 is β, among them, but the primary phase of ZA22 and ZA27 is a. (2) Through the test to the as-cast ZA alloy lines made in continuous casting by heated mold, it is found that the tensile strength and hardness increase greatly, but the elongation decreases. With the increase of aluminum amount from ZA 5 to ZA 12, ZA22 and ZA27, the tensile strength increases gradually. ZA27 has the best comprehensive mechanical properties in these four kinds of ZA alloys. (3) Heat treatment can decrease the dendritic segregation and improve the elongation of ZA alloy, but make their strength decrease slightly.

Color characteristics of Cu-Zn-Al alloys

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DIFFERENTIAL SCANNING CALORIMETRY AND X-RAY DIFFRACTION STUDIES ON AGING BEHAVIOR OF Zn-Al ALLOYS

Decomposition processes of the quenched Zn-Al alloys were studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the stabilities of supersaturated solid solution (SSS) of Zn-Al alloy and α' phase formed by quenching would reduce with the increase of Zn content and the precipitation of η-Zn phases even when aging at ambient temperature, so that the exothermic precipitation peak in DSC curve would disappear. The activation energy of the η-Zn precipitation and the reaction enthalpy were calculated and measured. The kinetics of α' decomposition or η-Zn formation was determined by XRD. The microstructure change during aging was observed by TEM.

MICROSTRUCTURE AND REVERSE SHAPE MEMORY EFFECT IN A Cu-Zn-Al ALLOY

The bainite structure in a Cu-Zn-A1 alloy related to the reverse shape memory effect has been observed by means of TEM.The reverse memory effect may be improved by up to one order of magnitude under applied constraint stress.The widespread propagation of bainite was confirmed to be the diffusion controlled shear process by the parabolic configuration of side interface of bainite plate and the twisting of intersected bainite plates.