NiTi alloy is produced by mechanical alloying(MA). It becomes amorphous after milling for enough time, such as 100 h in this paper. DSC measurement shows that the crystallization temperature is 676 K for the amorphous...NiTi alloy is produced by mechanical alloying(MA). It becomes amorphous after milling for enough time, such as 100 h in this paper. DSC measurement shows that the crystallization temperature is 676 K for the amorphous powder. Activation energy of crystallization is 199.98 kJ/mol for MA powder, which is lower than that of amorphous prepared by magnetron sputtering. Avrami parameter of crystallization is 1.07.展开更多
Based on the local canning compression,severe plastic deformation（SPD） is able to lead to the almost complete amorphous nickel-titanium shape memory alloy（NiTi SMA）,in which a small amount of retained nanocrystall...Based on the local canning compression,severe plastic deformation（SPD） is able to lead to the almost complete amorphous nickel-titanium shape memory alloy（NiTi SMA）,in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix.Crystallization of amorphous NiTi alloy annealed at 573,723 and 873 K was investigated,respectively.The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-MehlAvrami-Kolmogorov（JMAK） equation.NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K,where martensite phase transformation is suppressed due to the constraint of the grain boundaries.Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample,where（001） martensite compound twin is observed at room temperature.It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains.SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.展开更多
The powders of pure Al, Fe, and Zr for preparing Al78Fe20Zr2 were subject to a high-energy planetary ball milling.The microstructure evolution of the mixtures at the different intervals of milling was characterized by...The powders of pure Al, Fe, and Zr for preparing Al78Fe20Zr2 were subject to a high-energy planetary ball milling.The microstructure evolution of the mixtures at the different intervals of milling was characterized by X-ray diffraction（XRD）, transmission electron microscopy（TEM） and differential scanning calorimetry（DSC）.It was found that a nearly complete amorphization could be achieved in the mixtures after ball milling for 23 h.Further ball milling led to the crystallization of the amorphous powders.A long time ball milling, e.g., 160 h, led to a complete crystallization of the amorphous powders and the formation of Al3Zr and Al13Fe4.The crystallization products caused by ball milling are almost the same as that produced by isothermal annealing of the amorphous powders in vacuum at 800 K for 1 h.展开更多
Ni_(100-x)Mo_x(x=20,30,38)alloys have been synthesized by mechanical alloying.The structure evolution of powders in mechanical alloying process has been monitored by X-ray diffraction,scanning electron microscopy and ...Ni_(100-x)Mo_x(x=20,30,38)alloys have been synthesized by mechanical alloying.The structure evolution of powders in mechanical alloying process has been monitored by X-ray diffraction,scanning electron microscopy and transmission electron microscopy analyses.The results show that the Ni_(62)Mo_(38)sample becomes partially amorphous after high energy ball milling,while the Ni_(80)Mo_(20)and Ni_(70)Mo_(30)become non-equilibrium nanocrystals.展开更多
Rapidly solidified Al87Ni7Cu3Nd3 metallic glasses, prepared by using melt spinning, were treated under both isothermal and non-isothermal regime. The amorphous ribbon and the annealed samples were closely examined by ...Rapidly solidified Al87Ni7Cu3Nd3 metallic glasses, prepared by using melt spinning, were treated under both isothermal and non-isothermal regime. The amorphous ribbon and the annealed samples were closely examined by means of differential scanning calorimetric, conventional X-ray diffraction and high resolution transmission electron microscopy with selected-area electron diffraction, with special interest in primary crystallization into α-Al nanocrystalline particles, in order to understand structural characteristics of Al-based amorphous/nanocrystalline alloys, and nucleation and grain growth mechanism on the nanometer scale during primary crystallization. The results show that, the as-prepared ribbons are fully amorphous and homogeneous in the micron scale, but contain high density of quenched-in clusters or crystallite embryos. Primary crystallization mainly leads to formation of two-phase mixture of a-Al crystal and residual amorphous phase. The annealed ribbons exposed isother-mally at HOP C for 5, 130 minutes and heated continuously up to less than 310℃ at 40℃ C/min consist of large amount of α-Al fcc crystal nanoparticles dispersed uniformly in an amorphous matrix. However, a very little amount of finer orthorhombic Al3Ni intermetalics particles exist in the annealed ribbons heated up to 310℃. During primary crystallization, the leading kinetic mechanics to impede growth of the α-Al crystal is soft impingement, instead of geometric impingement.展开更多
Rapidly solidified Al 87 Ni 7Cu 3Nd 3 metallic glasses were prepared by using melt spinning. Its calorimetric behavior was characterized by using differential scanning calorimeter. The metallic glasses were partially ...Rapidly solidified Al 87 Ni 7Cu 3Nd 3 metallic glasses were prepared by using melt spinning. Its calorimetric behavior was characterized by using differential scanning calorimeter. The metallic glasses were partially crystallized under continuous heating regime. Primary crystallization was studied through structural characterization of the amorphous and partially crystallized ribbons by means of conventional X ray diffraction and transmission electron microscopy with selected area electron diffraction. The results show that, the as spun ribbons are fully amorphous and homogeneous on the micron scale, but contain high density of nanoscale quenched in clusters or crystallite embryos. Primary crystallization mainly leads to formation of two phase mixture of α Al nanocrystalline and residual amorphous phase. Precipitation of α Al nanoparticles is limited by build up and overlapped diffusion field of solute atoms with low diffusion rate. At the earlier stage of primary crystallization the crystal nuclei exhibit high density and growth rate. With the α Al crystal growing, the crystal growth rate decreases, and even at the later stage further crystallization into α Al crystal becomes difficult to occur due to thermal stabilization of the residual nickel and neodymium enriched amorphous phase, the saturated values of crystallized volume fraction and α Al crystal diameter getting to 20%30% and 515 nm.展开更多
Crystallization kinetics of Fe55-xCrzsMo7B10C4Nbx（x= 0, 3） bulk amorphous alloys were analyzed using X-ray diffraction and differential scanning; calorimetric （DSC） tests. In practice, crystallization and growth m...Crystallization kinetics of Fe55-xCrzsMo7B10C4Nbx（x= 0, 3） bulk amorphous alloys were analyzed using X-ray diffraction and differential scanning; calorimetric （DSC） tests. In practice, crystallization and growth mechanism were evaluated using DSC tests at four different heating rates （10, 20, 30, and 40 K/rain） and kinetic models. Two-step crystallization behavior was observed when Fe55Cr18MOTB16C4 and Fe52Cr18MoTB16C4Nb3 bulk amorphous alloys were annealed, where Fe36Cr12M010 phase was crystallized in the first step of crystallization. Results show that Fe36Cr12Mo10 and Fe3C phases were crystallized in the structures of the alloys after further annealing： process. Activation energy for the crystallization of Fe36Cr12Mozo phase was measured to be 543 kJ/mol in Fe52Cr18MoTB16C4Nb3 alloy and 375 kJ/mol for Fe55Cr18Mo7B16C4 alloy according to Kissinger-Starink model. Moreover, a two-dimensiona nucleation rate was found in Fe52Cr18Mo7B16C4Nb3 diffusion controlled growth mechanism with decreasing alloy whereas a three-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in crystallization of Fe36Crz2Mo10 phase during annealing of Fe55Cr18MoTB16C4 alloy. TEM （transmission electron microscopy） observations reveal that crystalline Fe36Cr12M010 phase nucleated in the structures of the alloys in an average size of 10 nm with completely mottled morphology.展开更多
Elemental powders of Al-24. 4 % Fe (mole fraction) alloy were mechanically-alloyed using a conventional type ball-milling. Not only Fe particles gradually disperse in Al matrix but also Al and Fe crystal grains are re...Elemental powders of Al-24. 4 % Fe (mole fraction) alloy were mechanically-alloyed using a conventional type ball-milling. Not only Fe particles gradually disperse in Al matrix but also Al and Fe crystal grains are refined to the order of ten nanometers after ball-milling for 180 h. The am0rphization starts at the early stage of ball milling and proceeds gradually. Complete amorphization is confirmed by ball-milling for 720h. The amorphization of the Powders after ball-milled for 180 h can be promoted by heating up to 673 K,which suggests a possibility of producing amorphous material by appropriate ball-milling and subsequent heating.展开更多
The nanocrystallization behaviour of a bulk Zr-based metallic glass subjected to compressive stress is investigated in the supercooled liquid region. Compared with annealing treatments without compressive stress, comp...The nanocrystallization behaviour of a bulk Zr-based metallic glass subjected to compressive stress is investigated in the supercooled liquid region. Compared with annealing treatments without compressive stress, compressive deformation promotes the development of nucleation and suppresses the coarsening of nanocrystallites at high temperatures.展开更多
文摘NiTi alloy is produced by mechanical alloying(MA). It becomes amorphous after milling for enough time, such as 100 h in this paper. DSC measurement shows that the crystallization temperature is 676 K for the amorphous powder. Activation energy of crystallization is 199.98 kJ/mol for MA powder, which is lower than that of amorphous prepared by magnetron sputtering. Avrami parameter of crystallization is 1.07.
基金Project (51071056) supported by the National Natural Science Foundation of ChinaProject (HEUCF201317002) supported by the Fundamental Research Funds for the Central Universities of China
文摘Based on the local canning compression,severe plastic deformation（SPD） is able to lead to the almost complete amorphous nickel-titanium shape memory alloy（NiTi SMA）,in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix.Crystallization of amorphous NiTi alloy annealed at 573,723 and 873 K was investigated,respectively.The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-MehlAvrami-Kolmogorov（JMAK） equation.NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K,where martensite phase transformation is suppressed due to the constraint of the grain boundaries.Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample,where（001） martensite compound twin is observed at room temperature.It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains.SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.
基金financial support of the National Natural Science Foundation of China (No. 50371072)the Hunan Provincial Natural Science Foundation (No. 09JJ3086)
文摘The powders of pure Al, Fe, and Zr for preparing Al78Fe20Zr2 were subject to a high-energy planetary ball milling.The microstructure evolution of the mixtures at the different intervals of milling was characterized by X-ray diffraction（XRD）, transmission electron microscopy（TEM） and differential scanning calorimetry（DSC）.It was found that a nearly complete amorphization could be achieved in the mixtures after ball milling for 23 h.Further ball milling led to the crystallization of the amorphous powders.A long time ball milling, e.g., 160 h, led to a complete crystallization of the amorphous powders and the formation of Al3Zr and Al13Fe4.The crystallization products caused by ball milling are almost the same as that produced by isothermal annealing of the amorphous powders in vacuum at 800 K for 1 h.
文摘Ni_(100-x)Mo_x(x=20,30,38)alloys have been synthesized by mechanical alloying.The structure evolution of powders in mechanical alloying process has been monitored by X-ray diffraction,scanning electron microscopy and transmission electron microscopy analyses.The results show that the Ni_(62)Mo_(38)sample becomes partially amorphous after high energy ball milling,while the Ni_(80)Mo_(20)and Ni_(70)Mo_(30)become non-equilibrium nanocrystals.
文摘Rapidly solidified Al87Ni7Cu3Nd3 metallic glasses, prepared by using melt spinning, were treated under both isothermal and non-isothermal regime. The amorphous ribbon and the annealed samples were closely examined by means of differential scanning calorimetric, conventional X-ray diffraction and high resolution transmission electron microscopy with selected-area electron diffraction, with special interest in primary crystallization into α-Al nanocrystalline particles, in order to understand structural characteristics of Al-based amorphous/nanocrystalline alloys, and nucleation and grain growth mechanism on the nanometer scale during primary crystallization. The results show that, the as-prepared ribbons are fully amorphous and homogeneous in the micron scale, but contain high density of quenched-in clusters or crystallite embryos. Primary crystallization mainly leads to formation of two-phase mixture of a-Al crystal and residual amorphous phase. The annealed ribbons exposed isother-mally at HOP C for 5, 130 minutes and heated continuously up to less than 310℃ at 40℃ C/min consist of large amount of α-Al fcc crystal nanoparticles dispersed uniformly in an amorphous matrix. However, a very little amount of finer orthorhombic Al3Ni intermetalics particles exist in the annealed ribbons heated up to 310℃. During primary crystallization, the leading kinetic mechanics to impede growth of the α-Al crystal is soft impingement, instead of geometric impingement.
文摘Rapidly solidified Al 87 Ni 7Cu 3Nd 3 metallic glasses were prepared by using melt spinning. Its calorimetric behavior was characterized by using differential scanning calorimeter. The metallic glasses were partially crystallized under continuous heating regime. Primary crystallization was studied through structural characterization of the amorphous and partially crystallized ribbons by means of conventional X ray diffraction and transmission electron microscopy with selected area electron diffraction. The results show that, the as spun ribbons are fully amorphous and homogeneous on the micron scale, but contain high density of nanoscale quenched in clusters or crystallite embryos. Primary crystallization mainly leads to formation of two phase mixture of α Al nanocrystalline and residual amorphous phase. Precipitation of α Al nanoparticles is limited by build up and overlapped diffusion field of solute atoms with low diffusion rate. At the earlier stage of primary crystallization the crystal nuclei exhibit high density and growth rate. With the α Al crystal growing, the crystal growth rate decreases, and even at the later stage further crystallization into α Al crystal becomes difficult to occur due to thermal stabilization of the residual nickel and neodymium enriched amorphous phase, the saturated values of crystallized volume fraction and α Al crystal diameter getting to 20%30% and 515 nm.
文摘Crystallization kinetics of Fe55-xCrzsMo7B10C4Nbx（x= 0, 3） bulk amorphous alloys were analyzed using X-ray diffraction and differential scanning; calorimetric （DSC） tests. In practice, crystallization and growth mechanism were evaluated using DSC tests at four different heating rates （10, 20, 30, and 40 K/rain） and kinetic models. Two-step crystallization behavior was observed when Fe55Cr18MOTB16C4 and Fe52Cr18MoTB16C4Nb3 bulk amorphous alloys were annealed, where Fe36Cr12M010 phase was crystallized in the first step of crystallization. Results show that Fe36Cr12Mo10 and Fe3C phases were crystallized in the structures of the alloys after further annealing： process. Activation energy for the crystallization of Fe36Cr12Mozo phase was measured to be 543 kJ/mol in Fe52Cr18MoTB16C4Nb3 alloy and 375 kJ/mol for Fe55Cr18Mo7B16C4 alloy according to Kissinger-Starink model. Moreover, a two-dimensiona nucleation rate was found in Fe52Cr18Mo7B16C4Nb3 diffusion controlled growth mechanism with decreasing alloy whereas a three-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in crystallization of Fe36Crz2Mo10 phase during annealing of Fe55Cr18MoTB16C4 alloy. TEM （transmission electron microscopy） observations reveal that crystalline Fe36Cr12M010 phase nucleated in the structures of the alloys in an average size of 10 nm with completely mottled morphology.
文摘Elemental powders of Al-24. 4 % Fe (mole fraction) alloy were mechanically-alloyed using a conventional type ball-milling. Not only Fe particles gradually disperse in Al matrix but also Al and Fe crystal grains are refined to the order of ten nanometers after ball-milling for 180 h. The am0rphization starts at the early stage of ball milling and proceeds gradually. Complete amorphization is confirmed by ball-milling for 720h. The amorphization of the Powders after ball-milled for 180 h can be promoted by heating up to 673 K,which suggests a possibility of producing amorphous material by appropriate ball-milling and subsequent heating.
基金Supported by the Science Foundation for Excellent Young Scholars of Heilongjiang Province under Grant No JC-05-11, the Program for New Century Excellent Talents in University of China under Grant No NCET-04-0322, and the Specialized Research Fund for the Doctoral Programme of Higher Education of China under Grant No 20040213049）
文摘The nanocrystallization behaviour of a bulk Zr-based metallic glass subjected to compressive stress is investigated in the supercooled liquid region. Compared with annealing treatments without compressive stress, compressive deformation promotes the development of nucleation and suppresses the coarsening of nanocrystallites at high temperatures.
