IN718镍基合金激光喷丸微观组织特性及其高温稳定性

Micro-structural Characteristics and High-temperature Performance of IN718 Nickel-based Alloy Treated by Laser Peening

摘要:

为了研究激光喷丸对IN718镍基合金微观组织特性的影响及其在高温下的稳定性,首先在常温下对标准拉伸试样关键区域进行激光喷丸处理,随后将喷丸样置于700 ℃下保温300 min,作为对比,分别测试了未喷丸样、喷丸样和喷丸高温保持样的表层残余应力和纳米硬度,并采用光学电镜和透射电镜对其微观组织特征进行观测。结果表明,激光喷丸后试样的表层出现了高幅值的残余压应力,最高值出现在喷丸中心,约为-706 MPa；激光喷丸后试样的表层硬度显著提高,平均纳米硬度提高了约67.4%；高温保持后试样的残余应力和纳米硬度均有不同程度的降低,但与未喷丸试样相比,强化增益效果仍然存在；显微组织观察结果显示,激光喷丸后,试样深度方向上晶粒形态均出现了显著的“分层”现象,塑性变形层晶粒尺寸细化明显,其影响深度可达80 μm；保温后喷丸样晶界处可见析出条状δ相；TEM图像进一步表明,激光喷丸诱导的晶粒细化是条状孪晶和位错组织混合交织的结果；保温后喷丸样晶界处发现了沿同一方向向晶内延伸的位错阵列,这很好地验证了位错在高温下的活性,且是激光喷丸抑制晶粒在高温下快速增长的直接证据。

Abstract:

The aim of this study was to investigate the effects of laser peening (LP) on the micro-structural characteristics and high-temperature performance of IN718 nickel-based alloy. Firstly, LP process was performed on the standard tensile specimens at ambient temperature, then the treated specimens were exposed at 700 ℃ for 300 min. As comparison, we detected the surface residual stress and nano hardness on the untreated, LPed and heat-exposure specimens respectively. The corresponding micro-structures were also observed. The results indicate that high amplitude compressive residual stress distributed in the LP treated surface, the maximum value was -706 MPa, appearing in the central region of the LP area. LP process significantly increased nano hardness of treated surface, the average nano hardness was enhanced by 67.4%. Both the residual stress and nano hardness decreased after heat exposure, however, the strengthening effect still existed while compared with untreated specimens. An obvious “layer-seperating” phenomenon on the grain morphology in the depth direction was observed after laser peening. The grain size in the plastic deforming layer refined greatly, the affecting depth of which reached a value of 80 μm. The precipitated δ phase was found in the grain boundaries for the heat-exposure samples. A further observation verified that the interaction of mechanical twins and dislocation resulted in the grain refinement. After heat exposure, the single- directional-distributed dislocation array in the grain well verified the dislocation activities, which indicates that LP can inhibit the grain growing of IN718 alloy at high temperature.