Hiromoto Kitahara

Deformation Behavior of Magnesium Single Crystals in C-Axis Compression

In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals, c-axis compression was performed. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression is bigger than that of a-axis tensile. {10-13} twin and {11-24} twin occurred at 77293K and 77473K respectively. Fracture surface at 77293K was {11-24} and at 473K was {11-22}.

Temperature Dependence of Deformation Behavior in Magnesium and Magnesium Alloy Single Crystals

It is important to research activation of the slip systems in magnesium crystals to understand deformation behavior of magnesium. In this study, pure magnesium, Mg-7.0at%Li and Mg-0.1at%Zn single crystals were stretched in the [11-20] direction in the range of 77K to 573K to investigate the deformation behavior by non-basal slip. The active slip system was investigated by the observation of slip bands, etch pit bands and dislocations by TEM. {11-22} <-1-123> second order pyramidal slip is activated in all magnesium and magnesium alloy single crystals, and its yield stress shows anomalous temperature dependence in the range from 77K to 293K, however, the yields stress decreased rapidly with increasing temperature above 293K. The yield stress due to the pyramidal slip in Mg-Li and Mg-Zn alloy were lower than that of pure magnesium about 20MPa whereas the stress of Mg-Zn at 77K was about two times higher than pure magnesium.

Deformation Behavior of Magnesium Single Crystal in c-Axis Compression and a-Axis Tension

In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals by non-basal slips and twins, c-axis compression and a-axis tension tests were performed in the range of 77K-573K. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression was bigger than that of a-axis tensile. In compression, fracture surface were (11 4) under 293K and were {30 4} above 373K, and fracture strain was smaller than the case of tension test. {10 1}-{10 2} double twin were activated at higher temperature and the crystal, therefore, fractured along the twin interface.

Influence of Impurities to Deformation Behavior in Magnesium Single Crystal

Generally, plastic deformation of magnesium alloys is difficult at room temperature. In order to improve formability of magnesium, impurity elements in magnesium were reduced by vapor deposition technique. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was applied to the determination of trace elements in refined magnesium. To investigate influence of impurity element to deformation behavior, high purity magnesium single crystals were prepared. When the magnesium single crystals are stretched in <11-20> direction, {11-22} <-11-23> pyramidal slips were activated just after yielding in the range of 77K to 293K. The yield stress of high purity magnesium was a half of the stress in raw magnesium.

Martensite transformation of ultrafine grained austenite in Fe-28.5at%Ni alloy

Martensite transformation of the ultrafine grained (UFG) austenite fabricated by the accumulative roll bonding (ARB) process was studied. The Fe-28.5at.%Ni alloy sheet was severely deformed in austenite state by the ARB process up to 5 cycles. The ARB processed sheet had the ultrafine lamellar boundary structure. The mean lamellar spacing was 230 nm in the 5 cycles specimen. The sheets ARB processed by various cycles were cooled down to 77 K to cause the martensite transformation. Martensite transformation starting (Ms) temperature decreased with increasing the number of the ARB process. The Ms temperature of the ultrafine lamellar austenite in the 5 cycles specimen was 225 K, which was lower than that (247 K) of the conventionally recrystallized specimen with mean grain size of 22 μm. The martensite having characteristic morphologies appeared from the UFG austenite, although the martensite transformed from the coarse-grained specimen showed typical plate (or lenticular) morphology. The strength of the nano-martensite transformed from the UFG austenite was about 1.5 times higher than that of the UFG austenite, and it reached to 970 MPa.

Tensile deformation of magnesium and magnesium alloy single crystals

According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip and alloying effect for the non-basal slip, pure magnesium and Mg-Al-Zn single crystals were stretched in the [110] direction. While {112}<23> second order pyramidal slip was activated at room temperature in pure magnesium, {101}<23> first order pyramidal slip became active slip at higher temperature. In Mg-Al-Zn alloy single crystal, {101} twin also activated by adding aluminum. These results indicate that active non-basal slip systems and twin in magnesium strongly depend on deformation temperature and alloying elements.

Variant Selection of Plate Martensite in Fe-28.5at.%Ni Alloy

Variant selection rule of plate martensite in a coarse-grained Fe-28.5at.%Ni alloy was investigated. Crystallographic analysis of martensite plates was carried out by electron back scattering diffraction (EBSD) analysis in a scanning electron microscope with a field emission type gun (FE-SEM). Certain variant selections were recognized for martensite plates in the Fe-28.5at.%Ni alloy. Equivalent strain, which was calculated from the shape strain of each variant, was used to evaluate the accommodation of the shear strain induced by martensite transformation. Variant selection was ruled not only by the shear strain accommodation but also by the ngle between habit planes of adjacent martensite plates.

Plastic Deformation Behavior in Magnesium Alloy Single Crystals

Zn and Al are major alloying elements of Mg alloys. Main slip system of Mg is a basal slip and the CRSS increases with Zn or Al content. According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip, Mg-Zn and Mg-Al single crystals were stretched in the [110] direction and Mg-Zn single crystals were compressed in the [0001] direction. {112}<23> second order pyramidal slip was activated in Mg-0.1at%Zn and Mg-0.5at%Al. On the other hand, {101} twin was mainly activated in Mg-1.0at%Al alloy. Yield stress due to the pyramidal slip of magnesium decreased by 0.1at%Zn addition, however they increased with addition of aluminum..

Crack Orientation Dependence for Fatigue Behavior of Magnesium Single Crystals

Magnesium alloys are very attractive, because they are extremely light, and possessing excellent specific tensile strength. It is important to understand fatigue crack growth mechanism in magnesium. The fatigue tests have been performed using two types of CT specimens with different notch orientations in laboratory air at ambient temperature. The notch plane and direction in the C-specimens were (10 ― ,10) and [0001], E-specimens were (0001) and [10 ― ,10] respectively. Fatigue fracture surfaces and da/dN∆K (stress intensity factor range) curves showed strongly dependence of notch orientations in magnesium. In the C-specimens, basal slip bands were observed near the crack and the fatigue crack growth rate was faster than the other specimens. There were many twins near the crack of the E-specimen. Fracture surface in E-specimens and higher ∆K were similar to C-specimens. da/dN of the E-specimen was lower than the E-specimen. Introduction Magnesium alloys are very attractive as structural materials, because they are extremely light, and have excellent specific tensile strength. Recently, it paid attention to auto mobils and structures. Therefore it is important to investigate fatigue crack propagation behaviors. There are many studies about bcc and fcc, however there are a few studies about hcp metals such as magnesium doesn’t have about fatigue crack propagation. Therefore we have been studied about fatigue crack propagation characteristic in the pure magnesium single crystals [1.2]. In the present study, fatigue test have been performed using CT specimens of pure magnesium single crystals to investigate orientation dependence of fatigue crack propagation characteristic, and fatigue behaviors. Experimental Procedure Single crystals were made from pure magnesium by the Bridgeman method. CT specimens with 20mm and 5mm thickness were prepared and a notch (a: notch length, w: width of specimen) was introduced on one side of the specimen. As show in Fig.1 in the C-specimens the notch plane and the direction were (10 ― ,10) and [0001], respectively E-specimens were (0001) and [10 ― ,10]. After the specimens were annealed for 8cycles at a temperature range of 673 to 523 K with 21.6 ks. The test were performed at R=0.1 using an electro servohydraulic fatigue testing machine operating at frequency of 10Hz in laboratory air at ambient temperature. Stress intensity factor range, ∆K, was determined from measuring crack length at surface by optical microscope. Results Key Engineering Materials Online: 2007-08-15 ISSN: 1662-9795, Vols. 345-346, pp 307-310 doi:10.4028/www.scientific.net/KEM.345-346.307

Mechanical Properties of Ultra-Fine Grained Fe-Cr-Ni Alloy Fabricated by ARB

Microstructures and mechanical properties of Fe-15wt.%Cr-10wt.%Ni alloy ARB processed by various cycles were studied. The starting material showed lath martensite structure. However, the austenitic phase became stable by the high straining (ARB) above Af temperature. The volume fraction of austenitic phase greatly increased to around 90 % by 2 ARB cycles and nearly saturated at about 95 % after higher ARB cycles. The grain refinement progressed during the ARB, which leaded to the formation of ultrafine lamellar UFG austenitic microstructure with mean lamellar spacing of about 300 nm. The UFG Fe-Cr-Ni alloy performed both high strength and large elongation. Especially, the yield strength of the alloy ARB processed by 5 cycles reached to 900 MPa, and the total elongation was 40 %. The good ductility of the present specimens was attributed to the occurrence of transformation-induced plasticity (TRIP).