Hot Deformation Behavior Of TiBw/TC4Composites With Network Microstructure

In this paper,5vol.%TiBw/TC4composites with a network microstructure werefabricated by traditional powder metallurgy method through low energy ball milling andhot-pressing sintering process based on large and spherical TC4powders and fine TiB2powders. Then hot deformation behavior of the composites was investigated and plasticdeformation ability was characterized by means of high temperature compression testsand hot tensile tests on Gleeble-1500D simulator and Instron-1186, respectively.Meanwhile, microstructural evolution of the composites after hot deformation wasanalyzed by optical microscope and scanning eletron microscope. The effects of hotdeformation parameters on microstructures and high temperature fracture characteristicswere studied.Hot compression testes were carried out in the temperature range of900¹000℃and in the strain rate range of0.001¹0s^-1with a reduction of60%in height.Thestress-strain curves of the composites displayed peak flow stresses, after flow softeningthe curves attained steady-state flow period gradually. Besides, flow stresses decreasedwith elevated temperatures and declining strain rates. Oscillation and discontinuousyielding could be observed from the curves at higher strain rates. The flow curves ofTC4alloy were similar to the ones of the composites while the flow stresses were muchlower.Constitutive equations under peak flow stresses and steady-state flow stresses wereconstructed on the basis of hot compression data, respectively. Deformation activationenergies were also calculated with the value of822.3kJ/mol in+β phase region and thevalue of209.4kJ/mol in β phase region under peak flow stresses. Processing maps werealso established by using the dynamic material model and different regions inprocessing maps were analyzed. The optimal parameters for hot working of thecomposites were900⁹80℃and0.05⁰.5s^-1. The flow instability region ranged from900℃to1000℃at strain rates higher than1s^-1with a true strain higher than0.5.From the results of hot tensile tests, all the specimens of the composites exhibitedsuperplasticity in900¹000℃with a nominal strain rate of0.001s^-1. At940℃with0.001s^-1, the maximum elongation214%was obtained.From the microstructures observation, different deformation regions corrspondedto various microstructures. In deformation center region with maximum plastic strain,the spherical TC4particles were torn and crushed dramatically, TiB whiskers broke upand directionally oriented. At the edges of the specimens with minimum strain,spherical TC4particles deformed a little, TiB needles hardly broke up. The range ofdeformation center region expanded with the increased deformation reduction. Moreover, deformation promoted the transformation from phase to β phase. Adiabaticshear bands can be seen from the deformed specimens of TC4alloy. However,instability characteristics such as cracking of TiB whiskers, interfaces cracking betweenthe reinforcements and matrix, pores occurring at triple junctions of the networkmicrostructure can be observed from the deformed specimens of the composites.The fractographs of the hot tensile specimens indicated that the fractures of thecomposites were ductility fractures with lots of ductile dimples and tearing edges.