Papers by Author: Xu Jun Mi

Abstract: Microstructure characterization and hardening behavior of a new designed Ti-12.1Mo-1Fe alloy during solution and aging treatment was investigated in the present study by OM, Vickers hardness. The results showed that the beta transus temperature of new Ti-12.1Mo-1Fe alloy was about 780°C. Further observation of ω phase should be performed by TEM later. It is also found that ω phase played a more important role than α phase in hardening. The hardening due to ω phase can lead to a high hardness about 470 Hv but the coarse α phases result in a hardness below 300 Hv.

Abstract: The microstructure evolution of a new designed metastable beta Ti-9.2Mo-2Fe alloy during aging treatment was investigated by TEM. The results revealed that athermal ω phases were observed in SAD pattern during water quenching from above the β transus temperature for the alloy. As well, isothermal ω phases were found during aging at 500 °C even though they have a limited time of stability. The coexistence of ω and α phase was found in the early aging time and the pre-formed ω phase during aging undergoes coarsening and act as uniformly distributed nucleation sites for α precipitation during subsequent isothermal aging treatment. A consequence of such ω- assisted nucleation is that relatively large amount of α precipitations are formed and fine distributed in β matrix. In addition, the α precipitations are of a relatively finer size scale (nanometers to sub micrometers).

Abstract: Transformation behaviors and texture of Ni47Ti44Nb9 cold-rolled plates were studied by differential scanning calorimetry and X-ray diffraction test. R phase transformation does not occur in Ni₄₇Ti₄₄Nb₉ cold-rolled plate annealed at 350°C-750°C followed by quenching into the water. Martensite transformation temperature first increases and then decreases with increment of annealing temperature, and the maximum achieves at 700°C. The heat of reverse martensite transformation increases, while hardness decreases as annealing temperature increases. The major texture of cold-rolled plate is {332} and spread from {332} to {110}. When the annealing temperature is above 600°C, the major textures are {332} and {111} recrystallization texture in secondary cold-rolled plate.

Abstract: Ni-Ti-Nb wide hysteresis shape memory alloys of three Ni/Ti ratio components were manufactured by vacuum induction melting. The transformation temperature, mechanical properties and recovery strain were studied by using differential scanning calorimeter and material testing machine. It shows that with Ni/Ti ratio increase, the transformation temperature and mechanical properties decrease. Shape recovery strain is higher when Ni/Ti ratio is 1.068, with recovery strain range from 6.8 to 7.5. The faster the cooling rate after annealing, the higher is the transformation temperature, and the lower are the mechanical properties and recovery strain.

Abstract: The objective of this work is to investigate the influence of measuring frequency on the internal friction of Ti50Ni48.5Fe1.5 shape memory alloy. The DSC, DC electrical resistivity and internal friction measurements were employed to characterize the sample in this study. Comparing the DSC result, the two internal friction peaks were confirmed which are corresponding to the R-phase transformation and martensitic transformation of Ti50Ni48.5Fe1.5 alloy during the cooling process, respectively. The height of internal friction peak corresponding to martensitic transformation increased with decreasing of measuring frequency. But, the height of the internal friction peak corresponding to R-phase transformation exhibited little measuring frequency dependence. Meanwhile, the value of the relative modulus peaks of the specimen increased with measuring frequency increasing. Furthermore, there was an interesting phenomenon that the martensitic transformation finish temperature shifted to higher temperature side with increasing of measuring frequency. It could be associated with the variation of applied vibration energy.

Abstract: The homogenization treatment, hot extrusion, and annealing treatment are carried out to study the effects of these processes on the mechanical properties and structure of AM140 magnesium alloy. The parameter of homogenization treatment (420°C× 24h) determined by DSC and metallurgical photo is sufficient which ensures the eutectic phase melt into magnesium matrix greatly. The Rm, Rp0.2 and A of billets after hot extrusion are 355MPa, 305MPa and 3.5% respectively. The variation of structure and mechanical properties during the process of aging is also studied by optical observation and tensile strength .

Abstract: The effect of Al contents on the properties and structures of AZ80, AZ91，AZ131 and AZ151 magnesium alloys has been studied. The experiments of homogenization treatment, hot extrusion, and annealing treatment are carried out in order to study the effects of these processes to the properties and structure of AZ80, AZ91, AZ131 and AZ151 magnesium alloys. The parameters of homogenization treatment (420°C×24h, 420°C×36h) determined by DSC and metallurgical photo are sufficient which ensure the eutectic phase melt into magnesium matrix greatly. The strength of extruded bars shortly after homogenization treatment increase (Rp0.2 from 232 to 310MPa), but elongation decrease (A from 12 to 2.5%) with Al contents. The differences and causes of structure and mechanical properties of extruded bar after the process of aging are analyzed in detail by tensile experiments, hardness measurement, and optical observation.

Abstract: Surface preparation is potentially important to the mechanical and biomedical properties of NiTi shape memory alloys. In this paper, the effect of surface preparation on fatigue life of a Ti-55.2wt%Ni alloy has been studied. Surface of the NiTi wires is prepared by five methods of chemical etching, electropolishing, oxidizing, mechanical polishing A and mechanical polishing B, and then examined by the self-developed fatigue evaluation device. Under scanning electron microscopy (SEM) the fractures of all specimens show dimple morphology. It is found that compared with the samples after chemical treatment, mechanical polishings and thermal oxidation, electropolished sample has longer fatigue life for its lower surface roughness despite a lack of face-hardened layer.

Abstract: In this paper, Al-10.8Zn-2.8Mg-1.9Cu alloy was synthesized by the spray atomization and deposition technique. The microstructure and mechanical properties of the spray deposited alloy at various aging conditions (T6, T73 and RRA treatment) were studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII zones are major precipitates for the alloy under peak-aged condition. The peak UTS and YS values achieved at an ageing temperature of 120 °C was 818 MPa and 793 MPa. Under two-step aging condition, the GP zones and η’ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa(about 7%) comparing to the peak-aged condition. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.

Abstract: The experiments of hot extrusion, hard drawing and annealing treatment were carried out in this paper in order to study the effects of these processes to the mechanical properties and structure of MB5B wrought magnesium alloy. The experimental results showed that, the eutectic phase was to melt into magnesium matrix greatly during the heating process and precipitated from matrix with point-like state in transverse section after extrusion. The tensile strength and elongation of billets after above mentioned treatment were equal to those after homogenization treatment. The as-received billets are then drawn continuously under the circumstance that the percent reduction in area(ψ) of each pass was no more than 15% and the total reduction in area was no more than 50%. As the results the tensile strength increased and elongation decreased sharply at the same time. The grains and eutectic phase of alloy after drawing were prolonged significantly. The subsequent heat treatment had little effects on the properties of the hard drawn bar.