Al-20%Cu多晶粒竞争生长的原位观测及汇聚型枝晶淘汰模型构建

采用同步辐射成像技术研究Al-20%Cu(质量分数)亚共晶合金的晶粒竞争生长动力学,并建立与实验结果相一致的数学模型。结果表明,对于发散型竞争生长,遵循“正常淘汰”规则。对于汇聚型竞争生长,成分过冷的作用加速非择优取向晶粒中一次枝晶臂的生长速度,导致非择优取向枝晶出现“异常淘汰”现象,淘汰择优取向枝晶。探讨汇聚型竞争生长的“正常淘汰”与“异常淘汰”之间的临界条件,建立枝晶生长速率、枝晶臂间距和枝晶倾斜角之间的数学关系,又称“二次枝晶臂阻挡模型”,可以快速预测晶界位相和淘汰类型。

Microstructure and mechanical properties of TIG/A-TIG welded AZ61/ZK60 magnesium alloy joints

The dissimilar joints of AZ61 and ZK60 magnesium alloys were obtained by tungsten inert gas arc(TIG)welding and activating tungsten inert gas arc(A-TIG)welding processes.Microstructure characterization shows that,the fineα-Mg equiaxed dendrite crystals contained Mg17Al12 and MgZn2 particles in the fusion zone.The average size of theα-Mg grains in the fusion zone was refined to 19μm at welding current of 80 A,which resulted in the largest tensile strength of 207 MPa.The tensile strength and the width of the beam of the A-TIG welded AZ61/ZK60 joints showed strong dependence on the amount of TiO2.However,the inhomogeneity of the heat-affected zone near different base metals presented no significant effect on the mechanical properties of the welded joint.

Experimental determination of 800℃ isothermal section in Al-Zn-Zr ternary system

The 800℃ isothermal section in the Al-Zn-Zr system was investigated using the equilibrated alloy method and the solid/liquid diffusion couple approach by means of scanning electron microscopy equipped with energy dispersive spectroscopy, X-ray powder diffraction and electron probe microanalysis. Thirteen three-phase regions are experimentally confirmed in the ternary system. A ternary compound Zn50Al25Zr25 named as T phase exists stably in the 800℃ isothermal section. Its composition range varies widely (16.84-55.1 at.% Zn, 18.02-56.3 at.% Al and 26.0-28.53 at.% Zr), and it can be in equilibrium with all the binary compounds. The maximum solubilities of Zn in Zr3Al, Zr2Al, Zr3Al2, Zr4Al3, ZrAl, Zr2Al3, ZrAl2 and ZrAl3 are 7.5, 0.84, 0.33, 0.89, 0.91, 1.12, 0.64 and 3.8 at.%, respectively. The maximum solubilities of Al in Zn3Zr, Zn2Zr and ZnZr are 1.6, 1.3 and 13.6 at.%, respectively.

Phase equilibria of Zn-Al-Ti ternary system at 450 and 600℃

The phase relationships in the Zn-Al-Ti system at 450 and 600℃ were experimentally determined using equilibrated alloys method.The specimens were investigated by means of scanning electron microscopy coupled with energy dispersive spectroscopy and X-ray diffractometry.Eleven and eight three-phase regions are confirmed in the 450 and 600℃ isothermal sections,respectively.The Ti2Al5 which only exists at high temperature(990-1199.4℃)in Ti-Al binary system is confirmed in two isothermal sections due to the dissolution of zinc.The T phase is confirmed as a ternary compound rather than an extension phase of TiZn3 at 450℃.The T2 phase is a new ternary phase stable at 450 and 600℃ in Zn-Al-Ti ternary system.

Phase equilibria of Co-Mo-Zn ternary system

To experimentally determine the isothermal sections of Co-Mo-Zn ternary system at 600 and 450℃,the equilibrated alloy and diffusion couple methods were adopted by using scanning electron microscopy coupled with energy-dispersive spectrometry,X-ray diffractometry and electron probe microanalysis.Experimental results show that there are six three-phase regions on the Co-Mo-Zn isothermal section at 600℃and nine three-phase regions on the Co-Mo-Zn isothermal section at 450℃.No ternary compound is found in these two isothermal sections.Both the maximum solubilities of Mo in the Co-Zn compounds(γ-Co5 Zn21,γ1-Co Zn7,γ2-Co Zn13 andβ1-Co Zn)and that of Zn inε-Co3 Mo are no more than 1.5 at.%.The maximum solubilities of Zn inμ-Co7 Mo6 are determined to be 2.1 at.%and 2.7 at.%at 600 and 450℃,respectively.In addition,the maximum solubilities of Co in MoZn7 and MoZn22 are 0.5 at.%and 4.7 at.%at 450℃,respectively.