Zhongwu Zhang

Improvement of magnetic properties of an Fe-6.5 wt. % Si alloy by directional recrystallization

We report that magnetic properties of an Fe-6.5 wt. % Si alloy can be improved through texture control by using directional recrystallization. Columnar grain structures with column sizes of ∼0.38×1.2 mm2 were developed during directional recrystallization. It was found that there are low energy boundaries between columns and main textures of the specimen were {110}⟨111⟩ and {111}⟨110⟩. As a result, the coercivity of a directionally recrystallized specimen is reduced by a factor of 5 when measured along 60° away from the growth direction, as compared to a specimen consisting of ∼77 μm equiaxed grains.

Determination of temperature dependence of full matrix material constants of PZT-8 piezoceramics using only one sample

Resonant ultrasound spectroscopy (RUS) was used to determine the temperature dependence of full matrix material constants of PZT-8 piezoceramics from room temperature to 100 °C. Property variations from sample to samples can be eliminated by using only one sample, so that data self-consistency can be guaranteed. The RUS measurement system error was estimated to be lower than 2.35%. The obtained full matrix material constants at different temperatures all have excellent self-consistency, which can help accurately predict device performance at high temperatures using finite element simulations.

Temperature and frequency dependence of the coercive field of 0.71PbMb1/3Nb2/3O3–0.29PbTiO3 relaxor-based ferroelectric single crystal

The hysteresis loop of ferroelectric materials becomes narrower with the increase in temperature due to energy barrier reduction, while the coercive field level increases with frequency due to the inertia of polarization reversal. These two competing effects determine the limiting operation field of medical imaging transducers at high frequencies. We have measured the coercive field of the 0.71PbMb1/3Nb2/3O3–0.29PbTiO3 single crystal as functions of both temperature and frequency. It was found that the coercive field linearly decreases with temperature at all frequencies. Related theoretical analysis was also performed to understand the physics behind the observed phenomena.

Influence of Austenitizing Temperature and Time on Microstructure and Mechanical Properties of an YP460 Grade Crack Arrest Steel

The influence of austenitizing temperature and austenitizing holding time on the microstructure and mechanical properties of a YP460 grade crack arrest steel was investigated. The first group specimens were austenitized at several temperatures ranging from 900 to 950 °C followed by water cooling. The second group specimens were austenitized at 900 °C with austenitizing holding time from 0.25 to 5 h followed by water cooling. Microstructure was characterized through optical microscopy. Tensile properties, hardness and plane strain fracture toughness of all these materials were determined and correlated with the microstructure. The results indicated that the austenitizing temperature influences the volume fraction of bainite and ferrite and then the mechanical properties. The volume fraction of bainite and ferrite and grain size are also affected by austenitizing time.

Effects of Matrix Microstructure on the Nanoscale Precipitation and Precipitation Strengthening in an Ultra-high Strength Steel

Matrix microstructure and nanoscale clusters are the two main factors influencing the mechanical properties of nanocluster strengthened steels. Here, an ultra-high strength steel with a tensile strength of ~1.64 GPa and an elongation of ~14% has been developed through a combination of fine matrix microstructure and precipitation strengthening. Matrix microstructure was primarily controlled by annealing treatment. After annealing treatment at 750 °C for 1 h, the hot-rolled microstructure changes to the layered sorbite-like structure. The precipitation strengthening contributes a similar yield strength of ~494 MPa in both hot-rolled and annealed steels. The results indicate that there is no effect of matrix microstructure on the subsequent precipitation of nanoscale clusters and precipitation strengthening. The matrix microstructure and the precipitation of nanoscale clusters are independent and can be controlled separately.

Effects of Ce Addition on the Microstructure and Mechanical Properties of Accident-Tolerance Fe-Cr-Al Fuel Cladding Materials

Fe-Cr-Al alloys are promising materials for accident-tolerance fuel cladding applications due to their excellent performance of oxidation and corrosion resistance under elevated temperature. In this study, effects of the addition of a small neutron absorption cross section rare-earth element Cerium (Ce) on the microstructure and mechanical properties of Fe-Cr-Al alloys with 0–0.1 wt% Ce have been investigated. As Ce content increased, the grains became size-refining obviously and number of precipitates increased. The results of EDS showed that the precipitates were mainly consisted of intermetallic compounds. Notably, the ultimate tensile strength and elongation reached the optimized values when the content of Ce was 0.02 wt%. However, the tensile properties decreased when Ce content was above 0.05 wt%, which may be due to the excess of intermetallic compounds.

Effects of Oxygen on the Density of States and Elastic Properties of Hafnium—First Principles Calculations

Due to its excellent comprehensive properties, Hf has been the preferred material for control rods in the nuclear reactors. There is a very strong influence of even small additions of oxygen element on the mechanical properties of Hf alloy. This work we report results of first-principles calculations of structure stability, density of states and elastic properties including the full set of second order elastic coefficients, bulk moduli and shear moduli, Young’s moduli, and Poisson’s ratio of Hf as a function of positions and concentration of oxygen atoms. Oxygen atom prefers to occupy octahedral and hexahedral interstitial sites due to the lower formation energy and less lattice distortion. Oxygen content has very weak influence on the density of states. The effects of oxygen content on the elastic parameters were estimated with the conclusion that approximately high oxygen addition decreased elastic and plastic properties.

Influence of Hot Rolling on Mechanical Behavior and Strengthening Mechanism in Boron Carbide Reinforced Aluminum Matrix Composites

Boron carbide reinforced aluminum matrix composites are widely used as neutron absorption materials. Here we report that mechanical alloying has been successfully employed to synthesize metal matrix composite powders with Al as the matrix and B4C, Al4Gd and Al4Sm as the reinforcement. The effects of hot rolling on the morphology, mechanical properties as well as the strengthening mechanisms are investigated. Hot rolling results in improving particle distribution and less agglomeration, improving the bonding between particles and matrix and decreasing voids. Thermomechanical processing can increase the density and remove the defects. As increasing rolling deformation to 50%, both YS and UTS of composites are enhanced significantly, showing 25.8 and 27.0% improvement in comparison with composites after sintered, but the elongation changes little. The increase in the yield strength after hot rolling can be attributed to two primary strengthening mechanisms in this work: The coefficient of thermal expansion (CTE) mismatch between B4C, Al4Gd and Al4Sm reinforced particles and Al matrix and the existence of load transfer from Al matrix to the hard reinforced particles.

Influence of Strain Path Change on the Microstructure and Mechanical Properties of Duplex Mg–Li Alloy

The microstructures, texture evolution, and mechanical properties of unidirectionally-rolled and cross-rolled Mg–9Li–6Er alloy were investigated in this paper. The results show that the Mg–9Li–6Er alloy mainly consists of α phase and β phase along with Er5Mg24 eutectic. The strain path was changed between rolling passes during the cross-rolling process, which led to a weaker texture development compared to the conventional unidirectional-rolling method. At the same time, cross-rolling process makes the alloys have a much significant deformation strengthening effect than that of the unidirectionally-rolling process, which can be mainly attributed to the change in strain path, making the α phase distributes disorderly in the β phase and interacts with each other into a network. On the other hand, due to the disordered distribution of matrix phases, uniform plastic deformation is blocked, and thus the ductility is greatly lowered.

Influences of Thermomechanical Processing on The Microstructure and Mechanical Properties of a HSLA Steel

High strength low alloy (HSLA) steels with high strength, high toughness, good corrosion resistance and weldability, can be widely used in shipbuilding, automobile, construction, bridging industry, etc. The microstructure evolution and mechanical properties can be influenced by thermomechanical processing. In this study, themomechanical processing is optimized to control the matrix microstructure and nano-scale precipitates in the matrix simultaneously. It is found that the low-temperature toughness and ductility of the steels are significantly the matrix microstructure during enhancing the strength by introducing the nano-scale precipitates. The effects of alloying elements on the microstructure evolution and nano-scale precipitation are also discussed.