Bing Ye

Texture memory and strain-texture mapping in a NiTi shape memory alloy

The authors report on the near-reversible strain hysteresis during thermal cycling of a polycrystalline NiTi shape memory alloy at a constant stress that is below the yield strength of the martensite. In situ neutron diffraction experiments are used to demonstrate that the strain hysteresis occurs due to a texture memory effect, where the martensite develops a texture when it is cooled under load from the austenite phase and is thereafter “remembered.” Further, the authors quantitatively relate the texture to the strain by developing a calculated strain-texture map or pole figure for the martensite phase, and indicate its applicability in other martensitic transformations.

Finite-Element Modeling of Titanium Powder Densification

A powder-level, finite-element model is created to describe densification, as a function of applied stress during uniaxial hot pressing, of CP-Ti and Ti-6Al-4V powders with spherical or spheroidal shapes for various packing geometries. Two cases are considered: (1) isothermal densification (in the α- or β-fields of CP-Ti and in the β-field of Ti-6Al-4V) where power-law creep dominates and (2) thermal cycling densification (across the α/β-phase transformation of Ti-6Al-4V) where transformation mismatch plasticity controls deformation at low stresses. Reasonable agreement is achieved between numerical results and previously published experimental measurements and continuum modeling predictions.

A Novel Method to Achieve Grain Refinement in Aluminum

A significant grain refinement of pure aluminum is achieved upon addition of TiCN nanoparticles (NPs). Unlike the conventional inoculation, NPs can induce the physical growth restriction through the formation of NP layer on the growing grain surface. An analytical model is developed to quantitatively account for the NP effects on grain growth. The NP-induced growth control can overcome the inherent limitations of inoculation and shed light on a potential method to achieve grain refinement.

Compressive Creep Behaviour of Extruded Mg-10Gd-3Y-0.5Zr (wt.%) Alloy

The compressive creep behaviour of extruded Mg-10Gd-3Y-0.5Zr (wt.%, GW103) alloy was investigated at temperatures from 200 °C to 300 °C and under stresses from 50 MPa to 120 MPa. The peak-aged alloy exhibited minimum creep rates ranging from 1.90×10-9 s-1 to 6.14×10-6 s-1 and the aging treatment exerted a positive effect on its creep performance. The measured stress exponent of the peak-aged extruded alloy varied from 2.0 to 3.4, while the activation energy was 83.4 kJ/mol and 184.3 kJ/mol at low temperature and high temperature regime, respectively. This suggested grain boundary sliding was the primary creep mechanism at low temperature but dislocation-controlled creep dominated at high temperature. XRD patterns and SEM micrographs indicated precipitates increased with creep time, and further dynamic recrystallization occurred. Precipitate free zones (PFZs) were clearly observed near the grain boundaries parallel to the loading direction.

Effect of Cooling Rate on the Microstructure and Mechanical Properties of Cu/Al Bimetal Fabricated by Compound Casting

Cu/Al bimetal was fabricated successfully by compound casting at different cooling rates in the range from 0.8 K/s to 1.83 K/s (0.8xa0°C/s to 1.83xa0°C/s). Interfacial cooling curve, microstructure evolution, bonding strength, and interfacial formation mechanism of Cu/Al bimetal were investigated simultaneously. Results show that the transition zone consists of the intermetallic compounds (IMCs) and the remelting zone. The IMCs are identified as Al4Cu9, AlCu, and Al2Cu, types of which are not affected by the cooling rate. The cooling rate primarily influences the thickness of IMCs, the microstructure of the remelting zone, and the morphology of the remelting zone/Al interface. The Cu/Al bimetal produced at the cooling rate of 1.02xa0K/s (1.02xa0°C/s) has relatively higher bonding strength than those at the cooling rates of 0.8 K/s and 1.83 K/s (0.8xa0°C/s and 1.83xa0°C/s). Shear fracture primarily occurs on the hard brittle IMCs rather than on the remelting zone. Based on the cooling curve and diffusion analysis, a new bonding mechanism of Cu/Al bimetal is proposed.

Tribological Behavior of Carbon Nanotube-Reinforced AZ91D Composites Processed by Cyclic Extrusion and Compression

Reciprocating wear tests were conducted to assess the wear resistance of CNT-reinforced AZ91D composites prepared by cyclic extrusion and compression (CEC). Effects of CEC, CNTs, and wear parameters on the tribological behavior of the composites were discussed. Results show that the matrix grain of the 0.5xa0wt% CNTs/AZ91D composites is largely refined from ~u2009112xa0µm to 126.6xa0nm after eight passes of CEC. Accordingly, the hardness of the composites is increased by more than 82.0%. The wear rate of the CNTs/AZ91D composites decreases with the implement of CEC and the addition of CNTs. The lubrication effect of CNTs diminishes after CEC. Besides the reinforcing effect, the incorporated CNTs help to liberate the friction heat of the CNTs/AZ91D composites and reduce the welding of the wear debris due to their extraordinary thermal conductivity.

Bonding of Aluminum Alloys in Compound Casting

The influence of the coating materials, coating thickness, and casting process on the interfacial microstructure and mechanical properties of the overcast A6061 bars with aluminum A356 and A6061 alloys was studied by OM, SEM/EDS, and mechanical testing. Results indicate that Ni coating has better thermal stability than Cu coating that heavily reacts with liquid Al alloy and forms a reaction zone around 130–150xa0μm during gravity casting. In the gravity casting, coarse and cracked Al3Ni phase distributes along the interfacial region and degrades the mechanical properties of the overcast joints. In squeeze casting, however, fine and dispersed Ni-rich strengthening phases form uniformly in the interfacial zone and improve the metallurgical bonding of the joints. The heat transition and application of pressure during solidification are two key factors in determining the integrity and mechanical properties of the overcast joints.

Effects of Melt-to-Solid Volume Ratio and Pouring Temperature on Microstructures and Mechanical Properties of Cu/Al Bimetals in Compound Casting Process

Cu/Al bimetallic composites were fabricated by compound casting where aluminum melts were cast on Ni-coated Cu substrates. Effects of the melt-to-solid volume ratio (VR) and the pouring temperature on interfacial microstructures and mechanical properties of Cu/Al bimetals were investigated systematically. Results show that a continuous and compact interface can be formed when the pouring temperature exceeds 953 K (680xa0°C). When the VR or pouring temperature increases, the solidification time of α(Al) increases, resulting in an increase in the dissolution content of Cu substrate. The transition zone consists of α(Al)u2009+u2009Al2Cu eutectic layer and intermetallic compounds (IMCs) layer, and the IMCs are identified as Al2Cu, AlCu, and Al4Cu9. Ni coating participates in the formation of AlCuNi phase between the Al2Cu layer and AlCu layer when the pouring temperature is 973xa0K (700xa0°C). The presence/absence of Ni-containing phase has a close relationship with the dissolution of the Cu substrate. Shear-strength tests show that shear fracture mainly occurs at the hard brittle IMCs layer, and the highest shear strength of 36.01 MPa is obtained for the samples fabricated at 993 Ku2009+u2009VR 49.24 (720xa0°Cu2009+u2009VR 49.24).